Accuphase A-20-V Brochure
m Pure Class-A operation delivers quality power: 20 watts × 2 into 8
ohms 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 sound
quality with totally stable operation m Bridged mode allows use as a
monaural amplifier m Balanced inputs m Heavy-duty speaker terminals
Pure Class-A goes straight for the heart of the music. The output stage uses
Current NFB
network
I/V
converter
Trans-impedance amplifier
(+) input
(–) input
Buffer
Buffer
Current
adder
Amplifier
Output
power MOS-FET devices arranged in a triple parallel configuration f or each
channel. Ultra-linear power progression reaches down to v ery low impedance
loads: 80 watts × 2 into 2 ohms. Current feedback topology assures stability
and creates an utterly convincing sound stage. Gain control makes the
amplifier perfectly suited also for use as midrang e/high-range amplifier in a
multi-amplifier system.
Accuphase power amplifiers are designed to
realize two major goals: very low output impedance
(Note 1), and constant drive voltage (Note 2). As a
result, Accuphase amplifiers are capable of driving
any kind of speaker load with optimum results,
which is one of the reasons for the high praise that
these products invariably receive. The low
impedance not only ensures accurate speaker
drive but also absorbs the counterelectromotive
force 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 design
principles. Another adv antage is the use of po wer
MOS-FET devices for further enhanced sonic
definition. In a pure class-A amplifier, the power
supply delivers a constant amount of power
regardless of the presence or absence of a music
signal. This means that the amplifier remains
unaffected by fluctuations in voltage and other
external influences. As a consequence of this
design, the output stage produces considerable
amounts of thermal energy, but in the A-20V this
is dissipated by extra-large heat sinks, to eliminate
the possibility of problems caused by internal heat
build-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. Three
pairs of these devices are arranged in a parallel
push-pull configuration for each channel. The result
is stable operation with ideal power linearity even
down to ultra-low impedances.
The current feedback principle developed by
Accuphase requires only minimal amounts of
negative feedback to ensure outstanding phase
characteristics in the upper frequency range. This
approach combines operation stability with excellent
frequency response. A gain control is provided which
operates by modifying the NFB amount. This is
useful for example in m ulti-amp systems where even
minimal amounts of noise in the medium and high
frequency bands could be a problem.
Note 1 Low amplifier output impedance
The load of a power amplifier, namely the
loudspeaker, generates a counterelectromotive force that can flow back into the
amplifier via the NF loop. This phenomenon is
influenced by fluctuations in speaker
impedance, 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 the
impedance of the load. A conventional
amplifier can easily be made to operate in this
way down to a load impedance of about 4
ohms. However, at 2 ohms and below much
more substantial output reserves will be
needed, which can only be sustained by an
extremely well designed and capable output
stage and a highly robust and powerful power
supply section. To build such an amplifier is a
task that requires not only considerable
experience and resources, but also a thorough
reappraisal 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 pairs
of these devices are arranged in a parallel pushpull configuration for each channel. The result is
stable operation with ideal power linearity even
down to ultra-low impedances. The parallel
connection cancels out impedance differences of
individual devices, thereby minimizing residual
noise. It also allows using the MOS-FETs in their
Power 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 that
current increases linearly. Actual
measurements of clipping power
have yielded
the following
Output current (A)
figures, which
impressively
demonstrate
the more than
ample performance of the
Output voltage (V)
* 1-ohm operation possible with music signals only
Figure 2 Load impedance vs. output power
(output voltage/output current)
A-20V: 1 ohm: 156 watts, 2 ohms: 126 watts, 4
ohms: 85 watts, 8 ohms: 50 watts.
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 perf ormed.
A trans-impedance amplifier then converts the
current into a voltage to be used as the feedback
STABILIZER
CIRCUIT
Figure 1 Circuit diagram of amplifier section
BIAS
REGULATOR
REGULATOR
Figure 3 Principle of current feedback amplifier
signal. 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 trans-
Gain (High)
parency,
coupled with utterly natural energy balance.
Figure 4 Frequency response with current feedback
(response remains uniform also when gain changes)
Frequency (High)
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