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Pure Class A power amplifier using power MOS-FET devices Fully
balanced signal paths as found in high-quality instrumentation
amplifiers. Further refined MCS+ topology and current feedback design
result in even better S/N ratio, distortion, and other performance
parameters. Robust power supply and power MOS-FET devices in triple
parallel push-pull configuration sustain an amazing 150 watts per
channel into ultra-low 1-ohm impedance loads (with music signals).
Pure Class A power amplifiers from Accuphase
have long been blending the purity of class A
operation with the superior performance of power
MOS-FETs. While building a string of outstanding
amplifiers, Accuphase has accumulated a store
of technical know-how that is second to none. The
latest in this series, the A-30 is a pure class A
stereo power amplifier based on the advanced
technology of the model A-60.
The A-30 employs the so-called instrumentation
amplifier principle throughout. The signal handling
stages feature further improved MCS+ topology
and the renowned current feedback approach. This
has resulted in electrical characteristics that
surpass even the demanding standards set by its
predecessors. Employing only highest grade
materials and strictly selected parts, the A-30
pursues the two most important goals of an
amplifier: very low output impedance (Note 1) and
constant drive voltage (Note 2).
The output stage of the A-30 features power MOS-
FET devices renowned for their excellent sound
and superior reliability. Because they have negative
thermal characteristics, there is no danger of
thermal -runaway- as exists with bipolar transistors.
Operation remains totally stable even when theamplifier is running hot. Driving these devices in
pure class A assures high-definition sound that
brings out the finest nuances in the music.
In pure class A operation, the power supply delivers
a steady current, regardless of the presence or
absence of a musical signal. This means that the
amplifier remains unaffected by fluctuations in
voltage and other external influences. On the other
hand, it also means that the output stage generates
considerable thermal energy. In the A-30, this is
dissipated effectively by large heat sinks which
provide ample capacity to remove the heat
produced by the internal circuitry.
The heavy-duty power supply easily sustains
output levels of 120 watts into 2 ohms, 60 watts
into 4 ohms, or 30 watts into 8 ohms (per channel).
This linear progression demonstrates that the
amplifier will be capable to drive even speakers
with very low impedance ratings or with
pronounced impedance fluctuations. Stability
remains excellent at all times. The amplifier also
has the necessary reserves to handle musical
transients that require considerable power in an
instant. If even higher power is required, bridged
mode turns the A-30 into a high-output
monophonic power amp.
Output current (A)
Output voltage (V)
OUTPUT
+ B3
- B3
-INPUT
+INPUT
NFB
NETWORK
NFB
NETWORK
GAIN CONTROL
CIRCUIT
+
-
-
+
REGULATOR
REGULATOR
+ B1
- B1
- B2
+ B2
Q1
Q3
Q2
Q4 Q8
Q6
Q7
Q5 Q9
Q17 Q19
Q18 Q20
Q21
Q22
Q23
Q24
Q25
Q26
Q13
Q11 Q15
Q10 Q14
Q12 Q16
NFB
NETWORK
MCS+Multiple Circuit
Summing( ) Bias stabilizer
circuit
Bias stabilizer
circuit
Bias stabilizer
circuit
Bias stabilizer
circuitBias stabilizer
circuit
Fig. 1 Circuit diagram of amplifier section (one channel)
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: 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-30. It can be clearly
seen that output voltage is virtually 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-30: 1 ohm: 175
watts, 2 ohms: 142 watts, 4 ohms: 94 watts, 8 ohms: 58 watts.
-1-ohm rating is for music signals only.
Fig. 2 Load impedance vs. output power
(Output voltage/output current)
Instrumentation amp configuration allows fully
balanced signal paths
The A-30 features a new -instrumentation amplifier-
principle whereby all signal paths from the inputs to
the power amp stage are fully balanced. This results
in excellent CMRR (Common Mode Rejection Ratio)
and minimal distortion. Another significant advantage
is that external noise and other external influences
are virtually shut out. The result is a drastic
improvement in operation stability and reliability.
Further refined MCS+ topology for even lower
noise
Accuphase's original MCS (Multiple Circuit Summing)
+
-
-INPUT
NFB
NETWORK
GAIN CONTROL
CIRCUIT
NFB
NETWORK
OUTPUT
+INPUT +
-
+ -
Signal input stage Power amplifier stage
Instrumentation amplifier configuration
principle uses a number of identical circuits
connected in parallel to achieve superior performance
characteristics. MCS+ is a fur ther refined version of
this approach. Improvements in the bias circuitry of
the input-stage buffer amplifier result in greater
stability. This in turn makes it
possible to extend the parallel
operation approach to the class A
drive stage of the current/voltage
converter, thereby further lowering
the noise floor.
Power MOS-FET output stage
with triple parallel push-pull
power units delivers 120 watts
into 2 ohms, 60 watts into 4 ohms,
or 30 watts into 8 ohms with
outstanding linearity
The output stage (Figure 1) uses
power MOS-FETs. 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 maximum power dissipation of
one MOS-FET is 130 watts, but the
actual power load per pair is much
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lower, so that each device is driven only in its low-
power range where 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-30 is set to 50 watts per channel into 8
ohms (sine wave output).
Current feedback circuit topology assures
excellent phase characteristics in high range
In the A-30, the signal current rather than the more
conventionally used voltage is used for feedback.
Since the impedance at the current feedback point
(current adder in Figure 3) is very low, there is almost
BufferCurrent
adder
I-V
converter
Trans-impedance
amplifier Amplifier Output
Current NFB
network - Input
Buffer + Input
Gain(Large)
(High)
Frequency
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 transparency,coupled with
utterly natural
energy
balance.
Figure 4 shows
frequency
response for
different gain
settings of the
current
feedback amplifier. The graphs demonstrate that
response remains uniform over a wide range.
Robust power supply with large power
transformer and high filtering capacity
In any amplifier, the power supply plays a vital role
since it acts as the original source for the output
delivered to the speaker. The A-30 employs a large
and highly efficient 400 VA power transformer. It ishoused in an
enclosure filled with
a material that
transmits heat and
absorbs vibrations.
This completely
prevents any
adverse influences
on other circuit
parts.
Two aluminum
electrolytic
capacitors rated for
47,000 µF each
serve to smooth out
the pulsating direct
current from the
rectifier, providing
more than ample
filtering capacity.
Fig. 3 Principle of current feedback amplifier
Fig. 4 Frequency response with current feedback(Response remains uniform also when gain changes)
n Power amplifier assembly with three
parallel push-pull power MOS-FET
pairs per channel mounted directly
to large heat sink, MCS+ circuitry,
and current feedback amplifier
