4
* U101b (U201b) form an adjustable gain amplifier. The gain is set by a series of link positions.
* The signal then passes to the output connector J105, (J205) PIN2 and is sent to the main
amplifier on PCB710.
* The signal is also passed to the limiter control circuit.
* On Ch. A ONLY, the signal is inverted by U101a and passed to Ch. B via the bridge switch
for bridged operation.
3. LIMITER CIRCUIT and POWER REDUCTION CONTROL (PRC) – PCB701
* The audio signal from U101b (U201b) is passed via the PRC switches, which attenuate the
signal, to U101c, where it is buffered before being rectified by the circuit of U102b. The d.c.
voltage (relative to the audio signal strength) is applied to PIN3 of U102a (U202a) and also
to PIN2 (6) of U5a (U5b). PIN2 of U102a (U202a) and U5a (U5b) PIN3 (5), are connected to
a reference d.c. voltage derived from ‘Vcc +ve’, the high voltage rail that supplies the output
amplifiers. This reference, called ‘V-threshold’, is adjustable (on PCB710) and is used to set
the limiter threshold.
* The d.c. control voltage out of U102a (U202a) controls Q101 (Q201) via a fast attack, slow
decay circuit and controls the gain of the audio channel via LED OPTO LDR, LDR101
(LDR201).
* U102a (U202a) PIN1 will only start to move +ve when PIN3 exceeds the reference set on
PIN2. Hence the limiter threshold is set.
* The PRC switches control the gain and therefore the voltage into PIN3 of U102a (U202a).
thus the threshold of limiting can be lowered to reduce the maximum output of the amplifier.
4. OUTPUT AMPLIFIER – PCB710
The unique MC
2
Audio sliding op-amp drive circuit combined with multiple bi-polar output
transistors form the power output stage. The rail switching is performed by a bank of FETs.
* Bias adjustment is via VR104 (VR204).
* VR103 (VR203) allows minor gain adjustments for alignment.
* OPTO coupler, OPT103 (203) is used to detect onset of clipping.
* Q114 (Q214) and Q115 (Q215) form a current detecting circuit which will trigger the
protection circuit via OPTO101 (OPTO201) and OPTO102 (OPTO202), also clamping the
drive signal at R168 (R268) and R169 (R269).
* R140 (R240), ZD101 (ZD201) and R141 (R241), ZD102 (ZD202), develop the ±15V supplies
for the 'sliding' op-amp relative to the output rail.
* Q51, Q52, Q53, Q54 provide the gate drive for the FET switching rails.
5. PROTECTION
The amplifier has built-in protection against various conditions:
a. Excessive temperature: heatsinks and PSU
b. Excessive drive: limiting circuit at onset of clipping
c. Output short circuit or low impedance
d. DC shift on output.
a. TEMPERATURE
* Thermistors Thxa and Thxb sense the output stage heatsink temperature, whilst Th4 and
Th5 sense the FET bank temperatures.
