This circuit protects the power transistors in case of overload, the speakers in case of power amp
malfunction, protects amplifier in case of short circuit in loudspeaker or speaker cable or too low
impedance of connected speakers, and also performs a muting function when the power supply is turned
ON or OFF. It is composed of four sections.
MUTING
When the power supply is turned ON, T4 base is reverse biased through D5 & 1k resistor, turning T4 OFF.
T5 base potential rises as 220uF/10V electrolytic charges from +36V through 56k & 100 ohms, and T5&T6
turn ON several seconds later. At the moment when power is turned OFF, the reverse bias of T4 base
disappears momentarily because it is coming directly from transformer secondary and not from some DC
rail which will remain for a while even after power is turned OFF. T4 remains ON however, due to residual
supply voltage in +36V rail. 220uF capacitor very rapidly discharges, T5 base potential drops and T5&T6
turn OFF. The relays releases and speakers are disconnected.
OPERATION BY DETECTOR CIRCUIT COMMAND
Command from detector circuits pass through one of D1, D2, D3 or D4 and are applied in the form of a
current flow. T3 is normally reverse biased through it's 150k base resistor, but when a large current flows
through one of these diodes, T3 base potential declines according to the voltage drop at 150k resistor. T3
then turns ON, T4 base potential rises and T4 turns ON. 220uF capacitor rapidly discharges and T5 base
potential drops, turning T5&T6 OFF. The relays release and the connection between amplifier and
speakers becomes cut off.
OVERLOAD DETECTOR CIRCUIT (see schematic below)
Shorting of the power amplifier load or the load impedance below the specified value causes a command
to be sent to the relay drive circuit. With the output stage in class AB or B operation, when "Ta" transistor
is operating in the positive half cycle, "Tb" transistor becomes cut off and the signal current flows as
indicated by the solid heavy arrows. Point "D" potential at this time is the point "A" potential divided by
R1 and R2. Also, point "C" potential is the point "A" potential divided by Re1 and Rload. Point "D" is
connected to T8 base and point "C" to T8 emitter through R3 and Re2. When Rload is extremely small,
the point "C" potential becomes considerably lower than point "D". This potential difference forward
biases T8 , hence it turns ON and current flows in D3.
"Tb" operates in the negative half cycle and "Ta" becomes cut off. The signal flows is indicated by the
broken heavy arrows. T8 is biased by the potential difference between point "C" and point "E". If Rload is
extremely small, the point "C" potential becomes considerably higher than that of point "E". T8 turns ON
and current flows in D3.
If large current flows in "Ta" and "Tb", T8 becomes ON due to the Re1 and Re2 voltage drops, and current
flows in D3. 220nF capacitor between base and emitter of T8 prevents faulty operation due to external
noise.
CENTER POINT POTENTIAL DETECTOR CIRCUIT
If a DC potential is produced at the speaker output of the power amplifier, a command is sent to the relay
drive circuit. T1 and T2 compose a differential amplifier. When the same input is applied to both input
terminals (T1 and T2 bases), no output is present. However, if there is a difference between the terminal
inputs, that difference is amplified and becomes the output between two collectors. During normal
operation, an AC signal only is present at the speaker output of the amp. As the reactance of two 470uF
capacitors is sufficiently low, practically the same signal is applied to T1 and T2 bases, resulting in
absence of output at the collector sides. If a DC potential is produced at the amp's speaker output, it
becomes the input of T2 only because DC can't pass through 470uF capacitors. If that DC voltage is
negative, T2 collector current declines, and at T1 the collector current increases and the collector
potential drops, causing current to flow through D2.
If that DC voltage is positive, T2 collector current increases and the potential drops, while at T1 collector
current decreases and the potential rises. Current therefore flows through D1.
SHORT CIRCUIT ON SPEAKER TERMINAL
AND LOW IMPEDANCE DETECTOR CIRCUIT
(refer to the main schematic)
The heart of this detector subsystem is IC1. It is a dual OPAMP working as a voltage comparator.
Noninverting inputs of both comparators (pins 3 & 5) are connected to the same reference point formed
by the voltage divider consisting of two resistors 15k and 3,3 ohms. Inverting inputs (pins 2 & 6) are also
connected to the similar dividers consisting again of 15k resistors but with left and right speaker as a
lower legs instead of 3,3 ohms resistors. Nature of comparator operation is simple: as long as potential at
inverting (-) input is higher than the one at noninverting (+), output will be clamped to the negative
supply rail and vice versa. Thus, if there is a short in speaker cable or in the loudspeaker or connected
impedance is lower than 3,3 ohms, potential at inverting input will be lower than potential at
noninverting, which will cause output to jump high (practically clamped to the positive supply rail +15V),
and that will cause T10 to turn ON causing current to flow through D4 and ... the rest of the process is
pure magic...
Even though it might looks like overload detector and this one with IC1 are doing the same thing
regarding short at the output, it is not entirely true. Overload detector (with T8&T9) will "monitor" the
situation during the normal operation of the amp, while IC1 protection will detect the problem during the
short period from the moment of turning the amp ON, 'till speaker relays activates. If there is short or too
low impedance from the beginning , speaker relays will never turn ON. If short circuit happens later
during the listening, IC1 detector won't sense it but overload detector will!
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This circuit protects the power transistors in case of overload, the

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