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A monophonic power amplifier with impressive muscle: 2000 watts into
1 ohm MCS topology for input stage assures high S/N ratio. 22 wide-band
high power transistors in parallel push-pull configuration. Power supply
with massive 3 kVA toroidal power transformer realizes constant voltage
speaker drive and delivers linear power down to impedances as low as one
ohm. Teflon PCBs with low dielectric constant and minimum loss.
When developing the M-8000, Accuphase took a fresh
look at the entire concept of the power amplifier. As a
result, the M-8000 was designed to realize the ideal
of constant voltage drive, which is best implemented
in a monophonic configuration. In order to bring out
the full performance potential of any loudspeaker,
unaffected by the often drastic fluctuations in speaker
impedance, the amplifier must have very low output
impedance (Note 1), and it must be able to supply a
constant drive voltage (Note 2).
In the M-8000, a complement of 22 output transistors
with a collector dissipation (Pc) of 150 watts each is
used in the output stage. Connected in parallel, these
devices have a combined collector dissipation of 6,600
watts. At the extremely low load impedance of 1 ohm,
the amplifier is rated to deliver an amazing 2,000 watts.
Constant voltage drive enables linear progression of
output vs. load impedance. This performance is
sustained by a massive Super Ring toroidal
transfor mer housed in a diecast enclosure with directly
mounted heat sinks, and by large filtering capacitors.
The transformer is rated for 1,5 kVA, max. 3,0 kVA,
and there are two capacitors of 40,000 ÂµF each. This
assures more than ample reser ves and allows the
M-8000 to meet even the most demanding and rapidly
fluctuating power requirements. Use of two units in
bridged configuration is also possible, resulting in a
mono amplifier with even higher capabilities.
The impor tant input stage also has been given due
attention. Another Accuphase innovation called MCS
(Multiple Circuit Summing) helps to minimize noise.
The predriver stage features a DC stabilized power
supply. This results in drastically improved S/N ratio,
minimum distor tion, and superb performance in all
other aspects. Stable output is achieved regardless
of fluctuations on the AC side. Current feedback
topology makes it possible to combine stable
operation with impeccable frequency response. The
circuit boards of the M-8000 possess a Teflon base
with low dielectric constant and minimum loss.
Balanced inputs help to shut out external noise. The
copper foil side of PCBs and all input and output
terminals as well as all major signal carrying points
are gold plated. The overall result of these measures
is musical purity that leaves nothing to be desired.
* Teflon is a registered trademark of DuPont USA.
Ultra-powerful output stage with 22 parallel push-
pull transistors delivers 2,000 watts into 1 ohm,
1,000 watts into 2 ohms, 500 watts into 4 ohms
and 250 watts into 8 ohms
The M-8000 uses a complement of 22 high-power
transistors with a collector dissipation (Pc) of 150
watts and a collector current of 15 amperes each.
These devices are excellent in every regard,
including frequency response, current amplification
linearity, and switching characteristics. The 22
devices are connected in a parallel push-pull
configuration and mounted to immense heat sinksmade of diecast aluminum. This assures efficient
dissipation of thermal energy and provides plenty of
performance margin. As a result, the power amplifier
is capable of delivering enormous output power in a
linear progression towards lower load impedances:
2,000 watts into 1 ohm, 1,000 watts into 2 ohms,
500 watts into 4 ohms and 250 watts into 8 ohms.
The M-8000 also is able to drive reactive loads with
ease.
Figure 2 is a graph plotting the output voltage versus
current characteristics. Even when the load changes,
the output voltage remains almost constant, showing
linear current progression. Actual measurement of
clipping power at the extremely low load impedance
of 1 ohm yields 2,330 watts. At 2 ohms, the figure is
1,230 watts, at 4 ohms 630 watts, and at 8 ohms
310 watts. This demonstrates the impressive
performance reserves of this amplifier.
MCS topology in input stage drastically improves
S/N ratio
The input stage features Accuphase's original 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.
Stabilized power supply in driver
stage assures outstanding
operation stability
The MCS circuitry and predriver stage
employ a DC stabilized power supply.
When the power stage amplifies a
signal to large amplitudes, this could
cause noise in the input stage via the
power supply. This is prevented by
fixing the voltage of the predriver
stage, to improve the quality of the
power supply for the low-level
amplification stages. Outstanding
S/N ratio and stable operation
unaffected by ambient temperature
and by AC line fluctuations is
guaranteed at all times. Note 1: Low amplifier output impedance
When forming the load of a power amplifier a 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 output circuitry. The internal 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 when the impedance of a load fluctuates drastically, 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 propor tional to the impedance of
the load. A conventional amplifier can be easily 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 reser ves are
needed. This can only be achieved by a thorough redesign of all
basic amplifier aspects.
Bias stabilizer
circuit Bias stabilizer
circuit
Constant current
circuit
Bias stabilizer circuitBias stabilizer circuit
Fig. 1
Circuit diagram of amplifier section
Fig. 2 Output power vs. load impedance(output voltage/output current: actual measurements)
* 1-ohm oper ation possible
with music signals only Output voltage (V)
Output current (A)

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Power amplifier assembly
with MCS circuit, current
feedback amplifier circuitr y,
and output stage with 22
parallel push-pull transistors
mounted directly to two large
aluminum diecast heat sinks
Current feedback circuit
topology prevents phase shifts
in high frequency range
The M-8000 employs the original
Accuphase current feedback pr inciple.
Figure 3 shows the operating principle
of this circuit. At the sensing point of the
feedback loop, the impedance is kept low and
current detection is performed. An impedance-
converting amplifier then tur ns the current into a
voltage to be used as the feedback signal. Since the
impedance at the current feedback point (current
adder in Figure 3) is ver y low, there is almost no
phase shift. Phase compensation can be kept to a
minimum,
resulting in
excellent
transient
response and
superb sonic
transparency.
Figure 4 shows
frequency
response fordifferent gain settings of the current feedback
amplifier. The graphs demonstrate that response
remains unifor m over a wide range.
Use of two M-8000 in bridged configuration
possible, resulting in a mono amplifier with four
times the power
Bridged operation means that two amplifiers are
driven by the same signal voltage but with opposite
phase. The speaker load is then connected between
the positive output terminals of the amplifiers. When
used in a bridged configuration, two M-8000 units for m
a single mono amplifier with awesome power
capabilities: 4,000 watts into 2 ohms, 2,000 watts into
4 ohms, or 1,000 watts into 8 ohms.Printed circuit boards made from Teflon with low
dielectric constant and low loss
The printed circuit boards for the signal-carrying
circuits are made of Teflon, a glass fluorocarbon resin
material. Teflon has extremely low specific inductive
capacity which is desirable for fast signal transmission
and a low dielectric dissipation factor which results in
minimal transmission losses. High-frequency
characteristics and heat resistance are also excellent.
For further improved sound quality, the copper foil side
is gold plated.
Fig. 3 Current feedback amplifier principle diagram
Fig. 4 Frequency response with current feedback
(Response remains unifor m even when gain changes)
-“ Input
BufferCurrent adder
I-V
converter
Trans-impedance
amplifier Amplifier Output
Current NFB
network + Input
Buffer