This is a simple peak limiter - performance is quite respectable, and it can be used with conventional amps using bipolar transistors, MOSFETs. The gain control element is a Light Dependent Resistor (LDR). These are blessed with a few very useful features for our purposes, one of which is low distortion even at quite high signal levels. This is the figure of the circuit.
The operation of the circuit is 10k resistor selected for the input, and although this is lower than I would like, many power amps have a relatively low input impedance and too much signal would be lost. The LDR simply shunts the signal to earth when it is illuminated. A single unit should control both channels of the power amp as shown. If only one channel is needed, then delete the components for "Right", including the associated light pipe. The value of R3 must be selected based on the amplifier power. For a 100W amp, a value of 1.8k is about right, but it is likely that a little experimentation will be needed. As a rough guide, the table below will be helpful, and it is probable that the value from the table will be OK. The idea is to limit the current through the LED to a sensible maximum.
The operation of the circuit is 10k resistor selected for the input, and although this is lower than I would like, many power amps have a relatively low input impedance and too much signal would be lost. The LDR simply shunts the signal to earth when it is illuminated. A single unit should control both channels of the power amp as shown. If only one channel is needed, then delete the components for "Right", including the associated light pipe. The value of R3 must be selected based on the amplifier power. For a 100W amp, a value of 1.8k is about right, but it is likely that a little experimentation will be needed. As a rough guide, the table below will be helpful, and it is probable that the value from the table will be OK. The idea is to limit the current through the LED to a sensible maximum.
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