The delay circuit is built around the well-known 555 timer, shown as Q4 in the schematic. P2+R14 and capacitor C5 define the time delay. With the values shown the delay can be set between 6 and 12 seconds. I think a reasonable delay is about 10 seconds. If you would like to allow more time for your power amp to settle before the speakers are connected you can increase the value of P2+R14 and/or capacitor C5. R15 and C6, connected to the trigger pin (pin 2), simulate a reset pulse and allow the timer to start counting. Pin 5 is decoupled through a 0.luF/100 MKT capacitor.
The relays are connected between the output pin 3 and the + supply. Because this circuit is meant to be a universal one, I am using two relays in parallel, in order to cope with high current/high-power amplifiers. The relay contacts are connected in anti-parallel to minimize rectifying effects. A green LED is connected across the relays and can be mounted on the front panel to indicate proper operation of the power amp.
Q1A and Q1B, a dual, FET-input opamp, is configured as a window detector. The +input of Q1B is connected to a reference voltage of approx. +2V, while the -input of Q1A is referenced to -2V. The -input of Q1B and the +input of Q1A are connected together and form the DC sensing input. As long as the DC input is within the "window, defined by the +2V and -2V references, both outputs are positive and both level-shifting Transistors Q2 and Q3 are off.
When the DC input moves outside the "window, i.e. more positive than +2V or more negative than -2V, Q1B or Q1A output will change to -12V, pulling current through the resistor chain R7-R8. Transistor Q2 turns on, which turns Q3 on, and pins 6/7 and pin 2 of the timer are pulled to zero Volt through D7/D8. This resets the timer, and the relays disconnect the speaker from the amplifier. When the DC offset is removed from the input the opamp output, which was at -12V, returns to +l2V and Q2/Q3 turn off. The timer is now ready to start counting again and after the predefined time delay the relays will switch the speaker on.
The output of your power amp has audio signals present, which have to be removed from the input of the DC sensing circuit. This is done with a single-pole RC network, consisting of R1 and C1/C2. The -3dB point of this network has to be selected low enough to avoid triggering the circuit by low frequency audio signals. However, it should not be too low, because it makes the circuit too slow. If you select C1=1uF, the circuit will activate at 50Hz with 100W into 8 Ohm. This is not acceptable in most applications. With 6.8uF I measured full power down to below l0Hz, without triggering the circuit. With C1 equal approx. 11uF (two 22uF caps in series) the -3dB point is at 0.15Hz and full output is available down to 5Hz. The circuit stays permanently shot down when you drive the amp with 2Hz at full power. You might want to experiment with this: if you want to prevent subsonic signals from reaching your speakers you might want to increase the -3dB point.
The purpose of Dl and D2 is two-fold: It prevents the caps from charging to a very high voltage, which would take a long time to discharge. More importantly, it is a protection for the opamp inputs: it prevents the input voltage to rise above the +l2Volt rail or below the -12V rail, which would destroy the ICs. A series resistor R2 limits the input current to the opamps.
There are two extra DC inputs to the circuit: S1 and S2. These can be used as extra logic inputs from other protection circuits. The logic level is the same as the output of the opamps: +12V (or no connection) and -12V. With +12V (or no connection) the circuit is simulating a normal operation, i.e. the circuit is not activated. With S1 or S2 equal -12V, Q2 and Q3 turn on instantly (there is no time delay here) and the speakers are switched off. These inputs can be used for example for thermal breakers (the contact of the thermal breaker has to be normally open and connected to -12V), for circuits testing for short circuit at the output, for power sensing circuits, etc.
Power is supplied by two 12V TO-220 regulators. AC input is 2x12-15V, with center tap. This can be supplied by the normal power transformer, or by a separate transformer of 5-10VA rating.
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