ASHLY
MOS-FET POWER AMPLIFIERS
OPERATING INSTRUCTIONS
ASHLY AUDIO INC,
TABLE OF CONTENTS
Introduction
Unpacking, Installation
Input & Output. Connections, Stereo Operation
Mono operation
Bridging Operation
Meter Display
Thermal Status Indicator
Specifications/Block Diagram
Definition of Terms
age
Pue
INTRODUCTION
The áshiy FET-200 and FET-500 represent an entirely new breed of power
amplifiers which are destined to replace conventional amps in virtually every
application. Far from being just another variation on well worn designs, these
amplifiers have been engineered from the ground up to take full advantage of
au exciting new semiconductor technology which has recently become
available_the Power MOS-FET (metal-oxide-semiconductor, field-effect-
trausistor). Combining many of the desireable operating characteristics of
vacuum tubes with the efficiency of transistors, the MOS-FET offers superior
audio fidelity and introduces an entirely new level of field reliability.
Ashly power amplifiers put all of these features together into a carefully
thought out, roadwortay, user-oriented package which will provide years of
maintenance-free service.
WHY MUS-FET'3?
Couventional power amplifier design as typically produced a circuit which
consists of two parts; 1) the amplifier itself, and 2) a collection of
protective circuits which keep the amplifier from destroying itself and the
speakers it is driving. Some of the more common protective circuits include
VI dlimitiuz, cuermai feedback, detectors that monitor DC at the output
terainals, short circuit protection and turn-on delay circuits that keep the
Speakers from jumping out of their cabinets when power is applied. The result
has typically been a compromise in audio performance, greatly compounded
electronic aud mechanical complexity, and amplifiers which are intolerant of
less than ideal operating environments.
The basic weakness of these amplifiers has always been the power output
transistors themselves, which are prone to failure as a result of “thermal
runaway”. this breakdown occurs when the amplifier is delivering a lot of
power and begins to heat up. Increasing temperature produces an increase in
coilector current in the output transistors, which ın turn produces more heat.
If this cycle is unchecked, the transistor will be destroyed.
Provably the greatest advantage of power MOS-FET's for professional audio use
is their complete immunity to thermal runaway. Not only does this remove the
primary fuilure mechanism of the conventional amplifier and eliminate the need
for elaborate thermal protection circuits, it also allows the design engineer
tv use a much nigher quiescent current in the amplifier's output stage,
thereby insuring that the output devices are always operating in their most
linear region. This results in consistently lower distortion.
In addition, the simpler input drive requirements of the MOS-FET devices makes
possible a clean, concise amplifier design. The resulting reduction in
component count and internal wiring provides for a compact design and enhanced
reliabiiity.
THE ASHLY DESIGN
Our goal in designing the FET-200 and FET-500 was to produce amplifiers that
combine the sonic excellence of a high-end stereo amp with the ruggedness and
stamina necessary in pro audio.
ELECTRONIC
doth amplifiers are fully complementary, push-pull type with totally discreet,
high voltage, wide bandwidth electronics. This approach inherently assures
low noise, low distortion, and excellent transient response. Inputs are
briaging, active balanced (transformerless), equipped with both 1/4" jacks and
ALik-type connectors of both sexes to facilitate linking several amplifiers
together. All input connections can, however, operate as balanced or
unbalanced. This is determined by the connector used in conjunction with the
input. Stereo, mono and bridging modes are user selectable by means of rear
panel switches, requiring no internal modifications or additional components.
All electronic components are conservatively rated and are mounted on a rugged
giass-epoxy circuit board. An efficient U-I laminated power transformer saves
space and minimizes magnetic flux leakage, permitting other low-level
equipment to be mounted above or below the ampiifier with no danger of induced
hum. Large value, computer grade electrolytic capacitors assure long service.
[wo 1U-segment, 27 dB range LED meter displays provide an easily viewed
indication of power levels, and a seperate LED indicates thermal overload.
