Thunderchunk

[PBE6]

I should post a slight revision, this should fix the input impedance issue:


You may see some wacky values on Vref resistors throughout the schematic, the values themselves aren't so important as long as they're large (470k is a safe lower limit). Also, I would suggest trying different clipping diodes. 1N914's would make good substitutes for the doubled 1N34's, or you could try introducing asymmetrical clipping by combining different singles/pairs.

[PBE6]

@Frank_NH here is a link to a post from RG about the pros and cons of buffering Vref:

http://www.diystompboxes.com/smfforum/index.php?action=printpage;topic=90947.0

[antonis]

You may see some wacky values on Vref resistors throughout the schematic, the values themselves aren't so important as long as they're large (470k is a safe lower limit).

Quite right, but you also have to modify some cap values...

(C7 & R7 form a subsonic filter... it doesn't make any harm but is completely useless...)

[PBE6]

I had an issue with high-pitched whining on another "diodes-to-Vref" build so I decided to isolate the diodes and send them to ground instead. C6 and C7 are there because of that choice, but as you say they introduce filtering effects of their own (luckily not in the audio range for C7/R7).

[Frank_NH]

@Frank_NH here is a link to a post from RG about the pros and cons of buffering Vref:

http://www.diystompboxes.com/smfforum/index.php?action=printpage;topic=90947.0

Thanks!  Sounds like a buffered Vref can be more stable than a simple voltage divider but at the price of an extra op amp and potentially some op amp noise (probably not an issue for an overdrive).