Philips 22 ah 572 service manual

Page 2

Phono Pre-Amp Assembly

The Phono Pre-Amp assembly receives, and when necessary
amplifies, the input signals to the unit. The input circuits
are designed with a high input impedance, and a low output
impedance. This prevents loading of the source and
provides a high current output to the next stage to help
eliminate hum and noise.

The input circuits for the Tuner, Aux., and Tape Play input
jacks consist of a capacitively coupled emitter follower,
whose output is applied to the switching circuits.

The input circuit for each channel of the Phono 1 and
Phono 2 jacks is identical. This circuit (a differential input,
complementary output amplifier) provides a high input
impedance and a low output impedance, as did the emitter
followers mentioned above. However, since the signal level
at the phono jacks may be extremely low, depending upon
the cartridge being used, amplification may also be necessary.

The operation of this circuit is as follows, using the right
channel of the Phono 1 input. The signal from the input
jack is coupled through C201 and R209 to the base of
0201, R201 and R205 provide the input impedance of
approximately 47K ohms, while R209, 0257, and 0205
form an RF suppression network. Capacitor, C201, keeps
any DC potential from being present at the input jack,
but passes the input audio signal. 0201 and 0205 form a
differential amplifier. The gain and frequency response of
this amplifier are determined by the feedback networks in
the base circuit of 0205. By varying the amount of feed-
back present at the base of 0205, R225 establishes the
overall gain of the amplifier, and therefore, the sensitivity of
the circuit.

The output of the differential amplifier is direct coupled
from the collector of 0201 to the base of the driver, 0209.
The output at 0209 collector is applied to the bases of
0213 and 0217 which form a complementary symmetry
push-pull amplifier. The output of the complementary
symmetry amplifier is taken from the junction of R253 and
R257. It is then coupled through C229 and pin 2 of
P/J214 to the Selector Switch assembly. C229 passes the
audio signal, but blocks any DC potential from the switching
circuits, thereby eliminating any audible 'pop during the
switching operation.

Selector Switch Assembly & Tape Monitor & Mode Assembly

The switching circuits contained on the Selector Switch
assembly and the Tape Monitor and Mode assembly are used
to-distribute the signals to the appropriate places and light
the correct function and mode indicators.

The Selector Switch assembly receives the tuner, aux., and
phono inputs from the Phono Pre-Amp assembly. The
Function Selector switch, $201, is rotated to select which
input signals will be coupled to the Tape Monitor and Mode
assembly. A portion of this switch also provides a current
path to light the appropriate function indicator. The Tape
Play inputs are coupled directly from the Phono Pre-Amp
assembly to the Tape Monitor and Mode assembly. Here,
they are connected to the Tape Monitor switch, 8202, along
with the signals from the Selector Switch assembly. The
Tape Monitor switch connects the appropriate input signals
to the Tape Record jacks and K201, one of the Tape
Monitor relays. Part of 8202 also forms a current path to
light the correct Tape Monitor indicator.

The other Tape Monitor Relay, K202,is connected to the
input signals from the Selector Switch assembly. The two
Tape Monitor relaysare connected so that when the contacts
of one are open, the contacts of the other are closed.

C862 011

Contacting the Tape Monitor Touch Control changes the
state of the relays. Therefore, with the Tape Monitor
circuit activated K201 is energized and the signals from the
Tape Monitor switch are coupled through the relay contacts
to the Mode switch and on to the Volume and Filter
assembly. When the Tape Monitor circuit is deactivated
K202 becomes energized, and the signals from the Selector
Switch assembly are now coupled through the relay contacts
to the Mode switch. The Mode switch, S203, determines
upon which output channel the input signals appear. The
signals leaving the Mode switch are applied to the left and
right channel circuits on the Volume and Filter assembly.
Part of the Mode switch also provides a current path to
light the correct mode indicator.

Volume and Filter Assembly

The Volume and Filter assembly establishes the level of the
units output signal, and determines the balance between the
right channel level and the left channel level.

Also included on the Volume and Filter assembly are
circuits which change the frequency content of the signal
to provide low-cut filtering, high-cut filtering, and loudness
compensation.

The operation of this assembly is explained using the left
channel circuits. The signal at pin 3 of P/J303 is coupled
through R372 to the top of the Volume control, R3258.
The signal is also supplied to the filter network in the base
circuit of 0302. Due to the action of this filter network
the mid-range frequencies of the signal at the base of 0302
are attenuated more than the high and low frequencies.
When the Loudness control is activated, the positive potential
(about 25V) at the gate of 0304 is reduced to nearly
ground by saturated 0553, on the Touch Control Assembly
(see Touch Control Assembly for further information).
This action causes 0304, which acts as an open switch when
Loudness is not activated, to now act as a closed switch.
The outputof the emitter follower, 0302, is now coupled
through 6310 and 0304 to the tap on the Volume Control,
R325B. Thus, the high and low frequencies are boosted at
volume settings below the tap.

From the wiper on the Volume Control, the signal is
coupled through C314 to a differential amp, 0306 and
0308. The output of the differential amp is direct coupled
from the collector of 0306 to the base of 0310. Being a
common emitter amplifier, 0310, amplifies and inverts the
signal, and applies it through C320 to the top of the
Balance control, R3268. A portion of the signal at the
collector of 0310 is applied as negative feedback, through
R336, to the base of 0308. The divider network made up
of R336 and R338 determines the amount of feedback
present at the base of 0308. This in turn determines the
gain of the differential amp, 0306 and 0308. Capacitor,
C318, has no effect on the signal.

To allow for better filtering action, a two-stage filter is
included on the Volume and Filter assembly, with Darling-
ton amplifiers acting as buffers between stages.

With the high and low cut filters not activated, 0312 and
0318 are turned off. This condition effectively removes
these transistors from the circuit, since they appear as a very
high resistance. Under these conditions, the signal at the
wiper of the Balance control is coupled through C322,
R342, and C326 to the base of 0314. 0314 and 0316 are
connected as a Darlington amplifier with its output coupled
from the emitter of 0316 through C330, R358, and C334
to the base of 0320. 0320 and 0322 are also connected as
a Darlington amplifier with its output coupled from the
emitter of 0322 through C338 and pin 5 of P/J301 to the
Tone Control assembly.