Oscillator mod filter. Got this idea from looking at a picture of Micro Moog. Feed oscillator 1 into filter modulation like so :

This enhances the sound and makes it more peaky and wild especially at certain frequencies near the filter cutoff point. Variation1.wma was recorded with filter being modulated with osc1.
The diode is absolutely necessary. I managed to blow up a PIC chip when 12V from analog section spiked into PIC pin!