[sdiy] TR-909 schematic/PCB error

Richie Burnett rburnett at richieburnett.co.uk
Mon Nov 14 21:51:15 CET 2016

Bruno Afonso wrote:

> Do you mind sharing a bit about your digital modelling?
> Are you modeling the analog circuits using spice like equations?

Yes, sort of.  I can;t go into the details now, but it's like a spice model 
optimised for speed (realtime calculation on DSP) and audio quality.

> Seems impressive your model could pick up on these details given even 
> people making clones missed out on those

You can easily see the difference in the rimshot waveform when it is 
high-pass filtered.  There seems to be some discussion about it here (the 
diagrams are perfect for this discussion although I haven't read the 


The RS waveform looks like the very top waveform when viewed directly at the 
output of the VCA (before the HPF).  (The second thick waveform is just a 
magnification of a sketch of the RS waveform in the TR-909 service manual.)

The RS waveform looks like the third waveform down at the output of IC50b 
(after the HPF.)  It is quite an aggressive high-pass filter at about 230 
Hz, and it's LF rolloff has the effect of introducing a "tilt" or "droop" on 
the otherwise "flat" portions of the clipped RS waveform.


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