## TR-909 Tom and Snare oscillator frequencies

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Wiggling with Experience
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### TR-909 Tom and Snare oscillator frequencies

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.
Licenced to solder since 1993

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Super Deluxe Wiggler
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Joined: Mon Aug 19, 2013 11:49 am
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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