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Aiming for the ne plus ultra Experience the peerless sound of pure class A
and MOS-FETs. Input stage with MCS topology assures impeccable
performance in all aspects with minimum noise and distortion. 10-parallel push-
pull power MOS-FETs operating in pure class A, and power supply with
massive 1 kVA toroidal transformer deliver linear power into ultra-low
impedances down to 1 ohm. 4-step gain selector further reduces residual noise.
The monophonic power amplifier M-8000 as
well as the stereo power amplifiers P-7000 and
P-5000 from Accuphase are highly acclaimed
milestones in the history of high-class amplifiers.
Featuring similar design technology as these
models, the A-60 is a no-holds-barred pure class
A stereo amplifier. Accuphase also has created
a long and distinguished line of high-output
pure class A amplifiers. The musical qualities of
our A-100, A-50, and A-50V models have won
the admiration of audio connoisseurs the world
over. The A-60 is a worthy heir of this tradition.
In the input stage, the inventive MCS principle
pushes the noise floor down to amazingly low
levels. Current feedback topology combines
operation stability with excellent frequency
response. Only minimal amounts of negative
feedback are needed, which is highly beneficial
in terms of sound quality. At the same time, the
A-60 is designed to realize two major goals:
very low output impedance (Note 1), and
constant drive voltage (Note 2).
The power MOS-FETs used in the output stage
are renowned for their superior sound and high
reliability. Ten pairs of these devices are
arranged in a parallel push-pull configuration
for each channel. These superb devices are
driven in a pure class A circuit configuration.
Reflecting Accuphase's vast expertise and
unwavering dedication to sound quality, the
A-60 brings out even the most delicate nuances
in the source with full authority.
Pure class A operation means that the circuit
always draws the same amount of power from
the power supply, regardless of the presence
or absence of a music signal. It is impervious
against external influences and has high
stability. The output stage produces
considerable amounts of thermal energy, but
because the MOS-FET devices used in the
A-60 have negative thermal characteristics,
there is no danger of thermal runaway as exists
with bipolar transistors. In addition, extra-large
heat sinks on both sides of the amplifier help to
prevent internal heat buildup.
Another attractive feature of the A-60 is the
sophisticated power meters with digital readout
and bar graph indication. A dedicated DSP
performs arithmetic operations that allow the
meters to always show the true power levels of
the constantly changing music signal.
BIAS
STABILIZER
BIAS STABILIZER
BIAS
STABILIZER
CONSTANT
CURRENT
CIRCUIT
Output current (A)
Output voltage (V)
Note 1 Low amplifier output impedanceThe 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. This absorbs the counterelectromotive
force generated by the voice coil and prevents the occurrence of
intermodulation distortion.
Note 2 The constant drive voltage principleEven in the presence of a load with wildly fluctuating impedance,
the ideal power amplifier should deliver a constant voltage signal
to the load. Figure 2 shows the results of actual output voltage/
output current measurements at different load impedances for the
A-60. It can be clearly seen that output voltage is almost constant
at various loads, which means that current increases in a linear
fashion. Actual measurements of clipping power have yielded the
following figures, which impressively demonstrate the more than
ample performance of the A-60: 1 ohm: 513 watts, 2 ohms:
343 watts, 4 ohms: 202 watts, 8 ohms: 110 watts.
Fig. 2 Output power vs. load impedance
(output voltage/output current)
* 1-ohm operation possible
with music signals only
Power MOS-FET output stage with 10-parallel push-
pull power units delivers 480 watts into 1 ohm, 240
watts into 2 ohms, 120 watts into 4 ohms, or 60
watts into 8 ohms with outstanding linearity
* 1-ohm loads to be driven with music signals only
The output stage (Figure 1) uses power MOS-FETs.
10 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 maximum power
dissipation of one MOS-FET is 130 watts, but the
actual power load per pair is only 6 watts, so that
each device is driven only in its low-power rangewhere linearity is excellent.
A music signal consists of a continuous succession
of pulse waveforms. To prevent clipping on occasional
momentary high-level pulses, the maximum clipping
level of the A-60 is set to 100 watts per channel into
8 ohms.
MCS topology in input stage reduces residual noise
and drastically improves S/N ratio
The input stage which has an important role to play
features Accuphase's innovative MCS (Multiple Circuit
Summing) design. Three separate unit amplifiers for
the input signal are connected in parallel, which
minimizes noise and
distortion and greatly
improves other performance
parameters as well. This
manifests itself in further
improved sound quality.
Gain control also switches
NFB for reduced noise floor
The gain control of the A-60
has four positions: MAX,
-“3 dB, -“6 dB, -“12 dB.
Reducing the gain also results
Fig. 1 Circuit diagram of amplifier section (one channel)
Power MOS-FETs
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in correspondingly lower noise,
which can be very beneficial when
driving high-efficiency speakers
where residual noise can be a problem.
Current feedback circuit topology
assures excellent phase characteristics
in high range
In the A-60, the signal current rather than the
more conventionally used voltage is used for feed-
BufferCurrent
adder
I-V
converter
Trans-impedance
amplifier Amplifier Output
Current NFB
network -“ Input
Buffer + Input
back. Since the impedance at the current feed-
back point (current adder in Figure 3) is very low,
there is almost no phase shift. Phase compensa-
tion therefore can be kept at a minimum. A minimal
amount of NFB results in maximum improvement of
circuit parameters. The result is excellent tran-
sient response
and superb
sonic transpar-
ency, coupled
with utterly
natural energy
balance. Figure
4 shows fre-
quency re-
sponse for dif-
Gain
(Large)
(High) Frequency
ferent gain settings of the cur-
rent feedback amplifier. The graphs dem-
onstrate that response remains uniform over a
wide range.
Bridged operation mode creates true monophonic
amplifier with 960 watts into 2 ohms, 480 watts
into 4 ohms or 240 watts into 8 ohms
* 2-ohm loads to be driven with music signals only
Bridged connection turns the A-60 into a monophonic
amplifier with four times more power than during
stereo operation. The result is an extremely capable
performer with almost unlimited reserves.
Robust power supply with large "Super Ring"
toroidal transformer and 82,000 µF filtering
capacity
The power supply section is a critical aspect of
any power amplifier. The A-60 features a large
toroidal power transformer with a rating of about 1
kVA. The transformer is housed in a non-resonantaluminum enclo-
sure filled with
damping material
that has excellent
heat transfer char-
acteristics. Toroi-
dal transformers
which use heavy-
gauge copper wir-
ing on a ring-
shaped core have
important advan-
tages for audio ap-
plications, such as
very low imped-
ance, small size,
and high conver-
sion efficiency.
Power amplifier assembly with 10-
parallel push-pull output devices
mounted to large heat sink, MCS
circuit, and current feedback
amplifier circuitry (one channel)
Fig. 4 Frequency response with current feedback(Response remains uniform also when gain changes)
Fig. 3 Principle of current feedback amplifier
