[sdiy] 303 resonance was: emulating bi-polar capacitors

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Thu Feb 17 12:30:42 CET 2022

Hi Adam,

This was a long time ago when I discovered the backwards fitted cap and 
messed about with it.  From what I can remember its effect seemed to be 
to make the maximum resonance setting of the VCF slightly less resonant, 
than when replacing with a brand new polar electrolytic from a reputable 
manufacturer fitted the right way around.  Steve thankfully gave a good 
technical explanation for why this would be so.  It basically decreases 
the gain inside the VCF and that makes it less resonant when you apply 
feedback around it...

With the resonance pot turned up to the maximum it sounded a little bit 
more shrieky or whistley at the top end when the backwards cap was 
replaced with a new good one.  But it was easy to get back to a more 
controlled and pleasing resonant sound by just turning down the 
resonance pot by a small fraction.

But the key important thing here is that there are literally dozens of 
components inside the TB-303 filter circuit who's tolerances influence 
what the maximum amount of resonance is, and how close the TB-303 filter 
comes to self-oscillation when the Resonance pot is turned up to 
maximum.  (For example there are many mods online to increase the 
maximum resonance by messing with the resistors surrounding the 
long-tailed pair signal recovery diff-amp, or even some dubious 
recommendations to turn up the 5.333v supply rail !!!)

I personally don't like the sound of the TB-303 filter if it approaches 
self-oscillation because it starts to sound whistley and annoying at the 
very top end, but that is my preference, (Mike doesn't like it full 
stop! :-))  But I think it would be misleading to suggest that Roland 
might have deliberately chosen to fit that capacitor backwards in order 
to ensure that the filter never got close to self oscillation.  I might 
be wrong, and maybe their analogue engineers know something about 
electrolytics of that vintage in reverse bias that might achieve this 
goal.  But as an experienced electronics design engineer everything I 
know tells me that this would be a risky strategy at best.  Tweaking the 
value of one of umpteen other resistors in the feedback loop around the 
filter circuit would achieve a similar effect, and should be much more 
predictable and reliable.

Specifically it is not clear to me that the reverse biased electrolytic 
would even start to leak current immediately, or whether it would 
degrade gradually for prolonged application of reverse bias?  So it is 
possible that although the reverse fitted cap seems to have a slight 
(but clearly noticeable) effect on the sound of my TB-303 at maximum 
resonance setting when I tested it (several years ago,) it might not 
have had this effect at the time of production.  It's also possible that 
it might continue to degrade and leak more if I actually had the spare 
time to power it up and use it more often!


On 2022-02-17 01:36, Peter Pearson wrote:
> Wait... Do I need to recap my 303?  Or should I put a bi-polar cap in
> there??
> On Wed, Feb 16, 2022 at 7:29 PM Adam (synthDIY)
> <synthdiy at adambaby.com> wrote:
>> OK, sorry, I can hear the groans already, but I’m gonna take the
>> bait here….
>>> On 16 Feb 2022, at 10:35 pm, rburnett at richieburnett.co.uk wrote:
>>> My impression is that leakage current through the cap fitted the
>>> wrong way round was decreasing the loop gain around the filter and
>>> making it less resonant.  So it might account for the fact that
>>> the TB-303 filter never quite manages self-oscillation without
>>> some other modification.
>> So Richie, the 303 filter has a more “musical” high resonance
>> than many other synths (some acid/electronic tracks only use it in
>> high resonance settings, rather than as a ‘true’ bass sound). Do
>> you think this is contributing to that?
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