Common Filter Types for Audio Applications I'm beginning to write a basic "virtual analog" synthesizer and want to implement a lowpass with variable cutoff frequency and resonance. Does anyone have insight into filter types commonly used to mimic the sound of traditional analog gear?

More generally, what are common filters types found in audio applications such as synthesizers, equalizers, etc. I took a basic DSP class many years ago but never gained an appreciation for design choices to consider for audio. I would appreciate any references along these lines.
a lowpass with variable cutoff frequency and resonance.


RBJ has a cookbook on how to do this:

http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt

Here the cutoff frequency is called w0, and the resonance is called Q:

LPF:        H(s) = 1 / (s^2 + s/Q + 1)

            b0 =  (1 - cos(w0))/2
            b1 =   1 - cos(w0)
            b2 =  (1 - cos(w0))/2
            a0 =   1 + alpha
            a1 =  -2*cos(w0)
            a2 =   1 - alpha


If you want to vary these parameters continuously while passing signal, you are not supposed to use biquads (because updating the coefficients causes glitches?) and you should use state-variable filters instead:

http://www.earlevel.com/main/2003/03/02/the-digital-state-variable-filter/

This is also a 2nd-order lowpass with variable Q.


  Does anyone have insight into filter types commonly used to mimic the sound of traditional analog gear?


Generally any filter transformed to a digital IIR using the bilinear transform is an "analog-like" filter.  Bessel is an exception because the important property of a Bessel filter is its group delay, which is not preserved by the bilinear transform.  Bilinear transform attempts to preserve the frequency response primarily.