Hi,

I'm trying to emulate the sound of this analog processor with Aida DSP, but I'm facing some problems,

so I ask the community

The full schematic is available on the net, for simplicity I will post only the problematic part.

Remove 1u-10k network, Cc on the feedback and the diodes. Also consider D is varying from 0.0 to 1.0.

This is a non inverting amplifier with an high pass characteristic (shelving). More precisely this is a first order

transfer function, and can be discretized in the following way

R2 = 51000 + 500000*D;

a1 = (R1 + R2) * Cz * 2 * Fs;

a2 = R1 * Cz * 2 * Fs;

B0 = (1 + a1) / (1 + a2)

B1 = (1 - a1) / (1 + a2)

A1 = (1 - a2) / (1 + a2)

tf1z = (B0+B1*(z^-1))/(1+(A1*(z^-1)));

with Fs=48kHz

My problem is that this filter has a tremendous gain and coefficients rapidly overflow 5.23 fixed point dynamic.

For example with D=0 you have

B0 = 11.3624;

B1 = -11.2724;

A1 = -0.9099;

but just with D=0.25

B0 = 36.7605

B1 = -36.6704

A1 = -0.9099

Any idea?

Thanks

Hello,

I think David is on to something, it might make more sense to model the effect via stock SIgmaDSP blocks instead of calculating filter coefficients. The arrangement below could make a good starting point:

As you have noted, the original op-amp circuit has a shelf at about 750 Hz, with variable gain and a diode clipper. As the parameter D is increased, ultimate gain at 750 Hz and above increases faster than the gain at lower frequencies. My SigmaDSP version uses the state-variable filter and multiplier to simultaneously adjust gain and frequency response, thus providing a roughly similar function. The cubic clipper provides the soft-clipping distortion characteristic of overdriven vacuum tubes.

Best regards,

Bob