50 - Accuphase
STEREO POWER AMPLIFIER
-50
® Pure Class-A Operation Provides 50 W x 2 (into 8 Ohms)
of Quality Power @ Output stage with Power MOSFETs in a
10-Parallel Push-Pull Configuration @ Hybrid IC Module Input
Stage for High Performance and Outstanding Reliability
@ Digital Power Meters Showing True Power Values
The A-50 is truly a genuine class-A operation
stereo power amplifier built by Accuphase, without
any compromise, by pursuing its ideal for repro-
duced music. Thus, Accuphase incorporates its
audio technologies that have been accumulated
since its foundation as well as the highest grade
electronic devices that are available today.
The advantage of the class-A amplification
method is that power fed from the power supply is
always constant, regardless of the presence of
musical signals or not. This means that the ampli-
fier exhibits high stability, while being subjected to
no external interference. Thus, the amount of heat
generated from the output stage becomes ex-
tremely high. In general, the power consumption
of any class-A stereo power amplifier with a rated
output of 50W is approximately 200W (about 350W
in the case of the A-50), which is dissipated as
heat. To counter this heat problem, the A-50
employs an exceptionally large heat sink for both
the left and right channels to enhance the effect of
heat dissipation, thereby eliminating any trouble
due to the concentration of heat in the internal cir-
cuitry.
The output power per channel of the A-50
becomes as high as 400W at a 1-ohm load (actu-
ally measured value), 200W at a 2-ohm load,
100W at a 4-ohm load, or 50W at an 8-ohm load.
Thus, the output power can produce an ideally
linear output value despite the fluctuation of the
load impedance of a speaker system. For this
reason, the A-50 is capable of large output opera-
tion even when a low-impedance loudspeaker is
connected. This amplifier never appears to be
lacking power, having the capability to handle
occasional high pulse music signals. Its clipping
level is factory-set to approximately 100W at an 8-
ohm load.
For its output stage, the A-50 employs power
MOSFETs that have an established reputation for
their contribution to excellent sound quality and
performance reliability. Driving 10 pairs of these
field-effect transistors in a parallel push-pull circuit
configuration, the A-50 can maintain stable opera-
tion even with loads having an extremely low
impedance.
The input stage of this power amplifier is a bal-
anced differential input type in a pure complemen-
tary push-pull configuration, which is an Accu-
phase original. This makes it possible to provide
for not only a directly connected balanced trans-
mission, but also a direct connection even in the
case of normal unbalanced signal transmission
without resorting to any conversion circuit. This is
done by connecting one input side of a normal un-
balanced signal transmission to the ground.
Thanks to this input circuit configuration, there is
no difference in the quality of sound between the
balanced and unbalanced inputs. Also employed
in this input circuit is Accuphase's original hybrid
module, with all FETs, transistors, and ICs
mounted on an alumina ceramic substrate. This
hybrid module technology makes it possible to
secure high performance, high reliability, and ex-
tended stability without being affected by thermal
changes.
The power supply unit of the A-50 consisting of
a power transformer and filter capacitors is a
supply source for all energy. If an amplifier has an
inadequate power supply margin, it is apt to have
problems, not only in the quality of sound, but also
with its basic performance characteristics. De-
spite the A-50's output capacity of 2 x 50W/8
ohms, the power transformer used is a toroidal coil
Pure Class A — The Sound of Excellence. Enjoy Music tl
Power MOSFET Devices in the Output Stage in a 10-Par
Even at Load Impedances as Low as One Ohm.
Enjoy 50 Watts of High-Quality Power per Channel (into
as Monophonic Power Amplifier Delivering a Stunning 2
type with a power capacity as large as approxi-
mately 1,000VA and the two filter capacitors em-
ployed are very large capacitance types. Each of
them has a rating of 56,000uF (electric charge ca-
pacitance of 3.5 coulombs). The transformer has
been housed in an aluminum die-cast enclosure,
which is additionally filled with a resin having an
excellent vibration-suppressing coefficient of elas-
ticity. This measure successfully isolates the
power transformer completely from external elec-
trical and mechanical interference.
