Yamaha P2050 Owner's manual

P2050
OPERATING MANUAL
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ABOUT THIS MANUAL
SCOPE
The P2050 is a versatile, high quality audio power
amplifier. It may be used as the sole amplifier in small
sound systems, or as a complement to larger power
amplifiers in bi- and tri-amplified systems. Like any power
amplifier, the P2050’s performance depends on system
design and installation, in addition to its own capabilities.
Thus, the P2050 Operator's Manual contains more than
simple hook-up and operating instructions; it also covers
system design parameters, installation techniques and
applications.
Additionally, this manual reviews a few of the basic
mathematic tools used in system design, from dB to
Ohm's law.
ORGANIZATION
We recommend that you read the entire Operator's
Manual. However, if you are using the P2050 in an
existing system, and you are familiar with professional
power amplifiers, the BRIEF OPERATING INSTRUC-
TIONS (Page ONE 1) contain all the information
necessary for basic connections and operation.
The SPECIFICATIONS in sections THREE and
FOUR, are highly detailed, including oscilloscope photos,
and discussions of the P2050's excellent performance
specifications. The last part of Section FOUR is a
discussion of the advantages of genuine professional
equipment, like the P2050, compared to hi-fi or pro-
fessional consumer equipment.
The INSTALLATION AND DETAILED OPERATION
section, which begins on Page SIX 1, includes more
complete instructions, special considerations for using
the P2050 “on the road,” as wel! as in permanent
commercial and studio installations. This section also
covers grounding and shielding concepts, cabling
considerations and several other topics.
The APPLICATIONS section, which begins on Page
SEVEN 1, discusses the use of the P2050 in several
typical setups, and includes wiring diagrams. This
section also covers other devices that are normally
associated with a power amplifier, from graphic
equalizers to compressor/limiters.
The APPENDIX, on Page EIGHT 1, discusses
definitions of a number of the terms used in the manual,
and reviews some of the basic mathematic tools used in
system design, such as the dB, Ohm's law, voltage
division, and power formulas.
INDEX
THE P2050 — BRIEF OPERATING INSTRUCTIONS
INTRODUCTION
GENERAL SPECIFICATIONS
PERFORMANCE GRAPHS & A DISCUSSION OF SPECIFICATIONS
THE DISTINCTION BETWEEN PROFESSIONAL AND HI-FI EQUIPMENT
IMPEDANCE . .
OPERATING LEVELS
DYNAMIC RANGE . .
GAIN OVERLAP AND HEADROOM
INPUT SENSITIVITY RATINGS . .
PROFESSIONAL EQUIPMENT ADVANTAGES
INSTALLATION AND DETAILED OPERATION
PHYSICAL MOUNTING . .
CABLING AND IMPEDANCE MATCHING . .
ACTIONS OF THE P-2050 PROTECTION CIRCUITS
GROUNDING AND SHIELDING . .
AC: POWER, FUSES, ACCESSORY OUTLETS, WIRING, SAFETY
MONO OPERATION . . . . . .
APPLICATIONS
BIAMPLIFICATION AND TRIAMPLIFICATION .
ECHO, REVERB AND DELAY . .
COMPRESSION AND LIMITING . .
EQUALIZATION, HIGH AND LOW PASS FILTERS
SPEAKER PROTECTION . Coe ee
SPECIFIC APPLICATIONS
APPENDIX
DEFINITION OF TERMS: dB, dBV, dBv, dBm and dB SPL
SPECIAL USE OF dB (VOLTS) | IN THIS MANUAL .
OHM'S LAW . . . .
POWER . .
IMPEDANCE . .
SERIES AND PARALLEL IMPEDANCE CONNECTIONS
VOLTAGE AND CURRENT DIVISION.
BALANCED, UNBALANCED, AND FLOATING CIRCUITS
TRANSFORMERS . 2. о.
SECTION SEVEN
SECTION ONE
SECTION TWO
SECTION THREE
SECTION FOUR
SECTION FIVE
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SECTION SIX
1
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Fig. 1 — P2050 Front Panel
POWER
PUSH ON/PUSH OFF
YAMAHA
PROFESSIONAL SERIES
NATURAL SOUND POWER AMPLIFIER
MODEL P2050
SECTION ON
BRIEF OPERATING INSTRUCTIONS
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A. Power Indicator
Glows when the Power Switch is
B. Power Switch
Controls AC power to the P2050.
“on.”
Fig. 2 — P2050 Rear Panel (U.S. Model)
CHANNEL 8 so — a
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Input Attenuators
Calibrated, stepped input attenuators lower input
signal levels ahead of any amplification stages.
NOTE:
The P2050 is made to be mounted in a standard 19”
wide electronic equipment rack. It takes up 3-1/2” of
vertical space, and extends 11-1/:
” behind its front panel.
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A. Input Connectors
The XLR input connector on each channel is un-
balanced and is wired in parallel with the two adjacent
phone jacks (tip/sleeve type).
B. Stereo/Mono Mode Switch
When using the P2050 as a stereo power amplifier,
set this switch to STEREO position. If higher power
output is desired and the program material is monaural,
set the switch to MONO position and feed only the
Channel A input. Then, by connecting the speaker cable
across the red output terminals of the two channels
(rather than the red and black terminals of one channel),
the available power output to the speaker is more than
doubled. (Use only CH A input attenuator.)
C. High Pass Filter Switch
With this switch set to FLAT, the P2050's output
ty pically is down less than 1dB at 20Hz and less than
3dB at 10Hz, relative to the midband output. A 12dB/
octave high pass filter (low cut filter) may be switched
into the signal path of both inputs, with the 3dB down
point set at 20Hz or 200Hz (actually 190Hz).
D. Input Polarity Switch
Determines the polarity of the two XLR input
connectors (Pin 2 or Pin 3 “hot”); does not affect the
two phone jacks. See diagram on P2050 rear panel.
NOTES:
1. Input impedance is 25k-ohms minimum, OdB
(0.775V RMS) produces 45 watts output into 8 ohms
(19V RMS).
2. Input channels may be paralleled by connecting
them together with phone plug to phone plug cables, as
shown on Page S1X 7; not required for bridged mono
operation, as STEREO/MONO MODE switch serves
this purpose.
C D E F G
3. Input transformers for matching or isolation, should
be located several inches from the P2050's power trans-
former for maximum hum rejection.
E. Fuse
3 amp, 250 volt, type AGC (3AG)*; fuse should
always be replaced with same size and type. If the fuse
blows consistently, the amplifier should be checked by a
qualified Yamaha service technician.
F. Output Connectors
Standard 5-way binding posts {3/4" spacing) accept
banana plugs or direct-wired connections. {U.S., Canadian
and Australian models)
Conventional binding posts accept direct-wired
connections. (other territories’ models)
NOTES:
1. Continuous average sine-wave power output into
8 ohms is 45 watts per channel in STEREQ mode, 95
watts total in MONO mode; power output rises at lower
impedances.
2. Protection circuitry lowers power output when load
impedance falls below 2.5 ohms in STEREO mode, or
below 5 ohms in MONO mode.
G. AC Power Cord*
The P2050 requires a 120V AC50 or 60Hz line.
(105V min., 135V max.; 1.8 amps max. at 120 volts)
NOTE:
For European model, an internal voltage selector
(220V/240V switchable) is provided near the front panel.
In this case 220V is factory-preset. If you want to change
into 240V line, ask your Yamaha dealer.
* For U.S. and Canadian models only. For other territories’
models, see the rear panel of the P2050.
S
INTRODUCTION
The P2050 complements Yamaha's higher powered,
field proven power amplifiers. Its highly accurate sound,
sophisticated design, professional features, and uncom-
promising performance make the P2050 an ideal
companion to the P2100, P-2200 or P2201 in bi- or
tri-amplified sound systems. The P2050 is also an
excellent stand-alone power amplifier for smaller mono
and stereo sound systems, as well as headphone
distribution systems.
CALIBRATED INPUT ATTENUATORS
The P2050 has log-linear INPUT ATTENUATORS.
The input attenuators are marked in 22 calibrated dB
steps, detented for extra accuracy. The attenuators
provide a smooth, noisefree transition from the highest
to the lowest audio level. dB-calibrated input attenuators
have numerous advantages: on the road, they allow
predictable and repeatable setups; in commercial sound
applications, they allow easy, accurate input sensitivity
adjustments; in studios or discos, they let operators
simultaneously adjust the level of two channels, or two
programs on separate amplifiers, with precise tracking.
INPUT AND OUTPUT CONNECTIONS
INPUT CONNECTORS for each channel include one
“female” XLR connector (unbalanced) plus two phone
jacks, all wired in parallel. This provides the flexibility
necessary for convenient bridging to another amplifier,
as well as for adapter-free connection to almost any
mixer. A POLARITY switch allows either pin 2 or pin 3
of the XLR to be chosen as the “hot” lead, satisfying
DIN/JIS or USA standards. Outputs are standard five
way binding posts, usable with reliable “banana” plugs
or direct wired connections on U.S., Canadian and
Australian models only. On other territories’ models
output binding posts accept direct wired connections.
MONAURAL OPERATION
The P2050 may be converted to a higher power
monaural amplifier with the flick of a handy rear-panel
MODE switch. Placing the unit in MONO mode estab-
lishes a transformerless balanced output, the speaker
load “bridged”” across the “hot” terminals of both
channels. In this mode the P2050 delivers twice the
voltage to the speaker load. Since the minimum load
impedance is also doubled when switched to MONO
mode, the power available to a single output load 1s
about twice that available in STEREO mode.
PERFORMANCE
The P2050's performance is impressive, audibly and
in laboratory measurements. The unit is conservatively
rated at 45 watts into 8 ohms (for each channel). Even at
full output power, this amplifier sounds clean and open.
Because the P2050 has the ability to sustain full power
output, it is well suited to use in live rock or disco sound
systems, where an amplifier can really “cook” all night
long. While prolonged full power operation does not
qualify an amplifier for professional acclaim, doing so
with truly low noise and distortion is another story.
The P2050 is very nearly the ideal “straight wire with
SECTION "WO
gain.” It measures less than 0.05% THD at full rated
power (typically less than 0.02% THD and less than
0.03% IM). This ultra-low distortion is under the residual
noise floor of many analyzers, and is virtually undetect-
able by even the most critical listeners.
A high damping factor of better than 100 at frequen-
cies below 1kHz reduces the tendency for speaker cone
overshoot, giving tighter and better defined speaker
response; a high damping factor (i.e., low output source
impedance) also ensures efficient power transfer to the
speakers. Because the P2050's frequency response ex-
tends well beyond 50kHz, the unit will accurately
reproduce the most complex musical waveforms — even
the tortuous output of today’s synthesizers. However,
high frequency response has not been achieved at the
expense of stability; in fact, the P2050 is rock steady.
Even when connected to highly reactive speaker loads,
there is no tendency to shut down or “take off’” into
spurious oscillation.
MECHANICAL DESCRIPTION
The P2050 is constructed to withstand the high “G”
forces encountered on the road. The unit weight 16
pounds (7.2kg) and has a solid front panel that mounts
in any standard 19-inch rack. Front panel controls are
recessed to avoid damage or accidental setting changes,
and are further protected by a pair of sturdy carrying
handles. Inside and out, the P2050 is extremely reliable.
Still, should service ever be required, the unit is
designed for easy access. Massive side-mounted heat
sinks are designed for efficient cooling, making fans
unnecessary in all but the most severe thermal operating
conditions. Four non-conductive feet ensure proper air
flow when the amplifier is shelf mounted, and avoid
inadvertent ground loops. Multiple protection circuits
make the amplifier nearly abuse proof and eliminate the
need for troublesome DC power supply fuses.
S
SECTION THREE
GENERAL SPECIFICATIONS
Power Outper Per Channel: (Refer to Figure 3)
45 watts continuous average sine wave power into
8 ohms with less than 0.05% THD, over a bandwidth
of 20Hz to 20kHz, both channels driven.
Frequency Response: (Refer to Figure 5)
+0dB, —1dB, 20Hz to 50kHz.
Total Harmonic Distortion: (Refer to Figures 6 —8)
Less than 0.01% @ 25 watts, 8 ohms, TkHz.
Less than 0.02% @ 25 watts, 8 ohms, 20Hz to 20kHz.
Intermodulation Distortion
Less than 0.03% using frequencies of 70Hz and 7kHz,
mixed in a ratio of 4:1, single channel power output
of 25 watts into 8 ohms.
Input Sensitivity
An input of OdB* (0.775V), +0.5dB, produces an
output of 45 watts into 8 ohms, INPUT attenuator
set for maximum level.
Input Impedance: (Refer to Figure 10A)
25kohms, minimum (unbalanced).
Actual Output Impedance: (Refer to Figure 108)
Less than 0.08 ohms from 20Hz to 1kHz; less than
0.18 ohms from 20Hz to 20kHz.
Damping Factor: (@ 8 ohms} (Refer to Figure 9)
Greater than 100 at any frequency from 20Hz to
1kHz; greater than 45 at any frequency from 20Hz to
20kHz.
Hum and Noise
At least 110dB signal-to-noise ratio (I.H.F./A.S.A.
#724.3-1944).
Rise Time
3.8 microseconds, or better (10% — 90% of 1 volt @
1kHz square wave output).
Slew Rate
15 volts per microsecond, or better (at 30 watts into
8 ohms, 200kHz square-wave input).
Channel Separation: {Refer to Figure 11)
At least 82dB at 1kHz, at least 70dB at 20kHz.
Phase Shift: (Refer to Figure 12)
20Hz to 20kHz, £10 degrees.
Offset Voltage
Less than +30mV DC.
Unit Step Function Response
See scope photo (Figure 20, Page FOUR 4), and
discussion (Page FOUR 6).
Thermal Characteristics
Massive black anodized heat sinks are thermally
joined with the chassis, thereby utilizing the entire
amplifier as a heat sink.
Protection Circuits
A self-resetting thermal switch shuts down the AC
power if the power transformer winding temperature
reaches 130 degrees Centigrade. See Page SIX 13 for
power overload circuit discussion.
Turn On/Turn Off Characteristics
There is no turn off transient; the turn on transient
is minimal (see Page SIX 13). Warm up time is less
than 0.2 seconds.
Power Requirements (Refer to Figure 13)
AC, 120 volts nominal, 50-60Hz (105V min., 135V
max.); 1.8 amperes maximum at 120VAC; 216 volt-
amperes maximum at 120 volts; approximately
25 volt-amperes at idle.**
Efficiency: (Refer to Figure 13)
As high as 52%.
Input Connectors
One “female” XLR connector, pin 2 “hot”, pin 3
connected to pin 1 (shield); switchable for pin 3
“hot”. XLR is unbalanced and in parallel with two
tip-sleeve (standard) phone jacks.
Output Connectors
Standard 3/4-inch spacing, ‘‘b-way’’ binding posts.
(U.S., Canadian and Australian models)
Conventional binding posts. (other territories’ models)
Indicator
“Power ON” indicator LED.
Controls
INPUT ATTENUATORS (one per channel)
22-position, log-linear, detented and dB-calibrated;
they attenuate input signal in 2dB steps from 0dB
attenuation to —34dB, then steps of —37dB, —42dB,
—50dB, infinity.
POWER switch (ON/OFF).
INPUT POLARITY switches.
HIGH PASS FILTER switch; FLAT, 20Hz low cut or
200Hz low cut @ 12dB/octave.
MODE switch (MONO/STEREO).
Fuse
AGC (3AG) type, 3-amp fuse for the AC line input. **
Dimensions
Mounts in a standard 19-inch (48 cm) rack. 3-1/2”
high (8.8 cm); maximum depth behind front panel
is 11-1/4” (28.5 cm); maximum depth including front
handles 12-5/8" (32.0 ст).
Weight
16 Pounds (7.2 kg).
Color
Semi-gloss black.
* In these specifications, when dB represents a specific voltage,
OdB is referenced to 0.775V. “dBm” denotes a power level,
whereas ‘dB’ denotes a voltage level which is referenced to the
voltage measured across 600 ohms. OdBm is referenced to 1mW
(0.775V RMS driving a 600-ohm termination). For example,
when 12.3V is fed to a high impedance, the level is designated
“+24dB”. When +24dB (12.3 volts) drives a 600-ohm termina-
tion, the level is designated “+24dBm”. The level in “dB” is
specified, wherever applicable, to avoid confusion when the
input is fed by various low and high impedance sources. See the
APPENDIX beginning on Page EIGHT 1 for a further discussion
о .
** For U.S. and Canadian models only. For other territories”
models, see the rear panel of the P2050.
THREEZ
MONAURAL MODE SPECIFICATIONS
Power Output: (Refer to Figure 14)
~ 95 watts continuous average sine wave power into
16 ohms with less than 0.05% THD, 20Hz to 20kHz.
Frequency Response
+0dB, —1dB, 20Hz to 50kHz.
Total Harmonic Distortion: (Refer to Figures 16 & 17}
Less than 0.01% € 50 watts into 16 ohms at 1kHz.
Intermodulation Distortion
Less than 0.05% using frequencies of 70Hz & 7kHz,
mixed in a ratio of 4:1, at a power output of
50 Watts into 16 ohms.
Input Sensitivity
An input of OdB (0.775 volts), +0.5dB, produces an
output of 95 watts into 16 ohms, INPUT attenuator
set for minimum attenuation (maximum levei).
Input Impedance
25 kohms minimum (unbalanced).
Damping Factor: (@76 ohms)
Greater than 200 at any frequency from 20Hz to
1kHz; greater than 90 at any frequency from 20Hz to
20kHz.
Hum and Noise
At least 110dB signal-to-noise ratio (1.H.F./A.S.A.
4724.3-1944).
Slew Rate
25 volts per microsecond, or better, at 30 Watts into
16 ohms, 200kHz square wave input.
NOTE: All performance specifications are made on
U.S. and Canadian models at an AC line voltage of
120 volts 1%, using a £1% non-reactive load resistor
at an ambient room temperature of 25-degrees Centi-
grade. Also effective for other territories” models.
Specifications subject to change without notice.
ECTION FOUR
A DISCUSSION OF SPECIFICATIONS „Ц
NOTE: In the discussion beginning on Page FOUR 5, soo
references to specific specifications assume normal stereo vo
operation (not mono operation) unless otherwise indicated. =
Normal (Stereo) Graphs
OUTPUT POWER (WI
Lad
OUTPUT POWER (W)
2
14 16
6 B 10
LOAD IMPEDANCE {{))
100 5K 10K 20K
FREQUENCY (Hz) Fig. 4 — Output Power vs Load Impedance (Resistive)
Fig. 3 — Power Bandwidth 892 @ 0.5% T.H.D. (stereo mode single channel driven)
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20 50 100 200 500 1K 2K SK 10K 20K TOOK 1 2 5 10 20 100
FREQUENCY (Hz) QUTPUT POWER (WI)
Fig. 5 — Frequency Response Fig. 6 — T.H.D. vs Output Power at 8 {) Load Impedance
(both channels driven)
T.H.D. (%)
1 10 20 50 100 20 50 100
500 IK 5
OUTPUT POWER (W) FREQUENCY (Hz)
Fig. 7 — T.H.D. vs Output Power at 8{) Load Impedance Fig. 8 — T.H.D. at 8 © Load Impedance
(single channel driven)
2 5
10K 20K SOK 100K 200K
8
X
8
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DAMPING FACTOR
INPUT IMPEDANCE (£2)
=
x
901
10K 20K 50K 100K 200K
20 50 100 10K 20K 50K 100K 200K 20 50 100 500
1K sx 1K si
0 FREQUENCY (Hz) FREQUENCY (Hz)
Fig. 9 — Damping Factor vs Frequency at 8%2 Load Fig. 10 — Impedance vs Frequency at 82 Load Impedance
Impedance
OUTPUT IMPEDANCE (£2)
dB RELATIVE TO 25 WATTS QUTPUT
FOURZ2
DRIVEN OUTPUT {25 WATTS)
EFFICIENCY (%)
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POWER CONSUMPTION (WI
UNDRIVEN OUTPUT
1K 2K 10K
500
FREQUENCY (Hz)
Fig. 11 — Crosstalk (Input Attenuator OdB at 80
Load Impedance)
20 SK
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PRIMARY CURRENT (A)
8 AC INPUT (VA)
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20 30 40 50 60 70
OUTPUT POWER (W) PER CHANNEL
80
Fig. 13 — Output Power/Power Consumption
(dotted line = 412 / solid line = 82)
RELATIVE PHASE IN DEGREES (INPUT VS OUTPUT)
500 1K 2K
FREQUENCY (Hz)
TOK
at BO
Fig. 12 — Phase Response RL
Mono Mode Graphs
OUTPUT POWER (Wi
30
28
2 4 6 B 10 12 14 16 18 20 22
LOAD IMPEDANCE (£2)
24 26
Fig. 15 — Output Power vs Load Impedance (resistive)
OUTPUT POWER (W)
500 = 50K 200K
FREQUENCY (Hz)
5K 10K
Fig. 14 — Power Bandwidth Mono 16 {2 at 0.05% T.H.D.
=
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T.H.D. (%)
© 20kHz.
3 Зону”
2 10
20
OUTPUT POWER {W)
Fig. 16 — T.H.D. vs Output Power (Mono Mode) at 16 {2 |
load Impedance !
T.H.D. (%)
50K
100K
10K 20K 200K
IK 5K
FREQUENCY (Hz)
Fig. 17 — Total Harmonic Distortion (Ry at 16 (2)
FOURS
The following are actual oscilloscope photographs
made by an independent testing laboratory. The close
vertical alignment of input and output traces in Fig. 18C
through 19A depicts very low phase shift, so the amplifier
will not alter musical wave shapes.
QUTPUT
20V/div
OUTPUT
20V/div
52 Ez
a 23
z2 z>
10u S/div 0.2mS/div
Fig. 18A — 20,000Hz Square Wave Response Fig. 18B — 1,000Hz Square Wave Response
The extremely fast and symmetrical rise and Near perfect response is evident in the
fall times of the amplifier are evident, demon- duplication of the input waveform by the out-
strating the ability to reproduce accurately put waveform. There are no ‘squiggles’ or
musical waveforms and harmonics well beyond spikes, meaning there is no ringing or overshoot.
the range of human hearing.
OUTPUT
20V /div
OUTPUT
20V/div
INPUT
1V/div
INPUT
1 V/div
20mS/div 0.2mS/div
Fig. 18C — 10Hz Square Wave Response Fig. 18D — Square Wave Response into a
The output waveform displays very respect- Highly Inductive Load (at 1kHz)
able tow frequency response. The slight “'tilt"” The P2050's ability to maintain a sharply
shows a DC gain of unity, which prevents defined square wave output into a reactive load
damage to speakers in the event any DC offset demonstrates stability under the worst con-
is fed to the amplifier input. ditions. There is still a complete lack of
unwanted ringing, as well as virtually unmea-
surable phase shift.
