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!