PAIA vcf question

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PAIA vcf question

Postby EuphAural » 06 Jan 2013, 01:49

I'm looking to make this PAIA vcf, ir something like it.
I don't quite get how it functions though. Looking at it, there are two passive lowpass filters in series, with diodes in series with the capacitors. The CV is split with a voltage divider and injected between the caps and diodes. So how exactly would this modify the cutoff frequency?

The series diodes look like they'd block any voltage to ground unless their forward voltage is exceeded, which would mean they charge the caps, right? Does the degree of charge in a cap affect its impedance at different frequencies?

Thanks in advance for any help.
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Re: PAIA vcf question

Postby jmraney317 » 14 Jan 2013, 01:27

I have an old book from TAB that discusses a lot of the old PAiA circuits in detail. The one you posted works like the voltage controlled bandpass filter in the 2700 series. The odd part of the circuitry is the diodes - you have to look at the current through them as being an impedance equivalent to a resistor. Looked at that way, the circuit makes a lot more sense... or you could try modeling it in LTSpice or similar software and doing an AC analysis to make the circuit operation a bit clearer.
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Re: PAIA vcf question

Postby sbernardi » 22 Jan 2014, 07:00

One of my hobbies was designing and building analog synths.

This PAIA circuit is a pretty crappy VCF. There is a whole lot of control voltage bleedthru. It is a very simple circuit, however.
A better design would be a state variable filter using LM13600 or LM13700 OTA's for the control elements. You can get simultaneous lowpass, bandpass, and highpass out of it plus resonance.

The PAIA 4730 is a later version that uses the state variable design. Here's a link to the manual
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