The digital peak power meter of the A-50 dis-
plays a true power value. The meter first detects
the current and voltages and then calculates the
power value with a built-in analog multiplier. The
output will be further processed by a microcom-
puter, which incorporates an A/D converter to in-
dicate the true
power value of the
constantly chang-
ing musical sig-
nals. This digital
power meter can instantaneously read the impe-
dance of the speaker which varies complexly
along with different frequencies to display the true
power value. The power supply for the digital
power meter has its own exclusive transformer to
eliminate any adverse effect to the amplifier cir-
cuits.
Guaranteed Linear Power of 200W/2 Ohms,
100W/4 Ohms, or 50W/8 Ohms per Channel
thanks to Power Unit with Power MOSFETs in
10-Parallel Push-Pull Configuration.
Fig. 1 is a circuit diagram illustrating the amplifier
output stage of the A-50. This output stage fea-
tures an extravagant circuit configuration in which
10 pairs of power MOSFETs are parallel-con-
nected for each of the N and P channels (Qs, Qs,
… Q»4 for the N channel and Qz, Qs, ... Qo5 for the
P channel). Because the permissible power loss is
120 watts per MOSFET employed, the power
burden per parallel pair of the MOSFETs becomes
only 5 watts. This means that the amplifier can use
a small power range of MOSFETs, which have ex-
cellent linearity. In the pure class-A amplification
operation of the A-50, a large current always flows
through its circuitry, but thanks to this circuit con-
figuration, the current becomes only 1/10 and thus
tremendously raises the safety feature of the entire
circuitry.
The A-50 provides huge aluminum die-cast
heat sinks to cope with the heat generated by the
class-A operation. All of these design considera-
tions for ideal power amplification make it possible
not only to maintain an extremely stable as 50
watts per channel into 8 ohms operation to the
utmost limits, but also to guarantee an ideal power
amplification as high as 100 watts per channel/
4 ohms, or 200 watts per channel/2 ohms. Thus, 4
the amplifier is capable of delivering large output =>
power even when connected with low-impedance
loudspeaker systems.
A musical signal consists of a succession of
pulse waves. To prevent occasional instantaneous
pulse signals from clipping in normal operation,
the maximum clipping level of the A-50 is factory-
set to 100W at 8 ohms.
Modularized Balanced Differential Input Cir-
cuits
As shown in Fig. 2, the input stage of the A-50 has
Accuphase's original balanced differential, pure
complementary push-pull circuit. Its inverted and
non-inverted signal input circuits employ FET
buffers respectively with a high-input impedance
and low-output impedance.
The important parts of this input stage have
been made modular by developing a hybrid IC.
NFB POWER
NETWORK SUPPLY
OUTPUT
E a e
—INPUT =
oie A Qu
LOW NOISE
PRECISION AMPLIFIER
+INPUT
o—0 +
POWER
SUPPLY
Be Q | Qo| Qu] Qis| Qis| Qiz| Qua] Qazi] Q2a| Qos
; |
Fig. 1 Circuit Diagram of the Amplifier Unit (One Channel)
-B,
inputs. The output side generates non-inverted (+)
and inverted (-) signals, which have the same
voltage but opposite phase (inverted by 180
degrees), while the input side receives and mixes
these two opposite-phased signals through its +
and — amplifiers. The noise components of the
signals generated in the cable enter both termi-
nals with the same phase, whereby they cancel
out each other and disappear when mixed in the
input amplifiers.
PREAMPLIFIER
POWER AMPLIFIER
=
; i BUFFER
|
I
Fig. 5 Principle of the Balanced Outputs
to Inputs
The longer the cable between two audio
equipment units, the greater the possibility to ad-
versely effect the sound quality due to possible
external noise. Thus, a balanced connection pro-
vides a possibility for amplifiers to be completely
free from such external signal disturbance and to
be able to transmit signals of high quality.
