ok, here is the first of the finalized mod sheets: the percussion voice
http://www.openmusiclabs.com/files/rw_perc_mod_lg.png
i made the decision for my box to completely cut the noise tone. it didnt sound good, was a lot of work to get sounding ok, and duplicated the same sounds i got from other voices. instead, i used the mix pot to mix between the hi/lo tones. this gives a much wider range of sounds. this involved disabling the noise source so it wasnt splattering all over the circuit, and cutting the trace to the mix pot. for mine, i cut both ends of the pot, and wired directly from the outputs of U13. but, its easier to just cut one side and adjust the volume on U8B. C120 is removed to keep the opamps from having to drive a large capacitive load when the pot is at the extremes. R296 needed to be reduced as the levels were much hotter now. 100ohms was a good value to put it just at the edge of distortion across the whole range. you can go a bit smaller if you want it perfectly clean, but the SNR gets worse.
the next thing i did was mod the hi/lo tones so they are louder with more accent. i think it was originally done as a fixed volume because the mute circuit also did envelope control. but, once i got the mute function going, i decided i liked the variable volume effect anyways. this is done by removing the diodes at the inputs of U13, and replacing the capacitors with resistors. i picked the values to give a bit of distortion at the highest level. the resistors can be increased by 20% or so to eliminate this, or reduced by 20% to increase this distortion. due too these capacitors being replaced with resistors, the decay time was made shorter. its ok, but the notes lost a bit of that hollow sound, so i removed R279 and added another 1uF cap to bring the decay time back to where it was.
next up was the accent circuit, which just needed a 10k drain resistor for C216 to discharge through. this gives a click at the beginning of the notes, which is louder with more accent. im not sure i fully like it, but it can be easily switched out by disconnecting the drain resistor (or any other method of blocking the signal).
finally, the mute circuit, which probably worked on their bench, but not in production due to the wide spread of JFET parameters. so i measured the JFET active range for my VCA compressor, and it was -4V for full off, and -3V for 1/2 gain. full gain was up at -0.5V. so i modded R253 so the output was around -8.5V with no signal. this, mixed with the 0V output of the fullwave rectifier, gives -4.2V, which ensures that the VCA is off when there is no signal. this greatly reduces the background hiss of this voice. next, i decreased R6 to 36k to increase the envelope. this allowed the compressor to open up, giving longer notes that were louder, and acted more like a compressor. if you put a 1.2k from the inverting pin of U22A to ground, this increases the gain even more, and basically gets rid of the envelope effect entirely, just gating the VCA.
other things i tried, but didnt want to add switches/pots for at this time:
1. put a pot or switch on the signal going from the output to the compressor. a pot here is great. for testing, i just wired from the volume pot wiper, and was tempted to leave it like this. this allows a really wide range of sounds from the voice. without the compressor, they are very much just resonant ringdowns, very sharp and short. with the compressor, they get a nice hollow sound to them.
2. put a diode across R304. this makes the compressor turn on instantly, which sounds pretty good when there is no accent added. it also increases the compression effect, but does also distort the beginning of notes some.
3. i tried higher compression gain settings, and they were fun, but ultimately i decided it was too much for a fixed setting, and they distorted a lot.
4. fixing the noise source. this is possible, but complicated. basically, the JFET needs to be wired differently. the noise source should go to the gate, and the the envelope should go to the drain. since JFETs vary a lot, you will need to test values to get the response you like. the way to do this, is to take a 100k or so resistor, and tie one end to the noise source, and the other to the gate. with a variable 100k resistor with a 50k in series, connect it to -12V. adjust the pot until the noise source is sounding right. then check the value and solder that value in. the other thing you will need to do is to adjust the volume level. it will most likely be too high and distorting. to fix this, you can either use a smaller drain resistor, or attenuate the noise input. the latter is probably easier, as the envelope will need adjusting elsewise. with a 100k drain resistor to the envelope, the capacitance will need to increase at the envelope by another ~1uF. the attack time become a bit unreasonable above this point, and R295 might need to be shorted out. putting a few capacitors around the circuit would allow shaping of the noise a bit.
