m 3-parallel push-pull output stage delivers linear power into
ultra-low impedance loads m Current feedback circuit topology
combines excellent sound quality with total operation stability
m Bridged connection mode allows upgrading to truemonophonic amplifier m Massive Super Ring toroidal transformer
m Balanced inputs m Two sets of large speaker connectors

The embodiment of musical power. Witness a stereo power amplifiercapable of delivering 400 watts of linear power into 1-ohm loads. Current
feedback topology guarantees stable operation up to ultra highfrequencies. 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 (Note1), 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 ofdriving any kind of speaker load with optimum
results, which is one of the reasons behind thehigh praise that these products invariably
receive.
The P-370 is a stereo power amplifier which
fully implements these advanced circuit designprinciples. The output stage uses three pairs ofhigh-power transistors in each channel,
arranged in a parallel push-pull configuration.
These devices are mounted to massive heatsinks, 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 theenergy 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 amountsof negative feedback. Balanced inputs make theamplifier impervious to externally induced noise
during signal transmission. The front panelfeatures two large analog power meters whichgive the amplifier an elegant look. For use in an
audiovisual system, the meter lights can beswitched off so as not to interfere with themonitor image.

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 thesupplied 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 ofabout 4 ohms. However, at 2 ohms and below, much more substantial output
reserves are needed. This can only be achieved by a thorough redesign ofall 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 dissipationof 150 watts and collector current of 15 amperes.

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

These transistors are arranged in a 3-parallelpush-pull configuration (Figure 1) and mounted
on massive heat sinks, for efficient heatdissipation. This enables the P-370 toeffortlessly 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

BIAS STABILIZER CIRCUIT– INPUT

BIAS STABILIZER CIRCUIT+ INPUT

BIAS STABILIZER CIRCUIT

Figure 1 Circuit diagram of amplifier section (one channel)

the feedback loop, the impedance is kept lowand current detection is performed. An
impedance-converting amplifier then converts
the current into a voltage to be used as thefeedback signal. Since the impedance at thecurrent feedback point (current adder inFigure 3) is very low, there is almost nophase shift. Phase compensation canbe 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 graphsdemonstrate that response remains
uniform over a wide range.

Robust power supply with "Super Ring"
toroidal transformer and high filtering
capacity
The P-370 featuresa large toroidal
power transformer
with a rating ofabout 700 VA. The
transformer
ishoused in anenclosure filledwith a material thattransmits heat andabsorbs vibrations. A toroidal transformer uses

heavy-gauge
copper wiring on adoughnut-shaped
core. This results in
low impedanceand high efficiency,
while allowingc o m p a c tdimensions. In
particular,the"Super Ring" transformer used in the P-370 has
various advantages, such as the near-circularcore caliber, allowing near-circular coil windings
with high packing density.
Two ultra-large aluminum electrolytic capacitors

rated for 47,000 uF each serve to smooth out
the pulsating direct current from the rectifier,providing more than ample filtering capacity.
Balanced connection reliably blocks induced
noise
Balanceds i g n a ltransmission
means that
twosignallines are usedwhich carrythesamesignal with
oppositephase. On thereceiving side,
the signals areUnbalanced and balanced input
mixed, whichconnectors
cancels out
any noise, leaving only the pure original signal.
Bridged mode creates a true monophonicamplifier with 600 watts into 4 ohms or 300
watts into 8 ohms
Bridged operation means that two amplifiers are
driven by the same signal voltage but with
opposite phase. This turns the unit into a highgrade monaural amplifier with even higher
output capability.

n Output stage of one channel with
3-parallel push-pull transistors
mounted directly to large heatsinks. Power amplifier assemblywith current feedback amplifier
circuitry.