As shown in Fig. 6, the A-50 has the most ideal
input circuit configuration for a balanced signal
transmission by feeding signals directly without
any intermediary to the non-inverted (+) and in-
verted (-) input terminals of its differential input
circuit. To achieve this, the A-50 employs two 2-
ganged high-quality, precision attenuators, which
control the input level of the left and right channels
individually in 1dB steps with minimal gang
errors. The attenuators are a very high sound
quality type. Its resistor element is finished with a
‘mirror-like surface, thus featuring a low distortion
factor and an outstanding resistance to wear and
tear.
Cu
9
2 =
INPUT
Y
OUTPUT
3 e
XLA CONNECTOR al
#
=
E
Fig. 6 Balanced-Input Circuit of the A-50
All Signal Paths Gold-Plated
For the signal paths, through which audio signals
flow, Accuphase has since long been using high
purity copper. In the A-50, all such signal paths
are additionally gold-plated to minimize surface
resistance and to ensure smooth current flow.
Since power amplifiers must handle very large
currents, the A-50 employs oxygen-free copper
(OFC) for each signal path, which is additionally
gold-plated. The shielded lead wires to the attenu-
ators use a copper material with much higher
purity for further improved sound quality.
Rigid Aluminum Die-Cast Enclosure Used to
Accommodate Large Power Transformer and
Large-Capacity Filter Capacitors.
The power supply unit of a power amplifier is an
important source of energy. Especially with a
class-A power amplifier, its power supply unit
becomes more important due to the constant flow
of large current. Despite the output capacity of
50W per channel at
8 ohms, the power
transformer used in
the A-50 is a huge
capacity, highly effi-
cient toroidal type
with a current capac-
ity as large as
1,000VA. The hous-
ing of this power
transformer is alumi-
num die-cast, which
is filled with resin
with a high vibration-suppressing coefficient of
elasticity. This measure makes it possible to shield
external interference from the power transformer
and assures absolutely quiet operation.
The two filter capacitors employed are a large-
capacitance electrolytic capacitor type, each of
which has a rating of
56,000uF (an elec-
tric charge capaci-
tance of 3.5 coul-
ombs). Conventional
type capacitors are
normally insulated
with a vinyl sleeve,
but the capacitors
used in the A-50
have a special soft
coating of polyure-
thane material with
high elasticity. Thus, it can effectively suppress ex-
ternal vibrations, while maintaining a large capac-
itance.
Digital Peak Power Meter for True Power Value
Indication.
The A-50 is equipped with a power meter, which
displays its readings in digital format for easy
output monitoring. The power meter circuit dis-
plays a true power value by reading the actual
impedance loads of the speaker systems con-
nected. Fig. 7 shows a block diagram of this
power meter. The digital power meter first detects
the voltage and the current, which are then calcu-
lated by an analog multiplier to produce a power
value. This value is further processed to show a
digital readout by way of a microcomputer con-
taining an A/D converter.
Because a musical signal is a succession of
pulse waves, it is difficult to read the signal accu-
rately with an analog meter. This newly employed
digital power meter can, however, read instanta-
neously the impedance of the speaker, which
varies complexly with frequency, and displays the
true power output value to the speaker systems
connected.
The power meter can display the peak holding
time in two different modes, i.e. “1 sec” or “infinity
(c0)". The digital display can be totally switched
off when not required.
Ultra-Large Speaker Terminals Allow Banana
Plug Insertion.
The A-50 provides ultra-large speaker terminals
that can handle very thick speaker cables. Each of
these terminals is made by shaping solid bronze
material, then gold plated and finally covered with
a molded cap for insulation. In addition, each
speaker terminal can accept a banana plug.
