I have a little more testing / tweaking to do. I put the Flanger after a pedal containing a charge pump and the charge pump's switching freq was beating with the BBD's sample rate and making it "squeal like a piggy."
Strymon might have to look at this!!!Investigation & Findings:
The pedal that contained the charge pump mentioned above was a modified Chela as described here and here. I tried another pedal with a charge pump in it, the Crunch Captain Deluxe and observed the same squeal. Next, I replaced the 'lectric Mama with a commercial Flanger, a Monoprice yellow Flanger which appears to be a relabeled Mooer E-Lady. It exhibited the same squeal behavior.
The cause of the squeal is some of the charge pump's 40KHz switching noise sneaks out of the pedal via the output jack. When that 40KHz noise gets into a sampled circuit, like a BBD delay, chorus or flanger, the 40KHz signal is aliased down into the audio range when the sample clock is within ~10KHz of the charge pump noise. When the flanger clock is swept toward the longer delay end on the range, that's exactly what happens. Noise problems like this can be attacked on two fronts: the transmitter and the receiver.
We can make the transmitter end less noisy by filtering the signal before it gets out of the box. The modded Chela, which Cooder aptly named "Rippa," has a PRESENCE switch at the end which varies the high freq cutoff. Either the up or down position of the PRESENCE switch provides enough high freq attenuation to kill the squeal. The Crunch Captain Deluxe has no output filter on-board, so I will have to kludge one in.
We can make the receiver less susceptible by filtering the signal coming in to remove as much of the 40KHz switching noise as possible. I improved the 'lectric Mama's filtering upstream of the BBD by increasing R2 and C4, and adding another filtering stage with R7 and C5. I had to retune the filtering on the output side of the BBD (C9, C10 & R12) to restore the overall freq response. While I was at it, I tweaked the Vref voltage (increased R19) to maximize the BBD's headroom. While all of this helped, it was not sufficient to kill the squeal. It is also necessary to make sure that any pedal containing a charge pump does not transmit excess switching noise.
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I would have thought so, but I just tried a ModTone Space Flanger and a Moen Chorus after the Crunch Captain Deluxe and the modded Chela. No squeal. One possibility is that neither one sweeps the clock freq low enough. The Space Flanger may have more filtering, I'll have to open it up and see.Thanks, that answers a lot of questions (I think). If the charge pump is the source, wouldn't all circuits utilizing similar charge pumps, BBDs and clock chips exhibit similar results?
Late to this party, but what were the “lazy” techniques?The 3-day course I attended was a subset of that material. Part-way into the first day, I turned to the engineer sitting next to me and said "You know, I was already doing some of this stuff and I just chalked it up to laziness." He said "Yeah, me too." It felt good to have our "lazy" techniques validated.
I'm going to download all of this stuff, thanks!