m Pure Class-A operation delivers quality power: 20 watts × 2 into 8ohms m Power MOS-FET output stage features 3-parallel push-pull
configuration and delivers linear power even into extremely lowimpedance loads m Current feedback design combines superb soundquality with totally stable operation m Bridged mode allows use as amonaural amplifier m Balanced inputs m Heavy-duty speaker terminals

Pure Class-A goes straight for the heart of the music. The output stage uses
power MOS-FET devices arranged in a triple parallel configuration for eachchannel. Ultra-linear power progression reaches down to very low impedanceloads: 80 watts × 2 into 2 ohms. Current feedback topology assures stability
and creates an utterly convincing sound stage. Gain control makes theamplifier perfectly suited also for use as midrange/high-range amplifier in a
multi-amplifier system.

Note 1 Low amplifier output impedance
The load of a power amplifier, namely the
loudspeaker, generates a counterelectromotive force that can flow back into theamplifier via the NF loop. This phenomenon is
influenced by fluctuations in speakerimpedance, and interferes with the drive
performance of the amplifier. The output
impedance of a power amplifier should
therefore be made as low as possible by using
output devices with high current capability.
Note 2 Constant drive voltage principle
Even in the presence of a load with wildly
fluctuating impedance, the ideal power
amplifier should deliver a constant voltage
signal to the load. When the supplied voltage
remains constant for any impedance, output
power will be inversely proportional to theimpedance of the load. A conventional
amplifier can easily be made to operate in thisway down to a load impedance of about 4ohms. However, at 2 ohms and below much
more substantial output reserves will beneeded, which can only be sustained by anextremely well designed and capable output
stage and a highly robust and powerful power
supply section. To build such an amplifier is atask that requires not only considerable
experience and resources, but also a thoroughreappraisal of basic tenets.
Power MOS-FET output stage with two units in
3-parallel push-pull configuration delivers 80
watts into 2 ohms, 40 watts into 4 ohms, or 20
watts into 8 ohms with outstanding linearity
The output stage (Figure 1) uses power MOS-FETs
with negative thermal characteristics. Three pairsof these devices are arranged in a parallel pushpull configuration for each channel. The result isstable operation with ideal power linearity evendown to ultra-low impedances. The parallel
connection cancels out impedance differences ofindividual devices, thereby minimizing residual
noise. It also allows using the MOS-FETs in theirPower MOS-FETs

most linear low-power range, which further
contributes to sound quality.
Figure 2 shows the output voltage/current characteristics at various load impedances. Output voltage is almost
constant at
various loads,
meaning thatcurrent
increases linearly. Actual
measurements of clipping power
have yielded
the following
figures, which
impressively
demonstrate
Output voltage (V)* 1-ohm operation possible with music signals only
the more thanample perfor- Figure 2 Load impedance vs. output power(output voltage/output current)
mance of theA-20V: 1 ohm: 156 watts, 2 ohms: 126 watts, 4ohms: 85 watts, 8 ohms: 50 watts.
Output current (A)Accuphase power amplifiers are designed torealize two major goals: very low output impedance(Note 1), and constant drive voltage (Note 2). As aresult, Accuphase amplifiers are capable of driving
any kind of speaker load with optimum results,
which is one of the reasons for the high praise thatthese products invariably receive. The low
impedance not only ensures accurate speaker
drive but also absorbs the counterelectromotiveforce generated by the voice coil, thereby
eliminating a major source of intermodulation
distortion. The overall result is a significant
improvement in sound quality.
The A-20V is a pure class-A amplifier which fully
implements these advanced circuit designprinciples. Another advantage is the use of power
MOS-FET devices for further enhanced sonicdefinition. In a pure class-A amplifier, the power
supply delivers a constant amount of power
regardless of the presence or absence of a musicsignal. This means that the amplifier remains
unaffected by fluctuations in voltage and otherexternal influences. As a consequence of thisdesign, the output stage produces considerable
amounts of thermal energy, but in the A-20V thisis dissipated by extra-large heat sinks, to eliminate
the possibility of problems caused by internal heatbuild-up.
The power MOS-FETs used in the output stage are
renowned for their superior sound and high reliability.
Because they exhibit negative thermal
characteristics, there is no danger of thermal"runaway" as exists with bipolar transistors. Threepairs of these devices are arranged in a parallelpush-pull configuration for each channel. The result
is stable operation with ideal power linearity evendown to ultra-low impedances.
The current feedback principle developed by
Accuphase requires only minimal amounts ofnegative feedback to ensure outstanding phasecharacteristics in the upper frequency range. Thisapproach combines operation stability with excellent
frequency response. A gain control is provided which
operates by modifying the NFB amount. This isuseful for example in multi-amp systems where evenminimal amounts of noise in the medium and highfrequency bands could be a problem.

