[sdiy] Increasing resonance sound in 2164 VCO?

ColinMuirDorward colindorward at gmail.com
Tue Jan 28 05:46:28 CET 2020

Hi Neil,
Does increasing that 750R to ground make the resonance louder?
I've been after something similar, but came up with the idea to simply use
a non-resonant 4pole lowpass in parallel with a resonant bandpass (outputs
summed). On the frontpanel, the user sees three controls: Frequency (which
controls both VCFs), Resonance (which is actually an attenuator at the
output of the bandpass), and then a "Q-width" control (which is actually
the BPF resonance). It seemed promising on the breadboard, but maybe once I
get it in the rack I'll find out why others don't seem to be doing this.

On Tue, Jan 28, 2020 at 1:14 AM Neil Harper <metadata at gmx.com> wrote:

> hey everyone,
> I've breadboarded a few of the vcf's in the 2164 datasheet, trying to
> find the sound that I like the best. It's been fun, learning a lot.. I
> like the sound of 4-pole filters, but find the resonance to sound very
> nasal/robotic on the implementations that i built... where the resonance
> is taken from the output of the 4th pole and fed back in.
> my latest breadboard is "Figure 33:  Resonant 24db per Octave 4-Pole
> Low-Pass Filter with 2nd Order Bandpass Resonance" from SSI's AN701
> document ( http://www.soundsemiconductor.com/downloads/AN701.pdf )
> as I had hoped, the 2nd order bandpass resonance source sounds exactly
> like what I'm looking for. Unfortunately, the prominence and volume of
> the resonance subtle. with a 10Vpp square wave coming out, the resonant
> wave is 1.8Vpp just before self-resonance (I actually don't care about
> self-resonance.. not a big thing for me).
> I'm attempting to include a picture of the schematic I've built, it's
> modified from Figure 33, where I've added voltage-controlled resonance
> using an LM13700 (sort of bodged in from the Ripples schemo). it works,
> but you can see from the badly drawn scope "shots" that the resonant
> wave is pretty small.
> I noticed that if I change R1 (first input resistor to the filter) and
> make it larger, the outputted test square wave will get smaller but the
> resonant wave stays the same. As shown, if I increase R1 from 15k to 22k
> the square wave goes down to 7Vpp and the resonant wave riding on the
> square stays at 1.8Vpp.
> I can keep changing that R1 until the resonance sound is very prominent
> and to my taste in relation to the output, but then my filter is only
> outputting half the volume of what goes in. I'm OK with bumping that up
> with an extra op-amp stage afterwards, but wouldn't I just be increasing
> the noise floor as well? Is there another way to get the same effect and
> keep my filter at unity gain?
> --
> /// Neil Harper
> /// Every Wave is New Until it Breaks
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