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_ Fig. 19A — 1,000Hz Sine Wave Shown Fig. 19B — 20,000Hz Sine Wave Shown
ah Highly Magnified Noise and Distortion with Highly Magnified Noise and Distortion
omponents Components
‚| At 30 watts output (8 ohms) the P2050's Even at 20kHz, the P2050 yields clean,
distortion is so low (0.0034%) that it is almost symmetrical low distortion amplification
buried in the noise. The sine wave is clean and (0.0106% THD @ 30 watts into 8 ohms).
symmetrical.
OUTPUT
20V/div
INPUT
not to scale
Fig. 20 — Unit Step Function Response
POWER OUTPUT
Types of Power Ratings
Peak power refers to the maximum undistorted power
output of an amplifier. Most amplifiers cannot sustain
their peak power ratings for long periods of time with-
out external cooling fans. Because there are many
different methods of rating an amplifiers peak power,
it is hard to objectively compare the peak power ratings
of two amplifiers. The peak power rating is primarily
useful for determining an amplifier’s ability to reproduce
the peaks and transients in a musical program, peaks
which may be 20dB or more above the average power
level. The ability to accurately reproduce these high
power peaks in a musical program is one of the most
important advantages of the P2050 as compared to a
smaller power amplifier.
“RMS” power is actually a misnomer for average
power. Average power is usually measured with a sine
wave input signal, and is equal to the amplifier's rms out-
put voltage squared and then divided by the load imped-
ance (see Appendix). Because rms voltage is used in the
formula, the resulting power rating is commonly called
“rms power.” To be more accurate, the P2050 is rated
in watts of “continuous average sine wave power,” which
Is calculated from the rms voltage across a known load.
Since the P2050 is a professional power amplifier,
not sold for home hi-fi use, it is not required to meet
the power rating standard set by the FTC (Federal Trade
Commission), a standard meant for consumer power
amplifiers. However, the P2050 is measured under
severe conditions which simulate the most demanding
professional usage. Thus, the P2050 would easily meet
the FTC ratings for consumer amplifiers. In addition,
the P2050 user has the benefits of professional features
and reliability.
Power Output versus Load
Within its maximum limits, the P2050 acts like a
perfect voltage source (see Appendix). That is, its power
output rises with decreasing load impedance. When the
load impedance drops below 2.5 ohms, the P2050"s
protection circuits begin to limit the power, resulting
in the curve shown in Figure 4 (normal operation) and
Figure 15 (mono operation).
DISTORTION (Refer to Figures 6, 7, 8, 16 & 17)
The P2050 is designed to have the lowest possible
distortion. There are many different forms of distortion,
however, and comprehensive distortion ratings offer a
means to compare the performance of different
amplifiers.
Harmonic distortion is characterized by the appear-
ance at the amplifier output of harmonics of the input
waveform which were not present in the original input
waveform. Total Harmonic Distortion, or T.H.D., is the
sum total of all of these unwanted harmonics expressed
as a percentage of the total signal.
Harmonic distortion in an amplifier can be created in
any of several ways. The T.H.D. rating of a power
amplifier refers to creation of unwanted harmonics by
the amplifier during “linear” operation (normal input
and output levels, impedances, etc.). Harmonic dis-
tortion is also created by “clipping,” a form of ‘’non-
linear” operation which occurs when the signal level at
an amplifier's input is high enough to drive the amplifier
beyond its rated maximum output. The amplifier, in
attempting to reproduce this signal, reaches its maximum
output voltage swing before it reproduces the top of the
signal waveforms. Since the output voltage cannot rise
FOURS
any farther, the tops of the waveform are ‘squared off,”
or clipped, as that shown in Figure 58, Page SEVEN 1.
Clipping distortion adds odd upper harmonics {3rd,
bth, etc.) to the original signal. Input clipping, where
the input stage of the amplifier is overdriven by a high
level input signal, would be similar. The P2050 has wide
input headroom and extremely high peak power output
capabilities (headroom) to help avoid the problems of
clipping distortion.
Another form of harmonic distortion that occurs in
some power amplifiers is called crossover distortion. *
Crossover distortion is caused by improper bias in the
output transistors of an amplifier. The amount of cross-
over distortion stays the same whether the signal is
large or small, so the percentage of distortion goes down
as the signal level goes up. Thus, an amplifier with cross-
over distortion may sound relatively distortion free at
high output levels, yet sound “fuzzy” at low levels. Some
amplifiers have internal adjustments which enable a
service technician to control the amount of output
transistor bias, and therefore control the distortion. The
P2050 has automatic biasing circuitry which needs no
adjustment and avoids crossover distorticn under all
operating conditions.
Fig. 21A — Large Amplitude Sine Wave with Crossover
(notch) Distortion.
TDR >>>
Fig. 218 — Smaller Amplitude Sine Wave with same amount
(higher %) of Crossover (notch) Distortion.
Intermodulation distortion, or I.M., is characterized
by the appearance in the output waveform of frequencies
that are equal to sums and differences of integral
multiples of two or more of the frequencies present in
the input signal, The difference between intermodula-
tion distortion and harmonic distortion is that two or
more different frequencies must be present to produce
intermodulation distortion, and that intermodulation
distortion products may not be harmonically related to
the original frequencies. (Only one frequency is needed
for harmonic distortion to appear.) Like its harmonic
distortion figure, the intermodulation distortion in the
P2050 is low enough to be virtually inaudible even in
the most critical situations.
Dynamic Frequency Response Shift is related to both
harmonic and intermodulation distortion. When high
level low and high frequency signals are present in the
same waveform, the high frequency signals “ride” on
top of the low frequency waveforms (See Figure 58,
Page SEVEN 1). If amplifier headroom is inadequate,
the low frequencies may “push” the high frequencies
above the output limits of the amplifier, clipping them
off the waveform (Figure 58C). The low frequencies may
*“Crossover,” in this case, refers to the transition between the
positive half and the negative half of the output voltage wave-
form in a “push pull” class B or AB power amplifier; it has
nothing to do with the crossover used to divide frequencies in
a speaker system. See Figure 21.
FOURG
remain unaltered, but the high frequencies are severely
reduced. At the same time, harmonics of the high
frequencies are produced which add to the super high
frequency content of the signal. Thus, along with the
distortion created by the clipping, the frequency
response of the original signal is drastically altered.
This type of distortion can be reduced by increasing
system headroom (using a more powerful amplifier,
and by biamplifying the system as discussed on Page
SEVEN 1.
The extremely low distortion figures of the P2050
indicate its overall quality and mean that its sound will
be precise and natural.
FREQUENCY RESPONSE (Refer to Figure 5)
The frequency response of the P2050 describes the
variation in its output signal level with frequency when
the input signal is held constant. The extremely “flat”
frequency response curve of the P2050 is an indication
of its overall quality and its ability to respond to upper
and lower harmonics of signals all the way to the
extremes of the audio spectrum.
Because good stability is necessary for some types
of commercial sound applications, notably constant-
voltage commercial sound speaker lines, some manu-
facturers restrict frequency response or allow relatively
high distortion in return for increased amplifier
stability. The P2050, on the other hand, has excellent
frequency response and ultra-low distortion, yet is
inherently stable under the most difficult loads, even
in the “mono’ mode.
The frequency response of the P2050 has been
intentionally limited, however, at very low, subsonic
frequencies. Because of this, severe low frequency
transients, or DC offset, appearing at the input to the
P2050 are unlikely to damage a speaker load. Other
amplifiers which are DC coupled throughout may have
a “flatter” subsonic frequency response, but this makes
them capable of amplifying dangerous DC input voltages
or sub-audio transients and delivering them (at high
power) to a speaker.
The P2050's rear-panel High Pass Filter switch permits
the user to restrict the amplifier's low frequency
response, a decided advantage in certain applications.
“Flat” position switches the filter out of the circuit
entirely so the amplifier's output is down less than 1dB
relative to its midband output. In “20Hz” position, the
filter cuts the 20Hz level by 3dB, and below 20Hz the
response rolls off at 12dB/octave; the 10Hz leve! would
be down approximately 15d8 (3dB + 12dB), preventing
inaudible sub-sonics from wasting headroom or damaging
woofers. Since there is seldom any program material at
20Hz, and most speakers do not function well in the
region, the “20Hz” roll off is more likely to improve the
sound than to detract from it. in “200Hz” mode, the
filter's “knee” moves up so that 190Hz is down 3dB.
Since the same 12dB/octave slope applies, 95Hz is down
15dB, 47.5Hz is down 27dB, 24Hz is down 39dB, etc.
This more drastic filter action is suitable when the
amplifier is used to reproduce mid or high frequencies in
a bi- or tri-amplified system because the filter further
protects high frequency drivers from over-excursion. The
“200Hz” high pass filter is also desirable in voice-only
paging applications, where little program exists below
200Hz, and exclusion of the low frequencies permits
higher average volume levels to be used with less wear
and tear on the speakers.
OFFSET VOLTAGE
This specification indicates the amount of DC
voltage naturally present at the output of the amplifier.
A high DC voltage could damage the loudspeaker load;
the +330mV (30 one-thousandths of a volt) level from
the P2050 is insignificant.
UNIT STEP FUNCTION RESPONSE (Refer to Figure 20)
A unit step function is like the leading edge of a
square wave; it goes up, but never comes down. The
response to this input indicates the output of the P2050
for a DC input signal which might come from a faulty
direct coupled preamplifier, or mixer. Note that the
P2050 will not reproduce a DC voltage fed to its input,
thus adding an extra measure of loudspeaker protection.
POWER BANDWIDTH (Refer to Figures 3 & 14)
The power bandwidth of the P2050 is a measure of
its ability to produce high power output over a wide
frequency range. The limits of the power bandwidth
are those points where the P2050 can only produce
1/2 the power that it can produce at 1000Hz. While the
frequency response is measured at relatively low power
output (1 watt), the power bandwidth is measured at
the P2050's full power output (before clipping). The
power bandwidth of the P2050 is quite “flat,” and
extends to 50kHz, well beyond the limits of the audio
spectrum.
The wide power bandwidth of the P2050 means that
it can reproduce high level upper harmonics of a signal
as easily as it can reproduce mid-range fundamentals.
It means that you get full power performance from the
P2050 over the entire audio frequency spectrum. This is
especially important when the amplifier is called upon
to reproduce musical material with high energy over a
wide frequency range, such as rock and roll.
PHASE RESPONSE (Refer to Figure 12)
The phase response of the P2050 is a measure of the
amount of time delay it adds to different frequencies.
An amplifier with perfect phase response would intro-
duce equal time delay at all frequencies reproduced.
The P2050's worst case phase shift of —10 degrees at
20kHz corresponds to a 1.4 microsecond (1.4 millionths
of a second) delay period which is insignificent in even
the most critical audio applications.
A
B
AVAVAVAVAV
Sine Wave
Sine Wave (Fundamental) {Fourth Harmonic of A)
С D
B added to A B shifted 90° {lagging} and
“In Phase” added to A ‘Phase Shifted”
Fig. 22 — Waveform of Amplifier with Poor Phase Response.
An amplifier with poor phase response would change
the shape of a waveform that was made up of a funda-
mental frequency and several harmonics by delaying
each harmonic differently. The effect might be similar
to that shown in Figure 22.
CHANNEL SEPARATION (Refer to Figure 11)
This specification indicates the output from one
channel when a signal is fed to the other channel. The
P2050's channel separation is very good, which means
that even critical stereo programs will be unaffected by
crosstalk between channels.
HUM AND NOISE
Hum or noise from a power amplifier disrupts a
program, and is irritating to a listener. Hum and noise
could be considered a form of distortion. The P2050's
hum and noise are so low that they are completely
inaudible under any normal listening circumstances.
RISE TIME
Rise time is a measurement of the amount of time an
amplifier requires to respond to a square wave at a
specified frequency. The rise time of an amplifier is an
indication of its frequency response. A fast rise time
corresponds to a wide frequency response. The P2050's
rise time specification is measured with a 1000Hz
square wave output signal of one volt peak-to-peak
amplitude. The rise time is the time the amplifier
requires to change from 10% (0.1 volt) to 90%
(0.9 volt) of its output. The first and last 10% are
normally not included in the test because any slight
non-linearities that occur in this portion of the test
signal or the amplifier could lead to measurement error.
Thus, a 10%-90% figure improves measurement accuracy.
SLEW RATE
Slew rate is a measure of the ability of the amplifier
to follow a fast rising waveform at higher frequencies and
higher power outputs than the rise time measurement.
The P2050’s slew rate is measured with a 200kHz
square wave input signal, at 30 watts output power into
8 ohms (stereo operation).
It might seem reasonable to assume that the fastest
slew rate for an audio waveform occurs at 20kHz. How-
ever, this is not the case. When one frequency is super-
imposed upon another, the combined waveform has a
slew rate that is greater than the slew rate of either
signal by itself. The actual value of the slew rate of one
of these waveforms, or of any waveform, depends not
only on the frequency, but on the amplitude of the
waveform as well. Thus, the criteria for a good slew rate
specification, which indicates that an amplifier can
reproduce these combination waveforms, varies with the
maximum power output capability of the amplifier.
The higher the power, the higher the required slew rate.
With a 15 volt/microsecond slew rate, the P2050 can
easily reproduce even the most extreme audio waveforms
at its full power output.
INPUT IMPEDANCE
The input impedance of the P2050 is high enough to
allow it to be used with most semi-pro devices, or to be
used as a “bridging” load for a 600-ohm source. Page
SIX 2 details input impedance and level matching for
the P2050.
INPUT SENSITIVITY
The P2050's input sensitivity indicates the input
drive voltage needed for the P2050 to produce 45 watts
into 8 ohms with input attenuators adjusted to maximum
clockwise rotation for minimum attenuation.
PROTECTION CIRCUITS AND
THERMAL CHARACTERISTICS
See the discussions under INSTALLATION, on
Page SIX 13.
GAIN
Gain 1s the ratio of the P2050's output voltage to its
input voltage. Maximum gain occurs when the input
attenuators are set for minimum attenuation. If the
input and output voltage are specified in dB, the volt-
age gain is equal to the difference of the two dB
numbers. As stated under INPUT SENSITIVITY, an
input voltage of OdB (0.775 volts) produces an output
FOURY
power of 45 watts into an 8-ohm load. 45 watts into
8 ohms implies an output voltage of 19 volts, which
corresponds to +28dB. dB is referenced to 0.775 volts,
as used in this manual. Thus, the voltage gain of the
P2050, with its input attenuators set for minimum
attenuation, is 28dB (+28dB — OdB = 28dB).
NOTE: 45 watts is +46.5dBm.
OUTPUT IMPEDANCE (Refer to Figure 10)
The output impedance of the P2050 is extremely
low. Thus, within its operating limits, the P2050 is a
good approximation of a perfect voltage source and
will deliver increasing power levels into lower impedance
loads in a linear fashion according to Ohm's law. The
appendix discusses Ohm's law and the concept of a
perfect voltage source.
DAMPING FACTOR (Refer to Figure 9)
Damping factor is a term that is derived by dividing
the load impedance (speaker or other load! by the
amplifier's output impedance. Thus, a high damping
factor indicates a low output impedance at a specified
load.
The cone/voice-coil assembly of a loudspeaker gains
inertia during its back and forth movements. This
inertia can cause it to “overshoot,” that is, to continue
movement in one direction, even when the amplifier is
trying to pull it back in the other direction. An amplifier
with a low output impedance can “damp’’ (reduce)
unwanted loudspeaker motions, as explained below.
Volts
No Voltage
a
Time
Fig. 23A — Speaker Cone at Rest
Volts Input
`` Voltage
`
x From
\ Amplifier
1
\
1
|
Time
Fig. 23B — Speaker Cone moved outward by Postive-Going
Voltage from Amplifier.
Volts
“Back EMF’
From
Speaker
Time N_ 7
x
—
Fig. 23C — Voltage from Amplifier has dropped to Zero but
Speaker Cone has moved back PAST its rest position (overshoot)
and is producing a voltage of its own: “Back EMF’
FOURS
During the “overshoot” movement, the voice coil of
the loudspeaker interacts with the loudspeaker’s
magnetic assembly to produce a voltage called “back
E.M.F.” (electromotive force). This action is similar to
the operation of a dynamic microphone. If the
amplifier's output impedance is low, this “back
E.M.F.” voltage is shunted through the amplifier's
output circuits to ground, and back to the voice coil.
Since the path from the voice coil, through the
amplifier's output circuits, and back to the voice coil
is a complete circuit, a current flows in the voice coil.
This current causes the voice coil to act like an electro-
magnet; the electro-magnet interacts with the magnetic
assembly of the loudspeaker, and the unwanted over-
shoot is reduced by a magnetic braking action.
Amplifier Lys
ie
1 (Am Current produced by “Back EMF”
o if
NS
“Back we
Speaker Voice-Coil
"HH /
Fig. 24 — Current produced by “Back EMF" follows path
through Amplifier's Output Impedance to speaker-coil.
If the amplifier's output impedance is low, con-
siderably less than the impedance of the loudspeaker
voice coil, this damping action is limited oniy by the
resistance of the voice coil combined with the resistance
of the speaker lead wires. While the value of a high
damping factor in reducing cone overshoot is disputed,
the P2050's high damping factor is evidence of good
overall engineering design.
THE DISTINCTION BETWEEN
PROFESSIONAL AND HI-FI
EQUIPMENT
In most applications, a variety of auxiliary equipment
will be connected to the P2050, including: mixers, tape
machines, compressors, graphic equalizers, echo, time
delay, and reverb units, and just about any other audio
electronics imaginable. Regardless of the function of
auxiliary equipment, it will undoubtedly fall into one of
two general categories, professional type or hi-fi type.
The following criteria place most “semi-pro” equipment
in the hi-fi classification.
The distinction between professional and hi-fi
equipment is important primarily because it affects
the way it will be used with the P2050. Brand name,
size, panel colors, durability and subtleties in function
are not the significant differences. What matters is that
professional equipment and hi-fi equipment usually
operate at different input and output levels, and require
different source and load impedances to function pro-
perly. The P2050 is designed to function well with
other professional equipment, although it has high
enough input impedance and sensitivity to yield excel-
lent results with hi-fi type equipment if a few pre-
cautions are observed. These precautions are outlined
in the Installation section of the manual. The following
paragraphs explain how the specific requirements differ
for professional and hi-fi (or semi-pro) equipment.
IMPEDANCE
The inputs of a piece of professional audio equip-
ment are usually designed to be driven from a low
impedance source, nominally 150 to 600 ohms, and its
outputs will drive low impedance loads (600 ohm or
higher). Power amplifier outputs are not considered in
this discussion. Professional input and output circuits
may be unbalanced, but they are often transformer
isolated (balanced or floating), and use dual conductor
shielded cables, with 3-pin XLR type connectors or
Tip/Ring/Sleeve phone plugs.
The P2050's inputs are unbalanced due to cost and
adaptability factors. Internally balancing the P2050's
inputs would require two matched input transformers
with heavy shielding to avoid hum pickup from the
P2050’s power transformer. Induced hum in low level
circuits, especially in low level transformers, can be a
problem with any power amplifier or other high current
device, such as a DC power supply. High quality external
transformers with less shielding can achieve the same
results with a substantial cost savings. In addition, the
user can choose the optimum impedance ratio for a
given situation, increasing the P2050's adaptability.
Either the “matching transformer box'’ or ‘step up
transformer box" described on Pages SIX 3, and SIX 4
are suitable, so long as they are kept several inches away
from the P2050.
Hi-fi and creative audio equipment generally is de-
signed to be driven from a 5,000-ohm or lower imped-
ance source, and its output will drive 10,000-ohm or
higher impedance loads. Hi-fi input and output circuits
are usually unbalanced, and use single conductor shielded
cables with 2-conductor connectors, either standard
phone plugs or phono plugs, also calied RCA or pin plugs.
SECTION FIVE?
Occasionally, the inputs of a piece of hi-fi or semi-pro
equipment are professional XLR connectors which have
been converted to a 2-wire, unbalanced circuit by
internally connecting either pin 2 or pin 3 to pin 1.
The nature of unbalanced, balanced, and floating
circuitry is discussed further in the Appendix of this
manual. For the purpose of this discussion, the most
significant point is that an unbalanced circuit is some-
what more susceptible to hum and noise, especially if
there is any irregularity in the grounding system.
NOTE: THERE IS NO CORRELATION BETWEEN
“BALANCED” OR “FLOATING” AND CIRCUIT
IMPEDANCE.
Low impedance and high impedance are relative
terms. À 150- to 250-ohm microphone is considered low
impedance, whereas a 10,000-ohm mic is considered
high impedance. A 600-ohm line is considered low imped-
ance, whereas 10,000-ohm, 50,000-ohm or 250,000-ohm
lines are all considered high impedance. Sometimes,
mics and lines with an impedance of 600 ohms to about
2000 ohms are considered “medium” impedance.
NOTE: THE IMPEDANCE OF A CIRCUIT SAYS
NOTHING ABOUT ITS LEVEL.
While the exact transition between low and high
impedance is not clearly defined, the distinction is still
important, primarily because the output impedance of a
source determines the length of cable that can be con-
nected between it and a load before a serious loss of high
frequencies occurs. The losses occur because all cables
have some capacitance between their conductors,
especially shielded cables. Some guitar coil cords may
measure as high as 1000 picofarads total capacitance!
The source impedance, such as a high impedance mixer
output, and the capacitance of a cable form a type of
low-pass filter, a filter that attenuates high frequencies.
This filtering effect can be reduced by using low
capacitance cable, by shortening the length of the cable,
by using a low impedance source or by some combina-
tion of these methods.
Shielded Cable
HL
ENEE
Capacitance in
a Shielded Cable
Mixer or Other
Source Device
Zo Source Device's Output Impedance
1 °
Cable Capacitance
(lumped)
| -0
Fig. 25 — The Source's Output Impedance and the Cable
Capacitance act as an “RC Lowpass” Filter which Attenuates
High Frequencies.
FIVE2
Cables from high impedance sources (5000 ohms
and up), should not be any longer than 25°, even if low
capacitance cable is used; shorten the cables if the
impedance is higher. For low impedance sources of
600 ohms or less, cable lengths to 100’ are usable. For
very low impedance sources of 50 ohms or less, cable
lengths of up to 1000’ are possible with minimal loss.
However, the frequency response of the source, the
desired frequency response of the system, and the
amount of capacitance and resistance in the cable all
play a role in any potential high frequency losses. Thus,
these values are meant as guide lines, and should not be
considered fixed rules.
For short runs and in smaller systems with fewer
components, the performance of an unbalanced circuit
may be adequate. In a long cable run, a balanced or
floating circuit tends to reject hum and noise pickup
better than an unbalanced circuit, and in complex
systems with several components separated by some
distance and running on different AC outlets, balanced
or floating circuits make proper grounding much easier.
