SSM2166 (Mic amplifier and compressor) as Audio AGC?

Henning Weddig DK5LV says he is in the process of using an op amp and SSM2166  on his “original” BITX40 as a way of achieving an audio AGC function.

This op amp has a  gain of 10 (20 dB) and an audio AGC system.   He purchased a pcb from ZL1CVD with the DIL chip via ebay years ago.  Unfortunately ZL1CVD does not sell this pcb any more…

The SSM2166 has a dynamic range of 60 dB, the opamp in front of it is intended to replace the first discrete amplifier after the demodulator and will amplifiy the audio into a range the SSM2166 can handle.  The SSM2166 has a provision for outputting an AGC voltage (an RMS output) and this may be able to be used for driving an S-meter.

Others are thinking along the same lines.  For example, Glenn VK3PE says:

“I’m thinking along the same lines. I built a Mic amp version using the SSM2167 and thought it might work also for the Receiver. It’s essentially the module seen on ebay but I’ve added a level pot on the output side.”

Meanwhile Alex PA1FOX comments:

“I am using the AGC from the original uBitx design, but found the time constant of 1uF and 100k to be far too low. This makes it a very fast AGC, not producing a nice sound with voices. I changed to 1.5 uF and 4M7 and now stronger stations are nice and clear (they ‘push’ the noise level down ) and the gain comes up nice and easy when the QRG is clear. I think I’ll keep it this way.”

Reference

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