Current feedback topology prevents phase shifts
The amplifying circuits in the A-20V use the current feedback principle for negative feedback. At
the input point of the feedback loop, the impedance is kept low and current detection is performed.
A trans-impedance amplifier then converts thecurrent into a voltage to be used as the feedback
Currentadder(–) input

I/V
converter

BufferOutputAmplifierTrans-impedance amplifier(+) input
REGULATOR

BufferCurrent NFB
network

Figure 3 Principle of current feedback amplifier

Gain(High)BIAS
STABILIZERCIRCUITsignal. Since the impedance at the current feedback point (current adder in Figure 3) is very low,
there is almost no phase shift. Phase compensation therefore can be kept at a minimum. A minimal amount of NFB results in maximum improvement of circuit
parameters. The
result is excellent transient response and superb sonic transp a r e n c y,coupled with utFrequency(High)
terly natural en- Figure 4 Frequency responsewith current feedback
ergy balance. (response remains uniform also when gain changes)

REGULATOR

Figure 1 Circuit diagram of amplifier section

Figure4
shows frequency
response for different
gain settings of the current
feedback amplifier. The graphsdemonstrate that response remains uniform over a wide range.
NFB switching type gain control
The current feedback principle, which ensures low
phase shift and high stability, allowed the designers to implement a gain control (–3 dB, –6 dB, –9dB, –12 dB) which operates by altering the NFB
amount. Consequently, when gain is lowered thenoise floor also becomes
lower, a desirable characteristic especially whendriving highly efficient
speakers. It also is advantageous when using
the A-20V as themidrange/high-range unit
in a multi-amplified system where separate amplifiers drive the individual
speaker units.
Robust power supply with larg e powertransformer and high filtering capacity
In any amplifier, the power supply plays a vital rolesince it acts as the original source for the output
delivered to thespeaker. The A-20V, in
spite of its conservative 20 W/8 ohms x 2rating, employs a large
400 VA power transformer housed in anenclosure filled with vibration-damping material. Two electrolytic capacitors, specially selected for their sonicproperties and each
rated for 47,000 µF,
provide ample filtering
capacity for the rectified current. The capacitors feature anelastic soft coating
which helps to makethem impervious to the detrimental influence of vibrations.
Bridged operation mode creates atrue monophonic amplifier with 160 watts
into 4 ohms or 80 watts into 8 ohms
Bridged mode means that the two channels of anamplifier are driven with the same signal voltage
but with opposite phase, and their output iscombined. The A-20V provides a switch
arrangement for bridged operation, which turns theunit into a high-grade monaural amplifier capableof delivering a full 160 watts into 4 ohms or 80
watts into 8 ohms.
Balanced connection reliably blocks induced noise
Balanced signal transmission means that theoutput stage of a component supplies two signallines which have identical voltage but opposite
phase. Since any noise interference that has arisen
during transmission will be present in both lines
with identical phase, such noise can be canceled
out, leaving only the pure original signal. Even withlong cable runs, the balanced connection principle
keeps the signal transfer completely free from any
kind of interference.
Large analog power meters
The large power meters have a peak holdfunction which lets the user easily monitor
the output level of the rapidly fluctuatingmusic signal. Thanks to logarithmic
compression, the meters cover a wide
dynamic range. A switch for meter
operation and illumination control is also
provided.

n Power amplifier assemblywith 3 parallel push-pull
MOS-FETs (total 6 devices for left and rightchannel) and current feedback amplifier circuitry
mounted directly to massive aluminum diecast
heat sinks

Major signal paths gold-plated

Extra-large speaker terminals

High-purity copper is commonly used in audio
components for signal path lines. The A-20V goes
one step further by providing gold-plating forprinted circuit board traces as well as for the input
jacks and speaker terminals. This approach results
in a distinct sonic improvement.

