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.