MECHANICAL
ithe Ashly MOS-FET power amplifiers are packaged in a rugged, one-plece welded
16 gauge steel chassis with an anodized aluminum front panel and an easily
removed steel top cover. The chassis and all internal components are
integrated into a single, rigid unit. The FET-500 requires 5 1/4" of rack
Space aud the FET-200 requires only 3 1/2". The power transformer is located
near tne front of each amplifier, reducing mechanical stress on the chassis.
Each channel's electronics and heat sink are combined into a compact, plug in
module which can be removed from the amplifier in seconds. Hard wiring is at
an absolute minimum.
Large heat sinks and an aerodynamic internal design permit the use of an
ultra-quiet, slow speed fan which pulls air in through the front panel
directly across tne heat sinks and power transformer, and out through the rear
panel. It is unnecessary to provide space or ventilation above or below the
amplifier, allowing for economical use of rack space.
The amplifier's output connectors are conveniently arranged on standard 3/4"
spacing tor quick interconnect via banana plugs. selecting the bridging output
involves only moving the banana plug to the two center (RED) connectors. Both
the AC line and the speaker terminals are fused protected. Fuses .can be
removed instantly without tools. There are no internal fuses.
IN USE
the result: better sounding amplifiers, remarkably un-fussy about how you use
them. They will tolerate low impedance, highly reactive loads and hot,
crowded racks. They are highly immune to RF interference and will not be
damaged by intermittent short circuits--sustained shorts will simply blow the
Output fuse. Due to their balanced circuit design, the use of delay circuits
and output relays are unnecessary. There is no turn-on thump and the volume
coutrols can remain up when power is applied and removed. Both amps will keep
on running right through a brown-out, even when line voltages fall to as low
as 20 volts!
This manual will explain the proper installation and use of Ashly amplifiers
and their controls. Please read it carefully and refer to it in case of
difficulty.
UNPACKING
As part of our system of quality control, every Ashily product is carefully
inspected before leaving the factory to ensure flawless appearance. After un-
packing please inspect for any physical damage. save the shipping carton and
all packing materials, as they were carefully designed to reduce to a minimum
the possibility of transportation damage should the unit again require packing
and shipping. In the event that damage has occurred, immediately notify your
dealer so that a written claim to cover the Gamages can be initiated.
THE RIGHT TO ANY CLAIM AGAINST A PUBLIC CARRIER CAN BE FORFEITED IF THE
CARRIER IS NOT NOTIFIED PROMPTLY AND IF THE SHIPPING CARTON AND PACKING
MATERIALS ARE NOT AVAILABLE FOR INSPECTION BY THE CARRIER SAVE ALL PACKING
MATERIALS UNTIL THE CLAIM HAS BEEN SETTLED. o
INSTALLATION
Ashly MOS-FET amplifiers are designed to wount in a standard 19" equlpticnt
rack. Since forced-air cooling is employed, it is not necessary to leave any
ventilation space above or below the ampiifier. This permits erficient use ot
rack space and multiple stacking of amplifiers. The only precaution to
observe is to leave the front panel air inlets and rear panel fan outlet
unobstructed. | |
The use of four front panei screws is recommended to evenly distribute the
stress on the panel. The amplifier chassis is sufficiently strong to support
itself by the front panel alone in permanent installations or where the rack
is moved only occasionally. For amplifiers which will spend much of their
time in transit, ict is suggested that some method of rear support be used.
Connect all amplifiers to a 3-wire grounded outlet supplying 120 Volts 50-60Hz
Maximum power consumption is 600 watts. Maximum current required is 5 amps.
INPUT AND OUTPUT CONNECTIONS
STEREO OPERATION
Connections for stereo operation are shown in Figure 4 below.
CATION: TO PREVENT ELECTRIC SIIC,
R L
C= OUTPUTS =
MIXING
CONSOLE
Figure 4 Stereo Operation
INPUTS
The input of Asnly amplifiers is a balanced bridging type, with an input
impedance of 10k ohms. Use of the balanced input provides superior hum and
noise rejection while eliminating ground loops. The input may also be used in
a single-ended, unbalanced mode by simply using unbalanced input connectors.
