Elektor compression overdrive

[Steben]

Well known old circuit of 1988.



I'm wondering how much gain there actually is. The second opamp doesn't have a lot of gain (11 max) and the clippers have a treshold of 0.4-0.5V.
Unity gain in first opamp will give at max drive settings just a slight overdrive.
In order to know that, you need the impedance range of the FET. Do datasheets of BF256C help?

Otherwise, cool design which gives a bit of bot worlds in one pedal: compression and drive. Power-amp-in-a-box pedal.

[maarten]

Decaying note (through IC1 and T2) ---> less voltage on base of T1, therefore less losses through T1 = increasing gain of IC1 = compression?
Maarten

[PRR]

> Unity gain in first opamp

It idles at substantial gain, as you imply:

> In order to know that, you need the impedance range of the FET. Do datasheets of BF256C help?

A number is right on the datasheet:

Small Signal Characteristics
gfs Common Source Forward Transconductance VDS = 15V, VGS = 0, f = 1KHz 4.5 mmhos

The zero-bias resistance and the zero-bias transconductance are very nearly the same, except one is quoted in Ohms and one is in Mhos. 4mMhos is 250 Ohms.

The two parameters are not really the same, but both are essentially the channel resistance when flowing freely. It will be 100 ohms to 1K for most general-use JFETs--- I often pencil 1K and may be pleased if it is lower.

This gives the first stage gain of 1.5 to 225, and 225 is a LOT of gain.