NONVOLATILE
MEMORYU FOR
METER DISPLAY OFF SWITCH
HOLD TIME SELECTOR SWITCH
VOLTAGE
SENSING \
CIRCUIT HIGH-SPEED HIGH- | W| ABSOLUTE-
L-ch PRECISION ANALOG VALUE
CURRENT MULTIPLIER CIRCUIT
SENSING
CIRCUIT
VOLTAGE
SENSING V
CIRCUIT HIGH-SPEED HIGH- |W] ABSOLUTE-
R-ch PRECISION ANALOG VALUE
CURRENT MULTIPLIER CIRCUIT
SENSING |
CIRCUIT
CALIBRATION
4-digit DISPLAY
for L-CH
HIGH-PRECISION B-Dit
PEAK HOLD | MICRO-
CIRCUIT PROCESSOR
4-digit DISPLAY
Tor A-CH
ANALOG |, 4.5-digit AD
SWITCH CONVERTER
HIGH-PRECISION
PEAK HOLD
CIRCUIT
B-bit D/A
CONVERTER FOR
CALIBRATION
Fig. 7 Block Diagram of the Digital Power Meter
Hl Power Amplifier Assembly
@ P-channel power MOSFETs in 10-parallel configuration
® P-channel drive MOSFET
© Stabilizer source resistor
© Drive circuit power supplies
@ Stabilizer source resistors
© N-channel drive MOSFET
@ N-channel power MOSFETs in 10-parallel configuration
© Predrive transistor
© Cover of input stage amplifier hybrid IC
O Gold-plated glass epoxy PCB
@ Large aluminum die-cast heat sink
@® Resonance suppression handle
e
"Eo
|
Large heat sinks with attached
10-parallel push-pull output stage
and amplifier unit
H INTERNAL LAYOUT
D Left-channel power amplifier unit
B Large toroidal power transformers
B Two 56,000 pF filter capacitors
BD PCB with meter drive and other circuits
9 Power meter display switch
3 Hold time selector switch
D Right-channel power amplifier unit
E Digital Meter Drive Assembly
@ 8-bit microcomputer @ Calibration potentiometer
O Offset calibration D/A converter O Hold-off MOSFET array
O Hi-speed A/D converter @ Sample-and-hold circuitry
O Power supply
@ Analog multiplier
© Analog switch
EH High-Quality Parts
inputs. The output side generates non-inverted (+)
and inverted (-) signals, which have the same
voltage but opposite phase (inverted by 180
degrees), while the input side receives and mixes
these two opposite-phased signals through its +
and — amplifiers. The noise components of the
signals generated in the cable enter both termi-
nals with the same phase, whereby they cancel
out each other and disappear when mixed in the
input amplifiers.
PREAMPLIFIER
o EA
POWER AMPLIFIER
Fig. 5 Principle of the Balanced Outputs
to Inputs
The longer the cable between two audio
equipment units, the greater the possibility to ad-
versely effect the sound quality due to possible
external noise. Thus, a balanced connection pro-
vides a possibility for amplifiers to be completely
free from such external signal disturbance and to
be able to transmit signals of high quality.
As shown in Fig. 6, the A-50 has the most ideal
input circuit configuration for a balanced signal
transmission by feeding signals directly without
any intermediary to the non-inverted (+) and in-
verted (-) input terminals of its differential input
circuit. To achieve this, the A-50 employs two 2-
ganged high-quality, precision attenuators, which
control the input level of the left and right channels
individually in 1dB steps with minimal gang
errors. The attenuators are a very high sound
_ quality type. Its resistor element is finished with a
mirror-like surface, thus featuring a low distortion
factor and an outstanding resistance to wear and
tear.
y ve
2 ;
1 7 i
INPUT ; > O
> LA OUTPUT
XLR CONNECTOR +”! A
Fig. 6 Balanced-Input Circuit of the A-50
All Signal Paths Gold-Plated
For the signal paths, through which audio signals
flow, Accuphase has since long been using high
purity copper. In the A-50, all such signal paths
are additionally gold-plated to minimize surface
resistance and to ensure smooth current flow.
Since power amplifiers must handle very large
currents, the A-50 employs oxygen-free copper
(OFC) for each signal path, which is additionally
gold-plated. The shielded lead wires to the attenu-
ators use a copper material with much higher
purity for further improved sound quality.
Rigid Aluminum Die-Cast Enclosure Used to
Accommodate Large Power Transformer and
Large-Capacity Filter Capacitors.
