CMR checkout procedure is as follows :
Inject a common-mode test signal at 1kHz and +4dBu to the channel under test. The common-mode test signal of
the Audio Precision test system is suitable, otherwise connect the signal to both pins 2 & 3 via 56 Ohm or 47
Ohm resistors. Observe the amplifier output (with level control at maximum) which should be less than -9dBu.
Power for the input stage is derived from the main +HT and -HT supplies via 1W resistors R24 and R25
and shunt regulated to a n18V, -18V by ZD3 and ZD4 respectively, all located on the main PCB. The out-
put of the input differential amplifier is fed via DC blocking capacitor C7A and 100R “Build-out" resistor R10A to
pin 5 of Header 2 (channel B - pin 2 of Header 3) and then exits the input board.
If LK1,2,3,4 are present and intact then the signal travels back onto the input board via pin 4 of Header 2 (channel
B - pin 5 of Header 3).
Sub-Sonic filter and Clip Limiter
The Sub-Sonic filter is based around the other half of the TL072 dual operational amplifier, IC1A on the
input board, configured as a Sallen & Key unity gain 2nd order Butterworth high pass filter. The two-pole three-
position switch, SW1A, selects the following -3dB cutoff frequencies:
1) 30Hz: only C8A and C10A connected;
2) 15Hz: C11A and C12A connected in parallel with C8A and C10A respectively;
3) Off position: C14A bypassing C8A and C10A.
Limiter switch SW2 in its OFF position shorts R2A and disconnects LDR1A to turn the Clip Limiter off.
In its ON position, SW2 connects R2A and LDR1A as a light-dependent voltage divider, controlling the
level of the input signal. LDR1A faces LED2A, which is lit when the amplifier reaches about 1dB below clip (at the
same time as the clip LED at the front). Because the resistance of LDR1A reduces as its cell area is illuminated,
pushing the amplifier into clip will, therefore, cause the input signal to be reduced at the R2A -LDR1A divider.
Thus the amplifier is Clip Limited.
The output of the filtering/limiting op-amp is AC coupled by C13A and built out by R11A, and continues to pin 3
of Header 2 (ch. B pin 4 of Header 3), at which point it re-enters the main board and makes its way to pin 1 of
CN2A, a three-pin header making connection by wire to the level potentiometers: pin 1 is Top, pin 2 is Wiper, pin
3 is Bottom. The signal emerges from pin 2 and is fed to the input of the Power Amplifier.
POWER AMPLIFIER
The power amplifier consists of a fairly conventional Class A driver stage driving a Class AB bipolar output stage.
Each stage will be dealt with individually.
Class A Driver
The input signal returned from the level control is fed via DC blocking capacitor C7A and R8A. DC bias
current for the Class A input stage is supplied via R9A, while 33pF capacitor C54A prevents any extreme high fre-
quency input signals from reaching the power amplifier and also provides a low source impedance at high fre-
quencies to ensure frequency stability.
The first stage of the class A driver consists of Q1A and Q2A configured as a long-tailed pair differential
