Vactrol based Envelope Follower

THE SSM "MIKE IRWIN" VCA

UPDATE. It seems the SSM2164 isn't available any longer. So this page may be of limited use. I leave it here because I still have a few of the boards and chips around, so this page is useful to me.

you may be able to use other VCA's instead, such as the one by THAT: here If anyone has a good source or alternate, please let me know.

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

The problem is to get the correct audio response out of the circuit. 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, 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, at about 30mV per DB. The 2018 is the same way. The bottom line is that a signal put through the stock circuit seems to fade away much too quickly when controlled by an ADSR or equivalent EG circuit.

There are several solutions to this problem, here is one by Mike Irwin. His circuit fragment was featured in EDN, and you should read that article here with figure 1 here if you are thinking about using this dual 2164 circuit. Basically Mike uses one stage of the 2164 to "linearize" the second, and programs the entire gain structure with a reference input that's below ground.

So following 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. (Note, I used to sell copies of this board but don't have any for sale any longer, sorry.)

There are or were other mod synth dudes 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 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.