14 Dec 2011
Recently Peter Thornton G6NGR sent me a number of example circuits for speech processors, some dating back to the 1960s. Anyone who operates QRP SSB on HF or VHF will appreciate how important "punchy audio" is when running low power. One circuit I remember working extremely well was one based on an idea by John Hay G3TDZ. Back in the 1970s I was using 100mW AM on 2m and wanted to be heard across Cambridgeshire. My audio stage consisted of a couple of pre-emphasis stages followed by a hard audio clipper and a low pass active filter. The result was an amazingly punchy signal with barely any change in audio level when talking. The 12dB/octave pre-emphasis helped to ensure that clipping of LF signals did not result in many artifacts within the audio passband. The LPF was essential because the heavy clipping results in many audio harmonics which would otherwise result in a broadened signal. With just 100mW and a dipole the QRP AM signal was excellent copy across the county. I keep meaning to try the same circuit (if I can remember it) with the FT817. It would produce a mighty punchy QRP signal!