Mosfet clipper questions, help please

[otsismi]

I've been reading through schematics for the OCD and egodriver and zendrive. Instead of the clippers shunting to ground like in dod250 and other simple hard clipping arrangements. The ocd and the zendrive have 2n7000's in a clipping arrangement but the pair gets some filtered voltage. Why is this? Why is this important? How can I learn more about the need for filtered voltage to these mosfets as clippers?

[midwayfair]

in the OCD, they're referenced to the bias voltage, which acts as common (ground) for the op amp. It's common to use Vb for a faux ground. This is still hard clipping, just like in the DOD250, with Vb substituted for ground. The Zen Drive uses soft clipping like in a Tube Screamer. Diodes act a little strange in a feedback loop of an op amp, where they never fully conduct. It's complicated and I don't completely understand the mechanism. It's okay to just know that the two clipping methods sound a little different.

[BTW, the OCD is kind of a bad example of implementing MOSFET clippers. MOSFETs conduct in both directions, one way being the generic body diode (acts like a silicon diode) and one being something special created by going gate+drain-->source. You could omit the 1N34A and the 2N7000 it's attached to and have just leave the remaining MOSFET by itself and have the same clipping structure. The Zen Drive has them both done correctly, where the extra diodes ensure that the MOSFETs only . Even more interestingly, had the MOSFETs both been implemented correctly in the OCD, the distortion would sound very similar to the Zen Drive because MOSFETs do not go into hard clipping until a fairly high voltage threshold is crossed (above 3 volts) but they start clipping around 1.2V.]

[otsismi]

Thank you for the response very informative! How come the mosfets need to be connected to a bias voltage or faux ground instead of actually ground?

[otsismi]

Also, I understand the zen has neg feed loop clip like screamer and the ocd has hard clip like dod250. But both use the bias voltage or faux ground. That's the thing that leaves me confused

[midwayfair]

How come the mosfets need to be connected to a bias voltage or faux ground instead of actually ground?

They don't. It's six of one half dozen of the other as far as the sound is concerned for diode clipping, and I'm not aware of an electronic reason either. "Ground" is just a convention used for the description of a common connection. It could have been done in the original OCD for something as simple as making the layout or schematic easier

Diodes in the feedback loop like in the tubescreamer or Zen Drive are the different ones. The Zen Drive has them directly connected from the output (pin 1) to the inverting input (pin 2) -- that's a feedback loop and the diodes affect the gain of the op amp. The DOD and the OCD both just connect them from the output pin (pin 1) to something that acts like ground (Vb) and has no effect on the gain of the op amp.

Read The Technology of Tube Screamers on Geofex for an explanation of the feedback loop diodes.

[gjcamann]

You should be aware of the difference in "Hard" clipping and "Soft" clipping. First forget about mosfet's for the moment and just consider the mosfet is being used like a diode for this whole discussion. Hard clipping is (usually) when you have the diodes clipping to ground, or Vcc or some fixed voltage - after the first opamp. Examples are the OCD, Rat, DOD250. Soft clipping is (usually) when you have the diodes clipping across the feedback path of an opamp, Examples are any Tubescreamer derivitave (aka Zen Drive, HAO Rust Driver, FullDrive2, Timmy, Paisley Drive, Kalamazoo, SD-1, etc). I generally associate soft clipping with an overdrive sound and hard clipping with a distorted sound. I think what makes the OCD so appealing, is that is uses hard clipping but really sounds like soft clipping - this gives it a different character than most overdrive pedals.

Hope this helps clarify things for you. And I second reading the Geofex article.

+1 for midway's comments on OCD diode clipping.

[otsismi]

I read an informative article on hard and soft called "cooking your own distortion". Thanks for the help guys.

One more question, I have the egghead circuit on my breadboard right now and it sounds great. Could I substitute the clipping diodes for mosfets like in the zendrive, and make no other changes?

[kothoma]

I have the egghead circuit on my breadboard right now and it sounds great. Could I substitute the clipping diodes for mosfets like in the zendrive, and make no other changes?

Sure, give it a try, it should work fine, but use mosfets and diodes in series (like the Zen).

[otsismi]

I tried the egghead with all sorts of diodes. Germanium with one silicon is nice but there's a little too much fizzies on the top end. What's a good passive rc low pass to keep all my mid range and get rid of those fizzies? Too lazy to do the math and I haven't found a tone control circuit I've liked more than cranking the mids up on my amp :-p