[sdiy] Increasing resonance sound in 2164 VCO?

[Neil Harper]

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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