The AM Forum

I am not certain if this has been reported here before, maybe by -QIX?
 
Analog Devices, one of my favorite semi companies (you gotta like the name at least), has an IC called the SSM2167. It is a complete mike preamp/compressor/limiter/downward expander, all in one 10 lead package.

All you need is 2.5-5.5 VDC, single ended, and a few caps, resistors, and a buffer amp if you are driving a long cable or 600 ohm input to a transmitter.

It has a variable 1:1 to 10:1 compression ratio, with a 40 dB range voltage controlled gain stage. There is a limiter built in for rollover of peaks, and a noise gate/downward expander function. It is rated about 0.2% THD, good to 20 KHz. Input is high Z and output can only drive 5 k or higher impedance, so it probably can use a line driver output stage. The RMS level detector inside has adjustable averaging time (with a cap) but fixed attack time.

It isn't studio pro-audio quality, but the specs are mighty good enough for AM transmission. It is a single ended part throughout, so not balanced audio circuit. If someone can build a small board for it, (its surface mount only), it could be placed inside a microphone base practically. This could perform all of the processing for microphone levels, except for the function of precise modulation limiter/control.

I just ordered two samples, to play with.

http://hightechham.com/proc.htm

There is a discussion of this on the Audio 101 thread with links to some designs.  If my memory serves me, the 2167 is the follow on to the 2166 which does not give access to the op amp feedback loop so Steve, WA1QIX's, circuit which works fine with the 2166 will not work with the 2167. 

Here's the circuit I use - comes with bass and treble controls :-)

(http://www.classeradio.com/ssm2166_compressor.gif)

Click Here for full-size Schematic (http://www.classeradio.com/ssm2166_compressor.gif)

Regards,

Steve