Rhythm Wolf Bass Synth Envelope
It seems like the intent here was to make both lin and exp envlopes, with the exp going to the VCA, and the lin for the VCF. Since there is no exponential converter on the VCF, the lin envelope makes for a really weak filter sweep. It spends a relatively smaller amount of time in the lower range, near the cutoff frequency. There are 3 accent levels (the 4th accent level is "off"). These are evenly spaced between 0 and 10V. The accent and trigger signals arrive at the same time, with the trigger ending first. i'm not sure why there are 2 signals here, but its handy; if you wanted to add a manual accent knob, you would drive U27B with your accent voltage. The gate signal is even more bizarre, in that it has PWM on the first 5ms. im not sure if this is intentional, or just some artifact of the way the signals are multiplexed. i'm assuming the former, as it gets multiplexed the same way as the other signals. This PWM signal has no effect on the remainder of the circuit, as its over-ridden by the accent/trigger signal where they come together at R312. But, if it were PWM'd after key-up, then you could have digital control of the decay time. The existance and size of C155 suggests that pwm was intended here. If you dont want your envelopes to be gated, you can just remove R311, and instead put a 10k to ground there. Q44 follows the accent voltage to charge C147, while at the same time Q47 is turned off to disengage the decay pot. This sets the min time before decay to 4ms (the trigger time).
The decay time on the VCA envelope is crazy long: 45s! Reducing C147 would fix this. Another odd thing here is that its a linear pot. Typically log pots are used for time and frequency control. There are mostly linear pots in the whole synth, but a number of different values, so getting a log wouldn't have been any extra cost. In this particular case, its a dual pot, so its an odd one out to begin with. Again, an interesting choice to have 2 different styles of envelopes, as it requires a dual pot.
The VCF envelope is less straightforward. First off, it has 2 comparators, each with startup pulses (Q38,43) on them to ensure a fixed state at startup. i'm not fully convinced these are necessary, and a quick pulse on the trigger would have done the same thing anyways. Both of these comparators merely follow the trigger signal, so its not too relevant. what it looks like is supposed to happen, is that U25B detects the trigger, and starts charging the integration capacitor, and U25A checks the envelope voltage, and shuts off U25B when it hits 10V. but, this doesnt happen, as the signal is not strong enough. there are a few different way to fix this. But, the upshot is U18B saturates at 11V, and there is a delay before decay begins as the integration capacitor needs to discharge quite a bit before U18B goes back into linear mode. R146 sets the attack time, and D48 ensures its only active during attack. D55 and D14 ensure that the gate signal does not discharge the capacitor. But, i'm not sure what this pathway is for anyways. It slightly adjusts the attack time, from 1ms to 1.5ms at higher accents. i'm not sure why you would want slower attacks for accented notes. Personally, i would re-route this signal to the comparison voltage on U25A, giving higher peaks on accented notes.
The two envelopes were mostly modded to fix the min/max decay times to have the knobs be in a more useful range. Although a linear envelope is a bit odd for the VCF, this was kept in order for the VCA and VCF to have different decay times. The VCF envelope could have been converted to exponential as well, but the way they change with settings is different, with the VCA extending much longer and it makes for some interseting sounds at long decay times. The most important fix was to change how the VCA envelope triggers, with it closing fully at the end of each note, rather than decaying out. This gives a difference between slid and normal notes that otherwise would not exist.
- Change C147 to 4.7uF to reduce max VCA decay time.
- Reroute R265 to C147 to shut off envelople immediately with note off.
- Add a 22k resistor to the VCA decay pot to set min decay time.
- Remove R209 to get rid of inverted accent control of VCF attack time.
- change R288 to a 1nF capacitor and C133 to a 150k resistor to make the envelope peak detection circuit actually function.
- Change R273 to 220k and R274 to 1M to set the envelope peak value to 10V.