The power supply unit of a power amplifier is an
important source of energy. Especially with a
class-A power amplifier, its power supply unit
becomes more important due to the constant flow
of large current. Despite the output capacity of
50W per channel at
8 ohms, the power
transformer used in
the A-50 is a huge
capacity, highly effi-
cient toroidal type
with a current capac-
ity as large as
1,000VA. The hous-
ing of this power
transformer is alumi-
num die-cast, which
is filled with resin
with a high vibration-suppressing coefficient of
elasticity. This measure makes it possible to shield
external interference from the power transformer
and assures absolutely quiet operation.
The two filter capacitors employed are a large-
capacitance electrolytic capacitor type, each of
which has a rating of
56,000uF (an elec-
tric charge capaci-
tance of 3.5 coul-
ombs). Conventional
type capacitors are
normally insulated
with a vinyl sleeve,
but the capacitors
used in the A-50
have a special soft
coating of polyure-
thane material with
high elasticity. Thus, it can effectively suppress ex-
ternal vibrations, while maintaining a large capac-
tance.
Digital Peak Power Meter for True Power Value
Indication.
The A-50 is equipped with a power meter, which
displays its readings in digital format for easy
output monitoring. The power meter circuit dis-
plays a true power value by reading the actual
impedance loads of the speaker systems con-
nected. Fig. 7 shows a block diagram of this
power meter. The digital power meter first detects
the voltage and the current, which are then calcu-
lated by an analog multiplier to produce a power
value. This value is further processed to show a
digital readout by way of a microcomputer con-
taining an A/D converter.
Because a musical signal is a succession of
pulse waves, it is difficult to read the signal accu-
rately with an analog meter. This newly employed
digital power meter can, however, read instanta-
neously the impedance of the speaker, which
varies complexly with frequency, and displays the
true power output value to the speaker systems
connected.
The power meter can display the peak holding
time in two different modes, i.e. "1 sec” or “infinity
(00)". The digital display can be totally switched
off when not required.
Ultra-Large Speaker Terminals Allow Banana
Plug Insertion.
The A-50 provides ultra-large speaker terminals
that can handle very thick speaker cables. Each of
these terminals is made by shaping solid bronze
material, then gold plated and finally covered with
a molded cap for insulation. In addition, each
speaker terminal can accept a banana plug.
METER DISPLAY OFF SWITCH NONVOLATILE
HOLD TIME SELECTOR swircH|] | MEMORYU FOR
VOLTAGE 4-digit DISPLAY
SENSING for L-CH
CIRCUIT HIGH-SPEED HIGH- | W| ABSOLUTE- HIGH-PRECISION 8-bit
L-ch PRECISION ANALOG VALUE PEAK HOLD MICRO-
CURRENT MULTIPLIER CIRCUIT CIRCUIT PROCESSOR
SENSING | 4-digit DISPLAY
CIRCUIT for A-CH
ANALOG | | 4.5-digit A/D
SWITCH CONVERTER
VOLTAGE т
SENSING М
CIRCUIT | HIGH-SPEED HIGH- | VW ABSOLUTE- HIGH-PRECISION B-bit D/A
R-ch PRECISION ANALOG VALUE — PEAK HOLD CONVERTER FOR
CURRENT MULTIPLIER CIRCUIT CIRCUIT CALIBRATION
С SENSING ATT CY a
CIRCUIT
Fig. 7 Block Diagram of the Digital Power Meter
E FRONT PANEL
EM REAR PANEL
O Left-channel digital power meter
O Right-channel digital power meter
O Input level control for left channel
(up to —20 dB in 1-dB steps and oo)
O Power switch
O Input level control for right channel
© Unbalanced input jacks
O Balanced
XLR-3-31
input connectors
or equivalent — (1): Ground
(2): Inverted (-)
(3): Non-inverted (+)
O Left-channel speaker output terminals
O Right-channel speaker output terminals
(O INPUTS/BRIDGE selector switch
UNBALANCED, BALANCED,
BRIDGE UNBALANCED, BRIDGE BALANCED
® AC power cord
@® Circuit breaker
Performance Guaranty
All Accuphase product specifications are guaranteed
as stated.