In any given situation, the decision to use a hi-fi
device or a professional one should be based on the
specifications of the inputs and outputs of that device
and on the requirements of the application. Here, reli-
ability and serviceability can be important factors.
OPERATING LEVELS
Nominal professional line level is usually +4dBm or
+8dBm; that is, the average program level is approxi-
mately 1.23V rms (+4dBm), or 1.95V rms (+8dBm)
terminated by a 600-ohm line. The peak level may ex-
tend to about +24dBm (12.3V rms). The line input
(high level input) of professional audio equipment is
designed to accept levels on this order of magnitude
without overdrive (clipping); most professional ampli-
fiers can be driven to full output by nominal +4dBm
input (source) levels, although a few units require
+8dBm (1.95V rms) at their input to yield full output.
See the discussion of “Gain Overlap’ on Page FIVE 4.
Hi-fi type equipment usually operates at considerably
lower line levels than professional equipment, often at
—16dB (0.123 volts) to —10dB (0.245 volts) nominal
level. Notice we use the expression “dB,” not “dBm.”
This is because ‘’dBm’’ denotes a power /eve/ relative to
1mW, or 0.775V rms across a 600-ohm impedance,
whereas “dB” denotes a voltage level (as defined in this
manual) relative to 0.775 rms. This is a subtle distinction,
and is explained in greater detail in the Appendix on
Page EIGHT 1, and on Page THREE 1 of the
specifications.
The nominal —16dB (0.123 volts) level of hi-fi equip-
ment is equal to 123mV rms (123 one-thousandths of a
volt) across a 10,000-ohm or higher impedance line.
Peak program levels may reach or slightly exceed +4dB
(1.23V rms across a high impedance line). Note that a
hi-fi unit capable of +4dB (1.23 volts) maximum output
into a high impedance, does not possess adequate drive
for 600-ohm circuits with nominal +4dBm level require-
ments. Thus, hi-fi equipment is usually incapable of
driving professional equipment to its full rated output,
at least not without first reaching a high level of distor-
tion. Moreover, when the output of hi-fi equipment,
which is almost always meant to be operated into a high
impedance, is connected directly to the low impedance
input of professional equipment, the hi-fi unit “sees” a
partial short circuit. Depending on the circuitry, this
may overload the hi-fi output, or it may simply drop
the output level by a few dB. The P2050's input
sensitivity and input impedance are high enough to allow
its use with most hi-fi or semi-pro equipment. However,
it is a good idea to check the specifications for each
situation. The point of this discussion is that impedance
and level are extremely important considerations when
connecting audio equipment.
DYNAMIC RANGE
Every sound system has an inherent noise floor which
is the residual electronic noise in the system equipment
or the acoustic noise in the room. The effective dynamic
range of a system is equal to the difference between the
peak output level of the system and its noise floor.
120dB SPL is the threshold of pain, and 30dB SPL is
the quietest environment one is likely to encounter out-
side of an anechoic chamber. A concert with sound levels
ranging from 30dB SPL to 120dB SPL has a 90dB
dynamic range. The electrical signal level in the sound
system, given in dB of voltage, is proportional to the
original sound pressure level, given in dB SPL, at the
microphone. Thus, when the program sound levels
reach 120dB SPL, maximum electrical levels at the
mixer's output might reach +24dB (12.3 volts), and
maximum power levels at the P2050's output might
reach 45 watts into an 8-ohm load. Similarly, where
sound levels drop to 30dB SPL, minimum electrical
levels will drop to —66dB (0.388 milti-volts) and power
levels will drop to 45 nano-watts (45 billionths of a
watt). These levels are not uncommon. The program
still has an electrical dynamic range of 90dB: +24dB —
(—66dB) = 90dB. This dB to dB correspondence is
maintained throughout the sound system, from the
original source at the microphone, through the electrical
portion of the sound system, to the speaker system out-
put. A similar correspondence holds for any other type
of sound system, a recording studio system, disco
system or a broadcast system.
Generally, the average electrical line level in the above
sound system is +4dB (1.23 volts) corresponding to an
average sound level of 100dB SPL. This average level is
usually called the nominal program level. The difference
between the nominal and the highest (peak) levels in a
program is the headroom. In the above example, the
headroom is 120dB SPL — 100dB SPL = 20dB (not
20dB SPL). Similarly, the electrical headroom is +24dB
— (+4dB) = 20dB. This corresponds to a power head-
room which is also 20dB.
In the above example, if the system had an electronic
noise floor of —56dB (1.23 millivolts), and a peak out-
put level of +18dB (6.16 volts), its dynamic range would
only be 74dB. If the original program had a dynamic
range of 90dB, then 16dB of the program would be lost
in the sound system. There may be extreme clipping of
program peaks, some of the low levels may be buriedin
the noise, or some of the program may be lost in both
ways. Thus, it is extremely important to use wide
dynamic range equipment, like the P2050 and Yamaha
PM-Mixers, in a professional sound reinforcement system.
In the special case of a tape recorder, where the
dynamic range is limited by the noise floor and distor-
tion levels of the tape itself, one way to avoid these pro-
gram losses due to clipping and noise is to “compress”
the program's dynamic range (see Page SEVEN 3). A
better way is to apply special “noise reduction” equip-
ment which allows the original program dynamics to be
maintained throughout the recording and playback
process. This improvement in the dynamic range of
recorded material again demands wide dynamic range
from every piece of equipment in the recording/play-
ACOUSTIC 44d8 2048 2848 6dB
TO ELECTRIC VOLTAGE INPUT VOLTAGE POWER
CONVERSION GAIN PAD GAIN GAIN
MICROPHONE 6" MIXER INPUT STAGE MIXER OUTPUT STAGE POWER AMP POWER AMP POWER INTO 1 SOUND PRESSURE SOUND PRESSURE
FROM SOUND INPUT STAGE OUTPUT STAGE 8-OHM SPEAKER LEVEL AT 1 METER LEVEL AT 1 METER
SOURCE FROM 1 SPEAKER FROM 4 SPEAKERS,
DRIVEN BY 45 EACH DRIVEN BY
WATT AMPLIFIER ITS OWN CHANNEL
OF 45 WATT
AMPLIFICATION
co CLIPPING LOUDEST MAXIMUM
' CLIPPING LOUDEST LEVEL PEAK POWER 45 WATTS
LEVEL EAK 4 $ +288 4
| | +24dB {19\)
140 | (12.3V) 20d8 20dB
HEADROOM HEADROOM
20dB
HEADROOM
NOMINAL
130 |- +4dB
(1.23) —— — —|-- - NOMINAL - — — — — — } NOMINAL
+8d8 0.45 WATTS
- FF 1 CLIPPING LOUDEST (1 avi
120 — LEVEL PEAK `
4 | 0dB
CLIPPING (0.775V) MAXIMUM SOUND
LEVEL LEVELS (PEAK)
10H . —— -11dB 20dB 13008 SPL
MAXIMUM VOCAL | (218mV} HEADROOM 90dB 9048 MAXIMUM SOUND 4 4
LEVEL AT 7” LOUDEST 9048 | SN " SIN LEVELS (PEAK) 0 0 ent
100 FROM MIC PEAK DYNAMIC LM LU RATIO o RATIO | {
-120dB SPL — 048 RANGE 49 | NOMINAL >
ú 1 * (78mv) 9 —20dB q
E 29d8 < {78mV) 5 20d8
T = HEADROOM ;
2 gol 20u8 HEADROOM = —— ——|- - — +NOMINAL
ui HEADROOM 7048 o
x S/N = s | 110d8 SPL
w NOMINAL RATIO g 110d8 > 110d8 — | — —|-- — NOMINAL
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Yamaha P2050
F1030
Frequency 120dB SPL
Dividing > Max.
Network — Highs
a DA 30dB SPL
120dB SPL Max. C Mic Min.
30dB SPL Min. Lows
120dB SPL
— Max.
yd Lows
30dB SPL
Min.
+28dB(18.9V) (45W/8%2)
+24dB (12.3V) +8dB (1.89V) (.45W/82)
| +4dB (1.23V) +
Maximum (Peak) — — — — —
~~ —
Levels —20dB (77.5mV) > Headroom = 20dB
> Dynamic Range
Nominal Levels —40dB (7.75mV) L = 90dB
—62dB (.6mV} (45nW/82)
—66d8B (0.388mV) Ÿ
Minimum Levels —110dB (2.45; V) NOTE: The P2050 has a specified signal to noise ratio of 110dB
(which is its dynamic range). The SYSTEM'S Dynamic Range is limited by
acoustic noise at the mic input, for the system shown, and by the maximum
dynamic range of the PM-700 Mixer (93dB), a very respectable figure
for a high gain device.
(System Noise Floor)
Fig. 26 — Dynamic Range in an Audio System
PIVE4
back chain, including the power amplifier.
The P2050 is designed for these wide dynamic
range applications. It has exceptionally low noise
figures, and high headroom capabilities (high power
output). In addition, its operating levels and impedances
correspond with professional requirements.
GAIN OVERLAP AND HEADROOM
Yamaha PM-Mixers have +24dB (12.3 volts) maximum
output levels. This high output level is advantageous in
many situations. One reason is that it assures adequate
headroom for driving the input of any professional
device. High headroom is also important for a mixer
that feeds a professional tape recorder, and in a concert
sound reinforcement system.
Occasionally a ‘passive’ device (no transistors or
tubes) is inserted between the mixer and the power
amplifier in a sound reinforcement system, or in a
studio monitoring system. Examples of passive devices
are passive graphic equalizers, passive low level cross-
overs (frequency dividing networks), pads and resistive
isolation networks. Passive devices always attenuate the
signal level somewhat. For example, a passive low level
crossover, when properly terminated, creates a 6dB loss
between the mixer and the power amplifier. Passive
graphic equalizers can create more than 6dB loss at some
frequencies. Because a mixer with +24dB (12.3V) out-
put drive has considerably more output level than is
needed to drive the inputs of most amplifiers, passive
devices may be used as desired. This extra output
capability above that needed to drive the power amplifier
is known as “gain overlap,” and is one of the most im-
portant advantages of a Yamaha PM-Mixer over other
mixers, especially non-professional mixers.
INPUT SENSITIVITY RATINGS
Some auxiliary devices have input sensitivites rated
like this: “nominal input sensitivity: OdB”. Others may
be rated like this: “input sensitivity: OdB for rated out-
put”. This later type of rating is typical of many power
amplifiers, including the P2050. The difference between
these ratings is subtle, but very important. The first
device, has a nominal input sensitivity of OdB (0.775
volts), and may be capable of peak levels far above OdB
(0.775 volts); the actual headroom may be stated in
another specification. The second device (the P2050 is
an example), has a peak input sensitivity of 0dB (0.775
volts). À OdB input signal to the P2050 drives it to full
output. Thus, the user must carefully select the system's
operating levels.
The gain overlap between maximum mixer output
drive capability (typically +24dB) and power amp input
sensitivity (0dB) lets the user choose a headroom figure
for the P2050; this will be typicaliy 10dB for speech or
concert reinforcement, 15 to 20dB for high quality
music reproduction or recording. The discussion on
Page SIX 5 illustrates the headroom adjustment process.
PROFESSIONAL EQUIPMENT ADVANTAGES
The many advantages of professional equipment
include: balanced lines for hum and noise rejection, low
impedance circuits for long cable runs, high operating
levels for maximum signal to noise ratio, high operating
headroom for low distortion and low noise, and reliable
XL-type connectors that are unlikely to be disconnected
accidentally and that tend not to hum or pop when
being attached. In addition, levels and impedances for
professional equipment are relatively standardized,
which, in many cases, eliminates the need for special
adapters, pads, transformers, or preamplifiers. For these
reasons, professional equipment, even though its initial
cost may be higher, will almost always benefit the user.
on a long term cost/performance basis.
The P2050 user realizes all of these professional
benefits. In addition the P2050 can be used with many
hi-fi or semi-pro devices, such as guitar preamps, semi-
pro or hi-fi tape machines.
27dB Gain
K
54dB Gain
> 6dB Loss
Mic
PM-Mixer Passive Graphic
Equalizer
Speakers
+25dB
(13.78V) ——
0dB
(0.775\) —
—25d8
(43.6mC) —+—
—50dB
(2.45mV} ——
+4dB (1.23V)
N
N
N
x
N
N
—50dB
(2.45mV) y
=. — — ——
id
+25.2dB (14.1V)
VIV
„”
_ ~~ (25W/842)
7”
—2dB (0.616V)
—— „
Fig. 27 — Typical Gains and Losses in a System
INSTALLATION AND DETAILED
OPERATION
PHYSICAL MOUNTING
Shelf Mounting |
The P2050 can be used on any surface, so long as
there is adequate ventillation. Do not remove the
P2050's feet, since this would prevent air flow below
the amplifier.
Permanent Installation Rack Mounting
Mount the P2050 in any standard 19” electronic
equipment rack as shown to the right. Leave adequate
space between the P2050 and other devices in the rack
for ventilation, and for expected cabling. Cooling fans
may be required when the P2050 must produce
extremely high average power output, or when it is
located in a high temperature environment, such as a
closed outdoor building in direct sunlight.
Rack Mounting for Portable Usage
Road cases must be durable enough to survive heavy
cartage, and airline travel. Brace the rear of the P2050,
and if the road case is small and ventilation is con-
stricted, install cooling fans. One possible design is
shown in Figure 28.
he 9.1/2" —
1
|
7-172"
ee -— o ый ное сн ИНЬ зн т = SR SD == === DR SR Dr OS стен Cr —— —— жк к че ешь ео = === пене к = E TE e == |
“>
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K—— 51/47 — ——
Front fan panel view before folding.
+ | } or
M———— 5-1 /4'! —
Front fan panel side-
view after folding.
3-1/2" | Rear panel after folding.
NOTE: Brace length assumes
a front-back rack depth of
15”.
Front View
Left braces for securing P2050 in
rack. (For right braces, use same
measurements and flop.
ECTION ©
P2050 Front view
Shelf mounting
Perforated panels for air flow
vn vv rw Y ” Ta wr "a a
a a o tata t tr ia ,. °, + 1
+, °, + * . .. * ” 2° 5000 я *,
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Rear fan
panel installed
Front fan
panel installed
Fig. 28 — P2050 with Cooling Fans
Extra rack strips mounted in
back of cabinet for mounting
special support brackets.
11
P2050 mounted in rack showing support brackets
made from bent pieces of 1/8" steel rod with nuts
welded to their ends.
X2
Regarding Input Impedance and Termination
There is sometimes a misunderstanding regarding the
nature of matching or bridging inputs, the use of termi-
nating resistors, and the relationship between actual
input impedance and nominal source impedance. Most
electronic outputs work well when “terminated” by an
input having the same impedance or a higher actual
impedance. Here, “terminated”” means “connected to.”
Outputs are usually overloaded when terminated by an
impedance that is lower than the source impedance.
When the actual input impedance of the following device
Is nearly the same impedance as the source, it is known
as a “matching” input. When the input of the following
device is ten times the source impedance, or more, the
input is considered to be a “bridging” input. There is
hardly any loss of signal level when an input bridges the
source device, but a matching input may cause a loss of
3 to EdB in level. Such losses, however, are normal and
usually present no problem.
It seldom is necessary to place a 600-ohm “termin-
ating resistor’’ across any high impedance input. The
P2050's input can be considered to be high impedance.
In fact, most 600-ohm outputs operate normally when
bridged by a high impedance; it is as though no load
where connected to the source device.
The only instance where a terminating resistor may
be required is when the manufacturer of the source
device specifically states that a terminating resistor is
necessary. In such cases, there is usually a special type of
output transformer in the source device, or the device is
constructed primarily of precision, passive components,
such as a passive equalizer. In these cases, the terminat-
Ing resistor assures optimum frequency response in that
device. Input terminating resistors are not needed for the
P2050 to operate correctly. If a 150 ohm or 600 ohm
resistor is specified for the source device, it should be
installed at the end of the cable nearest the P2050 in
order to minimize possible hum, noise or signal losses
in the cable.
Source Device Load Device
Input
Impedance
utput
Impedance
L
Fig. 29A — The Actual Voltage reaching the Load Device
is given by the Formula: (also see Appendix)
г,
V, =V o> 5
L S г, + 29
Cable
Ll [A
P2100
Termination
Termination
Resistor
Fig. 29B — Where to Insert a Termination Resistor when one
is required.
CABLING AND IMPEDANCE MATCHING
Attenuation Pads
A “pad” is a resistive network that lowers the leve! in
an audio circuit. The most common professionally used
pads are “"T-pads’ and “H-pads.” T-pads unbalance true
balanced lines and floating lines, but work well in un-
balanced circuits. H-pads are best for balanced or float-
ing lines, but should not be used in an unbalanced circuit
since they will insert a resistance in the return lead
(ground). For a discussion of other types of pads. refer
to the AUDIO CYCLOPEDIA by Howard M. Tremain
(Pub. Howard W. Sams).
Cabie
Fig. 30 — Where to Install a Pad when one is required.
Always install a T-pad near the input of the device it
feeds, with as short a length of cable as possible on the
low level side of the pad. This maintains a high signal
level in the longer transmission cable, minimizing any
induced hum and noise.
The low impedance pad values illustrated in Figure 31
are designed for 600-ohm lines. Commercially manu-
factured pads are available; consult your Yamaha dealer.
When connected between a 600-ohm, or lower, source
and a 600-ohm, or higher, termination, pad attenuation
values will remain fairly accurate. For higher impedance
circuits, resistor values must be changed. A 600-ohm pad
inserted in a high impedance circuit may overload the
device feeding the pad (the source device). Multiply the
given values by the output impedance of the source
device, and divide that answer by 600 to achieve the
desired value. The high impedance values listed for the
T-pads in Figure 31 are close approximations of average
hi-fi pads, based on 10,000-ohm nominal impedances.
For low level circuits, use 1/4 watt resistors. For
outputs with continuous sine wave levels above +24dBm,
dB Loss R1 T (ohms) R1 H (ohms) R2
0.5 300 16 150 8.2 180k 10k
1.0 560 33 300 18 82k 5.1K
2.0 1100 68 560 33 43k 2.7k
3.0 1710 100 820 51 27k 1.6k
4,0 2200 130 1100 68 22k 1.2k
5.0 2700 160 1500 82 16k 1k
6.0 3300 200 1600 100 13k 820
7.0 3900 220 2000 110 11k 680
8.0 4300 270 2200 130 9100 560
9.0 4700 270 2400 150 8200 470
10 5100 300 2700 150 68C0 430
12 6200 360 3000 180 5100 360
14 6800 390 3300 200 4300 240
16 7500 430 3600 220 3300 200
18 7500 470 3900 220 2700 150
20 8200 510 3900 240 2000 120
22 8200 510 4300 240 1500 91
24 9100 510 4300 270 1300 75
26 9100 560 4700 270 1000 62
28 9100 560 4700 270 820 47
30 9100 560 4700 270 620 36
32 9100 560 4700 300 510 30
34 10k 560 4700 300 390 22
36 10k 560 4700 300 330 18
38 10k 560 4700 300 240 15
40 10k 560 5100 300 200 12
50 10k 620 5100 300 62 3.6
Fig. 31 — Attenuation Pad Construction and Resistor Values
for High Impedance (10K-ohm) and Low Impedance (600 ohm)
[shaded areal circuits.
use 1/2 watt resistors; for continuous sine wave levels
above +30dBm, use 1 watt, low inductance resistors.
10% tolerance is acceptable for most pads.
It is possible to construct a pad within an XLR con-
nector, but the extremely tight fit can adversely affect
reliability. The Switchcraft model S3FM is a tube with a
male A3M (XLR) at one end, and a female A3F (XLR)
at the other end. Pads using 1/4 watt resistors can be
constructed inside this device. Cover the entire pad
with insulation tubing before final assembly into the
S3FM.
A “mini-box’’ fitted with male and female XLR con-
nectors is an easy to build, rugged housing for a pad.
Use stranded wire for best results.
Illustrated are three typical pad construction
techniques. For most applications, it will be sufficient
to construct only a few types of pads: 20dB, 24dB and
40dB pads cover almost any requirement. Consult
Figures 30, 31 and 32 for schematic, construction and
resistor value information.
“T” PAD
HI
O— vw
—O LO
Fig. 32A — Pads Constructed in Mini-Boxes
NVWYW O
Ri Rt
Ra
O
RI R1
Fig. 32B — Pad Constructed in Switchcraft Model S3FM
Transformers
Audio transformers, as distinguished from power
supply transformers, RF transformers or other trans-
formers, are primarily used for ground isolation,
impedance matching and level matching. The following
paragraphs detail several applications of audio trans-
formers at low signal levels. Speaker-level transformers
are discussed on Page SEVEN 6; the Appendix gives
further details on transformer operation.
Matching Transformer Box
Impedance matching transformers can be used to
connect a high impedance source to a low impedance
load, or vice-versa. See Page SIX 2 for a discussion of
matching versus bridging inputs. The box shown below
may be used to run a 600-ohm balanced or floating line
to the P2050's input, or it may be used between any
600-ohm source and high impedance input. Use a trans-
former capable of handling expected nominal and peak
operating levels.
The transformer should be mounted in a mini-box,
wired to the XLR connectors with stranded wire, and
connected to the auxiliary equipment with the appro-
priate audio cable. With suitable adapters, inline transfor-
mers, such as those manufactured by Shure Brothers,
Sescom, and others may be used.
P2050 INPUT LOW-Z LINE
MALE XLR FEMALE XLR
UTC-010
or equivalent
6 2 l
BOX GROUND LUG
Fig. 33 — Matching Transformer Box
Step Up Transformer Box
The step up transformer box illustrated here is
similar to a pair of matching transformer boxes. This
configuration provides voltage step-up for optimum drive
levels when connecting the output of a low impedance,
low level source, such as the headphone output of a
mixer, to the two inputs of the P2050. It has a stereo
phone jack input; if the input source is monaural use a
T.S. input jack, only one transformer, feed the P2050
X4
CH A input, and place the P2050 in Mono Mode.
Alternately, the box may be built with separate T.S.
phone jack inputs, or with XLR inputs. Two standard
T.S. phone jacks are provided for connection to the
“left” and “right” inputs of the P2050. Construct two
cables from dual conductor, shielded cable and T.S.
phone plugs to connect the transformer box output to
the P2050’s input. Locate the step up transformer box at
least 5 feet from the P2050 to avoid hum pickup from
the amplifier’s power transformer. However, the cables
from the transformer box to the amplifier should be no
longer than 10 feet, since this is a high impedance circuit.
Use low capacitance, coaxial, hi-fi type cable between
the box and the amplifier. Since the inputs of the P2050
are unbalanced, connecting two cables to its input forms
a short ground loop as shown in Figure 53 (see discussion
of grounding on Page SIX 13). To keep hum pickup at a
minimum, run the two cables close together; this
minimizes the area, and therefore the hum, enclosed
by the loop.
T.R.S.
