With the stock 440 envelope follower, the resonance of the filter is fixed. However, there is a simple modification you can do to create as sharp a peak as you want - even to the point of oscillation.

Open up the case and examine the circuit board; you'll see a TL022 dual bifet op amp. A 430k resistor (may be 470k in some units) runs parallel to pins 5 through 8 of the TL022.

To add a variable resonance option, simply add a 100k trimpot in series with the 430k resistor. The easiest way to do this is to disconnect one end of the resistor, and re-connect this lead to the middle terminal of the trimpot. Then, connect either of the other trimpot leads to where the resistor lead used to be.

Once the trimpot is in place, rotate the dial until you get the response peak you desire. That's all there is to it.

(David DiFrancesco is an engineer with DOD Electronics.)