Accuphase P-370 Brochure

m 3-parallel push-pull output stage delivers linear power into

ultra-low impedance loads m Current f eedback circuit topology
combines excellent sound quality with total operation stability
m Br idged connection mode allows upgrading to true
monophonic amplifier m Massive Super Ring toroidal transf ormer
m Balanced inputs m Two sets of large speaker connectors

The embodiment of musical power. Witness a stereo power amplifier
capable of delivering 400 watts of linear power into 1-ohm loads. Current
feedback topology guarantees stable operation up to ultra high
frequencies. Massive power supply with 700 VA toroidal transformer,
and wide-band high-power transistors in 3-parallel push-pull
configuration ensure constant-voltage drive.

In order for an amplifier to drive loudspeakers
to optimum performance, two major goals must
be realized: very low output impedance (Note

1), and constant drive voltage (Note 2). 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. Accuphase amplifiers are capable of
driving any kind of speaker load with optimum
results, which is one of the reasons behind the
high praise that these products invariably
receive.

The P-370 is a stereo power amplifier which
fully implements these advanced circuit design
principles. The output stage uses three pairs of
high-power transistors in each channel,
arranged in a parallel push-pull configuration.
These devices are mounted to massive heat
sinks, for efficient dissipation of thermal energy .
Power linearity is maintained down to extremely
low load impedances. This allows the amplifier
to easily drive even speakers with very low
impedance or uneven impedance curves . Using
the P-370 in bridged mode creates a mono
amplifier with even more impressive power
reserves.

The power supply section which acts as the
energy source for the amplifier employs a high­efficiency "Super Ring" toroidal transformer in
combination with large filtering capacitors.
Current feedback topology combines total
operation stability with excellent frequency
response, while requiring only minimal amounts
of negative feedbac k. Balanced inputs mak e the
amplifier impervious to externally induced noise
during signal transmission. The front panel
features two large analog power meters which
give the amplifier an elegant look. For use in an
audiovisual system, the meter lights can be
switched off so as not to interfere with the
monitor image.

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INPUT

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INPUT

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 proportional 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. Howe ver , at 2 ohms and below , much more substantial output
reserves are needed. This can only be achieved by a thorough redesign of
all basic amplifier aspects.

Triple-parallel power units with push-pull
configuration deliver ample linear power:
300 watts per channel into 2 ohms, 150 watts
into 4 ohms, or 75 watts into 8 ohms

The output stage uses high power transistors
with excellent linearity and switching
characteristics, rated for a collector dissipation
of 150 watts and collector current of 15 amperes.

These transistors are arranged in a 3-parallel
push-pull configuration (Figure 1) and mounted
on massive heat sinks, for efficient heat
dissipation. This enables the P-370 to
effortlessly drive even speakers with extremely
low impedance or with reactive loads.
Figure 2 shows the output/voltage
characteristics at various load impedances. It
can be seen that output voltage remains nearly

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Figure 2 Load impedance vs. output power

(output voltage/output current) of P-370

constant regardless of load, which means that
output current increases linearly. The actually
measured clipping power is an impressive 400
watts into 1 ohm, 330 watts into 2 ohms, 220
watts into 4 ohms, or 133 watts into 8 ohms.

Current feedback circuit topology prevents
phase shifts

The P-370 employs the so-called current
feedback principle. 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 converts
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 very 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 for
different gain settings of the current
feedback amplifier. The graphs
demonstrate that response remains
uniform over a wide range.

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Figure 1 Circuit diagram of amplifier section (one channel)