OUTPUT CONNECTIONS
The output of the amplifier is connected to Zobel Network R12A/C8A. This network presents a defined
load impedance to the output stage at high frequencies to ensure stability. Either of R12A or C8A being
faulty will result in the amplifier oscillating at high frequency, which may also be evidenced by mains
"hum" and/or distortion at the output. This signal is fed via output choke L1A which isolates any load
capacitance from the amplifier feedback to ensure stability.
The output is then fed through output relay RLY1A and on to the rear panel output connectors.
PROTECTION SYSTEM
The protection system is based around IC1, a TL074 quad op-amp.
The temperature of the heatsink is monitored by TH1, an LM35DZ temperature sensor integrated circuit
producing 10mV /
o
C. The temperature signal is then multiplied by 10 by one op-amp (pins 8,9,10) &
R16,R17. The output (pin 8) is fed directly to pins 6 & 13 serving as a temperature dependent (0.1V /
o
C) refer-
ence for two comparator circuits: one (pins 5, 6, 7) controls the relays and the other (pins 12, 13, 14) controls the
fan speed.
The Fan can run at two speeds, the changeover happening at about 55
o
C. R9 and ZD2 produce a reference volt-
age of 9.1V at the cathode of ZD2. This is divided by R18 & R19 to give about 5.5V at pin 12, the noninverting
input, which is compared with the temperature signal at pin13, the inverting input.
1) Temperature signal is less than 5.5V: the output of the op-amp will be high (+24V), turning Q1 off and there-
fore Q2 off. The fan speed is controlled by R21 which forces approximately half speed.
2) Temperature signal is more than 5.5V: the output of the op-amp will be low (-5.6V), turning Q1 on and there-
fore Q2 on. R21 is now effectively shorted out by Q2 and the fan runs at full speed.
At turn-on, C16 will charge through R9 and R10 towards the 9.1V reference (ZD2). The voltage is fed to
the non-inverting input (pin 5) of op-amp at pins 5,6,7 configured as a comparator with hysterisis (D9 and R11).
The reference for the comparator is set by the temperature reference which is about 2.5V at room temperature
(25
o
C), When the voltage across C16 exceeds the temperature reference, the op-amp output will swing high
(+24V) and turn Q3 on via current limiting resistor R13. When Q3 is on, it pulls current through the coils of RLY1
(softstart) and RLY1A, RLY1B on the output board. This also means that the collector of Q3 will swing low (close
to 0V) effectively shorting out R15 and LED2 to turn LED2 (Protect, Yellow) off.
The output of each channel is fed via resistors R2 (Channel A) and R3 (Channel B) into C9. The
combination of C9 with R2 and R3 forms a low-pass filter, and so at signal frequencies C9 will have no
voltage across it. In the event of a DC offset appearing at the output; however, C9 will charge to a DC
voltage. This voltage is limited to 3 diode drops (about 2V) by D3-D7 which also define a window
comparator with R5, R6 and op-amp at pins 1,2,3. Nothing happens while the voltage across C9 is between about
+1 and -1. If the voltage across C9 exceeds this then the output (Pin 1) of the comparator will swing low (-5.6V).
This will discharge C16 (see previous paragraph) through D8 and thus open the RLY1, RLY1A, RLY1B and turn
LED2 (Protect) on.
The network consisting of D1, D11, D12, R1, R4, C10 and C11 provides the rapid turn-off feature of the
