This Week on the Breadboard: Animals Diamond Peak Overdrive (with some mods, of course)

[Chuck D. Bones]

C6 provides a little extra top-end cut. The cutoff freq varies with the VOLUME setting, but is in the neighborhood of 10KHz when VOLUME is at noon. It's subtle.

 

[MattG]

@Chuck D. Bones - what are your thoughts on BJT vs JFET for Q1? I see the Animals/Skreddy circuit, and your initial design, used BJT, but then you switched to JFET.

 

[Chuck D. Bones]

Either way works well. There's not much difference in tone. The JFET version has fewer parts, but we have to deal with biasing Q1.

 

[jessenator]

A good starting point is Vp between 1 and 2V

I've been looking through some datasheets and I don't see that particular spec, "Vp". Am I not seeing something? Is that a value you calculate based on other specs in the datasheet? Thanks.

 

[Chuck D. Bones]

I have written extensively about JFETs in this forum.

Vp is the pinch-off voltage, the gate-source voltage where the drain current approaches zero. Most datasheets refer to it as Vgs,off and it is expressed as a negative voltage for N-channel JFETs. As a form of shorthand, we usually express Vp as a positive number, with the understanding that it is actually a negative value for N-channel JFETs.
In other words, Vp = -Vgs,off.

The other thing you need to know is that the datasheet just gets you in the ballpark. The datasheet Vgs,off range is large and in nearly every JFET circuit, you will need to measure and select the particular JFET for Vp or Idss.