Forsell sidechain modified for FET compressor

[dayvel]

I picked up some VCR2N FETs a while back, and with the discussion of FETs as gain control elements in these parts recently, it seems like a good time to mess around with them.

I "borrowed" Fred's opto sidechain and modified it to work with FETs -

It puts out two control voltages. CV1 is suitable for using with the FET as a series element in an L-pad arrangement or an opamp feedback-loop shunt type of doohickey. It should also work as a meter output.

CV2 is for using the FET as the shunt element in an L-pad, like in the 1176.

I put one together on a RatShak board, and testing with fingertip signal injection indicates that, amazingly, it does what it's supposed to.

So now I can start messing around in earnest. I'm especially curious to see what happens when you use one FET as a series element and another as a shunt element in an L-pad. Hope someone finds this useful.

 

[bcarso]

[quote author="dayvel"] I'm especially curious to see what happens when you use one FET as a series element and another as a shunt element in an L-pad. Hope someone finds this useful.[/quote]

It sounds logical at first blush but remember that FET's as voltage-controlled resistors get very nonlinear with more than about 100mV across. So the voltage-handling range of a series-shunt attentuator is necessarily quite small.

 

[dayvel]

Yep. I'm aware of that. What I actually had in mind was using and l-pad shunt stage followed by an opamp with the FET shunting the feedback loop. I figured the series-shunt arrangement would give me a quick and very dirty idea what would happen to the gain curves. I figure I'll end up using the l-pad shunt in any case. From what I've read, it's not only the simplest arrangement but probably works best. But as long as I have this kind of test bed set up, I might as well screw around with it a bit.