[sdiy] TR-909 schematic/PCB error
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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