Each channel provides a choice of either 1/4" jacks or XLR input connectors.
All four connectors are in parallel. Since only one of the four connectors is
used as an input at any one time, the others may be used to jumper other
amplifiers to the same source.
Signal connections are as follows:
Ihe (+) or in-phase connection is on the tip of the 1/4" stereo phone jacks,
and is on pin 3 of the XLR connectors. The (-) or out-of-phase connection is
on the ring of the phone jacks and on pin 2 of the XLR's. Chassis ground and
shield is on the sleeve of the 1/4" jacks and pin 1 of the XLR's.
Shield (Sleeve)
\
ip (+)
7,
Ring (-)
Figure 5
When feeding the amplifier from an unbalanced source, use either au XLi
connector or a standard stereo 1/4" phone plug and connect the signal "hot
connection to XLR pin 3 or to the phone plug tip. Connect signal ground to
the XLR's pins 1 and 2 or to the phone plug's ring and sleeve. The latter may
also be accomplished by simply inserting a standard 1/4" mono phone plug,
which automatically connects ring and sleeve together.
In the special case where the amplifier is physically mounted in the same rack
with the signal source that is feeding it, you may wish to use an alternate
counection system to avoid ground loops: connect tie signal hot to XLR pin 3
or to the phone plug tip. Connect signal ground to XLR pin 2 or to the phone
plug ring. Leave the XLR's pin 1 and the jack sleeve unconnected.
SPEAKER OUTPUTS
The speaker output connections are combination banana jacks/ binding posts
spaced on standard 3/4" centers. Speaker connections may be accomplishec
either by inserting banana plugs or by feeding bare stranded wire into tu
binding posts.
AGC BA
FUSE
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(CHANNEL 2)
OD
PRESS
AGC BA
FUSE
PRESS
Figure 6
ON
IL” Shield
MONO OPERATION
With the MONO switch depressed, the Channel 1 input will feed both sides of
the amp. Volume for both sides is controlled with the channel 1 LEVEL control.
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Figure 7 Mono Operation
BRIDGING OPERATION
Ashly amplifiers are in the bridging wode when both the MONO switch and
BRIDGING switch are depressed. In this mode, the channel one input feeds both
sides of the amp, but the phase of the channel two amplifier is inverted 180
degrees, so that the two amplifiers’ outputs are out of phase. The speaker
connection is then made to the two RED output jacks, as shown in Figure 8.
The black output jacks are not used.
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THIS APPLANCE TO RAIN OR MOISTURE MODEL FET-200
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20 AC
S040 Hz
400 Ww
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C= OUTPUTS ae?
MIXING
CONSOLE
Figure 8 Bridging Mode
METERS
The meters on the front panel respond to the peak output voltage of the
amplifier as measured at the speaker terminals. A ten segment, three color
LED display covers a twenty-seven decibel range.
If all of the LED's on one channel light up and stay on with only a small
input signal, a blown speaker fuse or faulty speaker wiring is indicated.
THERMAL STATUS INDICATOR
In the event that the amplifier overheats, it will automatically shut down and
the green THERMAL STATUS LED will turn off. The fan will continue to run, and
when the heat siak temperature returns to normai the green THERMAL STATUS LED
will come back on and the amplifier will turn itself back on.
It is extremely unlikely that any combination of musicai program material and
load conditions will activate the thermal shutoff mechanism in normal use.
Should overieating occur, check to be sure that the air flow in front of . and
behind the amplifier has not been blocked.
