less than about 3Vpk. Thus the amplifier is protected for short circuits because the base-emitter voltage of TR68A increases
when output current increases and when voltage across the output devices increases.
For output voltages exceeding about 3Vpk, ZD76 conducts connecting R55A to sense the output voltage. In this case, as
output voltage increases, the base-emitter voltage of TR68A reduces, thus the current limit is increased as the output voltage
increases, defining the third slope of the limiting characteristic.
“Peak” LED circuit
The “peak” LED (LED1A) is driven in series with the Limiter LED (LED2A) from the output of the amplifier via D13A with it’s
threshold controlled by ZD7A and R58A. With no signal present, ZD7A and R58A generate a reference voltage at the anode of
ZD7A, which is 18V below the +HT supply rail. All the current flowing through R58A comes from ZD7A. To turn the LED’s on,
the amplifier is required to produce an output voltage approximately 5V above the reference, at which point ZD7A is no longer
in breakdown and the current flowing through R58A comes from the output stage via D13A, LED1A and LED2A. Thus the
“peak” LED threshold and the “Clip Limiter” threshold vary with the +HT voltage and thus the output loading conditions.
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 /
ºC) reference 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 ºC. R9 and ZD2 produce a reference voltage of 9.1V at
the cathode of ZD2. This is divided by R18 & R19 to give about 5.5V at pin 12, the non-inverting 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 therefore 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 therefore 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 ºC), 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 (soft-start) 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.
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.
© 2002-2005 Crown Audio®, Inc.
