[sdiy] Pole Dancing

[Donald Tillman]

> 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
    http://www.timstinchcombe.co.uk/index.php?pge=poles#diode

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
http://www.till.com