søndag 30. desember 2018

Moog style ladder filter for the XM8

I have simulated a ladder filter for the XM8. It is based on various designs found around the net - I've studied the original moog circuit, the Schmitzbits one, the Yusynth one, the modified ASM-1 for emphasis voltage control and not least the Memorymoog (which is probably the source for the ASM-1) for the output VCA.

Cutoff CV is 1V/octave, using a 100k resistor to the mixing point at the negative terminal of U10. The point labeled cv in the schematics has a 0.5V/octave response, to allow 10 octaves of control using 5V CV.

U9 tunes tracking, it should be approx 18mV (17.5 - 18.2mV, depending on temperature) for a 1 octave increase.

U4 sets initial frequency (freq when CV is 0). Adjust to whatever you like.

U11 sets output symmetry.

I have suggested two alternatives for R16 and R24 - choose the one with the range you need for the input voltage. the 10Vp.p alternative will probably distort for higher amplitudes while the 20Vp.p is probably more noisy as it attenuates and amplifies more to run the core at the same amplitude.

Simulated circuit

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