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 Forum index » DIY Hardware and Software
Jim Patchell's Vocal Filter
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Psynth



Joined: Jul 18, 2018
Posts: 19
Location: UK
Audio files: 5

PostPosted: Thu Sep 10, 2020 2:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

Here is the schematic for the VCF core.

I've made a number of changes to Jim's circuit, partly to accommodate the LM13700 instead of CA3280. LM13700's don't like more than a 100mV or so on their inputs, so needed 100R resistors to ground on all inputs (acting as voltage dividers). (Various consequential values changes followed from this)

The other main change I made was to replace the exponential current source driving the resonant frequency with a linear source - by doing this I also (I think) removed the need for the quite complex temperature compensation which it didn't appear could be applied to an LM13700. The driver for Q is also linear rather than expo.

The filter resonant frequency goes from about 80Hz to about 4.3kHz, which is poor compared to most VCFs out there, but as human formant frequencies range from about 250Hz to about 3.7kHz, this actually suits this particular module.

I wasn't sure how to determine Q of a filter, but using a scope I measured the bandwidth at -6dB, and Q appears to range from ca 4 to ca 10. This means it doesn't go into self-oscillation, or even up into that characteristic squawking sound.

There is a lot of ripple in the pass band on the LP output.

I suspect improvements could be made to the circuit by someone more knowledgeable about fliter design, but I was pleased to end up with a modified version that works and uses commonly available parts.


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Patchell Vocal Filter core.png


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