Note that there is a very generous safety margin between the temperature
required to activate the thermal cutouts and temperatures which could damage
components in the amplifier. |
SPECIFICATIONS:
FET-200
RATED POWER 400 Watts/channel FREQUENCY RESPONSE +.5db 10Hz-50kHz
Both channels driven into HARMONIC DISTORTION < .05% 20Hz-20kHz, 100 watts,
8 ohms 8 ohms
160 Watts/channet IM DISTORTION (SMPTE) < .01% 100 watts, 8 ohms
Вой channels driven into DAMPING FACTOR > 400 20Hz-20kHz
ohms SLEW RATE: 50V/ usec
20 Watts into 8 ohms bridged
oats Into 8 ohms bridg RISE TIME 2 usec 10%-90%
FET-500
RATED POWER 250Watts/channel FREQUENCY RESPONSE +.5db 10Hz-50kHz
Both channels driven into HARMONIC DISTORTION < .05% 20Hz-20kHz, 100 watts,
400 Watts/channel IM DISTORTION (SMPTE) < .01% 400 watts, 8 ohms
ao channels driven into DAMPING FACTOR > 400 20Hz-20kHz
| | SLEW RATE: 50V/ psec
Watts into 8 ohms bridged
cow oS onms brid RISE TIME 2 usec 10%-90%
DEFINITION OF TERMS AS USED IN THIS MANUAL
ACTIVE
Electronic circuits which use devices such as transistors and integrated
circuits, and which are capable of voltage and power gain as well as
loss. Circuits using only resistors, capacitors, transformers, etc., are
referred to as passive.
AMPLITUDE
The voltage level of a signal. May be measured in volts or decibels.
Generally corresponds to the volume or intensity of an audio signal.
BALANCED |
À 3-wire circuit arrangement in which two conductors are designated as
signal lines (+ and -), and the third is a shield and chassis ground. The
signal lines are of opposite polarity at any given moment, and are of
equal potential with respect to ground. Balanced input amplifiers are
used on all Asnly SC series products to improve hum and noise rejection.
Jumperíng signal minus (-) to ground provides an unbalanced input.
BREATHING
A usually undesirable fluctuation of background noise resulting from
compressor action. (Also called "Pumping.”)
BUTTERWORTH
The name of a particular filter response shape. The response is
essentially “flat” within the pass-band, is 3dB down at the cutoff
frequency, and continues to attenuate at a constant slope. Also called a
"maximally flat" or "critically damped” filter shape, it is very popular
for crossovers and shelving filters.
CENTER FREQUENCY
The frequency (or pitch) at which a filter is most effective. In a
parametric equalizer, it refers to the frequency where a particular
boost/cut control has maximum effect. |
COMPRESSOR
An amplifier which reduces its gain as its input is increased beyond a
predetermined "threshold."
DAMPING
A force which opposes the tendency of a system to oscillate.
dB
A unit by which audio levels can be COMPARED. Often thoroughly
misunderstood are the concepts that decibels represent the level of a
signal compared to some reference level (15 dB cut means a certain level
less than a previous level -— the absolute level of the signal need not
be known), and that decibels are a logarithmic unit. Some handy numbers
to remember when dealing with decibels:
+3 dB
+6 dB
Double Power +10 dB
Double Amplitude, +20 dB
Quadruple Power
i
10X Power
10X Amplitude, 100X Power
il
H
dBm
A unit of measurement in decibels where 0 dBm = à power levei of 1
miliiwatt into a 600 ohm load. Originally defined by the telephone
company to measure line levels.
dBV
Decibel Volts, an update of the dim definition where U ¿BV = the same
voltage level as 0 dBm, but with no regard to power or impedance. O d3V =
0.775 Volts. This unit is much more appropriate for modern audic
equipment with high impedance inputs and low impedance outputs.
DISTORTION
Generally refers to ANY modification of an audio signal which produces
new frequencies which were not in the original. Examples are harmonic
distortion, where a circuit adds overtones to a fundamental sisnal, and
intermodulation or IM distortion, where two frequencies beat together to
produce sum and difference frequencies.
EQUALIZATION
Modification of the frequency response of an audio system for either
corrective or enhancement purposes.
FEEDBACK
Generally refers to any process where an output is in some form routed
back to an input to establish a loop. Negative feedback tends to be be
self stabilizing, while positive feedback causes instability.
FILTER
A circuit designed to pass some frequencies, but not others. There are
three general categories of filters: High-pass, band-pass, and low-pass.
