Omnifex Chorus mods

[Pedro Freitas]

Hello!

I've been asked by a friend to mod his Omnifex chorus because he though it was kinda thin sounding.
When I opened the thing up it had BBD in it!  
Then I compared the components against chorus schematics I had and
this thing is very similar to a Small Clone!!!
More, it's a delight to mod  :twisted:
In adition to the original Depth and Speed knobs know it has a
Feedback knob, a delay time switch, a Vibrato switch and now he
has pitch bending wierdness and lush chorus  
Don't frown your nose next time you see one of these on the thrift shop
window! Pick it up!

Pedro

[StephenGiles]

What BBD chip does it use and what clock chip - 4047?

[Pedro Freitas]

It uses a MN3102 clock and a MN3204 delay.

[Mark Hammer]

The thing you need to remember about BBDs is that they are a bit like tubes.  Once you decide to make a tube amplifier, there are a number of obligatory parts of the design that will always be there: power supply, output transformer, phase-splitter (if class A/B), etc.  Similarly, BBD-based effects all need to have a clock generator, an LFO, filters, a splitter stage and a mixer stage.

The MN3204 is a good candidate for "flangifying" a chorus.  It is 512 stages, compared to the usual 1024-stage chips.  Find the small cap beside the MN3102 and reduce its value by 4 (e.g., if 100pf, drop it to 22pf or nearest standard value) and you'll hear some nifty flanging as you decrease the delay range.

[Pedro Freitas]

The MN3204 is a good candidate for "flangifying" a chorus.  It is 512 stages, compared to the usual 1024-stage chips.  Find the small cap beside the MN3102 and reduce its value by 4 (e.g., if 100pf, drop it to 22pf or nearest standard value) and you'll hear some nifty flanging as you decrease the delay range.

Cool! I've only paraleled a 390p cap to the 100p one already there and got long delay times along with cool pitch bending. The worst part is that the clock filtering must be pretty steep in this pedal because almost anything above 100p makes clock audible. I already upped the filtering cap wich cured most of the clock whine but because the pedal is not true bypass I loose some treble in bypass mode too (a FET shunts wet signal to ground in bypass mode).
Gotta try reducing the 100p cap now

Thanks!
Pedro

[Mark Hammer]

Just keep in mind that:

a) The filtering already in there is based on an assumed slower clock speed suitable for chorus, so the bandwidth will be smaller than what might be feasible for flanging.  On the other hand, if you plan to switch back and forth between faster-than-stock and slower-than-stock delay ranges, sticking with the existing filtering is fine.

b) If it started out life as a chorus, there won't be any regeneration in there, which makes it less than perfect as a flanger.

c) Chorus units usually only need lfo sweep rates from medium fast to fast, and usually don't sweep slow enough for a flanger.  You may need to install another parallel cap to slow down the LFO a bit to get nicer flange sweeps.  On the other hand, you'llbe able to get some nice slow Leslie sounds just by changing the cap on the MN3102.