A question about diode clipping

[Vince_b]

Why does every distortion design always use 3 diodes (or 2 with different forward voltage) for asymmetrical clipping? Wouldn't a single diode also produce asymmetrical clipping?

[Mark Hammer]

You are correct, and there ARE many pedals which do so.  The venerable Tone Bender uses a single diode.

It's also important to recognize that there is no single form of asymmetrical clipping, and that players often want at least a bit of the compression of dynamic range that distortion pedals provide.  Use of a single didoe will provide clipping for only one half cycle, but what constrains the amplitude for the other half cycle?  Certainly the headroom and supply voltage will, but the sort of clipping that produces may not be the world's prettiest.  So, something lke a 2+1 complement, or functional equivalent (e.g., two FETs or MOSFets used as diodes, i combination with a Ge diode), is often used because it provides the compression and apparent sustain of a distortion pedal, but the clipping is still unevenly distributed between positive and negative half cycles.

[Vince_b]

Thanks for the answer Mark. I just din't knew about those pedals that use this configuration.
The reason that I asked is because I have tried it on a Tube Screamer and was pleased with the result. I'm not saying that it sound better than any other diode configuration but still, the sound was quite interesting.

[Seljer]

The tubescreamer even without the diodes goes into directly clipping the opamp rather quickly, mine only functions as a clean boost for the first third of the gain control

[kurtlives]

You are correct, and there ARE many pedals which do so.  The venerable Tone Bender uses a single diode.

It's also important to recognize that there is no single form of asymmetrical clipping, and that players often want at least a bit of the compression of dynamic range that distortion pedals provide.  Use of a single didoe will provide clipping for only one half cycle, but what constrains the amplitude for the other half cycle?  Certainly the headroom and supply voltage will, but the sort of clipping that produces may not be the world's prettiest.  So, something lke a 2+1 complement, or functional equivalent (e.g., two FETs or MOSFets used as diodes, i combination with a Ge diode), is often used because it provides the compression and apparent sustain of a distortion pedal, but the clipping is still unevenly distributed between positive and negative half cycles.

Isn't that Ge diode for stabilization?
Keeps the BE junction about ~0.3V and forward biased.