Continuous Average Output Power (20 to
20,000Hz)
Stereo mode (both channels driven)
200 watts per channel into 2-ohm load
100 watts per channel into 4-ohm load
50 watts per channel into 8-ohm load
Monophonic mode (bridge connection)
400 watts into 4-ohm load
200 watts into 8-ohm load
Total Harmonic Distortion
Stereo mode (both channels driven)
0.05%, with 2-ohm load
0.02%, with 4- to 16-ohm load
Monophonic mode (bridge connection)
0.02%, with 4- to 16-ohm load
Intermodulation Distortion
0.003%
Frequency Response
20 to 20,000 Hz, +0 dB, -0.2 dB
(for rated continuous average output, level control
at maximum)
0.5 to 160,000 Hz, +0 dB, -3.0 dB
(for 1 watt output, level control at maximum)
0.5 to 110,000 Hz, +0 dB, -3.0 dB
(for 1 watt output, level control at -6 dB)
Gain
28.0 dB (in stereo and monophonic modes)
Output Load Impedance
2 to 16 ohms in stereo mode
4 to 16 ohms in monophonic mode (bridge connec-
tion)
Damping Factor
160 in stereo mode
140 in monophonic mode (bridge connection)
Input Sensitivity (with 8-ohm load)
Stereo mode
0.80 V (for rated continuous average output)
0.11 V (for 1 watt output)
Monophonic mode (bridge connection)
1.59 V (for rated continuous average output)
0.11 V (for 1 watt output)
Input Impedance
Balanced: 40 k ohms; Unbalanced: 20 k ohms
Signal-to-Noise Ratio (A-weighted)
110 dB (input short circuit, rated continuous average
output)
GUARANTY SPECIFICATIONS (Guaranty specifications are measured according to EIA standard RS-490.)
e Digital Power Level Meters
Type:
Display range:
Hold time:
Meter disp
100 V, 117
Digital meter for true power value
indication
0.1 to 400.0 watts in stereo mode
1 to 2,000 watts in monophonic
mode
1 sec. and infinity (co) selectable
lay OFF switch: Provided
Power Requirements
V, 220 V, 240 V AC, 50/60 Hz
enrich life through technology
e Power Consumption
310 watts at zero signal input
550 W in accordance with IEC-65
e Maximum Outline Dimensions
475 mm (18-23/32 inches) width,
239 mm (9-7/16 inches) height,
550 mm (19-11/16 inches) depth
Weight
48.5 kg (106.9 Ibs.) net
53 kg (117 lbs.) in shipping carton
Load Tmpadance:
Gale fat ThHr):
20,580 JF
Haw Output Power
Z0Hr; 113,61H
Тк Нат 112.784
Zitz: 116.47
[el aio a À
Total harmonic ditiertien In E
вто
SE a jm
LA Ll]
Зи 1H
ee
Output power
Output power vs. total harmonic distortion
(at 8-ohm load, in stereo mode)
Guin Cat ТЕН: р:
20,00 di
Hax Output Power
d@Hss 411.074
IkHry 407, 65H
2047: 485,674
Total harmonie distertio
power
Output power vs. total harmonic distortion
(at 8-ohm load, in monophonic mode)
THO ws. I
Output Power Est ==
Hodal4 7 A-50
Ser i LUCE
Date i
Input y ML PNEU"
Output 1
Comsen ti
Load Tepedunce:
2 Ohm
Total harmonic distortion
=
=
Guim Cat 1kHr):
27.06 di
Max _Duiput Power 8.081 ==
ZH: 311, 43H
Jer; 344,456 = = =
2BaHr: 332. 63H Î
===. === DH |
t1&Hz eva
18H
Output peser
Output power vs. total harmonic distortion
(at 2-ohm load, in stereo mode)
THO v=. 1
Output Power
=
=
E
=
=
Tetal harmonic diztertion In X
Guin [at IkHrh:
27,04 «в
Мах Output Power p.eël
dëHr: 639,31H
IkHzs ES8.31H =
ZekHr: BIE, 38H
= ;
alo sol Ц 1H 18H LLL 1
Output power
Output power vs. total harmonic distortion
(at 4-ohm load, in monophonic mode)
ACCUPHASE LABORATORY INC.
E9310 Y
851-0113-00 (AD1)
PRINTED IN JAPAN
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