PHONE PLUG TIP A 7
RING Г 1
SLEEVE
TRIAD A-65J
OR EQUIVALENT
(2 TRANSFORMERS)
STANDARD
PHONE JACK
—e ~~ B LEFT
OUTPUT
|
‘
15K < riy
Y
RED/WHITE € CASE
GROUND
CASE
GREEN
YY
U Gi
GREEN/ORANGE
T.R.S
PHONE JACK BLACK/WHITE
,
я
4
INPUT ll Nee] .
| > “7 GROUND
DO NOT GROUND GREEN N _
TO CASE: JACK - и” RIGHT
MUST “FLOAT” OUTPUT
4
15К 0 е ei
Y 7
RED/WHITE € =
GREEN/ CROUND
ORANGE
BLACK/WHITE ! hd
“7 CASE
v' GROUND
TRANSFORMER AVAILABILITY
The matching and step-up transformers mentioned
In the preceding subsections are available from many
electronic parts dealers. Yamaha does not endorse
specific products by citing them herein; rather, these
transformers are mentioned for convenience only. If
you are unable to locate the transformers from your
local electronic parts dealer, contact the manufacturer
at the address shown below.
Sescom, Inc.
P. O. Box 590, Gardena, CA 90247
Phone (800) 421-1828 (213) 770-3510
Shure Brothers, Inc.
222 Hartrey Ave., Evanston, Illinois 60204
Phone (312) 328-9000 Cable: SHUREMICRO
Triad
305 N. Briant St., Huntington, Indiana 46750
Phone (219) 356-6500 TWX: 816-333-1532
UTC
150 Varick St., New York, NY 10013
Phone (212) 255-3500 TWX: 710-581-2722
A fine of very high quality transformers, suitable for the
most critical applications, is available directly from:
Jensen Transformer Company
1617 N. Fuller Ave., Hollywood, CA 90046
Phone (213) 876-0059
WHT. (RED)
«_ / BLACK
SHIELD”
INPUT A 8. ||
SIX 13). To keep hum pickup at a minimum, run the
two cables close together; this minimizes the area, and
therefore the hum, enclosed by the loop.
The two diagrams show circuits using a Triad A-65J
transformer, and a UTC A-24 transformer. Similar
600 ohm to 15k-ohm transformers are acceptable. The
1/4 watt, 10%, 15k-ohm resistors are used to terminate
the transformers for lower distortion and improved
frequency response.
Bridging Transformer Box
When a single, low impedance, balanced source which
must remain balanced feeds several P2050 inputs, the
bridging transformer box should be used. While matching
or step-up transformers like those just described would
maintain a balanced feed, several such boxes could over-
load the source device. By using a transformer which has
a high impedance primary and a high impedance
INTERCONNECTING CABLE FOR
PHONES OUTPUT TO STEP-UP
TRANSFORMER UNIT
T.R.S.
TIP PHONE PLUG
UTC A24
OR EQUIVALENT
(2 TRANSFORMERS)
STANDARD
PHONE JACK
LEFT
OUTPUT
L
Tv 7
,
«+.
15K 1
CASE
GROUND| To POWER
TRS. AMP INPUTS
PHONE JACK
o - -
DO NOT GROUND 1 7
TO CASE; THIS
JACK MUST “FLOAT” RIGHT
GUTPUT
CASE
GROUND
Fig. 34 — Step-Up Transformer Box
secondary, the source can feed several P2050 inputs
without being overloaded. Use one box for each P2050
input, paralleling the primaries. The primaries are then
fed from the single, balanced source, and the secondaries
are connected to the P2050 inputs. Construct the box
in a similar manner to the Step Up Transformer Box,
or the Matching Transformer Box.
GND Switch
Polarity Indicator
N |
Ÿ | + {Box)
|
|
|
|
|
|
|
|
|
Input Output
Optional
Polarity
Electrostatic Shields
(if available)
Fig. 35 — Bridging Transformer Box Schematic. Construction
is similar to photos in Figures 33 or 34.
Input Impedance Matching for the P2050
While the input impedance of the P2050 varies some-
what with the setting of the input attenuator, for
practical purposes, it is fixed at 25k-ohms. This means
that any source device feeding the P2050 must be
capable of driving a 25k-ohm load without overload,
distortion, or failure. Any professional device, most
semi-pro equipment, and most hi-fi devices meet this
requirement.
When a single source device feeds the inputs of
several P2050 amplifier sides, the effective load on
the source is equal to the parallel combination of all
the P2050 input impedances. To avoid overloading a
high impedance source, use a resistor matching network,
an impedance matching transformer, or insert a line
amplifier with a lower output impedance between the
source and the P2050's input.
Figure 36 is a voltage division diagram for the output
impedance of a source device and the input impedances
of several P2050's.
Generator 10dB
1000Hz “H” Pad
Sine Wave
XD
Level Matching and Headroom (also see Page FIVE 2)
Headroom is equal to the maximum undistorted
signal level capability minus the nominal signal level at
a given point. Noise floor is the average noise level at
that same point in the audio system. The difference in
level between the maximum undistorted output and the
noise floor is the available dynamic range. Judicious
setting of signal levels throughout the system can
optimize the dynamic range of the system, thus mini-
mizing the noise and maximizing the headroom.
First choose a headroom value. Bear in mind that
music often has peaks that exceed 20dB above
the nominal level. A 20dB headroom value represents a
peak level that is one hundred times as powerful as the
average program level. This means that for an 8-ohm
load and a 20dB headroom value, even an amplifier as
powerful as the P2050 has to operate at an average 0.45
watts output power. In some systems such as studio
monitoring, fidelity and full dynamic range are of ut-
most importance. Since studio monitor speakers
typically produce 100dB SPL at 4 feet with only 1 watt
input, an average power as low as 0.05 watts may be
adequate. In other situations, such as 70-volt background
music systems, a 20dB headroom figure is undesirable
and costly. For example, if 0.45 watts average power
and 10dB headroom are acceptable, then only a 4.5 watt
amplifier is needed. Thus, sacrificing 10dB headroom
allows the 45 watt amplifier to drive ten times as many
speakers.
For most sound reinforcement applications,
especially with large numbers of amplifiers, economics
play an important role, and a 10dB headroom value is
usually adequate. For these applications, a limiter will
help hold program peaks within the chosen headroom
level, and thus avoid clipping problems. For the extreme
situation where background music and paging must be
heard over high continuous noise levels, yet dangerously
high sound pressure levels must be avoided (i.e., in a
factory), a headroom value of as low as 5 or 6dB is not
unusual. With this low headroom value, and the extreme
amount of compression and limiting necessary to achieve
it without clipping distortion, the program may sound
unnatural, but the message will get through.
P2050
Optional Limiter to
restrict Headroom
past Graphic EQ with-
out allowing Clipping
4.3V
+15dB
. — ==
Graphic Dr To Speakers
у Equalizer bed
> re
ES
2, +28dB(18.9V} (45W/852)
+24dB (12.3V) +8dB (1.89V) (.A5W/89)
P2050 Maximum (Peak) ads 1.231 > J- 7
Sides > _
Levels —20dB (77.5mV) - Headroom = 20dB
? Dynamic Range
20 г Nominal Levels —40dB (7.75mV) < = 90а В
—62dB (.6mV) (45nW/852)
› — (5) = a —66dB (0.388mV)
2
VA Minimum Levels —110dB (2.45uV) 7 Fig. 37 — Headroom Adjustments
(System Noise Floor)
After choosing a headroom value, next adjust the
incoming and outgoing signal levels at the various
Fig. 36 — Voltage Division Diagrams
devices in the system to achieve that value. For the
simple system in Figure 37, the adjustments for a 20dB
headroom value would be made as follows:
1. Initially, set the attenuators on the P2050 at
maximum attenuation (ful! counterciockwise rotation).
Feed a sine wave signal at 1000Hz to the mixer input at
an expected average input level: approximately —b50dB
SIXO
(2.45mV) for a microphone, +4dB (1.23 volts) for a line
levei signal. The exact voltage is not critical, and 1000Hz
IS a standard reference frequency, but any other appro-
priate frequency can be used.
2. Set the input channel level control on the mixer at
Its rated “nominal” setting, and adjust the master level
control so that the output level is 20dB below the rated
maximum output level for the mixer. For the Yamaha
PM-180 Mixer used in the example, the maximum rated
output level is +24dB (12.3 volts), so the output level
should be adjusted to +4dB (1.23 volts), as indicated
either on an external voltmeter, or on the mixer's VU
meter (0VU).
3. Assume that the rated maximum input level for
the graphic equalizer in the example is +14dB (3.88
volts). Subtracting +4dB from +14dB leaves only 10dB
of headroom, so a 10dB resistive pad must be inserted
between the mixer output and equalizer input. Now,
the signal level at the input to the equalizer should be
—6dB (388mV), which can be confirmed with a volt-
meter.
4. Assume that the maximum rated output level of
the equalizer in this example is +18dB (6.16 volts).
Adjust the master level control on the equalizer so that
the output level is 20dB below this rated maximum, or
—2dB (616mV). Since the equalizer has no VU meter,
you need an external voltmeter to confirm this level.
5. Finally, starting with the attenuators on the P2050
at maximum attenuation (full counterclockwise rota-
tion), slowly rotate them clockwise, monitoring the
output level with a voltmeter. When the voltmeter
indicates 0.45 watts output from the P2050 (1.9 volts
rms into an 8-ohm load), there is 20dB headroom left
before clipping.
To operate this system, use only the controls on the
mixer, and avoid levels that consistently peak the
mixer's VU meter above the “zero” mark on its scale.
Any adjustments of the other devices in the system will
upset the headroom balance. However, the P2050's
calibrated attenuators allow easy setups and quick
changes, if you decide to change the headroom value.
They also allow you to momentarily fade the entire
program or a single channel and to later bring it back
up to exactly the same level.
To use this technique with any system, first design
the required speaker system, and calculate the number
of power amplifiers needed to safely operate the
speaker system with adequate headroom. Then, choose
the mixer and other devices that feed the power
amplifiers and set up the system according to the
above instructions.
In some cases, it may be useful to set up different
headroom values in different parts of a complex
system. For example, background music and paging
may need to be severely compressed in a noisy lobby
area, but the same program material would sound more
natural in less noisy office and auditorium areas of the
same installation if the headroom value were increased.
By placing a compressor/limiter in the circuit just
before the P2050 that feeds the lobby areas, the
headroom value can be lowered for that section only,
without affecting other parts of the system.
Cabling the System
Audio circuits may be divided into the following
classifications (by signal level):
1. Low level circuits carrying signals of —80dB
(77.5 microvolts) to —20dB (77.5 millivolts), such as
microphone lines.
2. Medium or line level circuits carrying signals of
—20dB (77.5mV) to +24dB (12.3 volts), such as mixer
outputs.
3. High level circuits carrying signals above +24dB
(12.3 volts), such as speaker lines.
4. AC power circuits, including lighting circuits.
5. DC control (or supply) cables to relays, from
batteries, etc.
Generally, each of these categories should be
physically separated from the others to avoid crosstalk,
oscillation, and noise spikes. One possible exception is
that DC control or supply cables and line level signal
cables can be routed together if the DC signal is
adequately filtered. Figure 38 shows the undesirable
results that can occur if line or speaker cables are
placed near microphone cables. This situation occurs
in concert sound when mixer outputs and mic inputs
feed through the same “snake” cable.
Any Mixer
-—
— To Power Amp Rack
| Crosstalk between Line & Mic Level
| Circuits in same “Snake” Cable
=
6
-
®
x.
=
o
E
=
©
x
Pa.
From Microphone
|
|
|
|
t
—
— — — ——— ———
Feedback Path
Fig. 38 — Example of Crosstalk
Figure 39 shows an equipment rack with a good cable
layout. Note that the different categories of cable are
carefully separated, and that where it is necessary to
cross two categories, they cross perpendicular to each
other. These suggestions apply to all types of systems,
portable as well as permanent.
LINE LEVEL & DC CONTROL CIRCUITS
MICROPHONE
CIRCUITS
(LOCAL OPTION <
\
(КС
O
x 77 7
Y
NY
%
dd dal L
DA
TA
A
и
GROUND
24 V DC LUG
EMERGENCY
SOURCE HIGH LEVEL 24 V DC EMERGENCY
= SPEAKER
CIRCUITS
POWER & GROUND
CIRCUITS
Fig. 39 — Cable Routing in Equipment Rack (Reprinted
from Sound System Engineering by Don & Carolyn Davis
published by H. W. Sams Co.)
Figure 40A shows the rear of a P2050 amplifier with
its two inputs ‘chained’ using a phone-to-phone cable.
This is done for mono operation into two separate 8f2
speaker loads. “Mono” mode, shown in Figure 408,
differs in that a 16 {2 speaker load is connected across
both “+” outputs, and only the CH A input is used.
For low and medium level balanced signal cables, use
good quality twisted pair shielded cable. For portable
use, a cable with rubberized insulation and braided
shield, such as Belden No. 8413 or No. 8412, will handle
easily, and survive road abuse. For permanent wiring, a
vinyl insulated cable with a foil shield, such as Belden
No. 8451, is easier to strip for terminations, and it pulls
through conduits with less drag.
Rear of P2050
rere]
From
Mixer
Output
Phone to Phone Cable
Fig. 40A — “Chaining” of Inputs for Mono Operation
into Two 82 Loads.
| эс ©
From
Mixer
Output
Fig. 40B — Input Connection for Mono Operation into
One 16 07 Load. (Use CH A in only)
For unbalanced, signal level cables, use low capaci-
tance shielded cable with a good quality, high-percentage
density, shield. Again, rubberized types work best for
portable use; vinyl types with foil shields are acceptable
for permanent installations, although the foil shield may
crack and split under the constant flexing of portable
usage. Many single conductor shielded cables have an
extremely fragile center conductor. To avoid this pro-
blem, use a higher quality dual conductor cable and
ground one center conductor.
For high level speaker cables and DC control cables,
use heavier gauge cable. The chart in Figure 41 shows
the effects of different sized wire gauge on power losses
in speaker cable. Except in extreme RF fields (radio
Wire Gauge vs Cable Length for 1dB Loss (21% loss) at
4, 8 8: 162 Loads (Dual Conductor Cable) |
AWG. ?
Wire
Gauge 4
{ 806) 6
1121 8
1204) 10
Ohms
Per
1000 32
Feet of
2-Wire 15.16) 14
Speaker
Cable 5,5, 16
(approx.)
(13.2) 18
(20.8) 20
133.0} 22
152 4) 24
10 100 1000 tkm 1 mile
Feet of Dual-Conductor Speaker Cabte — Solid Lines
Fig. 41 — Effects of Different Sized Wire Gauge on Power
Loss in Speaker Cables.
SIXT
frequency interference), speaker and control cables will
not need shields; when they do, use heavy-gauge shielded
cable, or place the cables in steel or aluminum conduit.
Connectors
In many cases, connectors will be dictated by the
types of equipment in the system. When you can make
choices, the following guidelines may help.
Phone Connectors are an audio industry standard con-
nector used for signal and speaker lines. T.S. (tip/sleeve)
types, like those used as inputs on the P2050 are used
for unbalanced signals; T.R.S. (tip/ring/sleeve) types are
used for balanced signals, or for stereo unbalanced
signals such as stereo headphones. Phone connectors are
generally easy to wire, and the metal types provide good
shielding. However, for high power applications, such as
the output of the P2050, many phone plugs do not have
rated current capacities high enough to avoid power loss.
Also, some phone plugs have a brittle insulator between
the tip and sleeve which can break if the connector is
dropped, resulting in a tip to sleeve connection which is
a direct speaker line short circuit. For this reason, phone
jacks have not been used for the P2050 output. If you
have another amplifier with a phone jack output,
military grade phone connectors, while mere expensive
and somewhat harder to wire, are the best choice for
avoiding these problems.
Phono Connectors are not usually considered profession-
al, and are not included on the P2050. If phono con-
nectors are part of a system, they should be the higher
quality types with a separate cover, such as Switch-
craft No. 3502.
XLR Connectors are another audio industry standard.
They come in several configurations for different types
of cabling, and can be used for either balanced or un-
balanced connections. Three wire types, like those used
as inputs on the P2050, are the most common. XLR
connectors are generally very durable, and are well
shielded. The three wire types have the added advantage
that pin # 1 always connects before pins # 2 or # 3 so
that the ground or shield wire connects before the
signal carrying wires. This allows any static charges
built up on the shields to equalize before the signals
meet, reducing pops in the system.
Banana Jacks are common in the audio industry, and are
a standard connector for test equipment. They do not
provide any shield, and can be reversed in their socket.
However, banana jacks like those used as output con-
nectors on the P2050 have high current ratings, and are
good speaker connectors, especially inside an equipment
rack where occasional disconnections must be made.
Other Connectors are occasionally used in audio work.
Standard, electrical twist-lock types have been used for
speaker connections, although there is always the dan-
gerous possibility of a mistaken connection to an AC
power line. Multi-pin “snake” connectors are common
for low level signals, but may be fragile and need careful
handling. For permanent installations, and for per-
manent connections in portable equipment racks,
crimp type spade lugs (as opposed to solder type) and
terminal strips may actually provide the best type of
connection since a properly crimped connection is more
reliable, and lower impedance than a solder connection.
A “hard wire” or direct connection is also reliable and
low impedance if properly made.
S48
CABLE AND CONNECTOR
WIRING CONFIGURATIONS
The preparation of complete cables, with connectors
properly installed, is the key to reliable and trouble-free
operation of any sound system. For this reason, the fol-
lowing illustrations are included. Experienced audio
technicians may wish to review these illustrations, even
if they already know how to wire connectors. A few
moments of extra care here can save hours of trouble-
shooting later on.
As a rule, the amount of insulation removed and the
length of exposed cable should be minimized. This
reduces the likelihood of short circuits and improves the
ability of the clamp to grip the cable firmly. Enough
heat should be used to obtain a free flow of solder, but
allow leads to cool quickly after solder flows to avoid
melting insulation. After each connector has been com-
pletely wired, the cable should be tested with an ohm-
meter or a cable tester. Continuity between the various
conductors and their associated connector pins must be
established, and there should be infinite resistance (an
open circuit) between all connector pins. In most cases,
especially in portable installations, XLR connectors
should not conduct at all between the shell and pin 1.
This avoids grounding problems from inadvertent touch-
ing of the shell to other devices.
Cables to be connected to terminal strips should be
prepared by stripping the ends and installing crimp-on
or preferably, solder type lugs. If there is any chance
the cable will be strained, use a cable that is constructed
with internal strain relief cord, such as Belden No. 8412.
Crimp a lug onto the cord, and secure the lug to an un-
used terminal. (The cord should be drawn slightly
tighter than the wire leads in order to take the strain
first.)
Shell
Center conductor connection
Cable clamp
Shell connection
WIRING AN RCA-TYPE PIN PLUG*
Parts identification and cable preparation.
Strip approximately 1/2" of outer insulation. Unwrap
or unbraid the shield and form a lead. Strip approxi-
mately 5/16" of insulation from the center conductor.
Tin both leads.
Solder the shield to the outer surface of the shell
connection, allowing enough free shield to wrap the
cable around to the center of the connector. Cool
the connection immediately with pliers.
Insert the center conductor in the hollow pin, and
fill that end with solder. Cool the connection immedi-
ately with pliers. Clean any solder splashes and inspect
for burned insulation. Pinch the clamp around the outer
insulation with pliers, firmly, but not so tight as to cut
the insulation.
Slide the shell forward and screw it tightly to the
threaded plug.
*Switcheraft Мо. 3502 connector illustrated. Many large
diameter cables are more easily wired to “simple” RCA
type pin plugs without a shell (Switchcraft No. 3501M, or
equivalent). The braid can then be soldered directly to the
shell of the plug.
Strain relief fitting Shell Insulating Collar
Male insert
T
Set Screw
Tracer cord
Braid (shield)
Strain relief cord (string)
Braid (shield) center
conductors
White wire
Pin 2
Black wire Pin 3 |
Shield Pin 1
Keying channel
WIRING A MALE XLR CONNECTOR
Parts identification (as the connector is usually
packaged).
Insert strain relief in rear of shell. Then slip shell
onto cable end, followed by insulating collar. Strip outer
insulation 1/2”. (No. 8412 cable illustrated here.)
Cut tracer cord, unbraid shield, cut cotton strain
relief cords.
Strip approximately 1/4" of insulation from center
conductors, tin, and trim to approximately 1/8" ex-
posed wire. Then twist shield, positioning it in the
correct orientation to mate with the insert. After tin-
“ning the shield, cut it to the same length as the center
conductors.
Solder the center conductors to their respective pins,
using just enough solder to fill the end of the pins.
Yamaha's wiring standard dictates that the black lead
mates with pin 3 and the white {or red) with pin 2
(see footnote on page 10 of this section). Then solder
the shield to pin 1. Clean any solder splashes and inspect
for burned insulation.
Slide the insulating collar forward, up to the flange of
the male insert. The outer cable insulation must be flush
with, or covered by the end of the insert. If any of the
center conductors are visible, the cable clamp may not
be able to firmly grip the cable. Then slide the collar
back into the shell.
Slide the shell forward, orienting its internal keying
channel with the raised lip (key) on the insert. Secure
the insert in the shell with the set screw. Place the cable
clamp over the rear of the shell, with careful attention
to the clamp’s orientation; a raised lip inside the clamp
should be aligned immediately over a lip in the shell for
thinner cable (No. 8451). The clamp should be turned
around for heavier cable (No. 8412) to provide clearance.
Insert the clamp screws and tighten fully.
X10
WIRING A FEMALE XLR CONNECTOR
Strain relief fitting Female insert
Shell
Insulating collar
Parts identification (as the connector is usually
packaged).
Locking tab
Cable clamp & screws 1
Set screw
Insert strain relief in rear of shell. Then siip shell
onto cable end, followed by insulating collar. Strip outer
insulation approximately 9/16”. (No. 8451 cable illus-
trated here)
Pull off foil wrap. Strip approximately 5/16" of
insulation from the center conductors, leaving approxi-
mately 1/4’ of insulation between the bare wire and
the outer insulation. Tin the center conductors, and
trim so that about 1/8” bare wire remains. Then tin the
shield conductor, orienting it with the center conduc-
tors so they are aligned with the proper pins of the
insert. Cut the end of the shield so that it extends 1/16”
beyond the center conductors.
Solder the center conductors to their respective pins,
using just enough solder to fill the end of the pin.
Yamaha's wiring standard dictates that the black lead
mates with pin 3, the white (or red) lead with 2 (see
footnote on page 10 of this section). Then solder the
shield to pin 1. Clean off any solder splashes, and inspect
for burned insulation. Insert the locking tab in the
female insert, as illustrated, with small nib facing front
of connector.