The oversize speaker terminals accommodate evenvery heavy-gauge
speaker cable. Theterminals are madeof extruded highpurity brass andare gold-plated for
utmost reliability
and minimum contact resistance.
Molded caps areprovided to assure
proper insulation.

mono position is usefulfor example to drive a
center woofer in mono,or to obtain the same
signal from both speaker
outputs for driving a biamped speaker setup
with separate amplifiers for the low and high frequency range.

Easy switching between dual mono operation
and bridged connection
A mode selector makes it simple to switch between
dual mono, stereo, or bridged operation. The dual

Unbalanced connectors and balanced inputs

n FRONT PANELAssembly with protection circuitry, etc.

A-20V Guaranteed Specifications

A[Guaranteed specifications are measured according to the EIAJ standard RS-490]

Bm Continuous Average Output Power (20 - 20,000 Hz)
180 watts per channel into 2 ohms
Stereo operation
140 watts per channel into 4 ohms
(both channels driven)
120 watts per channel into 8 ohms
160 watts into 4 ohms
180 watts into 8 ohms

Monophonic operation(bridged connection)m Total Harmonic Distortion
Stereo operation (both channels driven)

0.05%, with 2-ohm load
0.02%, with 4 to 16 ohms load

Monophonic operation(bridged connection)

CDEn REAR PANEL

FHG

m Intermodulation Distortion

0.003%m Frequency Response

At rated output : 20 - 20,000 Hz +0, –0.2 dB
At 1 watt output : 0.5 - 160,000 Hz +0, –3.0 dB

m Gain (GAIN selector in MAX position)

28.0 dB (in stereo and monophonic operation)m Output Load Impedance

Stereo operation :2 to 16 ohms
Monophonic operation : 4 to 16 ohms

m Damping Factor

Stereo operation :120
Monophonic operation : 60

m Input Sensitivity (with 8-ohm load)
Stereo operation
Monophonic operation9 JK

A Left-channel power meter (dB scale)
B Right-channel power meter (dB scale)
C Power METER OFF switch and rangeselector
OFFNORMAL–20 dBD POWER switch
E GAIN selector
MAX–3 dB

a GND
b Inverted (–)
c Non-inverted (+)

H Left/right channel speaker output terminals
I Mode selector
DUAL MONONORMAL–6 dB

F Unbalanced inputs–9 dB–12 dBUNBALANCEDBALANCED

1.00 V for rated output
0.11 V for 1 watt output
Balanced :40 kilohms
Unbalanced : 20 kilohms

m Signal-to-Noise Ratio (A-weighted)

110 dB with input shorted, at rated output

m Output Level Meters

NORMAL : –40 to +3 dB and direct watt reading–20dB :
–60 to –17 dB and direct watt reading
With logarithmic compression and on/off switch

m Power Requirements

120V/230V (Voltage as indicated on rear panel)
AC, 50/60Hzm Power Consumption

160 watts idle
250 watts in accordance with IEC-65

m Maximum Dimensions

Width 475 mm (18-11/16")
Height 170 mm (6-11/16")
Depth 426 mm (16-3/4")

m Weight

22.6. kg (49.8 lbs) net
27.0. kg (59.5 lbs) in shipping carton

BRIDGEJ Input selector

0.50 V for rated output
0.11 V for 1 watt output

m Input Impedance

HG Balanced inputs

0.02%, with 4 to 16 ohms load

K AC input connector (for supplied power cord) HRemarks
H This product is available in versions for 120/230 V AC. Make sure that the voltage shown on the rear panel matches the AC line voltage in your area.
H The shape of the AC inlet and plug of the supplied power cord depends on the voltage rating and destination country.
mSpecifications and design subject to change without notice for improvements.

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