Onkyo P 303 Service Manual

Page 2

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CIRCUIT DESCRIPTION
1. General Circuitry

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A basic condition for any good quality audio amplifier is to amplify audio signals without causing any change, or
coloration, in the original program source whatsoever. To achieve this goal. the model P7303 preamplifier employs only
high grade circuits and high performance circuit components throughout, and limits the signal path to the shortest route

possible.

Right
Channel

Left
Channel

BUFFER
AMPLIFIER

F LAT
AMPLIFIER

EQUARIZER
AMPLIFIER

MC
AMPLIFIER

The birds eye view (photograph) of the model P7303 shows
the completely independent left and right channels, with their
respective .+ and 7 stabilized voltage power supplies. Besides
the bus bar ground conductor, which has also been employed
in other Onkyo amplifiers, both left and right channels are also
equipped with separate power bus lines (bus bar power supply
lines) for direct supply of power to all individual circuits.
The constant voltage power supply in both channels is also
further stabilized by the incorporation of a second stabilizer
circuit.

The MC amplifiers (lower part of photograph) are designed for
direct connection to MC cartridge tonearms without the need
for extra stepiup transformers.

The faint input signals from MC cartridges are amplified up to
a level high enough for the equalizer amplifier to handle, but
importance has also been placed upon the reduction of noise
(Le. high S/N ratio). This is because the output level from
most MC cartridges in use today are extremely small, ranging
on the whole from 0.05mV to 0.3mVi Consequently, the
reduction of noise achieved in the P~303 has been very con-
siderable indeed. Employing a total of 6 transistors in a triple
pushipull circuit, S/N ratio has been improved to 70dB (e-
quivalent input of lSOdB/V). With an input impedance of 10
ohms, cartridges of specified cartridge load from 2 to 40 ohms
may be used. And in order to reduce the effects of induced
noise, and noise caused by external sources, very strict

,1_

1 .5V/600S2