Futzing with FETs

[drew.spriggs]

Howdy,

I've been on that FET binge while looking at amp/preamp emulation, and came across the original Foolin' With FETS article which includes a push-pull stage. With that in the back of my mind while I was looking through other schematics, I noticed pretty much just everything uses a SRPP (or incorrectly described mu-amp).

I got to thinking - is the only reason we don't see the push-pull stages more that it's harder to find p-channel FETs, or was there something else going on? PP stages are a lot less complicated, don't require potentially noisy vref (these stages always seem noisy) and no electros needed.

Fired up the obligatory LTSpice and tried to see what I could come up with using off the shelf parts using the first stage of a modified BSIABII.

Not absolutely identical, but we're definitely in the ballpark!

Anybody else tried this sort of stage?

 

[SpringReverbFan1234]

There's something like this in the SuryBear spring reverb, which is a recreation of the Fender 6g15. The reverb driver stage uses N and P channel mosfets.

 

[rossbalch]

How did you choose the P-chan JFET? Been thinking of getting some in for tinkering. Had my eye on the J177 as it's used in some of those AMT designs.

 

[drew.spriggs]

How did you choose the P-chan JFET? Been thinking of getting some in for tinkering. Had my eye on the J177 as it's used in some of those AMT designs.

Just tried both in the sim to see which got closer

 

[drew.spriggs]

So there you go. Not a massive difference, and using much less board space. The lower bandwidth of the PP stages is actually advantageous - the rolloff is similar to where the original design has a couple of staggered LPF's before the tone stack, so response is almost identical if I use staggered HPF's.