-12 or -24 dB per octave filter modification

This modification adds a second filter module to the Fatman. By switching from single to double filter modules, you can increase the slope of the Fatman filter from -12 dB per octave to a darker, fatter - 24 dB per octave filter.

Parts Required:

1) DPDT miniature toggle switch

1) LM13700

2) 15 K resistors

4) 1 K resistors

2) 560 polystyrene capacitors

2) 4.7 K resistors

2) 10 K resistors

2) 2N4126 transistors

2) 22 K resistors

1) 100 K trimmer

1) 500 K trimmer

This modification adds basically another filter to the Fatman. The design is exactly like the original Fatman filter, with the addition of a DPDT switch to bring the filter in and out of the circuit. My design makes the new filter the first module, so the resonance of the original circuit is added at the end of the filter chain.

I removed R57, the 100 K resistor, from the Fatman circuit board and re-soldered it to the center lug of R57, the front panel control for VCO mix. The schematic shows R57 replaced by a jumper, but it would be easier to just solder a jumper wire from the DPDT switch to the original R57 solder pad away from point "G" on the circuit board.

If you do not have much panel space, all you need to add is the DPDT switch for changing filter modes. If you would like to experiment more, add the 500 K resonance trimmer as a front panel resonance control, or replace the original 500 K resonance pot with a dual ganged, 500 K pot.

This mod is also compatible with other Fatman filter modifications. For example, if you implemented the John Sepulveda Band Pass mod, then switching to Band Pass with the -24 dB switch in place will create a dual filter, with a low pass filter followed by the band pass filter.

 I adjusted the 100 K trimmer so that switching the filter in and out will not add gain to the circuit. The output of the LM13700 is much hotter than the direct oscillators, so it is possible to overdrive the original filter. You might like this sound, and install a "filter overdrive" control, by making the 100 K output trimmer also a front panel control.