Slide insulating collar foward, up to rear edge of
female insert. The outer insulation of the cable must be
flush with, or covered by the end of the insert. If any of
the center conductors are visible, the cable clamp may
not be able to grip the cable firmly, and the connector
leads will soon fatigue. Then slide the collar back into
the shell.
Slide the shell forward, orienting the notch in the
shell with the locking tab in the insert. Secure the insert
in the shell with the set screw. Place the cable clamp
over the rear of the shell, with careful attention to the
clamp’s orientation; a raised lip inside the clamp should
be aligned immediately over a lip in the shell for thinner
cables (No. 8451). For heavier cables (No. 8412), the
clamp should be turned around to offset the lips and
provide more clearance for the cable. Insert the clamp
screws and tighten fully.
SLAM
Insulating collar Tip WIRING A STANDARD PHONE PLUG (2-conductor)
Tip connection
Shell
Parts identification.
Cable clamp Sleeve Sleeve
connection
Slide shell, then insulating collar over cable end. Strip
outer insulation for length equal to length of sleeve con-
nection. Unwrap or unbraid shield, twist to form lead.
Bend in this direction Position outer insulation just ahead of cable clamp,
strip center conductor from point just behind tip con-
nection. Tin center conductor and shield. Bend shield as
Hlustrated, solder to outer surface of sleeve connection.
(Cool immediately with pliers.) Insert center conductor
in tip connection, solder, cut end flush. Bend the end of
the tip connector (slightly) toward the sleeve connection
to help prevent the burr (from the cut wire) from cut-
ting through the insulating collar.
Using pliers, bend cable clamp around outer insula-
tion. Clamp should be firm, but not so tight as to cut
insulation.
Slide insulating collar forward, until flush with rear
of threads. Slide shell forward, screw tight to plug
assembly.
WIRING A TIP, RING &
Insulating collar Ring connection Ring SLEEVE PHONE PLUG (3-conductor)
Cable clamp
Sheli
Parts identification.
Tip connection
Sleeve connection Tip Slide shell and insulating collar over cable end. Strip
outer insulation for length equal to length of sleeve con-
nection. Remove any tracer cords and strain relief cords.
Form lead from shield. Hold cable with outer insulation
just ahead of cable clamp, and strip the red (or white)
conductor just behind the tip connection. Then strip
the black conductor just behind the ring connection.
Tin all leads, and cut the center conductors so approxi-
mately 1/8" of bare wire remains.
Solder the shield to the outer surface of the sleeve
connection, allowing enough free shield to bend around
to the other side of the cable clamp. Cool the connec-
tion immediately with pliers.
Insert the center conductor leads in their respective
connection points, and solder in place. Trim the leads
flush. Bend the end of the tip connection (slightly)
toward the ring connection to help prevent the burr
(from the cut wire) from cutting through the insulating
collar.
Bend slightly in this direction
Using pliers, bend the cable clamp around the outer
insulation. The clamp should be firm, but not so tight
as to cut the insulation.
Slide the insulating collar forward, until flush with
rear of threads. Slide the shell forward, and screw tightly
onto plug.
S112
Use of the Input Polarity Switch
The XLR input connectors on the P2050 are un-
balanced. In one position, the switch beside the con-
nectors attaches pin 2 to pin 1 (ground) leaving pin 3
“hot” conforming to US practice. In the other position,
the switch attaches pin 3 to pin 1 (ground) leaving pin 2
“hot” conforming to DIN/JIS standard. If the source
feeding the P2050's input is unbalanced, the switch
must be properly set to avoid shorting out the source. If
the source is balanced, the P2050's inputs will un-
balance the source. In many situations, this is acceptable;
however, the input polarity switch must still be set in
the position corresponding to the “hot” pin of the
balanced source. If the switch is set in the wrong
position, the signal effectively will be 180 degrees out-
of-phase at the P2050's output compared to the signal at
the source (reversed polarity).
Polarity Switch
2. HOT
XLR-3-31
To Amp
1
at
3. HOT
Standard
Phone _ I
I
Fig. 42 — P2050 Input Circuit
Output Impedance Matching
Within its rated power and voltage limits, the P2050
acts very much like a perfect voltage source (see
Appendix). Thus, as the impedance of the load goes
down, the total power delivered by the P2050 goes up.
Figure 4, Page FOUR 1 illustrates this action. Note that
when the impedance of the load falls below 2.5 ohms,
the P2050’s protection circuitry begins to limit the
total amount of power delivered.
For purposes of calculating the total load impedance
that is presented to the P2050, assume that speaker
impedances do not change with frequency. The
Appendix shows various series and parallel combinations
of speakers and the effective loads they present to the
P2050. Formulas for the power delivered to each
speaker in a parallel or series combination are included.
Note that a series connection of two speakers de-
grades the damping factor because each speaker looks
back at the amplifier through the impedance of the
other speaker (see Page FOUR 7). Thus the effective
output impedance of the P2050 as seen by one speaker
1/2 P2050
0.08 {2 Output Impedance
Fig. 43A — Speakers in Series
1/2 P2050
> 0.082 Output Impedance
882
Impedance of
Series Speakers
802
Fig. 43B — Equivalent Circuit: Speaker Impedances in Series
1/2 P2050
Effective Output Impedance 80
= 802+ 0.08(2= 8.080
Fig. 43C — Circuit as Seen by One Speaker of Series Pair
is equal to the actual output impedance of the P2050
plus the impedance of the other speaker.
Also, the impedance of most speakers lowers with
frequency, so that the effective load of two 8 ohm”
speakers in parallel across the output of the P2050
may be as low as 2.5 to 3 ohms at certain frequencies.
Thus, speaker loads much lower than 8 ohms nominal
impedance could overload the amplifier, especially if
the actual impedance drops far below the nominal
impedance. Figure 44 shows the variation of impedance
magnitude with frequency for one type of speaker
system.
сто 509 ||| iY]
“100 500 1K 5K 10K 50K
FREQUENCY (Hz)
Fig. 44 — Free-Air Impedance of Typical “872” Loudspeaker
NOTE: Impedance changes when loudspeaker is installed in a
cabinet.
The impedance of constant-voltage speaker transfor-
mers, such as those used on “70-volt” and “25-volt”
commercial sound systems, also falls with frequency.
This effect is exaggerated in lower quality transformers.
Note that a “perfect” transformer would not have any
impedance of its own. If low efficiency transformers are
used, the system will need more transformers and
speakers to achieve the same SPL than if higher quality
transformers were used. Thus, “economy” transformers
may actually cost more in the long run than higher
quality professional types. The P2050 is not intended
for use in 70 volt systems, but may be used in 25 volt
systems. In such installations a capacitor in series with
the output of the P2050 can limit the current at low
frequencies (see Page SEVEN 6), and thereby avoid the
possibility of constant protection circuitry operation, or
damage to the transformers from excessive output
power from the P2050.
FREQUENCY (Hz)
Fig. 45 — Impedance of Poor Quality 70-Volt Speaker
Transformer {Connected to 8 (2 Speaker, Tapped for “5 Watts,”
looking into Primary). 25-Volt Transformers Exhibit Similar
Behavior.
ACTIONS OF THE P2050 PROTECTION CIRCUITS
Several of the P2050's features contribute to the
protection of the amplifier and its loudspeaker load:
Fuse
The AC line fuse protects the P2050 from excessive
AC line voltage and, in the unlikely event of an internal
failure, the AC line fuse protects the amplifier from
severe damage. Always replace a blown fuse with the
same size and type. If the fuse blows consistently, the
P2050 should be checked by a qualified technician.
Grounding
The third wire on the AC line cord is a ground
wire, This wire connects the chassis of the P2050 to
AC ground for safety. Do not defeat this safety feature
unless other methods have been employed to ensure a
good AC ground.
Thermal Protection
There is a thermal fuse, located inside the P2050's
power transformer, that shuts down the AC power to
the P2050 if the temperature of the transformer
windings reaches 130° Centigrade. Special heat com-
pensating circuits in the P2050 insure that the amplifier
will perform properly within its operating temperature
limits.
Overload Protection
The P2050's overload protection circuits limit the
maximum power available to drive any load. The effect
of these circuits is to smoothly limit the power to loads
below 2.5 ohms. The overload protection circuit action
15 virtually inaudible, even when driving difficult, multi-
speaker loads. Figure 4, Page FOUR 1 and Figure 15,
Page FOUR 3 graph the power output of the P2050 for
varying load impedances.
Transients and DC Protection
The P2050 displays virtually no turn-off transient,
and the turn-on transient is minimal. A DC voltage at
the input will not be amplified (Figure 20, Page
FOUR 4), thus protecting speaker loads against
damage from DC at the output of the P2050.
13
GROUNDING AND SHIELDING
Definitions
Ground: A general term, used in various ways through-
out the audio industry. It can mean the same as
“common,” “earth,” “chassis” or “return.”
Earth: A connection made to the actual soil or dirt.
Also a connection made to a cold water pipe or any
other device that ultimately enters the soil, and that
can provide a very low impedance path to the soil.
Common: The “return” wire of an audio pair; any
point where several such return wires connect with
each other. There can be “signal commons,” “DC
power supply commons” or “AC power supply com-
mon” (neutral). A common wire may cr may not be
connected to ground or earth. Similarly, the AC power
supply ground may or may not be connected to the
audio system common or to earth.
Shield: A metallic shell around a cable, amplifier, or
other device that helps prevent the entrance of
unwanted interference.
Grounding: The process of careful connection of
common, shield, ground, and earth connections to
avoid unwanted hum and noise.
Ground Loop: If a common or return signal can travel
from one point to another via two or more paths, the
resulting circular path is called a “ground loop.”
Figure 46 shows two possible ground loops in an
audio system.
Mixer or other P2050
Device with
Unbalanced Output
Unbalanced Cable [
Ll D
al 3]
Hs ПЕ TL
Shield {
ls
Chassis
Connections
Ground Loop Path, follows Shield, through A
Device Chassis and AC Ground in Building Chassis
3 Wire Connections 7 3-Wire
AC Power | AC Power
Cable Ground Loop Path | Cable
——-—-—-—=-—_-—-—--——-—- —— — — — «= ===
AC Ground
(Part of Main AC System}
Mixer or other
Device with Stereo
Unbalanced Outputs P2050
Ground Loop Path
Shield \
L >
Chassis owe (- E = оп. = —— — LT
Connections
~~ — —<*
Shield
Ground Loop Path follows Shield of One
Cable to Chassis of P2050, back through
Shield of other Cable and through Mixer
Chasis,
Fig. 46 — Two Possible Ground Loops in an Audio System
RFI: RFI (radio frequency interference) comes from
any number of sources, including radio stations, CB
radios, SCR (electronic) light dimmers, neon lights and
others. RFI may show up in a sound system as a radio
program, as a hum or buzz, or as other noise. RFI often
enters a sound system at a low level preamplifier stage.
Many RFI problems can be cured by careful grounding
and shielding, and by the use of balanced, twisted pair
cables.
EMI: EMI (electro-magnetic interference) typically
comes from power transformers (either in a sound
system or a building's electrical supply), motors, or
cables carrying large amounts of current. EMI usually
shows up in a sound system as a hum or buzz. Twisted
1X14
pair, balanced lines effectively reject most EMI. When-
ever possible avoid placing sensitive equipment near
motors or transformers, and use twisted pair balanced
lines. Keep input transformers several inches away from
the P2050's power transformer.
Careful grounding and shielding can minimize
externally caused hum and noise. These techniques,
in essence, are to use balanced lines, use shielded cables,
and eliminate ground loops.
Use of Balanced Lines
Balanced lines are discussed in the Appendix. This
paragraph summarizes their advantages over unbalanced
lines for noise rejection. Balanced lines reject RF and
electromagnetic interference by phase cancellation
between the conductors; twisted conductors aid the
rejection. Balanced lines help avoid grounding problems
because the shield does not carry any signal current, as is
explained further in following paragraphs. Also, any
noise currents entering the shield cannot directly enter
the signal path because the shield is not part of the
signal path (in contrast to an unbalanced line, where
the shield is the signal “return” wire).
Use of Shields
An effective shield also aids noise rejection. The
shield effectiveness of many types of cable is specified
in percentage of density. A close braided shield can be
highly effective, but may be more expensive and is
harder to work with than foil shields. Foil shields, in
most cases, are more suitable for permanent cable con-
nections since they are easier to prepare. Many guitar
cables, especially the coil type, have poor shields and
are the source of much of the hum common in guitar/
amplifier systems. A poor quality cable may also
exhibit “microphonics,” a condition where movement
of the cable can cause noise in the sound system.
Noise Entering Center Conductor thru Shield
Dielectric
Center Conductor
Loosely Wrapped Shield
Noise Entering Center Conductor Thru Shield
Fig. 47 — Poor Quality Shielded Cable
Metal equipment racks and metal electrical conduits
are also effective shields against RF noise. However, few
shields offer really effective protection against electro-
magnetic interference (EMI). Solid iron conduit and,
possibly to a lesser extent, steel conduits and racks do
offer some protection. Fortunately, however, most
EMI can be avoided effectively by keeping sensitive
wiring and equipment away from large power transfor-
mers, electric motors, etc., and by using balanced,
twisted pair cabling whenever possible.
Ground loops are a common source of noise pickup.
Figure 55 shows the way noise enters a system through a
ground loop. One common source of ground loops in a
sound system is the double grounding path between
equipment caused by AC grounding the chassis of each
piece of equipment, and then making a second ground
connection between the two chassis via the signal cable
à rs Electromagnetic
Noise Source
shield. Figure 46, Page SIX 13 shows this problem.
Figure 49 shows a method of avoiding this type of
ground loop in a system by using what is known as a
“telescoping shield” connection where each piece of
equipment is AC grounded for safety, but a ground
loop is avoided by connecting the signal shield at one
end of the cable only. Traditionally, the shield is con-
nected at the “far” end of the cable, so that shield
currents “drain” in the same direction as the signals flow.
Figure 51 shows a similar connection using unbalanced
lines. The AC grounds on each device have been “lifted”
so that the only ground connection between two pieces
of equipment is the shield of the signal cable. Since, in
an unbalanced cable, the shield carries signal current, it
cannot be disconnected. Moreover, in this type of un-
balanced grounding scheme, if the shield becomes
disconnected inadvertently at some point along the
signal path, some pieces of equipment will not have an
AC ground, so safety is compromised.
Shield Carries Return Signal
CT
ae A
Center Conductor Carries Signal
4
[A
Noise Signal
Enters Return
Circuit
/
Second Return Path thru
AC GND (or other path)
Creates “Ground Loop”
Fig. 48 — Noise Entering System through Ground Loop
Devices with Balanced Inputs & Outputs
— >
— e
Shield Connected
at One End Only
3-Wire
AC Cable
Amplifier AC Grounds which are carried through main AC System
complete the shield without creating a ground loop.
Fig. 49 — Telescoping Shield
(Al Input Polanty Switch
in Normal” Pos:hon
“Grounds” Pin 3 of XLR
[connects to Pin 1)
CHAN A
Source
(8)
Balanced _
Source
Fig. 50 — Feeding the Input of the P2050 from a Balanced
Source without a Balancing {Bridging or Isolation) Transformer.
Unbalancing the line at the P2050's input (CHAN A
Diagram) will usually result in lower hum levels than unbal-
ancing the line at the source (CHAN B Diagram).
Devices with Unbalanced Inputs & Outputs
So is
‘Return Wire”
—
Shield Connected
TT
Two- Wire at Both Ends
AC Cable 3-Wire
or 3-Wire AC Cable
with Adapter
AC Ground
Since AC Ground is connected only at last device, Ground Loop
is avoided,
WARNING: Possible AC safety hazard exists if shield is broken:
see text.
Fig. 51 — Avoiding Ground Loops in an Unbalanced System
In any audio system, there are numerous ways by
which ground loops can be created. For example, if a
microphone feeds two mixers through a splitter device,
and the two mixers are AC grounded through their
power cables, a ground loop is formed. In this case, it's
better to lift the shield leading from the microphone to
one of the mixers than to lift the AC ground of one of
the mixers. This procedure not only preserves the safety
of the AC ground, but may actually provide better noise
suppression. If you learn to look for these potential
problems as a system is designed, you can avoid much of
the last-minute troubleshooting that is so often necessary
to get rid of hum and noise.
Balanced
Shielded Cable
dd.
М7
[Ground Loop!
| Path
Lift Shield Here to
Break Ground Loop
|
Balanced Mic \ |
С |
| Splitter и
Balanced Box +
Shielded
Cable
Building
AC Ground
Mixer 2
Fig. 52 — Avoiding a Potential Ground Loop when using two
Mixers and a Mic Splitter.
For safety reasons, the final ground point in a system
should actually be earth ground. Electrical codes always
require that the building's AC ground be connected to
earth ground at the building's AC service entrance. By
connecting the sound system ground to earth, instead of
connecting it to some arbitrary three-prong AC outlet,
you avoid any noise that may be traveling along the
building AC ground wire, and you are assured of a good
ground for safety, even if the AC ground wire at the
outlet is interrupted (See Page SIX 16). A good earth
connection can be obtained at a cold water pipe, or by
driving a long metal rod into moist ground. Hot water
pipes {which are usually disconnected electrically from
earth at the hot water heater), PVC pipes (which do not
conduct electricity), and connections to cold water
pipes which must travel through a water meter before
entering the earth are poor choices for earth grounding.
1X15
However a cold water pipe running through a water
meter that has been electrically bypassed does provide
a good ground connection.
It is worth mentioning that systems without ground
connections may be capable of interference-free
operation. Portable tape recorders and other battery-
operated, self-contained audio equipment are not earth
grounded. The electronics in airplanes are not grounded
to earth (at least not during a flight), yet the equipment
operates well. 7he purpose of earth grounding a sound
system is to keep the chassis of all equipment at the
same potential as the AC mains ground for safety.
Connecting the same unbalanced input signal to both
inputs of the P2050 causes a small, but unavoidable,
ground loop. To avoid hum pickup problems, keep the
area enclosed by this loop as small as possible by running
the two cables to the input close together. If the two
inputs are “chained,” keep the connecting cable as short
as possible.
P2050
Unbalanced
Source О
Device Unbalanced
“Y” Cable — — — — — 7
| Path of |
\ / | Ground Loup |
| —
/ A
\/
To minimize hum pickup, keep “Y” cable
branches physically close together.
Unbalanced
Source
Unbalanced Feed Cable
HT
0)
А
A
a
(TT T7
| Path of |
| смо |
| Loop
Unbalanced
Cable
|
Li
X
To minimize hum, keep cable as short as possible.
If practical, use “Mono” Mode instead of chaining
the inputs.
Fig. 53 — Minimizing Hum with Unavoidable Ground Loops
Grounding on the Road
Many of the above procedures are difficult to use on
the road. For example, the telescoping shield concept is
nearly impossible to use on a portable cable. Similarly,
it is a difficult and time consuming process to search for
a water pipe ground every time the system is moved
from one performance to another. Yet portable systems
can be extremely complex, and may have major ground-
ing problems.
The telescoping shield concept can be extended to
portable systems by installing a “ground lift switch” on
the output of each device, and on the inputs of devices
after the mixer. Since microphones are not grounded
except through the mixer, there is no need for an input
ground lift switch on most mixers. Figure 54 shows a
typical ground lift switch installation. By judicious use
of these switches, each piece of equipment can be AC
grounded for safety without causing ground loops.
Because of leakage currents from equipment in the
audio system, and in the house, some noise currents can
ride on the AC ground wire and are able to enter the
audio system. This problem is usually most noticeable
12X16
with sensitive equipment such as the mixer. Lifting the
AC ground at the mixer can often solve this problem.
However, lifting the AC ground on the mixer also lifts
the AC ground on the microphone chassis, causing a
safety hazard. Try connecting the mixer and any other
sensitive equipment to other AC circuits. The only
other apparent solution to this problem is to eliminate
the noise on the AC ground, which is not an easy task.
Since it has its own ground, a portable AC power distri-
bution system connected to the house service entrance
may be the most effective way to avoid all AC noises.
Such a system can be designed and constructed by a
qualified electrician; check local electrical codes before
each use.
Devices with Balanced Inputs & Outputs
DEI |
Cable with Shield Intact >
2
L |
L Land
$
Ground Lift
Switch Installed
3 Wire in Box or on
AC Cable
Rack Panel
Main AC Ground
Fig. 54 — Use of Ground Lift Switch
Perhaps the best answer to portable system
grounding problems, RFI, EMI, and AC noises, is to
develop a versatile grounding scheme. Ground lift
switches and adapters, and a portable AC power dis-
tribution system allow different grounding techniques
to be tried easily and quickly when a problem occurs.
AC: POWER, FUSES, ACCESSORY OUTLETS,
WIRING, SAFETY (Applicable on U.S. and Canadian
models only.)
The P2050 requires an AC voltage of 105V AC to
135V AC, 50 or 60Hz. If the voltage falls below 105V
AC or rises above 135V AC, the P2050 will not operate
properly, and may be damaged. At full power with both
channels operating into 8 ohms, the P2050 draws
approximately 216 volt-amperes, or 1.9 amps at 120V
AC (see Figure 13, Page FOUR 2). When a system uses
several P2050 amplifiers, check the current capacity
of the AC line, and distribute the amplifiers among
several AC circuits, if necessary. It is extremely im-
portant to always replace a blown AC fuse in the
P2050 with the same type and value.
The American Electrician's Handbook by Croft,
Carr and Watt, published by McGraw Hill, is a good
reference for an understanding of proper AC wiring.
Other smaller books, often available in hardware or
electrical supply stores, detail simplified residential
wiring. We do not suggest that you modify the AC wiring
in an auditorium or a club, or anywhere else. Such work
should be reserved for a qualified, licensed electrician.
But, if you understand proper AC wiring, you will also
understand the potential problems of improper wiring,
some of which are described below.
“Hot” (Black)
“Neutral” (White)
“GND” (Green)
Fig. 55A — Properly Wired 120VAC Outlet
CAUTION: In any audio system installation, govern-
mental and insurance underwriters’ electrical codes must
be observed. These codes are based on safety, and may
vary in different localities; in all cases, local codes take
precedence over any suggestions contained in this
manual. As set forth in the P2050 Warranty, Yamaha
International Corporation shall not be liable for
incidental or consequential damages, including injury
to persons or property, resulting from improper, unsafe
or illegal installation of the P2050 or of any related
equipment; neither shall the Corporation be liable for
any such damages arising from defects or damage
resulting from accident, neglect, misuse, modification,
mistreatment, tampering or any act of nature.
Lifted Ground
Broken, or disconnected AC ground wires in
existing AC outlets can create shock hazards; so can
older, two-wire sockets with no ground. Note that un-
less metal conduit connects the older, two-wire AC
outlet to ground (an uncommon practice except in
some public buildings), the screw on the outlet cover
plate is probably not grounded either. In this case, an
AC ground, or earth ground must be located some-
where else.
