THE 2164 IC based "MIKE IRWIN" VCA

I was trying to get a simple VCA going using SSM2164s and found I had trouble getting the right audio response from the chip.

The problem is to get the musical audio response out of the 2164 you have to screw with the incoming CV signal in some cases. Case in point: My ADSR generates 0 to 5V, and 2.5V should represent (to my ears anyway) 6db, i.e., when the ADSR CV is halfway to ground, So in this case, the VCA's output should half as loud relative to the signal at input. But as the stock 2164 stands, dbs are related to Voltage: about 30mV per DB. That means most unaltered control signal put through the stock circuit seems to fade away too quickly when controlled by an normal RC based ADSR or equivalent EG circuit if you just feed their outputs into the VCA chip's CV in.

Here I present a solution from Candian EE Mike Irwin. His circuit was featured in EDN, read that article here (go to the end of the PDF, pretty much)..... See figure 1 here if you are thinking about using his 2164 circuit. Basically Mike uses one stage of the 2164 to "linearize" the incoming CV, and programs the entire gain structure with a reference input that's below ground. This creates a log to linear response for incoming CV and a really good sounding dual VCA with a very low total parts count (although you also lose 2 of your 4 VCA's on a single 2164--but it's a good tradeoff).

Ffollowing this lead I put together a quick little Eagle PCB that has 2 "Mike Irwin" VCAs on a single board, along with provisions for dual CVs per VCA, and some glue components to help setup the REF signal. A zip file of the sch erc etc files is here.

There are or were other mod synth manufacturers making or designing this same VCA set up the same way. It might be easier to buy one of their vs. making your own, search the likes of Synthcube for "Irwin VCA" and see what you can find.

Finally, a couple of notes about this circuit. For one: it will work with DC voltages as well as audio; Very useful.

Second: this is a very good performing VCA; quiet and clean.

UPDATE. the SSM2164's availability has waxed and waned over the years. such as the one by THAT: here Small bear is selling a 2164 clone chip by Cool Audio. I have also found one from Sound Semiconductor here.I haven't tried either of these but I can't see why it wouldn't work as these are supposed to be exact clone copies of the original SSM chip.

Irwin VCA laid out on 3750 x 2000 mil PCB

To accommodate a lunetta project I've been working on, here's a PCB for the same thing. Make sure to use jumpers between the V--1 and V--2 and corresponding "REF" wirepads.

Eagle files are here.

PDF of board and schmematic are here.