qs to gurus about mosfet clippers...

[swt]

Hey R.G. Aron, and the rest of the pioneers!!. What leaded you guys to the choice of irf510+germaniuns in series, instead of for example bs170 and diode or led??. What's the difference in sound?. are they better in feedback loops or as clippers to ground?. thanks for your insights...and years of experience.

[R.G.]

I actually prefer the BS170.

Here's how that works. If you connect the gate of a MOSFET to its drain and slowly raise the drain/source voltage, nothing happens until you reach the threshold voltage. When you reach Vt, the gate makes the drain/source start conducting. It will only conduct enough to keep the voltage down on the gate. So the devices "traces out" the curve of the edge of the transconductance. This happens to be a very soft diode-like knee. Softer than any junction diode in fact.

Most power MOSFETs have a Vt of about 3-4V. The smaller MOSFETs like the TO-92 packages have a bit smaller one.

You have to put a junction diode in series with it, because there is a built-in body diode in the MOSFET that will conduct in the reverse direction at one silicon-diode drop. The external diode keeps this from happening, so you can put another MOSFET-diode in reverse-parallel and hear the MOSFET characteristic that way too. Germanium is a smaller addition to the already big MOSFET forward characteristic, so it gets to being germanium because we have small signals in effects. An LED would add 1.5 to 2V to the 3-4V breakover of a MOSFET, so you'd have to have a signal of 5-6V peaks before any signal clipping would occur, and that's very hard to do on a 9V battery supply. It's not all that easy to get 3-4V peaks on 9V anyway.

I don't know why aron settled on the IRF510. I like the sound of the smaller ones. De gustibus non est disputandem.

As to sound, MOSFETs never hard clip like silicons do. It's more of a severe squashing. It's much less harsh distortion.

All diodes sound different when in feedback loops (and therefore driven by a current-source version of the signal) versus through a resistor to ground (and hence in voltage-ish mode). Experiment.

[amonte]

interesting...if you were in a jam, could you get away with a 1N914 instead of the GE?  Don't they clip around .7 as opposed to .2 for the Ge? 

[amonte]

wait, I'm confused...this article doesn't show the extra diode in series...

http://www.muzique.com/lab/zenmos.htm

[R.G.]

wait, I'm confused...this article doesn't show the extra diode in series...

That's just because it's using the reverse body diode for one MOSFET as the blocking diode for the other. The diode is still there, just hidden inside the MOSFET. A germanium still cuts the total clipping voltage.

interesting...if you were in a jam, could you get away with a 1N914 instead of the GE?  Don't they clip around .7 as opposed to .2 for the Ge?

Yes. Yes.

[aron]

Hey R.G. Aron, and the rest of the pioneers!!. What leaded you guys to the choice of irf510+germaniuns in series, instead of for example bs170 and diode or led??. What's the difference in sound?. are they better in feedback loops or as clippers to ground?. thanks for your insights...and years of experience.

I am not in the same league as R.G. - however if pioneer means "have been doing it for super long and older" uh yeah!

OK, IRF was probably chosen because we could buy it at Radio Shack. Seriously! I chose the internal diode because I thought it sounded better (to me) than the other way of using the mosfet.

I think it sounds great in the Shaka 3. I did try the smaller MOSFETs and I don't know why I settled on the IRF - maybe because it was easy to solder to.

How anti-climatic!

[markm]

How anti-climatic!

Innovation happens! 

[swt]

thanks a lot gentlemen!!. quite informative. Guess i should try bs 170 o hear how that sounds....it's been a looong time since those shaka days, and experiments...