“Hot” (Black)
“Neutral” (White)
“GND” (Green) Y
Fig. 55B — 120VAC Outlet with Disconnected AC Ground
Wire creating potential shock hazard.
Reversed Polarity
Improper polarity connections, or polarity modifi-
cations, can cause reversal of the “hot” and “neutral”
AC wires. This can cause shock hazards, and noise in
some equipment.
“Hot” (Black)
“Neutral” (White)
“GND” (Green)
Fig. 55C — 120VAC Outlet with Polarity (Hot and Neutral)
Reversed creating shock hazard and causing possible noise.
Lifted Neutral
The “neutral,” or return, wire of a 120V AC circuit
should be connected to AC ground at the building
service entrance where the main AC power enters. How-
ever, this neutral is usually a center tap from a 240V AC
circuit; if it becomes disconnected at the service
entrance, a varying voltage will appear at the AC outlet,
which may rise as high as 240V AC, depending on the
load on each circuit. This poses shock hazards, and can
easily cause equipment damage.
Hot
E | Neutral
©
Е 120V
EC |
8 X
$ ! GND
120V Neutral
2 240V Hot |
E |
=
0
GND
Fig. 55D — 120VAC Outlets with Lifted Neutral. Outlets will
operate with voltage varying from 0 to 240VAC creating shock
hazard and causing possible equipment damage.
240V AC on 120 AC Outlet
It is possible, though illegal and dangerous, for a
240V AC circuit to be connected to a 120V AC outlet
as shown in Figure 55E. Fortunately, this rarely occurs.
In an older building, it may have been done to allow
120V AC wiring to carry the 240V AC voltage needed
to run lighting equipment. If the P2050, or some other
audio device, is plugged into such an outlet, the AC line
fuse will blow almost immediately, but some equipment
may still be damaged. In addition, this type of outlet
poses a shock hazard.
Hot No. 1
S
a 120V
©
all
8
>
|
= 120V
© 240V Neutral
= Hot No. 2 (No GND
co Connected)
Fig. 55E — 120VAC Outlet with a 240VAC Circuit
connected to it. This is a highly dangerous and illegal connection.
120V AC Outlet Connected to Dimmer Circuit
Possibly more common than the 240V-wired 120V
outlet is the connection of a 120V AC stage outlet to a
lighting dimmer circuit. This may have been done to
allow lighting to be controlled on stage from a remote
location. Connecting an outlet to a dimmer is a poor
practice, and the light dimmer can decrease the voltage
in the circuit. Some dimmers are capable of raising the
AC voltage. In either case, audio equipment connected
to the circuit may suffer damage, and shock hazards
are also possible.
Dimmer
Hot
Neutral
GND
Fig. 55F — 120VAC Outlet connected to a Light Dimmer
Circuit, a dangerous and illegal connection.
er
The best way to avoid all kinds of AC mains pro-
blems for permanent or portable systems is to check
the voltage and polarity of the outlet yourself — before
plugging in any audio equipment. Three wire AC circuit
testers are available at most hardware and electrical
stores, and will allow easy polarity and ground con-
tinuity checking of all outlets. While these testers may
show that an outlet has an extreme over-voltage con-
dition (the tester may burn out), the tester may not
show less extreme, but still serious, over-voltage con-
ditions. Also, even though such testers may display
continuity to ground at the third pin of the AC outlet,
the resistance in the ground may still be high enough to
warrant the use of a separate earth ground. Thus, it is
also a good idea to carry a small voltmeter for verifying
the actual voltage at an AC outlet, and to establish a
direct path to earth ground that does not rely on the
AC mains. Some commercially available AC plug strips
have an AC voltmeter built in, or you can install a
panel mount meter that reads voltage before equipment
is connected to the AC circuits in an equipment rack.
Even if the voltage and polarity of the AC outlet are
correct, the line may be “soft,” that is, it may not be
capable of sustaining proper voltage under load. Monitor
the AC line voltage when the P2050 is operating near
full power. If the AC line voltage falls below the
minimum rated for the P2050 (105 volts rms), the
P2050 will not operate properly, and could conceivably
sustain damage.
Lifting the AC ground to an audio device, while it
may solve some noise problems, also lifts the safety
feature for which the AC ground was originally designed.
If you must lift the AC ground, be certain that the AC
ground is carried through to that piece of equipment via
the shield of a signal cable, or by some other means.
Other Safety Considerations
While it may seem obvious, the P2050 does weigh
16 165 (7.2 kg), and should be adequately mounted to
prevent it from falling onto other equipment or people.
Also, while less obvious, the speaker output terminals
of the P2050 can deliver as high as 20 volts rms, and
under certain conditions, this could present a shock
hazard. It is common practice in the audio industry to
use “male” connectors to carry output signals, and
“female” connectors for inputs. For speaker level
signals, however, it may be safer to reverse this con-
vention, or to use “recessed male” type connectors as
outputs to avoid the possibility of coming into
contact with the output voltage of the P2050.
MONO OPERATION
Connections
With power OFF, move the rear-panel STEREO/
MONO switch to the MONO position. After placing the
amplifier in MONO mode, connect a mono input signal,
such as a single output from a mixer or other source, to
the P2050's Channel A input. Do not connect anything
to the Channel B input.
Connect the speaker load to the two red terminals
(+) on the P2050's outputs as shown in Figure 56. Do
not connect either speaker wire to ground as this would
short out one channel of the P2050, and would severely
cut the power available to the speaker load.
2X18
Use the Channel A attenuator to control the power
(signal) level. Keep the Channel B attenuator in its
“infinity” position (maximum attenuation).
In the “mono” mode, the P2050 will produce a full
95 watts into a 16-ohm load. The voltage output from
the P2050 in the mono mode is approximately 39 volts
rms; and since it can drive even highly reactive loads
with complete stability, it is suitable for driving constant
voltage 25V commercial sound speaker lines. The P2050
can offer cost savings when compared to multiple, low-
power amplifier installations. In addition, the P2050's
performance specifications far exceed most commercial
sound amplifiers.
Do Not Connect Speaker Wires to Speaker
Black (—) Output Terminals; Do Not Load
Connect either Red (+) Terminal
to Ground
} Е
(+) (+) |,
Red
Input
Fig. 56 — Output Connections for Operating P2050 in
“Mono” Mode
APPLICATIONS
BIAMPLIFICATION AND TRIAMPLIFICATION
Biamplification, or “biamping,” triamplification, or
“triamping,” all refer to the use of separate power
amplifiers to cover separate portions of the audio
spectrum.
The traditional, non-biamplified speaker system 1s
diagrammed in Figure 57A. The crossover network,
which routes the high and low frequencies to their
respective speakers, is located in the circuit between
the power amplifier and the speakers. A large system
may contain many power amplifiers, crossovers, and
speakers.
Figure 57B diagrams a biamplified speaker system,
and shows the crossover located in the circuit before
the power amplifiers, and a separate power amplifier
for the high and low frequencies. A triamplified system
has an extra crossover section, another power amplifier,
and a woofer, midrange and tweeter. Alternately, it has
a woofer, tweeter and super-tweeter.
The crossover for a biamplified system is a low level
crossover since it processes low power signals. It may
also be called an active or electronic crossover since it is
usually an active device using transistors, tubes, and/or
IC's. Some low level crossovers are passive, having no
transistors, tubes, or IC’s. All high level crossovers used
in non-biamplified speaker systems are passive and they
must process the full power of the power amplifier.
There are any number of good reasons for taking a
biamplified or triamplified approach to a professional
sound system. One reason is that a biamplified system
can actually provide more headroom per watt of
amplifier power than a system with a traditional, high
level, passive crossover.
Conventional Passive,
High-Level Frequency
P2050 Dividing Networks
— High Frequencies To Tweeter
> Low Frequencies To Woofer
> —— High Frequencies To Tweeter
Fig. 57A — System using Conventional, Passive/High-Level
Frequency Dividing Networks.
Low Frequencies To Woofer
Yamaha Optional Feed
F-1030 To Amplifier Driving
Electronic Super Tweeters
Frequency {Triamped System)
Dwviding
Network 1 12 P2050
High
| To Tweeters — Tweeters
Frequencies
Low
Frequencies
1/2 P2100
To Woofers
Fig. 57B — Biamplified System using Y amaha F 1030
Electronic Frequency Dividing Network, and Yamaha P2050
and 2100 Power Amplifiers.
ECTION SEVEN
Headroom
Program material (music or speech) is made up of
many different frequencies and their harmonics. Most
music, especially popular music, is bass heavy; that is,
there is more energy at low frequencies than at higher
frequencies. When both high and low frequencies, such
as a flute and a bass guitar, are present in a program, the
high energy bass frequencies can ‘use up’ most of the
power in a power amplifier leaving none for the high
frequencies. The result can be severe clipping of the high
frequency material. With an electronic crossover, the
high frequency material can be routed to its own power
amplifier, avoiding the clipping problem. This results in
an effective increase in headroom that is greater than
would be obtained by simply using a larger, single power
amplifier.
Figure 58A shows a low frequency waveform from a
power amplifier output. The peak-to-peak voltage of the
waveform is 84 volts, corresponding to 3C volts rms. If
this voltage were applied to an 8-ohm speaker load, the
power level would be 110 watts, which is equal to the
peak output of Yamaha's P2100 professional power
amplifier into an 8-ohm speaker load.
Figure 58B shows a high frequency waveform from a
power amplifier output. The peak-to-peak voltage, rms
voltage, and power into an 8-ohm speaker load are less
than shown in Figure 58A and correspond to a 7 watt
output into an 8-ohm load (21.2V P-P, 7.5V rms). The
levels of these high and low frequency waveforms are
typical of musical content.
Figure 58C shows the effect of adding the signals of
Figure 58A and Figure 58B, corresponding to a low
frequency note and a high frequency note being played
at the same time. Note that the total peak-to-peak
voltage (which would be 105 volts if it were not
clipped) is greater than the peak-to-peak voltage of
either signal by itself. For an amplifier to produce this
Fig. 58 — Advantages of Biamplification
SEVENZ
voltage into an 8-ohm load, it must be rated at 175 watts
(power is proportional to voltage squared). Since the
P2100 is rated at 110 watts peak, this waveform is
clipped, especially the high-frequency component.
If the same two waveforms in Figure 58A and
Figure 58B were reproduced by two separate amplifiers,
the total amplifier power needed would only be 117 watts
{the sum of the two powers 110 + 7), not 175 watts.
This power could be provided by one P2100 and one
P2050 amplifier. Thus, using two power amplifiers to
produce these two waveforms reduces needed amplifier
power capacity. Or, if you use two P2100 amplifiers,
there is a substantial increase in headroom.
NOTE: This headroom discussion presumes the waveforms
add coherently. In practice, biamplification may offer
less of a headroom improvement than suggested by these
mathematics.
Efficiency
A passive crossover is made up of resistors, capacitors,
and inductors. The resistors in the crossover “use up’
some power, as do the losses in the capacitors and
inductors. By removing the passive crossover, these
losses are also removed.
Damping
Damping was discussed on Page FOUR 7. With
reference to that discussion, any impedance inserted
between an amplifier's output terminals and a speaker’s
input terminals, reduces the damping factor; a passive
crossover is such an impedance. Thus, biamplification,
by removing the passive crossover improves the effective
damping factor.
Distortion
An electronic crossover avoids any possible non-
linearities that might be caused by a passive crossover.
This avoids one source of distortion. Also, as previously
explained, an electronic crossover reduces clipping dis-
tortion by adding headroom.
If clipping does occur, amplifier-caused harmonic
distortion may be less audible in the biamplified or tri-
amplified system. For example, if the power amplifier in
a conventional system clips during a very powerful, low
frequency note, unwanted harmonics are generated.
The conventional system would pass the harmonics
through the crossover to the tweeter, where they would
be audible. In a biamplified or triamplified system,
there is no crossover after the power amplifier. Thus,
the clipped low frequency note and its harmonics would
be restricted to the low frequency driver. Since the low
frequency driver is less sensitive to high frequencies than
the mid or high frequency drivers, the high frequency
harmonics would be attenuated, which would decrease
the audible distortion.
Dynamic Frequency Response Shift (also see Page
FOUR 6)
When the peaks of a complex waveform are clipped
off by inadequate headroom, two things happen. First,
since these peaks are usually high frequency informa-
tion, the high frequencies are lost, or reduced severely.
At the same time, the clipping creates new harmonics of
the input frequencies. These two factors can be con-
sidered to be changing the frequency response of the
system on a dynamic (changing) basis, depending on
the amount of clipping present.
When to Use a Traditional, Passive Crossover
In small sound systems, where high sound levels are
not needed and economy is a major consideration, a
speaker system with a traditional, passive crossover net-
work may be the best choice. For example, Yamaha's
S4115H, S0112T and SO110T are excellent as stage
monitors, or as main speaker systems for small to
medium sized clubs. For larger installations, a biam-
plified or triamplified system will not only perform
better than a system with passive crossovers, but it will
probably cost less too; the increased efficiency and
headroom allow fewer amplifiers and speakers to
produce the same sound level, and fewer crossovers
are required.
Realizing the Advantages
To realize the advantages of a biamplified or tri-
amplified system, the electronic crossover must be able
to work well with a variety of different power amplifiers
and speaker systems. In addition, because it plays a
critical role in the sound system, the electronic cross-
over must be highly reliable, and its performance must
be as good as, or better than, any other component in the
system. Yamaha's F-1030 electronic frequency dividing
network (electronic crossover) meets these needs. it is
an excellent choice for any biamplified or triamplified
system.
Criteria for Biamped Systems
Crossover Frequency and Slope
There is a freedom of choice available to the designer
of the biamplified or triamplified system that is not
available to the designer of a non-biamplified system.
The added advantage of being able to choose crossover
frequency and slope means that the system can be care-
fully optimized for a specific application, or it can be
made highly versatile for use in a wide variety of
applications.
Most manufacturers of quality speaker components
carefully specify both power capacity and frequency
range. The choice of crossover frequency can be based
on this information. For example, if a high frequency
driver's power capacity is rated at 20 watts of pink noise
from 2kHz to 20kHz, a crossover frequency of 2kHz or
higher is a good choice. A lower crossover frequency
might allow over-excursion of the driver's diaphragm,
leading to premature failure. If the system is biamplified,
the woofer will be chosen to complement the high fre-
quency driver's response. |f the system is triamplified,
both a woofer and a midrange driver or a super
tweeter must be selected so that the frequency ranges
of all the components complement each other. Pre-
ferably, there should be some overlap in the frequency
range of each successive driver.
The choice of crossover slope involves a tradeoff
between speaker protection and phase shift. A low
slope rate of 6dB/octave will produce a smooth system
response with minimum phase shift, but it may not
adequately protect high frequency drivers from exces-
sive low frequency energy or low frequency drivers
from excessive high frequency energy. A high slope rate
of 24dB/octave or higher will protect the drivers better,
but can introduce more phase shift than a crossover
with a lower slope rate. 12dB/octave and 18dB/octave
are widely used, and are good compromises. 12dB/octave
is the most common choice, but 18dB/octave can pro-
vide a little extra protection for sensitive components,
especially high frequency drivers. Again, decisions should
be based on a careful study of the abilities of the in-
dividual components, and of the system requirements. |
One common method of designing a three way
system, with woofers, midrange, and high frequency
drivers, is to biamp the system between the woofers |
and midrange, and to then use a passive, high level cross-
over between the mid and high frequency drivers. Since
there is generally less energy in the high frequency
range, the extra headroom and efficiency that would
be obtained by triamping may not be needed. This com-
promise will usually save money without adversely
affecting performance or reliability.
Selection of a Crossover (Dividing Network)
There are only a few passive, high level crossovers on
the market that are suitable for professional sound
systems. Those that are built into a finished speaker
system, such as Yamaha's S4115H, S0112T and
S0110T, are exceptions. Because of the limited
selection, a custom designed system with passive high
level crossovers usually has to be designed around the
crossover instead of around the drivers. Still, the cross-
over should meet certain criteria. It should have an
impedance equal to the desired speaker system
impedance (the impedance of the woofer, midrange
and tweeter must be the same for most passive, high
level crossover systems). If possible, choose the cross-
over frequency and slope by the criteria described in
the previous paragraphs. Also, choose a passive, high
level crossover with adequate power handling (for
reliability), good quality components (for low loss
and low distortion) and with rugged physical
construction.
The designer of a biamplified (or triamplified) system
must choose an electronic crossover from an expanded
set of criteria. A professional electronic crossover should
meet the professional criteria described on Page FIVE 1
for balanced inputs and outputs, and for input and out-
put levels and impedances. In addition, in order to be
usable in a variety of professional systems, an electronic
crossover should give the designer a choice of crossover
frequencies and slopes. Some electronic crossovers
restrict the choices, or require hard-wired changes, or
plug-in cards to choose different frequencies or slopes.
Yamaha's F-1030 is a two way or three way electronic
crossover which gives the designer a wide choice of
crossover frequencies selectable for each of three bands
by means of front panel controls. The controls are
recessed to avoid accidental setting changes. Either
12dB/octave or 18dB/octave slope rates can be selected
by internal switches. The F-1030 meets all the criteria
for a professional unit, and, in addition, has both XLR
and phone jack input and output connectors.
ECHO, REVERB, AND DELAY
Artificial echo is usually obtained in either of two
ways: with a tape delay similar to a standard tape
recorder, or with a digital! delay unit. Repeated echoes
are obtained by feeding some portion of the delayed
output back to the echo input (regeneration), or by
using multiple output taps along a tape or digital delay
path. In a tape recorder, the delay results from the time
it takes for the tape to travel from the record head to
the playback head or heads. In a digital delay unit, the
audio is converted to a computer-like digital code using
an analog-to-digital converter, delayed by shift registers,
and then reconverted to audio using a digital-to-analog
converter. Other methods of obtaining time delay are
available, from “bucket brigade” (analog) time delay
units to a technique where a microphone is inserted in
one end of a length of tubing and a speaker at the other
end.
Besides its use as an effect, a time delay device can be
a very useful tool in commercial sound systems. If two
speaker systems, which are fed by the same signal, are
separated by more than about 30 feet, a listener can
hear a distinct echo. By slightly delaying the signal to
the speaker system nearest the listener, such echoes
are avoided. This situation is presented in the Applica-
EVEN
tions section, in the diagram for a typical system in a
theatre. Here there is a primary speaker system at the
stage, and a secondary system under a balconv (which
cannot be covered directly by the stage speaker system).
COMPRESSION AND LIMITING
Dynamic range is the difference, in dB, between the
highest and the lowest volume levels in any audio pro-
gram (also see Page FIVE 2). A compressor is a device
that “shrinks” that dynamic range. The “threshold” of a
compressor is the level above which compression begins.
The “compression ratio” is the ratio of output level
change to input leve! change, in dB, for any program
material above the threshold. A limiter is a compressor
with a high compression ratio, usually 10:1 or higher.
Often a single device can be used for either compression
or limiting, since the distinction depends mainly on the
threshold and ratio settings.
Radio stations use compressors and limiters. Limiters
keep audio peaks from overmodulating and distorting
the broadcast signal (an FCC requirement), and com-
pressors keep the average modulation levels high in order
to reach the maximum audience.
The dynamic range of better quality magnetic tape
recorders is about 65dB. Since much live program
material has a dynamic range of 90dB or greater, a
recording studio can use a compressor/limiter to
restrict the dynamic range of a program to fit the
dynamic range of the tape medium. Special “noise
reduction” devices are available for tape recording, and
make use of complementary compression and expansion
to lower the noise levels on a tape recording and to
retain the original dynamic range of the program.
In a paging system, a compressor can keep the
average level of different announcers’ voices more con-
stant, so that paging can reach noisy areas of a factory
or airport more consistently. In addition, because of
reduced dynamic range, peaks are lowered, reducing the
chance of clipping distortion.
In concert sound reinforcement, or other large sound
reinforcement systems, a compressor/limiter can reduce
the chance of peak clipping, and can thus help avoid
amplifier or speaker damage from large turn-on/turn-off
transients, or from sudden, loud feedback. These uses of
compressor/limiters are valid for recording studio moni-
toring as well as for sound reinforcement, although
feedback should not be a problem in studio monitoring.
1/2 P2050
To
Tweeters
High Limiter
Yamaha Frequencies | optional
Yamaha F 1030
‚ Frequency
PM-Mixer Dividing To
Network Wooters
Low Limiter LL
Frequencies | optional)
1/2 P2100
Fig. 59 — Biamplified System showing Placement of
Optional Limiters.
While useful, compressors (compressor/limiters) are
not cure-all devices. The compressor “makes its decision”
to begin compressing by continuously monitoring the
program level. Unfortunately, the highest levels are
usually low bass notes. Thus the compressor/limiter
may compress the high frequencies needlessiy when it
detects a bass note that is too loud. One solution to this
problem is to use a compressor on each output of an
electronic crossover on a biamplified or triamplified
system so that the compressor acts only on the fre-
quencies in each band. This method requires two or
EVENS
three devices and is probably not applicable to broad-
cast. Another solution is to use a separate compressor
on each mixer input that receives excessive program
levels.
Another problem with a compressor is that, if it is
over-used, it can reduce the quality of sound in a
musical performance. Reduced dynamic range is often
audible, and a poor quality compressor can add
appreciable distortion to a program, especially at high
compression ratios.
EQUALIZATION, HIGH AND LOW PASS FILTERS
Equalization, originally, was the process of
‘’equalizing’’ the levels of the various audio frequency
bands for a “flat” system response. The term now
encompasses many different devices and techniques
that are used for effects purposes as well as to “smooth”
the response of a system.
Room Equalization
Whether it be a recording studio, concert hall, airport
lounge, or night club, a room has a frequency response
of its own. Carpeting, draperies and padded furniture
can soak up sound, primarily at high frequencies. The
high reverberation time of large concert halls usually
affects the low frequency sounds more than the high
frequency sounds. For these and other reasons, it may
be desirable to shape the frequency response of a sound
system to compensate for the response of the room.
Generally, acoustic solutions are the best answers to
acoustic problems, especially for severe resonances or
excessive peaks or dips in the room response. However,
for final smoothing of system response, or for portable
systems where acoustic solutions may be impractical,
electronic room equalization can be a valuable aid.
There are several different methods of room equali-
zation. Most methods use a specified sound source, such
as pink or white noise, or a tone burst which is played
through the system. The sound is monitored at one
point or at several points in the room using a “real
time” monitoring device. “Real time” means that the
monitor displays the system response on an instanta-
neous basis. A graphic equalizer, or other type of
equalizer is used to adjust the system response to com-
pensate for response irregularities displayed on the
real time monitor.
Equalization can also help to smooth the response
of a speaker system, a microphone, or most any type
of audio device. However, this can cause problems, as
explained below. Frequency response shaping techniques
can also be used for special effects: to increase the
sizzle of a cymbal crash, to sweeten the sound of a
violin or to add warmth to a singer's voice.
Equalizers come in all types and varieties. Some are
most suitable for a specific task, others have more
general uses.
