Accuphase A 20 Brochure

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STEREO POWER AMPLIFIER

-20

0 Pure Class A operation delivers qualily power : 20walls X 2
inlo 8 ohms 0 Power MOS-PET output stage lealures 3-parallel
push-pull configuration 0 Currenl leedback circuit topology en-
sures oparaliun stability Bridged mode allows use as a monau-
ral amplifier. Balanced inputs I Heavy-duty speaker Iermlnals

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The A-20 is a stereo power amplifier operating in
pure class A. It incorporates a wealth of sophisti-
cated Accuphase high-end technology. Each cir-
cuit device used in the A-20 has been strictly
selected to provide top-grade performance. Its con-
servative power rating combined with unsurpassed
musical realism make the A-20 ideal for use as a
stand-alone amplifier in a highly efficient, quality-
oriented system or as a midrange/high-range am-
plifier in a multi-amplification setup.

in pure class A operation, the power supply deliv-
ers a steady current, regardless of the presence or
absence of a musical signal. This means that the
amplifier remains unaffected by fluctuations in volt-
age and other external influences, allowing it to
reproduce even the most demanding dynamics and
delicate musical nuances with full fidelity. On the
other hand, it also means that the output stage
generates considerable thermal energy. The A-20
therefore uses power MOS-PET devices which
have negative thermal characteristics and remain
totally stable even under demanding operating con-
ditions In each channel, three ofthese devices are
configured in a push-pull arrangement and mounted
on massive heat sinks which provide ample capac-
ity to dissipate the heat produced by the internal
circuitry. Since the output stage remains linear with
any load, the amplifier delivers full output power
even when driving low-impedance speakers When
higher power levels are desired, it is also possible
to use the A-20 in bridged mode as a monaural
power amplifier.

An important feature of the A-20 is its use of cur-
rent feedback circuit topology which virtually elimi-
nates phase shifts in the upper frequency range.
This principle also assures uniform frequency
response which does not change with gain. In other
words, it combines total operation stability with
excellent frequency response. Thanks to this prin-
ciple, phase compensation can be kept at a mini-
mum, and high amounts of negative feedback with
their associated disadvantages are no longer re-
quired. Transient response is excellent, with su-

Pure Class A lets the sound emerge with sparkling clarity, like spring wa-
ter from a deep well. Sound that still carries the ambience of silence. The
output stage uses power MOS-FET devices arranged in a triple parallel
configuration for each channel. Current feedback topology assures stabil-
ity and creates an utterly natural sound stage. Perfectly suited also for use
as midrange/high-range amplifier in a multi-amplification system.

perb sonic transparency and detail, This is the rea-
son why music reproduced with the A-20 sounds
breathtakingly real.

To make optimum use of the current feedback prin-
ciple, the A-ZO uses a gain control that also ad-
justs the degree of negative feedback. When gain
is turned down, the noise threshold is further re-
duced, which is advantageous for example when
the A-20 is operating as the midrange/high-range
unit in a multi-amplification system where noise
could othenivise become a problem.

The driving energy delivered by an amplifier to the
speakers ultimately derives from its power supply.
In the A-20, a transformer with a capacity of a 400
VA is used in combination with two 47,000p F ca-
pacitors. This results in a wide performance mar-
gin and almost limitless current reserves. All
signal-carrying copper traces on the printed circuit
boards, as well as the input jacks, speaker termi-
nals and other important parts are gold»plated, to
assure utmost signal purity.

Output stage with power MOS-FETs in 3-parallel
push-pull contiguratlon and modular construction
delivers linear power: 80 watts/ch into 2 ohms, 40
watts/ch into 4 ohms, 20 watts/ch into 8 ohms

Figure 1 shows the output stage of the A-20 which
uses three parallel MOS-FET devices in each chan-
nel, mounted to large heat sinks. Output rating per
channel is 80 watts into 2 ohms, 40 watts into 4
ohms. and 20 watts into 8 ohms. This linear pro-
gression shows that the amplifier is capable of driv-
ing even low-impedance loads with perfect stability.

Power mos FETS In drlver stage and output slage

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The parallel connection ensuresthat individual char~
acteristics of the circuit devices, such as output
impedance and inherent noise are balanced and
kept to an absolute minimum. It also distributes the
thermal energy evenly and makes optimum use of
the excellent inherent linearity of MOS-FETs in the
low-power range. The overall result of this approach
is perfect operating stability combined with superb
musicality.

In order to prevent occasional transients in the
musical signal from causing undue clipping, the
maximum clipping level has been set to 50 watts
(8 ohms).

Current feedback circuit topology prevents phase
shifts

The open-loop gain of an amplifying circuit de-
creases at higher frequencies. Conversely, when the
gain is increased, frequency response, i.e. the band-
width that can be
handled by the
amplifier. be-
comes more nar-
row. To counter
this effect, a com-
monly employed
technique called _
sonata; sewer
part of the output signal back to the input. If phase
shift is disregarded, a circuit designed to have high
open-loop-gain can apply a high amount of NFB,
resulting in the wide frequency response of a closed-
loop circuit, as shown in Figure 2.

Conventional amplifiers employ voltage NFB,
whereby the output voltage is used for the feed-
back loop. In the A-20 however, the signal current
rather than the voltage is used for feedback. Fig-
ure 3 shows the operating principle of this circuit.

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At the sensing point of the feedback loop, the im-
pedance is kept low and current detection is per-
formed. An impedance-converting amplifier then
converts the current into a voltage to be used as
the feedback signal. Since the impedance at the
current feedback
point (current
adder in Fig. 3) is
very low, there is
almost no phase
shift. Phase com-
pensation can be
kept to a mini-
mum. resulting in
excellent tran-

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