Friday, October 04, 2024

Experimenting with an SSM2167 compressor limiter board

The home brew AM transmitter on the bench at the moment has thrown up the issue of either low or over modulation from my mic audio. Some time ago, I purchased a couple of interesting boards described as "SSM2167 Preamp Compressor Limiter Noise Gate Dynamics Processing Module DC 3V-5V Microphone Preamplifier". They are under AU$5 from AliExpress and really tiny.


To aid with experimentation, I've mounted mine on a carrier board.


R1 on the board controls the noise gate, which I'm not too interested in. R2 controls compression and came with a 1k resistor in place. The data sheet has this table:

1k would give almost no compression. A strange choice given how the boards are marketed. Happily there are big through holes available on the board for both resistors.



With the un-modified board I fed a 1kHz tone in and get peak to peak voltages:

50mV -> 440mV
100mV -> 880mV
200mV -> 1.43V

Higher inputs do not give higher output. The output waveform looks pretty good, so not clipping. Gain is rather low for my dynamic mic however.

I replaced R2 with a 100k resistor and it has much more gain.

5mV -> 880mV
10mV -> 960mV
20mV -> 1.06V

Output level stops there and again gain is being reduced.

Stephen, VK2BLQ, brought this idea to my attention as it has been discussed as helping the uBitx radios.

3 comments:

  1. I was thinking of trying this as AGC for a direct coversion receiver. What do you think?

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  2. It could be used to compress any audio but the time-constant might be a bit short to work as a normal AGC

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  3. Yes, that could be. One of the projects would be a fixed receiver for monitoring the International Beacon Project, especially on 28.2MHz, so the time constant should not be too critical. The only thing that could worry me a bit is the gate function, or maybe it could be used as a squelch function here.
    Just an idea for experimentation.

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