Graphic Equalizers
A ““graphic’’ equalizer is a multi-frequency, band
reject filter, or a bandpass/reject filter. Unlike the input
channel equalizers on a mixer, a graphic equalizer can
simultaneously operate at several 1-octave, 1/2-octave, or
1/3-octave frequency bands. Most graphic equalizers use
1.5.0. standardized center frequencies. (1.5.0. stands for
the International Standards Organization.) The units
are called “graphic” because most have linear slide con-
trols, and when they are set they create a visual image
that resembles the overall frequency response curve of
the unit. Some so-called graphic equalizers use rotary
controls. A graphic egualizer may provide attenuation
Graphic Equalization can be used to reduce resonant peaks
in the overall sound system {which consists of the microphones,
instruments, room and speakers. 1-octave EQ illustrated.)
NOTE: Shaded area represents sound level above which feed-
back will occur. If any frequency is reproduced at a level in the
shaded zone, then either the overall sound level must be turned
down (lower volume), or the graphic equalizer must be used to
reduce the level of the frequency band where the excess level
occurs. Proper selection and placement of microphones and
speakers can reduce the need for equalization. “He who equalizes
least equalizes best” (anon.).
A. A microphone picks up a vocal peak at TkHz, making it
necessary to reduce the average level (horizontal dotted
line) to some 5dB below the feedback point.
B. Lowering the 1kHz Graphic EQ slider about 5dB pulls down
the resonant peak and allows the overall volume to be raised
several dB. Any further increase of the volume control may
cause feedback to occur at several frequencies where lesser
peaks occur: an electric bass at 125Hz an acoustic guitar
resonance at b00Hz, and a stage monitor speaker peak at
2kHz that is being picked up by a nearby microphone.
С. То allow the average level to be raised further, the 125Hz,
500Hz, and 2kHz Graphic EQ sliders are pulled down
slightly. This smooths the overall frequency response and
allows maximum loudness throughout the audio spectrum.
A natural roll-off at the low and high ends remains, and 1s
preferred by many users. If flatter response is required, it can
be achieved.
D. If the input channel tone controls were used to bring up the
high and low ends of the spectrum, too much lift would
occur toward the middle, causing feedback. Also, too much
overall bass boost would waste amplifier power and might
lead to burned out speakers or excess distortion. By lifting
the 62.5Hz, 8kHz and 16kHz Graphic EQ sliders slightly, the
response is flattened without unwanted distortion, and
without creating feedback,
Relative Level in dB
=
>
L
Relative Level in de
с
Relative Level in da
O
Relative Lavel in dB
100 500 IK 5K 10K 20K
Frequency 10 Hz
Fig. 60 — How to Use 1-Octave Graphic Equalization
only (band reject), or attenuation and boost {band
pass/band reject).
Usually, each speaker feed requires its own channel
of professional-type graphic equalization which is
installed between the mixer output and the power
amplifier input. Stage monitor feeds, for example, may
require very different equalization than house feeds.
In recording and broadcast applications, the graphic
equalization applied to the recording is usually for tonal
considerations, and to avoid exceeding the frequency
response limits of the medium. At the same time, the
studio monitors or audience foldback system might
require graphic equalization to suit very different ends.
Professional graphic equalizers are usually more
durable than hi-fi type units, and they operate at
nominal +4dB (1.23 volts) line levels. The input of a
hi-fi type graphic equalizer will probably require padding
for use with professional type mixers such as the
Yamaha PM-Mixers, and the output level may be too
low to drive some common power amplifiers. The
P2050's sensitivity is high enough to allow it to be
driven by most hi-fi type graphic equalizers. Aside from
level and impedance criteria, some graphic equalizers
have characteristics that cause the overall response curve
to change drastically when one frequency band is ad-
justed, so two or more bands must be adjusted to pre-
serve a smooth response. Other equalizers maintain a
smooth transition to adjacent bands when just one
control is adjusted.
Parametric Equalizers
A parametric equalizer is one whose parameters can
be varied to suit the application. The parameters include
such factors as filter bandwidth (Q), center frequency,
and amount of boost or cut. Usually there are several
filters, and some parametrics are set up for stereo
operation. Each filter section in the equalizer can
either cut or boost frequencies within its band, and
the ranges of center frequencies available from adjacent
filters usually overlap.
Center Frequency
Center Frequency
Actions of a parametric equalizer
as the bandwidth is varied but
the amount of boost or cut
remains constant.
Actions of a parametric equalizer
as the amount of boost or cut is
varied but the “Q" is held
constant.
Fig. 61 — Actions of a Parametric Equalizer
A filter adjusted for wide band rejection character-
istics (low Q) can perform room equalization in a
similar manner to a graphic equalizer, or it can act as a
variable frequency cut or boost tone control. In a
narrow band reject mode (high Q), a parametric
equalizer can be used for feedback control, or to notch
out hum or feedback frequencies without subtracting
much of the adjacent program material.
Used carefully, a parametric equalizer can be an
RELATIVE LEVEL IN dB
20
EVEN
equalizer may “ring” at high-Q (narrow bandwidth)
settings. Ringing is caused when a filter begins to act
like an oscillator. While ringing may be usefu! as an
effect, it also may cause unwanted peaks in the
system's frequency response curve.
Other Equalizers
Tone controls are another type of equalizer. So are a
number of the special effects devices, like “wah-wah
pedals,”” “phasers,” “flangers,” etc. Each of these devices
was designed for a special purpose.
HIGH EQ
MAX
LOW EQ | HIGH EQ
CENTERED
CENTERED
RELATIVE LEVEL (dB)
HIGH EQ
MIN
100 1K
FREQUENCY (Hz)
10K
Fig. 62 — Actions of Tone Controls
High pass and low pass filters are special purpose
devices. They are sometimes called “horizontal” filters
because they do not boost or cut in the same manner
as a graphic or parametric equalizer, which would be a
“vertical” filter.
High pass filters, which pass frequencies only above
their cut off frequency, are used to cut low audio and
subsonic frequencies from a sound system. Using a 40Hz
or 80Hz high pass filter, for example, reduces dangerous
dropped-mic, or turn-on, turn-off transients, etc., but
allows all significant program frequencies to pass.
A low pass filter, which passes frequencies only
below its cut off frequency, can stop high frequency
oscillations and certain RF interference from reaching
the speakers.
In commercial sound systems, high and low pass
filters cut unneeded frequencies from the system, and
thus increase the total capacity of the system to repro-
duce the frequencies of interest.
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extremely helpful tool for sound reinforcement or for
recording. It should be remembered that, like graphic
equalization, excessive boost may reduce system head-
room, create clipping and make extreme power demands
on amplifiers and speakers. In addition, a parametric
00
500 IK 19% 5
FREQUENCY (Hz)
Fig. 63 — Actions of Typical High and Low-Pass Filters
NOTE: P2050 High Pass Filter is 12dB/octave at 20 or
200Hz.
SEVEN
Equalizer Problems
The previous discussions illustrate some of the many
uses of the various types of equalizers. Like any signal
processing device, an equalizer can also cause problems.
From the power amplifier’s viewpoint, the most signifi-
cant problem that can be caused by an equalizer is
clipping of frequencies that have been boosted to
extremes. If these boosted frequencies are in the treble
range, the clipping may sound like an irritating sizzle
that only happens on certain sounds. Similarly, clipping
in the low frequencies can cause bass notes to sound
fuzzy or muddy, or it can cause mid-range frequencies
to be harsh. Yet because the clipping only takes place
on certain sounds, it may not be immediately apparent
that clipping is the source of the problem. The choice of
a cut-only graphic equalizer, rather than a boost and cut
device, may help solve the problem; since boost
is not available, clipping problems are reduced. With cut
and boost graphics, parametrics, or other types of |
equalizers, the system operator must be aware of the
potential for clipping.
SPEAKER PROTECTION
The output power of the P2050 into an 8-ohm load
is 45 watts. Not all single speaker systems are capable of
absorbing that much power on a continuous basis. Most
speaker systems, however, are capable of absorbing short
duration peaks of considerably higher power than their
rated continuous power capacity. The speaker must be
protected against the abuses of excessive average power,
sudden large peaks, DC current, and frequencies outside
its range. The following are methods of achieving some
degree of protection against these abuses.
Fuses
Yamaha does not recommend the use of any type of
fuse as speaker protection. Fuses are slow-acting devices
of inconsistent quality, and do not offer adequate pro-
tection for speaker systems. They are mentioned here
only because they are used in some systems. Standard
fuses may be capable of protecting a speaker against
excessive average power, but they are too slow to
successfully protect a speaker against sudden peaks.
Fast-blow instrumentation fuses, with improved time
response, may blow on normal program peaks and
needlessly disrupt the program. Slo-blo fuses, on the
other hand, may not blow quickly enough to prevent
loudspeaker damage due to voice coil overheating. If
fuses are used, whenever possible, fuse each loudspeaker
separately so that a single fuse failure will not stop the
show.
A fuse will protect a loudspeaker against one com-
mon fault of a DC coupled amplifier: DC at the output.
The slightest DC offset from a direct coupled pre-
amplifier will be amplified and appear at the power
amplifier's output as a larger voltage with the power
amplifier's large current capacity behind it. Even though
there is no immediate audible affect (the extra power
draw may cause some amplifiers to hum slightly), the
loudspeaker absorbs the DC power output of the ampli-
fier. Since it cannot convert this DC power into acoustic
power, the speaker converts the DC to excessive heat.
Small amounts of DC voltage can shorten the life of a
loudspeaker, and any large amount of DC will cause sud-
den, catastrophic failure. Fortunately, the input of the
P2050 is not DC coupled so any DC voltages from pre-
amplifiers, etc., are not amplified and cannot reach the
speaker. The only time DC voltage could appear at the
P2050's output would be in the event of a severe
electronic failure inside the amplifier, a very unlikely
event.
Capacitors
Inserting a non-polarized capacitor in series with a
high frequency driver can protect the driver against
excessive low frequency energy. The capacitor acts as
a 6dB/octave high pass filter. Especially on a bi-
amplified or triamplified system, this kind of pro-
tection is desirable. In such as system, choose a
protection capacitor by the following formula:
500,000
Value (in microfarads) = Tx fx Z
(Where 7 = 3.14, “f” is the crossover frequency divided
by two, and Z is the nominal impedance of the driver.)
The same formula can be used to choose a capacitor
to insert in series with a low quality 25-volt speaker
transformer to avoid excessive current flow at low fre-
quencies (see Page SIX 13). With a speaker load con-
nected to the secondary, measure the impedance of the
transformer primary at the lowest frequency of interest,
which will probably be somewhere around 100Hz.
Choose the protection capacitor by the above formula
with Z = the measured impedance of the transformer,
and f = the lowest frequency of interest divided by two.
The voltage rating of the capacitor chosen must be
greater than the maximum expected total peak to peak
voltage that will ever appear at the driver's terminals.
For the P2050, this is equal to the sum of its positive
and negative supply voltages, which is 86 volts. The
most common types of capacitors used for driver pro-
tection are non-polarized electrolytics. Because of the
inductance associated with an electrolytic capacitor,
it may be paralleled with a mylar capacitor of about
1/10 the value in microfarads to reduce high frequency
losses.
Limiters
A limiter is not normally considered a loudspeaker
protection device, but it may be one of the best and
most practical. A “squared up’ or “clipped” waveform
causes a loudspeaker cone or driver diaphragm to move
to one position and stay there, then move back to the
extreme opposite position, and stay there, etc. Because
there is still power flowing through the voice coil, but
there is no voice coil movement, the power is converted
to heat. If a limiter is placed before the power amplifier
in a system, the limiter can be adjusted to prevent peaks
from reaching a level that would cause the power
amplifier to clip. This may avoid burned out loud-
speakers.
Transformers
The constant voltage transformers used in some com-
mercial sound systems lend a certain amount of pro-
tection to a loudspeaker. They will not pass DC current,
and most of them will not even pass subsonic frequencies
or very high frequencies, such as RF oscillations. Some
transformers have attached protection capacitors for
use with high frequency drivers.
Auto-transformers are sometimes used to match
speaker impedances. The auto-transformer provides
many of the same protections as a constant voltage
transformer, with the exception that it is possible for a
small amount of DC current to leak through to a loud-
speaker because the taps of an auto-transformer are all
from the same winding.
Passive Crossovers
Because a passive crossover usually inserts a capacitor
in series with the high frequency driver, and often
inserts an inductor in series with the lew frequency
driver (which limits the current reaching it), it can aid
Power
Amplifier
Auto
Transformer
Fig. 64 — Typical use of Auto-Transformer for Speaker
Impedance Matching which also helps protect the Speaker
from damage caused by DC at the Amplifier's Output.
in loudspeaker protection.
High Pass and Low Pass Filters
The functions of high and low pass filters were dis-
cussed on Page SEVEN 5. Because these filters limit the
subsonic and supersonic frequencies reaching the loud-
speakers, they can help prevent loudspeaker damage.
LEFT CHANNEL
Biamplified
Studio Monitor
Biamplified Control
Room Monitor
Left
Foldback
Out
Control Room Monitor Out
Studio Monitor Out
EVEN
SPECIFIC APPLICATIONS
The following diagrams illustrate a few of the many
possible applications of the P2050 in all types of sound
systems.
Studio Monitoring
The diagram in Figure 65 shows the P2050 used as a
studio monitor amplifier. Part of the system is biam-
plified. Alternately, the Yamaha F-1030 crossover
could be used for a triamplified system with a P2100
or a larger amplifier, such as the P2200, for the woofers.
The P2050's dB-calibrated attenuators are a distinct
advantage in this application. The operator can reduce
the level of a particular set of monitors, such as the
studio monitors during a “take,” and bring them back
up later to exactly the same setting.
Sustained power output, exceptionally low distortion
wide bandwidth and low phase shift, combined with
high reliability make the P2050 an ideal choice for a
studio monitor amplifier.
,
RIGHT CHANNEL
Biamplified
Studio Monitor
Biamplified Control
Room Monitor
Stereo
Headphone
Distribution
System
Right
Foldback
Out
Mixing Console
(Yamaha PM-2000, etc.)
Control Room Monitor Out
Studio Monitor Out
Fig. 65 — Recording Studio Monitor System (Biamplified with Yamaha F-1030 Frequency Dividing Networks)
SVENS
Concert Sound Besides having exceptional specifications, the P2050
Figure 66 illustrates the P2050 in a typical setup for is extremely reliable, and is built to take the abuses of
concert reinforcement. Note that there are a number of the road. Bracing the rear of the P2050 in a portable
completely separate feeds, with separate limiters, rack will “ruggedize” it for the most extreme cartage
equalizers, electronic crossovers and power amplifiers. requirements. (The P2050's AC coupled input will not
Individual channels can be easily checked during pass dangerous DC signals, further protecting speakers.
setup by turning down the calibrated attenuators on In addition, the P2050's protection circuits smoothly
all other channels. When check out is finished, it’s easy limit power during severe thermal and power demands
to bring back the levels to previous settings. (see Page SIX 13).
Amplifiers Operating in “Mono” Mode:
A separate bank of amplifiers is used
for each channel. Super Tweeter
| [11692
+
P2050 Rear Panel ED ¡do - | IN
Switches Set at A EL
“Mono” & “200Hz’’ 4-0 y High
Freguency
Horn
F1030 Yamaha Electronic Frequency Dividing Network
e
4 de { NOTE: P2100 Internal
Ares Stereo/Mono Switch
Set to Mono
ел |
1
J 1 16 {2
J | + Ur Low
To Other P2050, Optional Limiters Frequency
P2100 & P2200 | pie 4 Woofer
Main Amplifiers 14e A le
(As Required) 3 AI || |
Graphic
Equalizer
(Optional)
Transformer
Splitter
From Other Sources [ ©
Inputs
Yamaha PM-1000 Console
or PM-2000 Console
Outputs
To Other
- ———) Monitor
Program Out Right Monitor Out Right Yamaha Speaker
P2050
or
Program Out Left To Left P2100
to Other Dividing Monitor Yamaha
Network, Amps Channel S4115H
and Speakers Stage
Monitor
Speaker
Fig. 66 — Concert Sound System
Portable Instrument Amplifier
Figure 67 details possible connections for a portable
setup for a guitar amplifier. Ideal for this application,
the P2050 can easily reproduce the high power notes
that may be clipped off by lower power instrument
Electric
Guitar
SEVEN
amplifiers. Thus, it will ‘clean up’ the sound. In
addition, the P2050 is sensitive enough and its input is
of sufficiently high impedance that it can be driven by
the output of most hi-fi or semi-pro type preamps. The
P2050's calibrated attenuators can be turned down
during a break so that no preamp settings need be
disturbed.
All of these advantages apply when the P2050 is
used as a keyboard amplifier, with the additional
advantage of true stereo operation.
Guitar
Preamp (head)
Speaker
System
oh)
[©]
2
Fig. 67 — Instrument Amplifier
Headphone Distribution System
(Illustrations on next page.)
There is often a need to distribute audio to one or
more sets of headphones, sometimes with individual
level controls for each set. While many mixers and
consoles have phones outputs, some are capable of
driving only one or two sets of headphones, and others
require an external power amplifier to drive even one set.
The P2050 can power many sets of headphones, either
mono or stereo (Figures 68A and 68B).
It is important to determine the type of headphones
involved: high impedance (600 ohms or higher) or low
impedance (4, 8 or 16 ohms). A large number of high
Impedance phones may be connected in parallel across
the P2050 output; they draw little power, and their
combined impedances are still well above the amplifiers
2.5-ohm current-limiting protection circuit threshold.
If low impedance phones are used, their combined
parallel impedances must be considered; a good way to
ensure that the amplifier will not be overloaded is to use
10-ohm “build-out” resistors on each headset. Because a
substantial amount of power may be dissipated, the
resistors should be rated at a minimum of 5 watts (if
potentiometers are used, they should be ratedata
minimum of 10 watts).
Y -cables may be used for muiti-headphone distri-
bution, but a more reliable and convenient approach is
to use single input/multi-output distribution boxes. These
are easy to construct. Figures 68C and 68D illustrate two
types of stereo headphone distribution boxes, one with
and one without volume controls.” If the box is built
entirely with stereo phone jacks, then special Headphone
extension cables (with tip/ring/sieeve phone plugs) will
have to join the amplifier to the box(es). An alternative
IS to use stereo phone jacks for the headphones, but to
use XL connectors for the input and an extension output;
then standard mic cables can link the amplifier to the
box(es)
CAUTION: When using XL connectors pre-wired
in studio application, install a male connector in the wall
to carry the power amp output. This avoids any
possibility of accidentally plugging the power amp
output into a microphone input. Parallel-wired male
and female XL connectors mounted in each box facilitate
linking additional boxes using the same standard mic
cables.
*The schematics and photos for these boxes are provided
courtesy of Windt Audio, Inc., 1207 N. Western Avenue,
Los Angeles, CA 90029 (Phone (213) 466-1271).
A à
Jp
4
a
q
-
4
P2050
(Stereo)
Channel B Channel A
Use High Impedance Phones for Large
Systems (or a few low impedance
phones) unless resistive isolation ıs
employed (see ''8” below).
Two Single-Channel
Line-Level
Program Sources
Y
(Mono) Use Only High Impedance Phones Here
L AJ
Single-Channel
Line-Level
Program Source
Fig. 68A — Two versions of Mono Headphone Distribution Systems
бе
Without resistive isolation, With build-out resistors,
only high impedance, or 102 102 any number of low and/or
a few low impedance 102 100 high impedance phones
phones may be used. may be used.
Common ; |/
Right 9 9
Left 9 4
7 7
P2050
+ — (Stereo) — +
Fig. 68B — Stereo Headphone Distribution System
2-Channel Line-Level
Program Source
Phones Phones Phones Phones Phones Phones
Out
! Male XL |
|
— Ч - - — |
| с |
1
| 3 |
L—_ — J
о
|
|
|
!
| Stereo Phone |
Fig. 68C — Headphone Distribution Box for use with High Impedance Phones only.
Phones Phones
Right Left Right Left
Volume Volume Volume Volume
105
Out
| Male XL Ш
| | |
| |
| à |
1
| | e |
L Luz J
r ео
| I I
— _`роотоандия
| |
Р Î Left | |
[ Stereo F hone | {Stereo Phone |
Fig. 68D — Headphone Distribution Box with Volume Controls, for use
with Low and/or High Impedance Phones.
Discotheque
Disco systems, such as the one diagrammed in
Figure 69, really test an amplifier’s endurance. The
music from a record album may be highly compressed
so that its average power content is high, and the
amplifier may not get even a short rest during many
hours of operation each night.
Monitor
Speaker
Yamaha
SO110T STAGE AREA
VEN
With its massive heat sinks and sustained power
output capabilities, the P2050 is a highly reliable
amplifier for disco use. In addition, the P2056's low
distortion will produce clean sound, the kind of sound
that avoids listening fatigue — an important consideration
for high sound level operation. For larger floors where
higher power is required, we recommend using our
P2100 or P2200 amplifiers.
Monitor
Speaker
Y amaha
Sso110T
T -50dB
О) E
Direct Box
9) -50dB
Jr -50dB
-40dB
» Bar
©) Announce
a On-Off Mic
| | Inputs Switch
1-Channel PM-180 Mixer
Graphic EQ
(Optional) PGM Out NOTE:
Use right mix for
Line-Level Feed to Stage Monitors , +âdB stage monitor &
> left mix for program
Line-Level Submix to Main Mixer +4dB feed to PM-430.
Spare
Input
y RIAA Phono
- _ \
D. J. AREA
— 4 —
[= X eN -50dB__Preamp _20dB
| |
т RIAA Phono
Je о Z\B89N_50dB, CMP, _20dB
/ — |
From Phones | Hi-Level Inputs Low-Level Inputs
Out Jack
D.J. Cue
Headph
eadphones 2-Channel Graphic |
Equalizer (Optional)
For Making
Recordings
if a В
= A Cl
<
+
| D.J. Mic
PM-430 Mixer
PGM Out PGM or Mon Out
2 |
+4dB À Ts
|= , | +4dB
Left Right
Speaker DANCE Speaker
Y amaha FLOOR Yamaha
S4115H AREA S4115H
Speaker Level Lines
Right Left
Speaker Speaker
Yamaha Yamaha
S4115H
S4115H
Fig. 69 — Combination Disco/Live Club Sound System Setup
=VENT2
Commercial Sound Systems
The factory paging/background music system is
Figure 70 also shows the P2050 used in “mono” made.
One P2050 feeds the main factory areas with a highly-
compressed signal. The other P2050 feeds office areas
with a separate, less compressed signal that has been
equalized for a more natural sound.
The P2050 is exceptionally stable, even under highly
reactive constant-voltage line loads. In smaller systems
and in larger systems, the P2050's dB-calibrated attenu-
ators help the installer and operator achieve optimum
system performance. In fact, the P2050 can improve
just about any commercial sound system design, from
auditoriums and other reinforcement systems to
electronic church organs, shopping centers or airport
paging.