Large direct-reading analog power meters
Power levels can be easily monitored thanks tothe large analog power meters which cover awide dynamic range and provide direct readings.
Switches for meter on/off and illumination
control are also provided.
Easy switching between dual mono
operation and bridged connection
A mode selector on therear panel makes it
simple to switch
between dual mono,
stereo, or bridged
operation. The dualmono position is usefulfor example to drive a center woofer in mono,or to obtain the same signal from both speaker
outputs for driving a bi-amped speaker setup.
Two sets of speaker outputs
The large speakerterminals acceptheavy-gauge
speaker cable. Twosets of outputs with
an A/B speakerselector
areprovided.
Bothoutputs can also be
used at the sametime, which allows bi-wiring (supplying the same
signal via dual leads to speakers with separate
high-frequency and low-frequency inputs).

High-quality, high-reliability parts used inP-370

Assembly with protection circuitry, etc.

n FRONT PANEL

P-370 Guaranteed Specifications(Guaranteed specifications are measured according to EIA standard RS-490.)m Continuous Average Output (20 to 20,000 Hz)
Stereo mode
400
watts per channel into 1 ohms★
(both channels driven)300
watts per channel into 2 ohms
150
watts per channel into 4 ohms
75
watts per channel into 8 ohms
Note: Ratings marked ★ are for music signals only.
Monophonic mode(bridge connection)800
watts into 2 ohms★
600
watts into 4 ohms
300
watts into 8 ohms
Note: Ratings marked ★ are for music signals only.

m Total Harmonic Distortion
Stereo mode (both channels driven)
0.05% with 2-ohm load
0.02% with 4- to 16-ohm load
Monophonic operation (bridged connection)
0.02% with 4- to 16-ohm loadn REAR PANEL

m Intermodulation Distortion

0.003%m Frequency Response

At rated output: 20 - 20,000 Hz +0, –0.2 dB
At 1 watt output: 0.5 - 160,000 Hz +0, –3.0 dB

m Gain28.0 dB (in stereo and monophonic operation)m Output Load Impedance

Stereo operation:
2 to 16 ohms
Monophonic operation:
4 to 16 ohms
★ With music signals, load impedance of 1 ohm (stereo)or 2 ohms (mono) can be driven.

m Damping Factor

200 (stereo/monophonic operation)m Input Sensitivity(with an 8-ohm load)

Stereo mode
Monophonic modem Input Impedance

Balanced:
Unbalanced:

m Signal-to-Noise Ratio(A-weighted, input shorted)

120 dB (rated continuous average output)m Output Level Meters–50 dB to +3 dB, logarithmic scale,OFF switch provided

m Power Requirements

120V/230V (Voltage as indicated on rear panel)
AC, 50/60Hzm Power Consumption

44 watts idle
475 watts in accordance with IEC-65
335 watts at rated power output into 8 ohms

m Maximum Dimensions

Width:Height:Depth:

m Weight

24.4 kg (53.8 lbs) net
29.0 kg (63.9 lbs) in shipping carton★

1 Power meters for left and right channel(dB scale)
2 Meter operation/illumination switch
ONOFF
3 SPEAKER A/B switches
4 Power switch
5 Unbalanced inputs
6 Balanced input connectors
a Groundb Inverted (–)
c Non-inverted (+)

7 Speaker output terminals for left and rightchannel
8 Mode selector
DUAL MONO NORMAL
BRIDGE
9 Input selector
UNBAL
BAL
J AC input connector(for supplied power cord)★

0.98 V for rated output
0.11 V for 1 watt output
1.95 V for rated output
0.11 V for 1 watt output

Remarks

40 kilohms
20 kilohms

475 mm (18-11/16")
180 mm (7-1/16")
417 mm (16-7/16")★ This product is available in versions for 120/230 V AC. Make sure that the voltage shown on the rear panel matches the AC line voltage in your area.
★ The shape of the AC inlet and plug of the supplied power cord depends on the voltage rating and destination country.

Supplied accessory:• AC power cord

• Specifications and design subject to change without notice for improvements.

http://www.accuphase.com/

PRINTED IN JAPAN

C015Y 851-0115-00 (AD1)