Ticking is a potential problem with some guitar effects. Circuits with LFO's, such as Flangers and Phase shifters can run into problems if you're not careful with your layout.
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Advice needed: idea for addressing clock ticking in flangers
It`s not so much a case of keeping digital away from analog here, since you`re lopassing anyway. BTW: you will have A and D very close together right at the BBD.
The culprit here is to keep the 2 digital clocks apart from each other! (2 seperate digi rails - each well decoupled on their own!) This also means keeping distance between the 2 BBDs (with their clocks)... (so: 2 seperate decoupled analog rails for the 2 BBDs + LP, also);
(this is because the clocks themselves don`t harm - until they come close together, and create their difference-frequencies, which get lower in the audio-range, the closer you get to the TZ-point, which is just that situation, where almost nothing of the audio signal is left over to possibly mask any heterodyned beat frequency from the clocks!).
Put the LP filters right at the BBDs (every cm/inch of copper-trace is an antenna!);
use in- and put-lowpass filtering for each BBD separately!
Surround the critical traces with grounded areas (=groundplanes), in fact don`t leave any PCB space open, but fill everything out with ground.
Now if you use a 5th rail for the main audio, combine all supplies only at the big PS caps (shunted by 100n), you won`t need diodes or double-sided PCBs.
Don`t try wanting to put the TZF into a B*ss-sized pedal when not using 2-sided PCB!
To answer one (or 2) of Mark`s questions from a while ago:
IMO it`s the digital crap ("stairs") on the BBD`s in- & outputs, that is responsible for heterodyning before being filtered; the audio "left & right" of the BBD, "between" input-LP and ouput-LP, should be handled/dealt with like RF or digital signals;
"Synced" clocks of e.g.: 100kHz and 103kHz riding on the audio, will still give a 3kHz tone ("whine", when modulated...), when added or subtracted...
so you won`t wanna have a 3kHz lopass "hammer-remedy", but 2 individual ~50kHz "precaution-therapy" lopasses before combining audio to "overcome" the much-talked-about (incl. by me) TZF-howling.
Since BBDs chop/slice the audio right at their input (& bias-) pin (as opposed to A/D-converters, where this is technically happening some-where inside the chip after being buffered by a comparator or s.th.), I strictly recommend to individually LP-filter-decouple the input of each BBD, when fed from one single source... :wink:
(from this point on, it`s about time for me again to break back into silence)