Re: Xoxbox warble resonance
by antto on Thu Jan 05, 2017 3:31 pm
the warbling is not caused by the PSU
it's caused by instability in the VCF
there are about 4 or 5 high-pass filters inside the feedback path of the ladder filter
as a result, the filter becomes 9-pole (but you better read Tim Stinchcombe's analysis if you're interested about this in details) when the resonance is increased, and the response is more like a weird band-pass:
weird, because the resonant frequency of the lowpass ladder is variable (controlled by the cutoff/envmod/accent knobs) while the highpass filters are fixed, and create another resonant frequency which stays fixed too, at around 8Hz
the exact frequency and the "Q" factor of this low-frequency resonance depends on the cutoff frequencies of the highpass filters in the feedback path of the ladder
the circuit is sensitive to the exact values of those capacitors (or effective resistances)
usually it's stable (the Q factor is low, so it doesn't resonate much when excited), but if the values move away from "stable" - the Q factor can easily get high and then the low-frequency resonance would resonate for longer (when excited) and thus its amplitude will be able to get louder too - that's when it can get loud enough to start affecting the supply rails and by stressing the supply rails it causes the main VCF cutoff frequency to also go up and down (and if it goes stronger than that it starts affecting the VCO tuning too)
the low-freq resonance's Q factor can even go beyond self-oscillation, which is just terrible
so, to sum it up: this "warble" is not caused by the PSU or any mythical/rare/vintage components which are ommited from the x0xb0x
it's inherited by the design of the ladder filter, but it is normally in the "stable" region
if it warbles - then it's unstable and should be fixed
and since it's caused by the ladder filter design - any clone which copies that part of the circuit will also inherit it
i don't know why some people have praised the warble as a desired "effect" when it really isn't
i've listened to (and monitored) a whole bunch of acid tracks made with 303s from the 80s 90s, and on, and i've not seen any warbling in 97% of them
i've seen warbling on a few, i don't remember which exactly
i've also seen 303s going broken on recordings (unrelated to the warble, Armando - Land of confusion, in the first take, the 303 is playing broken, the slide disappears intermitently and at one point the sequencer crashes i think (the fart sound))
...but there's not any good examples of classic acid tracks with warbling 303s i can think of
PS: if someone reading this strongly disagrees and thinks the warble is great - i have a x0xb0x which warbles and i can't fix it, i could maybe trade it for a non-warbling one heh
now, if after all you still want to get your x0xb0x into the unstable region - desolder the highpass caps in the ladder, and replace them with sockets
then get a bunch of caps with different values and experiment
you should look up the value of each of them in the circuit and change it just slightly, not drastically
since they are already sitting close to each other in pairs and are very near to the unstable region