Q and D SSM2166 Compressor for Mic use.

[mersberg]

I built the board and was gonna use it for a microphone. I changed the input impedance by taking the input section and wiring up a 1k resistor and a 5k pot. I have a ringing effect during pauses and the gain seems to have no affect on the circuit. Besides matching the input impedance,do I need to change some values due to the board being setup for guitar usage?
http://www.muzique.com/tech/2166.htm

[Mark Hammer]

Well, quite honestly, the first  SSM2166 Q&D was set up for mic, rather than guitar.  Not deliberately.  It's just that the chip is intended to be most of a mic strip, and Jack O essentially took the appnote circuit, and threw a couple of things in to make it a pedal.  As we went along, folks realized there were a couple of things that needed tweaking to optimize it for guitar.

Depending on how you intend to use it, and what you want to use it with, that will determine what changes need to be made.  For example, it was more or less assumed that it would be fed a mic preamp output.  So, the input presumes a low impedance source, and the first op-amp stage is set for a gain of 2x with both R1 and R2 = 10k.  If you feed it with a mic preamp, great.  If not, you may wish to a) include suitable buffering, depending on the mic used, and b) up the gain of that op-amp.

[mersberg]

I was gonna build version 1 when it came out back in the early 2000 but I hesitated because all I had was the SOIC version and not the DIP that his layout originally called for. I had went back and forth over the last 10 years on ecthing the PCB. Wasn't until I seen the PCB available from Jack is when I finally went forward on it. I bought a few PCB's just to have and thought I could just modify the PCB for mic use.
I plan on using a SM58 into the compressor then into a dual band EQ designed for amatuer radio use. The input impedance to the EQ accepts a 600 ohm mic. Without the schematics for the EQ setup, I initially connected the SM58 to the EQ and got reports back that the audio was very muddy and bassy. I came to the conclusion that the mic with its 150 ohm impedance wasn't close enough to the 600 ohm requirement and the frequency response was altered therefore goofing up the audio. I wanted knock two birds with one stone A) match the impedance of the mic to the chain B) add a compressor to the mix. The EQ is built by a Ham guy W2IHY and has a built in noisegate. I've read different post and seen tweak others have done but they were for guitars and mostly for bass. I modified the input by removing R1,2,3,4,5, C1,2,13  and Q1 and replacing them with a 5k pot in series with a 1k resistor and a few coupling caps to try to match the impedance of the mic using the 10 to 1 method. I also changed the output resistor R10 to 560 ohms to match the EQ's 600 ohm requirement.

[Mark Hammer]

Well, keep in mind that any compressor or limiter is fundamentally an envelope-controlled device.  And that envelope has to have enough amplitude to be detectable such that it can provide the intended degree of control.  The SM58 will not have enough output, on its own, to provide that.  The op-amp stage in the 2166 (that uses R1 and R2 to set its gain) is part of what enables detection of a big-enough envelope signal.  You can either leverage that op-amp, and goose its gain beyond the 2x that the default values produce, or you can stick a preamp between the mic and the compressor.  Either way, do NOT expect much of anything from the 2166 if the envelope follower is supposed to be tracking the output of a mic directly.

[mersberg]

Mark, I appreciate all the insight you have given me. Could you recommend a  small package preamp that would be suitable for getting this SM58 to work with this compressor. I could probably get away with perfing it. I really want this 2166 to work, it has been in my to do list for a long time. I got a sample from AD back when they first came out with the P version. I do have lm324, lm386, tl082, 2n3904 and 2n3906 on hand if I could use those on a perf board would be very desirable.
Thanks