Speech Compressor

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David Mittelstadt:
Hi:  Does anyone know where I could find a simple circuit diagram for a speech compressor with a high impedance input and output?  I want to use it with a Heathkit HW-101 and a high impedance microphone.  I have seen one or two simple circuits on the internet, but they say nothing about input and output impedance, and I'm not smart enough to tell just by looking at the circuit.

David M

Steve Katz:
It doesn't have to be "high impedance input and output."

As a matter of fact, you'd be much better off using a low impedance dynamic microphone with an external speech compressor.  A low impedance (output) compressor can drive the high-Z input of the HW-101 perfectly, especially if the output voltage is adjustable.  If you find the compressor is overdriving the rig badly, you can always add a series resistor (like 50k) to step the output voltage down slightly for the high-Z input of the Heathkit.

The problem occurs in the "other" direction: A high-Z source cannot drive a low-Z load (usually).


Peter Chadwick:
A lot depends on what you want the speech compressor for. There are two basic types - the straight compressor, sometimes called a VOGAD (Voice Operated Gain Adjusting Device) and a processor. The VOGAD merely adjusts the drive level to the tx with differing voice levels: popular in some military applications because the guy can whisper but still modulate the radio. Not generally a lot of use in ham radio, it has a long time constant gain control. The other form of compression, the processor, reduces the peak to average ratio of the speech, and so you can run a higher average RF power output for the same PEP. Either type of processing taken to extremes sounds awful: you stop talking and the wife on the 'phone in the other room, the kids playing pop music in the bedroom and the fan on the amplifier all modulate up to full PEP output! Normally about 6 to 10 dB of gain adjustment is adequate.

Processors work by producing an SSB signal (can be by the phasing method) clipping the peaks, and then demodulating back to audio and filtering before feeding the tx. Clipping audio and feeding it into an SSB tx doesn't provide the performance. This type of processing can be as effective as small amplifier.

Bob Lewis:
Also be aware that because the compressor increases the average power output you may need to be concerned about the radio and power supply's ability to handle that power.

James Miccolis:
I would not recommend using a speech compressor with a stock HW-101. You'll lose more than you gain.

The HW-101 is a good rig for its vintage and price but you can't it to work miracles.

73 de Jim, N2EY


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