[sdiy] VCO that goes SHARP in highest octaves??

Roman Sowa modular at go2.pl
Tue May 10 11:07:16 CEST 2022

Are you sure C3 is Class I dielectric (that's C0G for example)?
All Class II caps (like X7R) show slight dependency of capacitance vs 
frequency. It's not that aparent in audio range, but you say a dozen of 
cents, so it's more or less what could be expected.

Put proper C0G cap and see


W dniu 2022-05-10 o 06:07, Neil Harper via Synth-diy pisze:
> so this one is pretty strange to me...
> i breadboarded a few of Thomas Henry's VCO-1 (i'm sure that design is
> familiar to most, if not it is here:
> https://www.birthofasynth.com/Thomas_Henry/Pages/VCO-1.html) with 3080
> replaced with a TI 13700 that worked worked exactly as intended, i was
> able to adjust the scale tracking and both needed a little bit of
> high-frequency compensation as expected - with C8 initially being about
> 10-20cents flat. no problem. built a few thru-hole pcbs, worked exactly
> the same.
> my most recent project was to design a SMT version using the exact same
> schematic - but with SMT equivalents. I received 5 boards back and all
> of them go SHARP at C8 (10-15 cents) without any hf-compensation.
> what's going on?
> my first suspicion was the hf-compensation trimpot never 'went to zero',
> so I removed the trimpot completely and the diode in the Rossom
> Compensation circuit - no change.
> as another test, I removed C1 ("C1 is a speed-up capacitor; its duty is
> to drain away any accumulated charge from the base of Q2. This permits
> the transistor to change states more rapidly"). the oscillator required
> re calibration, but C8 was now only 2-4 cents SHARP. i feel like this
> isn't the solution though, and that i was just crippling the
> high-frequency linearity of the oscillator to compensation for another
> non-linearity somewhere. but i can't think of anything that could be
> different going from breadboard to TH PCB and then to SMT PCB.
> -- 
> /// Neil Harper
> /// Every Wave is New Until it Breaks
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