(maybe OT) Third harmonic reduction?

[transient]

I've recently built the JFET preamp from this Tape-Op article:
http://members.cox.net/soma/hamptone.jpg

I'm trying to understand how it works, and this sentence has got me confused:
"A 50 ohm resistor has been added that is not bypassed by the 470 uf cap, which reduces the third harmonic by about 20dB."

What i'm curious about is, is it possible to selectively reduce an harmonic while not affecting the other harmonics? Or does it simply work like a LP, reducing all the higher order harmonics?

This might be a silly question, sorry if it's so. While i'm not a complete beginner, i'm not that far from being one

...
emre

[Joe]

It reduces the gain slightly, like a fuzz-face gain pot turned down a bit.

[Doug_H]

It's going to reduce the gain of the stage a little bit by introducing some degenerative feedback. I didn't read the whole article but my guess is he did a transient analysis of that stage (either by sim or by  function generator and scope) using a pure sine wave test tone at a particular frequency (e.g. 1Khz) and for that particular frequency it reduced the 3rd harmonic by 20db. A guitar signal is not a pure sine wave though and its frequencies/amplitudes vary. Although there may be a tendency for this circuit to produce less upper harmonics because the wave is more rounded rather than sharply clipped (or etc), due to the gain reduction for a particular freq and amplitude, I don't think you can conclude it is going to "cut all 3rd harmonics by 20db" and so forth.

So while it's not a low-pass filter, it's not cutting all 3rd harmonics by 20db either. That's what my intuition tells me, I haven't plugged through the math on it yet.

FWIW, a long time ago Gus Smalley introduced a small emitter resistor into a fuzz face first stage to smooth it out. He may have some more data to add to this. His comments might be searchable in the archives too, I don't know.

Doug

[transient]

Thanks

I knew that it would reduce the gain, but had no idea how such a simple mod would be able to reduce the third harmonic (i'm not even sure if you could do that with a highly complex circuit). So the bottom line is, what he stated isn't true except the certain conditions he tested the circuit, right?

It sounds great by the way, i'm using it to record the piezo sound of my guitar, sounds warm and punchy

...
emre

[aron]

Yeah, AFAIK it doesn't target the 3rd harmonic only.

Very useful article!