[sdiy] Moog 907 FFB with coils schematic

[Ingo Debus]

Am 01.01.2012 um 23:10 schrieb Oren Leavitt:

> Hi Anthony, Look for the Moog System 55/35, etc Technical Service Manual. There are pdfs here and there online. The 907/914 schematics and others are in there.

Too bad that the 914 schematics doesn't show the capacitor values. One can find out the capacitors of the bandpass and highpass filters from Juergen Haible's clone; but since Juergen implemented the lowpass completely different, these capacitors as well as the two resistor values are unknown. I have simulated both the Moog RLC lowpass as well as Juergen's Sallen-Key filter in LTSpice. With both capacitors of the Moog 914 lowpass filter at 1 uF, the resistors in parallel to the caps at 6.8 k and the input resistor at 10k the frequency responses match quite well, although not being 100% identical. Do these values make sense?
With Juergen's capacitor values, the bandpass center frequencies of the Moog 914 filter are quite spot on in simulation. But funnily this is not the case with the 907 bandpass filters using the cap values from the Moog schematics. The bandpass center frequencies in the simulation are slightly higher than those shown on the front panel. I guess it was intentionally done this way, so that the filters could be tuned by putting small capacitors in parallel with the existing ones.
Anyway I built my 907 clone with the capacitors from the schematics (which are used by Juergen too), and I'm happy with it. Hey, this is a musical instrument and not lab equipment.

Ingo