more diode questions

[comfortably_numb]

So I built a self contained clipping circuit.  It is simply input to a 1k resister followed by two diodes (LED and Si for asymmetry) in anti-parallel to ground and output.  I also connected a dpdt switch for bypass. 

It works, but requires all of the clean boost my boss bd-2 (modified of course  ) can give it.  I'm curious as to why this circuit requires so much gain before it to distort even the slightest amount, while a circuit like the distortion+ distorts freely.  Isn't it just a clipping unit like mine, after a boost unit like I'm using the bd-2 in this case?  I would think that having the boost at max would distort the diodes unbearably, and yet I'm getting the slightest amount of grit.  This is actually a very pleasant sound, but logistically speaking is impractical, as the boost has to be maxed.

Any thoughts?

[rockgardenlove]

I'm no expert, but I'd say its just the clipping threshold.

[jrc4558]

Absolutely right. The clipping threshold is higher than the RMS guitar signal. Especially if its a single-coil pickup equipped guitar.

BTW, which diodes takes the longest to start clipping? I know its a very plebeian formulation, but, to say, which diodes will react the slowest?

[comfortably_numb]

Yes, but my boss pedals use the same Si diodes that I did, and more of them, and they distort into angry compressed rhino territory, while this barely fizzles.  Is there that much gain difference on the first stage than the make-up stage in those pedals?  The boss is loud enough to overdrive my amp, but not a set of diodes?  Surely not.

[JimRayden]

The clipping headroom for a regular Si diode is approx. 0.7V. An LED's treshold is about 1V. An average humbucker gives out... hmm, about 0.5V-1V peak. You will need a booster in front and a vol pot after diode clippers. A 9V booster should amplify to up to 3-4V clean peak and that'll give you a nice amount of drive when you shunt it down to 0.7V.

Then there's little fellows called schottky diodes. They have a treshold voltage of about 0.2V and they do give a slight "crunch" when connected to the sound path. But they also take the volume down quite a bit.

After testing and breadboarding alot, I've come to a conclusion that there is no passive way to create proper distortion... nor any other guitar effect in that case.

So, here's the summary, not that it's been posted over and over.
LED - 1V (depending on the colour)
Si - 0.7V
Ge - 0.3V
Schottky - 0.2V

Search the Forum for "Black Ice" overdrive unit. That's about as good as you can get passively. Just try it out, the Schottkys are cheap.

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Jimbo

[WGTP]

If you look at the schematic, the last op amp before the output has a different set up than I have seen before.  There are a pair of diodes between the inverting and non-inverting input coming off the feedback loop.  That ??may?? limit the output to less than you need.  Can anyone explain what is going on there?  Try it with a different pedal that has more output.   

http://www.freeinfosociety.com/electronics/schemview.php?id=116

[JimRayden]

I just re-read the first post and got what the real question is.

Check if you got the LED and the diode back-to-back. Try flipping one of them around to be really sure. If you have them both facing the same way (by accident), you'll hear pretty much clean sound come through, with perhaps a little saturation, as you described.

Then, if it's still not enough drive, try Ge's or a Ge/Si setup, it will saturate a tad more.

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Jimbo