Other Uses
The P2050 is a basic tool for all types of sound
systems, yet it is not limited to sound systems alone.
Due to its own exceptional performance, the P2050
will not degrade the performance of even the highest
quality test oscillators, noise generators, tone burst
generators, function generators or other equipment.
ÉS
NU,
Main
Paging Paging
Mics from Mic
Other Areas
“Y” Cable]
or Mono _
Output
uted Factory Area
. Yamaha PM-180 Speakers with
Background Music Source Rack-Mount Mixer 25-Volt
(Tape, Tuner, etc.) Transformers
To
Other
Speakers
)
Office Area в hu
Speakers with Com- Limiter Relays
25-Volt Transformers pressor for Zone
Paging
P2050
©
©
©
©
LO
Fig. 70 — Factory Paging System
APPENDIX
DEFINITION OF TERMS: dB, dBV, dBv, dBm and
dB SPL |
The term dB, which means decibel (1/10th of a Bel),
expresses a ratio. The dB notation allows us to represent
very large ratios with small numbers, which are easier to
understand and use. The ratio in dB of two power
levels is equal to 10 times the logarithm (base 10) of
their simple numeric ratio:
dB = 10 log P, / Py
The ratio in dB of two voltages (V4 and Vo) or
sound pressure levels (P4 and Po) is equal to 20 times
the logarithm (base 10) of their simple numeric ratio:
dB = 20109 М, / Vo
Examples:
Power: The ratio in dB of 100 watts and 50 watts.
dB = 10 log 100/50 Answer: +3dB
Note that this means that 100 watts is 3dB above
50 watts. If we had compared 50 watts to 100 watts
(dB = 10 log 50/100), the answer would have been
—3dB. Similarly, any time the ratio of two powers is
2:1, their ratio in dB is +3dB. When the ratio is 1:2,
the ratio in dB is —3dB.
Voltage: The ratio in dB of 100 volts to 50 volts.
dB = 20 log 100/50 Answer: +6dB
Note that a ratio of 2:1 in voltage means a ratio in dB of
+6dB. If the ratio is 1:2, the ratio in dB is —6dB.
SPL: SPL ratios, expressed in dB, are similar to voltage
ratios. For example, two SPL levels with a numeric
ratio of 2:1 would have a ratio in dB of +6dB. SPL
ratios are seldom given as numeric ratios. Simple
numeric SPL levels would have units of dynes per
centimeter? or newtons/meter?.
The term “dB” implies a ratio. To express a single,
specific quantity in dB, there must be a reference
quantity. There are standards reference quantities for
SPL, voltage, and power, which extend the usefulness
of the dB notation system.
dBV or dBv express a voltage ratio. It is not directly
related to current or circuit impedance. OdBV is usually
referenced to 1 volt rms, and OdBv to 0.775V rms.
Example: The level in dBV of 10 volts rms:
dBV = 20 log 10/1 = 20 x 1 Answer: +20dBV
dBm expresses a power ratio. It is related to the voltage
or current across a low impedance. The 0dBm reference
is 1 milliwatt, which is equal to 0.775V rms in a
600-ohm circuit.
Example: The level in dBm of 1 watt:
dBm = 10 log 1/0.001 = 10 x 3 Answer: +30dBm
dB SPL expresses acoustic pressure (not power ratio).
The OdB SPL reference is 0.0002 dynes/square cm, which
is the approximate threshold of human hearing at 1kHz.
ECON
EIGHT
NOTE: Since SPL values in dynes/cm® or newtons/m?
uncommon, an example is not given.
dB expresses the difference between two levels (power,
voltage, sound pressure, etc.) and is a relative term. The
difference between +10dBm and +4dBm is 6dB. The
difference between —20dBV and —10dBV is 10dB.
dBV and dBm are not numerically equal when
dealing with 600-ohm circuits, although they are close;
OdBV is +2.2dBm at 600 ohms. As the impedance is
changed to other than 600-ohms, given a constant volt-
age, the value of dBV remains constant while the value
of dBm changes. For example, consider a +4dBm out-
put terminated by 600 ohms. The voltage level is
+1.8dBV. This circuit has a voltage drop of 1.23V rms,
and a power dissipation of 2.5 milliwatts. Assume that
the voltage now remains constant, but the termination
Is changed to 1200 ohms. The voltage level remains
+1.8dBV, but the power dissipation drops to 1.23mW,
+1dBm. Continuing this illustration, we raise the ter-
mination to 47,000 ohms. The voltage level remains
+1.8dBV (1.23V rms), but the power level drops to a
mere 32 microwatts, —15dBm.
The above illustration points out that the power
dissipation in high impedance circuitry is negligible.
Therefore, dBV is most often reserved to express signal
levels in high impedance lines and for microphone
specification. The term dBm is commonly used to express
signal (power) levels in low impedance lines, roughly
between 4 ohms and 1200 ohms. However, dBm is also
widely misused to express levels in high impedance lines,
rather than dBV. This is because at 600 ohms the
voltage for a given dBm is not the same as the voltage for
the same number of dBm in other impedances. If the
source is constant voltage or the input is high impedance,
then the voltage level is more important than the power.
Summary:
An increase of 3dB is equivalent to double the power.
An increase of 10dB is equivalent to ten times the
power.
A decrease of 3dB is equivalent to half the power.
A decrease of 10dB is equivalent to 1/10 the power.
An increase of 6dB is equivalent to double the
voltage or SPL.
An increase of 20dB is equivalent to ten times the
voltage or SPL.
A decrease of 6dB is equivalent to half the voltage
or SPL.
A decrease of 20dB is equivalent to 1/10 the voltage
or SPL.
SPECIAL USE OF dB (volts) IN THIS MANUAL
Assume that a Yamaha PM-Mixer has a constant input
voltage, such as the sinewave signal from a test generator.
The mixer output then acts very much like a perfect
sinewave voltage source because the mixer’s output
impedance is much lower than the load impedance. That
is, even when the load impedance varies from the lowest
rated load to an extremely high impedance, the output
voltage from the mixer will remain relatively constant.
However, the mixer's output power does vary with the
load impedance. dBm is a power rating, and if the
PM-Mixer’s maximum output were rated in dBm, that
rating would change with the load impedance. Thus,
a dBm-rated output level would be valid oniy at a single
load impedance, usually 600 or 150 ohms.
EIGHT2
It is common to rate a mixer's maximum output in -
dBm referenced to 600-ohms, and to treat this rating as
if it were a voltage rating, even though it is actually a
power rating. If a mixer’s maximum output is rated at
“+24dBm,” the rating really means “12.3 volts,” which
is the voltage produced by a power level of +24dBm
into a 600-ohm load. If you realize that by this rating
method, “+24dBm” means “12.3 volts,” and that
“+4dBm” actually means “1.23 volts,” etc., then you
can accurately interpret the specification. Of course,
there are mixers that will deliver 12.3 volts into a high
impedance, but cannot sustain this voltage with a
600-ohm load, and such mixers could not be honestly
rated at +24dBm. (NOTE: If the mixer’s output
impedance is 600 ohms or lower, it should be able to
sustain the rated output voltage into 600-ohm loads.)
One possible way to avoid the common, but often
misused, “dBm” output rating method would be to rate
the maximum output of a mixer in dBV or dBv. Since
the mixer acts like a voltage source, this would be an
accurate rating regardless of the load impedance. Un-
fortunately, the dBV rating is relatively uncommon in
audio, although it is used for some microphone ratings,
so it would be unfamiliar to most users and therefore
difficult to interpret.
To avoid confusion, we have rated outputs in “dB
(volts),”” where the dB value is equal to the voltage
produced by a numerically equal ““dBm’’ rating in a
600-ohm circuit. By this method, “OdB (0.775 volts)”
is exactly the same as “0dBm’’ for a 600-ohm circuit.
Since it is a voltage rating, however, “dB (volts)” is
accurate regardless of the mixer's load impedance, so
long as the mixer is not overloaded. Also, since the
actual output level, expressed in volts, follows the dB
rating, it is unlikely that the rating will be misinterpreted.
By this convention we have not created a new unit, we
have merely endeavoured to avoid the imprecision of
existing, widely used dB ratings. There is no substitute
for a thorough understanding of the mathematics
behind dB relationships, but such a discussion is
beyond the scope of this manual.
For additional information, refer to these books:
“Sound System Engineering”
“The Audio Cyciopedia”
OHM'S LAW
Ohm's law relates voltage, current and resistance in
a DC circuit by the following equation:
V=IR
(Where V is voltage, | is current, and R is resistance.)
Other forms of Ohm's law, derived by simple
algebraic manipulation, are:
| = V/R
R = V/l
“ For this Simple Circuit, the Voltage produced by the Source
Vs equals the Voltage across the Resistor Vg:
The Voltage across and Current through the Resistor are
related by Ohm's law:
Va=Ig XR
The Power Dissipated in the Resistor is:
2
PR=VR =R
OR
_ 2
PR Ig xR
OR
Pp=Vgpxig
Fig. 71 — Ohm's Law
POWER
In a DC circuit, the power absorbed by a resistor is
given by the following equation:
P=VI
(Where V and | are the voltage and current through the
resistor, and P is the power dissipation.)
By using Ohm's law and some algebraic manipulation
again, we come up with two alternate forms of the power
equation:
P=V2/R
P=12xR
A “perfect” voltage source would always produce
the same voltage, regardless of the load resistance, and
would be capable of an infinite current into a short
circuit (zero ohms resistance). A “perfect” current
source would always produce the same current, regard-
less of the load resistance, and would be capable of an
infinite voltage into an open circuit (infinite ohms
resistance, or no connection).
“Perfect”
Voltage
Source (
R =0 =
о ) ho о +
+
“) |
The Voltage across the Resistor is Equal to the
Source Voltage: _
VR Vs
because there is no voltage drop across the source's zero-ohm
output resistance.
Real World
Voltage
Source
The Voltage across the Resistor is Less Than the Source
Voltage due to the voltage drop across the output resistor R o:
R
= V —C—
R S В + Во
Vv
NOTE: X is the output resistance (or impedance) of the
source. Y is the voltage drop across the output resistance which
varies depending on other circuit parameters.
Fig. 72 — Voltage Sources.
IMPEDANCE
Impedance is the total opposition to the flow of
alternating current in a circuit. An impedance is made of
a pure resistance combined with a reactance. The reac-
tance itself consists of a capacitance, and inductance, or
some combination of the two. A “pure” resistance
maintains the same value, in ohms, whether the voltage
or current changes from a DC source to an AC source
at any frequency. On the other hand, the value of an
impedance, in ohms, changes with frequency, making it
more challenging to manipulate mathematically.
“Pure” circuit components, such as a perfect voltage
source, a perfect current source, or a pure resistance, do
not exist in the real world. However, the P2050 can be
considered to be a perfect voltage source because it
behaves in this manner within its specified operating
limits. Similarly, a source such as a mixer that is feeding
the P2050 can be considered to be a perfect voltage
source in series with a pure resistance, that pure
resistance being equal to the mixer's output impedance.
In some cases, even a speaker impedance can be
considered to be a pure resistance, although in other
cases the variation with frequency of a speaker's
impedance must be considered. The behavior of audio
circuits is more easily explained by making these and
other such assumptions.
To illustrate a typical simplifying assumption,
consider the fact that any impedance can be treated
as a pure resistance having a simple ohmic value, so
= АЛГЗ
long as only one frequency is used. However, single
frequencies are not representative of audio sources,
except for test tones. This is a good place to make a
simplifying assumption: assume the impedances that we
work with in audio can be treated like pure resistances
over the entire audio frequency range. In most cases,
this is a good assumption, and we have used it through-
out this manual. When the occasional exception shows
up, we have treated it separately. If we had to deal with
an actual impedance value, made up of a pure resistance
and a reactance, most of the formulas we use would be
similar, but would contain complex numbers with a
real and imaginary part. This would be a lot more
complicated, and not very much more accurate than
the simple ohmic values one works with in the
simplified equations.
VV V === В (Resistor)
— or + — C (Capacitor)
— 0 y OR NY NY \__ C (Inductor)
An “Impedance” is some combination (one, two, three or
more components connected together in a circuit) of Resistors,
Capacitors and Inductors.
Fig. 73 — Elements of an Impedance
SERIES AND PARALLEL
IMPEDANCE CONNECTIONS
Figure 74 diagrams the differences between series and
parallel connected impedances. The total impedance, ZT,
of a set of series connected impedances is simply their
algebraic sum. The total impedance of a set of parallel
connected impedances is given by the following formula:
1
ZT —
1/24 + 1/29 + 1/22 + 1/24 ... etc.
When there are only two impedances connected in
parallel, the formula can be simplified to:
; 2, X г,
T Z4 + Zs
If the two impedances are the same ohmic value, the
formula further simplifies to:
Zr = ZN
This final simplified formula is valid for any number
(N) of parallel impedances provided that their ohmic
values are all the same:
To calculate the power dissipated in any of the
impedances (any branch) of the circuits of Figure 74,
simply find the voltage across that impedance or the
current through the impedance using the voltage and
current division rules that follow. Alternately, find both
the voltage and the current in that branch. Then use
the power formulas developed on Page EIGHT 2. Note
that if all the impedances shown in any one of the
circuits in Figure 74 are the same, the power dissipated
in each of those equal impedances is also the same:
one-fourth of the total power dissipated.
SIGAN 4
O
21
22
7 ——.—
(Total Impedance)
Z3
Za
O-
Series Connected impedances.
2т = 2 + 2. + 2.4 + 2,
(See Text)
O in. "a
21 23
‘т —.
© + 4
Series/Parallel Connected Impedances.
2. 1 zy+2)) (25+ 2)
T 1 1 2. +2. +2. +2
z +2 )+ (z +2 1 2 3 4
1 2 3 4
OR
гт =(z,+22) Il (2,+2,) (IIMeans: “In Parallel With”)
O—
22
24
O
Parallel/Series Connected Impedances
Z, Z Za. Z
7 = 22), (22 ов 2т = 2; |! 2. + 2,4 || 2,
Za + Zo г. + Z,
Fig. 74 — Series and Parallel Impedances
VOLTAGE AND CURRENT DIVISION
When two or more impedances are connected in
series across a voltage source, they share the voltage
among themselves according to the following formulas,
where N is the number of impedances: (see Figure 75)
For two impedances:
a
V, =V
Z4 S 21 + La
For any number (N) of impedances:
Ca
21 + 45 + 43 +.... дц
Vz =Vg
1
If the ohmic value of all the impedances is the same,
they share the voltage equally:
V
Vo, = —S
21 N
When two or more impedances are connected in
parallel across a voltage source, they each receive the
same voltage, regardless of their ohmic value.
V> =V3 =....V, =V
La La ZN S
If the source is a current source, series connected
impedances all receive the same current: |
ly; =, =....V5, =V
21° 2. ZN Vs
Parallel connected impedances share the current from
a current source according to the following formula:
For two impedances:
22
I =1
Za S Z; + Z,
For any number (N) of impedances:
до + да +... ЖЕ
> =|
La S 21 + 25 + да +... +
Once voltage or current is known, power can be
calculated from the formulas on Page EIGHT 2.
In audio, most sources can be treated as voltage
sources, whether they are power amplifiers, microphones,
mixers, etc. By considering these devices as voltage
sources in series with their own output impedance, all
of the formulas for Ohm's law and voltage and current
division can be applied, with few exceptions.
+
<
N
7
N
Voltage Division between Two Series Connected impedances:
г Z,
\ = \/ ==; М = \ 55
Z, S г. + 29 Zo S Z, + Z,
Fig. 75 — Voltage and Current Division
{Continued on next page)
+ Vz,
a г,
Voltage is the same for Two Parallel Connected Impedances
regardless of their Impedance Values.
V> =V> =V
1 2 S
+
N.
—
-— — —
N
-—
Current is the same for Two Series Connected Impedances
regardless of their Impedance Values.
l> = |
Current Division between Two Parallel Connected Impedances:
2 2
=le 57-57) :1 =I
2, SNZ,+2,)' 2, S 2, +2,
BALANCED, UNBALANCED AND
FLOATING CIRCUITS
Unbalanced, balanced and floating circuits may all
be transformer isolated. The distinction between them
lies in the way the circuits are referenced to ground
(audio common). A FLOATING circuit has no ground
reference, as illustrated by the Yamaha PM-180,
PM-430, and PM-700 Mixers’ channel inputs and XLR
outputs. A BALANCED circuit requires either a center
tapped transformer, or resistors from each side of the
transformer to ground; either condition places both
sides of the transformer at equal potential with respect
to ground. In other words, the transformer is balanced
with respect to ground. Electronic balancing, done with
Unbalanced Circuit
with Shield
Unbalanced
Source
Device
Transformer
True Balanced Circuit
with Shield
EE
“differential” input or output circuits, can replace
transformers, with similar results. For example, the
output of the P2050 in the “mono” mode is balanced
electronically. Figure 76 shows transformer-created
balanced, floating and unbalanced lines.
Consider what happens if an RF source (radio
station, CB radio, SCR dimmer, etc.) causes a noise
current in the wires of a balanced circuit. Provided
that the source is physically distant from the circuit,
compared to the distance between the two wires, RF
will cut across both wires, creating equal noise voltages
in both wires. However, since the signals (wanted
voltage) in the two wires are out of phase with each
other, the in-phase noise (unwanted voltage) is
effectively canceled.
A balanced line may or may not have a shield. If it
does have a shield, the shield is usually at the same
voltage potential as the common or ground wire. Since
the phase cancellation of noise currents in a balanced
line is never perfect in the real world, most low level
balanced circuits, mic or line level, are shielded. Twisting
the two internal wires also helps cancel noise.
A floating line is also a two wire circuit, one which
is usually created by a transformer. However, unlike a
balanced line, the common or ground voltage has no
direct connection to the circuit. Even so, a floating line
will reject hum and noise as well as a balanced line, and
Is often used for audio applications.
TRANSFORMERS
Several applications of audio transformers are
discussed in specifics on Pages SIX 4 and SIX 5; the
following paragraphs concern general transformer
operation.
A transformer changes electrical energy at its input
(primary winding) into magnetic energy in ¡ts core.
This magnetic energy is transformed back into
electrical energy at the transformer's output
(secondary winding). If the transformer is wound
with a greater number of turns on its primary side
than on its secondary side, the voltage level at the
secondary will be lower than on the primary, and the
current level on the secondary will be higher than on
the primary. Since the impedance of a circuit is equal
to the ratio of that circuit's voltage level divided by its
current level, a transformer can transform impedances
as well as voltages and currents. These actions take place
in a precise, mathematical way described by the
equations on the next page.
Transformer
altos
Unbalanced Circuit
with Shield
Unbalanced Transformer
Source
Device Note:
No Center
Tap Ground
“Floating” Circuit with Shield
и
FLOATING circu]
| To Balanced, Floating or
| Unbalanced Device
|
Transformer
To Balanced, Floating
or Unbalanced Device
Note:
No Center
Tap Ground
Ll
Fig. 76 — Balanced vs Floating Circuits
SIGIANT6
NOTES:
“Ng” is the number of turns on the primary side of
the transformer, “No” is the number of turns on the
secondary side of the transformer.
Va” is the voltage level on the primary side of the
transformer, Vo is the voltage level on the secondary
side of the transformer.
“147 is the current level on the primary side of the
transformer, 15” is the current level on the secondary
side of the transformer.
“Z4" is the impedance of the circuit seen at the
primary side of the transformer, 29 is the impedance
seen at the secondary side of the transformer.
Equations:
1. V4 /V,=N, / No
2. [y /l9=N2/N,
3. Z, /Z2 = IN, / N91°
Equation 1 shows that the voltage ratio between the
primary and secondary windings of a transformer is
directly proportional to the transformer’s turns ratio.
This equation is applicable to voltage level matching
between two circuits.
Equation 2 shows that the current ratio between the
two windings of the transformer is inversely proportional!
to the turns ratio.
Equation 3 describes the impedance matching action
of a transformer. Note that the impedance ratio between
the primary and secondary of the transformer is directly
proportional to the square of the turns ratio.
Often, the transformer spec sheet gives its impedance
ratio, but not its turns ratio. A simple manipulation of
Equation 3 solves this problem:
4. N,/ N, = NZ,/2,
Consider the following example:
A transformer has a primary impedance of 15k-ohms,
and a secondary impedance of 600 ohms. If the input
(primary) voltage is —16dB (0.123 volts), what is the
output voltage?
From Equation 4: N, /N, = \/15.000/600
= N25=5
Since N, / № = \, / У, (Equation 1), then:
5 = (0.123 volts) / Vo, or Vo = (0.123 volts) / 5
Answer: Le = 24.6mV = -30dB
From this example, transforming a —16dB (0.123
volts) hi-fi output, with a source impedance of 15k-ohms,
to a professional input with an input impedance of 600-
ohms also drops the level a full 14dB to —30dB
(24.6mV).
Polarity Indication Mark
Core
Primary
(N4, Za)
Secondary
ML
| |
| | (No. Zo)
Hii
Electrostatic/Shields
Fig. 77 — Typical Audio Transformer
OTHER CONSIDERATIONS
A transformer actually doesn’t have any impedance
of its own. It merely transforms an impedance at its
primary to a corresponding impedance at its secondary
as described in Equation 3. Thus, the transformer in the
above example has an impedance ratio of 15,000:600
which equals 25:1. If a 150k-ohm circuit is connected
to its primary, the impedance seen at the secondary will
be 150,000/25 = 6000 ohms. Since this impedance
transformation works in both directions, if a 6000-ohm
circuit is connected to the secondary of the transformer,
the impedance seen at the primary will be 150,000-ohms.
Voltage and current matching also work in both
directions.
However, a transformer that is specified as having an
impedance ratio of 15,000 ohms-to-600 ohms has been
manufactured specifically to transform those impedances.
If it is used with circuits having considerably greater or
smaller impedances, its frequency response may be
degraded, or it can ‘ring’ (resonate). One way to over-
come this problem is to terminate the transformer with
its rated impedance as discussed on Page SIX 2.
It is also important to use a transformer at the volt-
age and power level for which it was planned. For
example, a mic level transformer will probably saturate
(distort) if it is used for line level circuit matching. Also,
a line level transformer will not perform properly if it
is used for mic level circuit matching. The magnetic
fields are so weak that non-linearity occurs. Whenever
possible, pick a transformer for each use according to
the voltage levels, power levels, and the impedances of
the circuits under consideration.
One other significant use of audio transformers is to
isolate the ground wire of one circuit from another to
prevent ground loops and reduce hum. The discussion
of balanced lines on Page EIGHT 5, and the grounding
discussion on Page SIX 13 expand this concept.
Speaker level transformers (constant voltage speaker
transformers, and auto-transformers) are discussed on
Page SEVEN 6.
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