The high-pass filter passes frequencies above a certain limit, the low-
pass passes frequencies below a limit, and the band-pass passes one group
of frequencies without passing those above or below. Our equalizer uses
band-pass filters, crossovers use high and low-pass filters.
FREQUENCY
The repetition rate of a waveform. Frequency is measured in Hertz. une
cycle per second (cps) is one Hertz (Hz). The higher a note on a musical
scale, the higher its frequency.
FREQUENCY RESPONSE
Refers to relative gain and loss at various frequencies across the audio
band. May be illustrated by a graph called a frequency response plot,
usually graphing decibels vs. ilertz or octaves.
HERTZ (Hz)
The unit of frequency measurement. (Formerly called Cycles-per-Second:
this explains it perfectly)
HEADROOM
Refers to the increase in level above normal operating level that can be
obtained without clipping. Usually expressed in dB.
10
IMPEDANCE
Essentially the AC equivalent of resistance. It describes the drive
capability of an output, or the amount of drive required for an input at
any given signal level.
Kilohertz. 1,000 Hertz.
LEVEL
The magnitude of a signal, expressed in decivels or volts.
LIMITER
An amplifier which reduces its gain as its input is increased beyond a
predetermined threshold. Usually used to protect audio systems against
sudden, high level signals, and possible overload.
LINE LEVEL
leaning "somewhere around OdBV” as opposed to MIC level of around -40dEV.
MILLISECOND
1/1000rn of a second
14 ICROSECOND
1/1,000,000th of a second
OCTAVE
OHM
PHASE
A logarithmic unit to compare frequencies. +1 Octave means double
frequency, -1 Octave means half frequency.
The unit of electrical resistance or impedance.
Describes how well two signals are in Step. In-phase means that positive
and negative peaks in two signals occur together, while out-of-phase
means they do not occur together. Variations in signal timing as well as
polarity can make two signals in or out of phase, or anywhere in between.
Phase is usually measured in degrees where O degrees is in-phase, 180
degrees is out-of-phase, and 90 degrees is in between (sometimes called
quadrature).
PREAMPLIFIER
The first stage of amplification, designed to boost very low level
signals to line level.
"О LA ‘
À measurement describing the sharpness or broadness of a filter.
RESONANCE
The tendency of an electrical or mechanical system to vibrate (or
oscillate) at a certain frequency.
SATURATION
The point at which the magnetic storage capability of a piece of
recording tape 15 exceeded.
11
SIBILANCE
The distortion caused by loud high frequency signals, such as the
"Ssss...” sounds in human speech.
SHELVING
Describes an equalization action where all frequencies above or below a
particular frequency are boost or cut.
SLOPE
In a filter or equalizer, a description of the rate of boost or attenua-
tion. Usually specified in dB/octave (6, 12, 18, and 24dB/octave slopes
are most common). The steeper the slope, the higher the "Q" in a filter.
THRESHOLD
An arbitrary signal level, above which a limiter or noise gate activates.
TRANSIENT
A sudden burst of energy in an audio signal, such as a breath blast in a
microphone, the sound of a snare drum, or a deep scratch in a record.
Transients frequently reach peak levels of 10 to 30 dB above standard
operating level, and may cause distortion or even damage to equipment.
UNITY GAIN
Output level = Input level.
VCA
Voltage Controlled Amplifier. An amplifier capable of both gain and
loss, controlled by a variable DC voltage.
WIRING, PHONE PLUG AND XLR
A stereo phone plug is wired + to the tip, - to the ring, and shield to
the sleeve. For a mono phone plug, combine - and shield, and connect both
to the sleeve.
An XLR (3 Pin) connector is wired + to pin 3, - to pin 2, and shield to
pin 1.
Sleeve
Mono Phone Plug:
(for unbalanced
) a Tip
inputs and outputs)
Sleeve Ring
Stereo Phone Plug:
(for balanced in-
puts and outputs)
2” Tip
XLR Type Connector:
(Male Shown)
12
17"
15”
Figure 3
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