TR-909 Tom and Snare oscillator frequencies

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dot matrix madness
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TR-909 Tom and Snare oscillator frequencies

Post by dot matrix madness » Sun Sep 22, 2019 8:29 am

Trying to get some different drum sounds based on the TR-909 tom circuit, I'm puzzled by the architecture of the used oscillators, which are similar in the toms and the snare.
Is there a way to compute their oscillator frequencies? I would like to get frequency ratios of 1.0, 1:1.5, and 1:1.98, respectively.
Obviously the 4069 inverters's strange VDD and VSS setup are part of the tuning as well. Currently, I see no way to simulate that in (LT)Spice.
Tanks in advance!
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Post by guest » Sun Sep 22, 2019 12:52 pm

the TUNE pot sets a voltage (V_tune) which goes to the inverters. this creates a current via the resistor at the output of the inverter (R). this then charges the capacitor (C) in the feedback loop of the opamp fed by this resistor. the charge time is given by:

t = C*V_thresh*R/V_tune

where V_thresh is the threshold it is rising to (in this case, 1/2 the supply rail).

since both oscillators get the same V_thresh and V_tune, you can only change the frequency via the feedack capacitor or the resistor. frequency is proportional to 1/t = 1/RC. so if you want some ratio x between them, make R1C1 = x*R2C2.

but, there is also an envelope applied to the frequency, so the percieved sound might be different. you might just want to put a small pot in series with the R, and tune it to your preference.
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Post by dot matrix madness » Mon Sep 23, 2019 3:19 am

Thanks very much! :hail:
Following that formula the frequency ratios of the oscillators in the TR-909 low tom are VCO3:VCO1:VCO2 = 1:1.5:2.77
Adjusting VCO2 to a ratio of 1.98 yields R=39k and C=20nF.
Still I will have to play around with the decay time of the envelopes, but now I know where to start on the bread board. (Not that I owe a real one)
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