| br>Here is a very simple mod that can be added for about $1 to most VCFs (not LPGs though.)
Basically, you have a switch to send some of fixed filtered input signal to the FM input. It gives as much tone variation as between different filter types (transistor ladder, ota svf, diode string, etc) for a fraction Of the cost, and better AB comparison.
Here is the story. I built one of Colin Muir's ladder VCFs, which use the Yusynth minimoog vcf circuit. His panels have an extra two holes for a dedicated LFO that I didnt install. What to do? So instead I have a little SPDT on-off-on switch. The output of the switch is hooked to the CV FM input summation node (through a 100k resistor). No components on the original PCB need to be altered, just the extra wire soldered in the appropriate place.
The switch selects between the input signal (for input 1 only) filtered lowpass at about 1.5kHz, or off, or the input signal filtered at 1.5kHz high pass, with a voltage divider. Just one switch, two 10n caps, and some resistors (20k, 68k, the details barely matter.) I attenuate by 2/3, but it is just taste how much self-FM you want.
So the effect? Enough to change the character if the VCF: for $1 I have two extra filters selectable, in effect. The low pass self-FM setting gives more bass, less thinness at high resonance, less buzzy. The high pass self-FM setting is not brighter but different. (At low resonance and low frequency of the filter, there is little effect.)
Anyway, I thought I would mention this as a low-hanging fruit hack. I guess if your panel had no room, you could put the switch in a 1U row underneath. Or if you have multiple input sockets, build it in so that input 1 is plain in, while input 2 is also LPFed to FM, and input 3 is also HPFed to FM or whatever.
Anyway you can easily preview the kinds of effect (and work out your own values) by feeding the signal input through a fixed filter and mixer into your VCF FM.. It seems too simple a function to waste rack space and cables using modules with: presets made by dedicated switched values gets the job done fine, I think. br> br>