[sdiy] TB303 Slide

Stewart Pye stewpye at optusnet.com.au
Sun May 27 01:36:34 CEST 2012

That makes sense. So the slide time is the same as a standard RC 
portamento circuit with the R being 100k and the C being 220n, the only 
difference is the way the C is switched and the fact that it is switched 
on briefly even when you don't want it.

I've simulated the RC circuit in LT spice and have proved T=RC 
regardless of the target voltage.

One thing that confused me was that the Arp Axxe service manual says for 
portamento speed:
Minimum speed 1.5 seconds per octave
Maximum speed 0.01ms per octave
Yet it looks like a simple RC circuit into FET buffered op amp buffer....

I'm designing a sequencer that will have slide and accent capabilities 
of the TB303 and am trying to decide whether to use a 12 bit DAC or an 
R2R ladder like the TB303. If I go the R2R route I'll use a decent 
voltage reference, and probably a 74ACT573 to drive the R2R ladder. This 
will allow me to drive it from a 3.3V micro, and should have enough 
current output to obviate the extra buffers in the 303 circuit. I just 
need to get one and measure the high and low output voltages.

Thanks for the advice.


On 5/26/2012 10:23 AM, Tim Stinchcombe wrote:
>> Does the R2R DAC in the TB 303 feeding the glide capacitor directly
>> actually achieve constant glide time regardless of the voltage span
>> between notes. (ie: glide time from 1V to 1.83V is the same
>> as from 1V
>> to 2V?)
> Yes. The output resistance of the DAC is always 100k, regardless of the
> voltage, and so the time constant with the 220n (C35) is 22ms - thus after
> about 60ms the slide is complete, no matter how large or small the voltage
> change was.
> I have spent quite a lot of time looking into this myself recently,
> including scouring the archives - there is lots of interesting stuff in
> there about how all this works: the timing of the slide signal, and that the
> slide is active for a short pulse on *every* note in order to load the 4174,
> but of course stays high for those for which 'proper' slide _is_ required!
> Tim
> __________________________________________________________
> Tim Stinchcombe
> Cheltenham, Glos, UK
> email: tim102 at timstinchcombe.co.uk
> www.timstinchcombe.co.uk

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