synths, and filter resonance

[Brian Marshall]

Ok, how do they do it?  lots of old analog synths had 12db, and 24db filters with adjustable resonance.  all i can think of is opamp, but i cant find much about it anywhere, and am not really that well versed in complex filter circuits.... can someone point me in the right direction.

Also anyone know any good books about active filter circuits that arent heavy on math.  I dont like books that are >math math math half assed examples, and nothing more.


I like books with pictures that show the relations of all the parts.

[Tim Escobedo]

Generally, VCFs have some kind of electronically varying resistance/impedance. Imagine a simple RC filter, where the R is electonically variable to vary the cutoff frequency. Usually, this is done using transistors, diodes or OTAs as the varying part. Or sometimes Vactrols.

There are at least three really common topologies in synths. The transistor/diode ladder, in which transistors or diodes are used as variable resistances. The Sallen-Key filter, implemented various ways, and state variable filters, which seem commonly implemented using OTAs or Vactrols.

Variable resonance is usually some kind of gain or positive feedback control that introduces ringing at the cutoff frequency, which is like a artificial harmonic, or kind of a weird distortion. That harmonic varies depending on the cutoff frequency, and gives a very distinctive color as the frequency is swept. Relatively high resonance is what makes every wah pedal go "wah".

Synth VCFs are usually at least 2nd order or higher. Learn more specifics about synth VCFs by going to http://musicsynthesizer.com and following the links. About filters in general by Don Lancaster's "Active Filter Cookbook".

[Brian Marshall]

Variable resonance is usually some kind of gain or positive feedback control that introduces ringing at the cutoff frequency, which is like a artificial harmonic, or kind of a weird distortion. That harmonic varies depending on the cutoff frequency, and gives a very distinctive color as the frequency is swept. Relatively high resonance is what makes every wah pedal go "wah".

I know that wahs have resonance, but i have yet to see a wah circuit where the resonance level (not the frequency) is variable, and stable.

maybe with a buffer/amplifier in the loop?

[Tim Escobedo]

Typical wah circuits are kinda hack quality if you want stable, consistent and independent control of resonance, cutoff frequency and gain.

In the typical Crybaby-style circuit, The second transistor IS a buffer. It'd certainly be possible to replace the second transistor with a op amp buffer, but I don't know how the circuit would be affected. Perhaps higher resonance capability due to the lower output impedance of the op amp? In this circuit, resonance seems mostly controlled by the resistor in parallel with the inductor which seems to introduce some damping. Though overall gain is also a factor. Which can be hacked some by the emitter resistor of the first transistor, and/or the input series resistor, and/or the collector resistor, and so on. Any of these adjustments also affect the range of the circuit. After a while, you realize how interconnected all these variables are.

Twin T/Bridged T filters and Multiple Feedback filters are similarly practically impossible to control parameters independently.

This is where some of the more complicated VCF designs come in. I think the state variable filter in particular is ideal where frequency, resonance and gain controls are easily implemented and independent. I'm sure there have been a few wah pedals released using this type of filter. The marketplace doesn't seem to tolerate much variation on the Crybaby-style inductor based wah, however.