Crazy Larry

[Crom]

I'm thinking of building the Crazy Larry.

http://img.photobucket.com/albums/v515/TheToneGod/crazy_larry.jpg

Can someone tell me what D1 and D2 do?

Thanks.

[rousejeremy]

Clipping Diodes?

[Crom]

I figured that for D1 just after I posted (sorry, it's late!) but how does D2 clip?

[Taylor]

(I don't consider myself a guru but I'm going to take a stab at analyzing this to build my chops)

The upper part of the circuit appears to be an envelope follower. D1 doesn't clip, it's there to remove the negative half of the waveform, then an RC filter, formed by R6 and C2, lowpass filters this rectified signal so that it is just a DC voltage that gets bigger with louder input signal.

This envelope controls a FET which kills the oscillator when you are not playing. This stops that oscillator bleeding that you often get with ring modulation and pseudo-sample-rate-reduction (which is what this circuit is).

I'm not as sure about D2, I think it is used to bias that opamp to a certain DC level. In any case I'm fairly sure that neither diode is used for clipping in the distortion sense.

[The Tone God]

Taylor pretty much nailed it. D1 half wave rectifies the signal which is then filtered by C2/R6 to make a DC voltage that is relative to the amplitude of the input signal. D2 is used to set the bias of the opamp stage.

The envelope circuit is based on the Dr. Q circuit so look into that to learn more. There should be plenty around here about it.

If you don't care about clock bleed you can leave that section out.

Andrew

[rousejeremy]

I'm glad I used a question mark