The circuit is designed to take almost any audio input but the output will have all peaks above a certain level, which can be ad­justed, eliminated.

Circuit in lines (Click to enlarge)

The circuit can either be wired into a receiver circuit or directly from a headphone socket. If wired into circuit permanently, RV1 should take the place of the nor­mal volume control and the output should be wired to the point which was previously connected to the volume control slider.

An extra control plus a switch will also have to be mounted on the receiver front panel. The circuit can either be left in permanently, as at most settings it will not affect the signal, or it can be switched.

The output of the receiver is taken to the input and amplified by Q1 which is connected in the com­mon emitter mode. This transistor will considerably increase the au­dio level and this is applied via a DC blocking capacitor, C2 to the two silicon diodes D1 and D2.

In the normal way these diodes will not have any bias voltage applied across them and so they will present a high resistance, and will not affect the output in any way. However, as soon as the output from the amplifier exceeds about OV6, the diodes will conduct and short the output to the negative line.

Two diodes are needed; one connected each way around so that both positive and negative going peaks are shorted out. The idea is to make sure that whatever the input level across RV1, it can be amplified so that at least OV6 can be applied across the diodes.

Since RV1 is adjusted so that the level is always the same a volume control has been included in the circuit so that the output level can be controlled in the usual way; this is accomplished by RV2.

To limit the noise, the input level is increased until the signal is just distorting, then backed off slightly so that no distortion is heard on the peaks. RV2 is then adjusted as a normal volume control. If RV1 is adjusted well below the limiting level and RV2 is adjusted for normal listening levels, the circuit has no effect.

However, it is a simple matter to include SW1 which will bypass the circuit. The supply voltage can be taken from a bat­tery as shown in the circuit, the current drain being very small, or from the receiver’s supply.