Audio Speech Compressor

There are many low power SSB rigs and kits don’t have any real speech processor circuitry, although there is a built in speech processor in most modern HF. Speech processing is an important thing, when the SSB is operated at low power to get through the QRM. Speech processing can be done by this circuit. This is a simple and low cost circuit but it can perform well. This circuit includes the microphone element. So, we can assemble complete speech processor microphone in an old defunct desk mike. Here is the schematic diagram of this circuit :

This circuit contains a feedback amplifier and an audio preamplifier. S1 should be closed, to use the speech processor. R5 is used as the gain control for basic preamplified microphone that is given by Leaving S1 open. Q2 is triggered to conduct by voice peaks from the feedback amplifier, when S1 is closed. It causing the gate of Q1 to become negative. The resistance between the source and drain of Q1 substantially increase due to the gate of Q1 become negative. During very large voice peaks, Resistor R1 allows a small amount of audio to pass. to smooth out the voice, this circuit uses capacitor C2. C6 gives a time delay, so we can do the fast attack, slow release action. R7 can be replaced by a potentiometer. So we have variable speech compression control. [Source: Radio Amateur Society of Norwich]