PWM pedal.

[Tony Allgood]

Michael wrote:
>The first draft of the schematic for my PWM distortion pedal is now on
the web at http://www.cs.ucl.ac.uk/students/M.Rogers/. Any comments,
criticisms or advice very welcome!



Can I make a few comments?

Your two ?? resistors can be 22k, but you need a 100uF cap in parallel
with the bottom one. This will ensure any ac is effectively grounded.

You really do need a buffer for your output. Why not use a standard
active baxandall tone network, instead of the passive one. This will
give you a better sound and make your output less dependant on input
impedance. Use a TL074 as your op-amp, and you won't notice the extra
space required. See the data sheet on the TL074 on the Farnell web site
for the circuit.

God knows what it will sound like. Rail to rail clipping, as introduced
by the schmitt will sound pretty fuzz-like. I prefer a softer knee to my
distortion. Put two green LEDs, each in parallel, in parallel with the
10K resistor in the mixer op-amp's feedback. One led points one way, the
other points the other way. This will give you a nicer sound. Works
great with my TB-3030. Mind you this will also distort your straight
through signal as well.

Ditch the 18K res on the non-inv input on the first op-amp. Not needed
with a FET op-amp. Use a 10uF cap on each of the opamps outputs to give
some dc blocking. Otherwise, your pots will crackle.

Have great fun and welcome to the world of wacky effects.

Regards,

Tony Allgood, Cumbria, UK

e-mail: oakley at techrepairs.freeserve.co.uk

Rack mounted Moog VCF module. Details to be found at...

http://aupe.phys.andrews.edu/diy_archive/schematics/effects/filter.html