[sdiy] Pole Dancing

Donald Tillman don at till.com
Wed Sep 19 22:30:51 CEST 2018

> On Sep 18, 2018, at 3:19 PM, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>> 'Anybody know the positions of the TB-303 real poles?
> To a simplification it is electrically just an *un-buffered* cascade of four R-C lowpass filter sections.  Whatever the resulting cutoff frequencies of those sections end up being.

It's a straightforward calculation... but it's *very* error prone.

> Tim Stinchcombe did a very detailed mathematical analysis of the TB-303 ladder in a document he put on his website some time ago.  He calculated the theoretical transfer function and possibly even predicted the feedback gain required to achieve self oscillation.  I think he also compared Spice simulation results of the actual circuit with the mathematical predictions but didn't do any measurements on a genuine TB-303 ladder filter.

Ah, here:

    Diode ladder, 4-pole low-pass

Tim says the TB-303 filter poles are at: (-3.53,0), (-2.35,0), (-1.0,0), (-0.12,0).

So you can pop those in and see what happens.

Now... I'm not a Roland fetishist, so I don't know anything about these guys.  Are the caps the same value?  
Are there variations?  Tim says his equation is for the same value caps.


And if anybody is looking for an interesting project; is there are set of cap values to use in a diode ladder that brings the poles closer together?  Is there an optimum?

  -- Don
Donald Tillman, Palo Alto, California

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