[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
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
More information about the Synth-diy
mailing list