Modifying the PAiA DrumTone CongaI've discovered that the method of controlling the Decay can be greatly simplified. Another discovery was that the transistor method of bringing about LFO/External modulation of the Pitch does in fact work -- as long as one doesn't expect too much from it.
I also found that green l.e.d.s produce a much better sounding Warp (cross-over distortion,) than red ones or diodes.
I worked out these changes on the Conga oscillator; they can be applied to the SynthDrum or any of the other voices. I recommend, however, that if the SynthDrum has been modified the earlier way and is working, that its Decay wiring NOT be converted to this new, simpler way.
Instead of the messy cut-and-jumpering around the trimpot to insert a front panel pot into that part of the circuit, I left that part mostly unchanged and placed a 250k pot between capacitor C10 and GROUND. If I had left the leads of C10 long enough I could've just clipped the one closest to the edge of the board, bent it up and soldered to it. Instead I had to desolder C10, then re-install it with one lead in the hole nearest R17.
Trimpot R21 is still used to adjust for the longest decay when the front panel DECAY pot is rotated fully clockwise. (The absolute maximum decay might be unstable and it would be best to back off a tiny bit from that setting.) One change I made there was to decrease resistor R20. This made the part of the DECAY pot's rotation that had little or no effect smaller. I decreased R20 by soldering a 4.7M in parallel with it, resulting in a total of approximately 2.18Meg. 2M, 2.2M, or 2.7M will work.
Pitch and Sweep
The value of the Pitch pot and the new value for R19 were chosen to give a range higher and lower than the fixed, non-modified pitch. Other pot values I tried were 50k and 250k.
To bring about modulation by LFO or an external CV (positive only), I replaced the diode used in the SynthDrum mod with an NPN transistor. (I had given up on that too easily, the first time I tried it on the SynthDrum.) It's possible to get just the right (wrong,) combination of Pitch, Decay and Sweep settings that the oscillator can be heard being weakly re-triggered by the LFO, or, once triggered, it never decays. Backing off the Sweep depth will prevent that.
Because of the 100k Sweep pot, a triggered Glide doesn't last as long as it did with a 0.2uF cap in the SynthDrum -- which normally would be fine with me since I dislike long glides. But it was too short even for my taste, so I increased the Glide cap to 1uF. Note that if you don't want LFO/Ext. modulation and only want Glide, the earlier method of diode plus 1M pot used in the SynthDrum modification is good enough.
The solid wire that brings Gnd from the center of the board to where it's needed can be held down with a dab or two of hot melt glue. Connections from it to the pots should be made with stranded wire.
Better than silicon or Ge diodes, or red l.e.d.s, GREEN l.e.d.s produce a much more pronounced change in the sound. Nothing subtle about it.
A final change was to increase R15. I'm sure there's a legitimate reason why the resistors' values at that location vary so widely among the different oscillators, but 330 ohms just didn't seem right to me. I used 22k, close to what's originally specified for the SynthDrum and Tom.
Triggers should be 5-20mS in duration. Too short will shorten the Decay or not fully charge the Glide capacitor. Too long will produce an annoying thump or a severe "Impact" click you'll have no control over.
Triggering may need to be "softened" with a cap to ground between C9 and R15.
When Warp is on Decay is shortened. (Actually, the envelope is truncated.) Therefore, when switching from a reasonably short non-warped sound to warped, Decay will sound too short. A not-so-simple refinement would be to make Warp a double-pole switch, with the 2nd pole putting a resistor in parallel with the Decay pot when Warp is on. Or you could just remember to readjust Decay when switching between non-Warped and Warped.
Demos of low/high pitch, short/long decay, etc., would pretty much sound the same as those for the SynthDrum modification. Instead, the following mp3's demo what's new for this mod.
1) One non-warped tone, at the longest decay I could obtain, followed by four short decay Warped tones.
2) various speed and depth LFO modulations. You can hear the modulation feed-thru isn't completely gone.
3) I couldn't get a truly "metallic" sound with audio-rate modulation, but combined with Warping I got a sound interesting in its own right.