Of rectifiers and synchronisation

[Tony Allgood]

Hi there,

I had a afternoon off on Sunday so I played around with some full wave
rectifiers for triangle and sawtooth multiplication. I tried a variety of
designs and the best one is the one from Horowitz and Hill. This is the
classic asymmetrical type, but good performance could be had so long as you
used a large amplitude triangle, 5v p-p or so, and if you keep the frequency
below about 5KHz. The op-amps used were TL072, but fundamental bleedthrough
could be reduced with a faster op amp like OP42. The diodes were 1N4148, but
schottkys would be better. The biggest thing that effects the quality of the
output is the symmetry of the input triangle. The rise and fall times must
be identical, otherwise the fundamental creeps in again. This is actually
very difficult to control since this is reliant on the VCOs output.

Has anyone seen the sync circuit in the Spectrum synth? The info is on
Anders' site. I think this will be my next Sunday project... to hack my
modular's CEM3340 VCOs and put in a new sync function. At the moment I am
using the standard extra transistor to sync VC01... but the Spectrum uses
two FETs to reset the timing cap on demand. There is a sync depth control as
well, which uses the PWM input of the 3340 of VCO1. Has anyone anything to
recommend on syncing CEM3340s?

Regards,

Tony Allgood, Cumbria, UK

e-mail: oakley at enterprise.net

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

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