Rhythm Wolf Hi-Hats
The RW utilizes both an analog and digital noise generator, and the tune doesnt actually tune anything, and rather sets the mix of these two. R118,113 limit the extent of this mix, and the relative volume of each is quite different. The digital noise is around 5Vpp max, whereas the analog is 500mVpp or so. There are two lowpass filters on the analog noise (C19,60) and the digital noise has a resonant lowpass and a regular RC lowpass (C48,54,67). This combined noise source goes to two pretty standard drum 'VCAs' (Q19,39 - one each for open/closed). The envelopes for these are generated with the same gate/accent setup as the bass synth. The envelope's peak value tracks the accent voltage. The decay time is half for the closed hat. The envelopes also go to comparators to shut off the digital noise source. The output of the VCAs go to a sallen-key highpass filter, using a single transistor. The output then goes to a mixer stage and the volume pot.
I find it odd that the digital noise source is connected to digital powersupplies. This means any noise in the digital side will be coupled into the hihat outputs. The amount of current drawn by the HC14 is really low, so i'm not sure it would have been splattering noise onto the analog supplies if it was connected to them. The other thing is that the comparators do not work as intended here. Since there are diodes and capacitors in the VCAs which hold their charge after the envelope is off, the emitters of the envelope buffers sometimes do not get down below the threshold to shut off the comparator. A pulldown resistor is needed at the emitters to ground. i'm not sure this matters much, though. This section could be modified to apply a sort of 'mute' to the voices, so they end more abruptly. This might be good for the closed hat. Another oddity is that as the decay knob is turned down, it loads down the charging circuit, and the envelope actually gets completely eliminated, so the voice pretty much stops sounding at really low settings. R208,202 could be increased to basically set a more reasonable limit on the decay time.
All in all, i like the sound of this, and fixing the decay times is a pretty simple mod. i would like to be able to alter the pitch in some way, and i think the easiest way would be to alter the supply voltage to the HC14. These can run from 2V to 5.5V, and the pitch of the oscillators change with the supply voltage, as the hysteresis increases with increasing supply voltage. A little modulation on it might be nice as well. If i felt that the mute function was needed, then this voltage would be applied to the emitter of Q72, otherwise it would be easier to just remove Q72 and apply it directly to the supply pin of the HC14.
i tried a number of different mods, and ended up using a small set of them. The ones i kept were as follows:
- Minimum decay time - change R208 to 10k and R202 to 4.7k. This is very much to your taste, so i would suggest trying a few different values to see how they sound.
- Tune knob rework - i eliminated the analog noise, as it didnt sound as good as the digital noise, and repurposed the tune knob to tune the digital noise. This is accomplished by varying the voltage to the 74AHC14 IC that creates the digital noise. To do this, remove Q72 and replace with an NPN transistor. The trace the 3.3V rail does not need to be cut if that pad is no longer used. Instead, run a trace to the +12V supply. i connected up at the comparator on the backside of the board, but any +12V point would do. Remove R363 and place a 1uF capacitor between the base of the transistor and digital ground. Digital ground can be found closest at C177. If you are not planning on doing pitch bending, i would suggest a 10uF capacitor instead, as it will be more stable. Run a wire to the wiper of P10.
- To rework the pot, cut the wiper trace and wire it to the transistor on the bottom of the board. Remove R118 and replace it with a 10k to +12V. Remove R113 and replace it with a 4.7k to ground. You can use a smaller value for the 4.7k, but it might not work. i installed a 3.9k, and it sounded great for a while, but then the digital noise started to cut out at low pot settings. The 74AHC14 is only rated down to 2V, and a 3.9k supplied 1.5V or so. i'm not sure if it was the low voltage or something else i had done that messed it up. A 3.6k is the lowest that will work at all. This will allow a wide frequency sweep. You can reduce the 10k value to get higher frequencies, but it seemed high enough to me at 10k.
- Reconnect the noise source to the VCAs with a 10k resistor from the output of U14A (pin1) to the input of the VCAs (C84,127). This value can be made smaller or larger to give varying amounts of clipping in the VCA output. Larger values will give a softer sound, but at some point will make the volume very quiet. Smaller values will give a harsher tone.
- Pitch shift - you can make the closed hats a different pitch than the open hats by adjusting R364 and R353. When one of these resistors is engaged, it pulls down the voltage on the 74AHC14, lowering the pitch. So, if you want the closed hat to be a lower pitch, put a resistor in R353 and not in R364. To make the open hat lower pitched, do the reverse. The value of the resistor determines the pitch depth, with values from 10k to 100k being reasonable. A 47k would be a good starting point. If you use a larger value than 1uF for the transistor base capacitor in (1), the pitch shift will have a slew to it. its interesting, but not something i wanted as a fixed option. See discussion below for tweaks to this mod.
Open hat closure - to close the open hat when the closed hat triggers, run a 22k resistor from the output of U36B (pin7) to C104. This will discharge the envelope of the open hat whenever the closed hat is on, very similar to how the 606 or 808 works. Smaller values of this resistor will close the hat faster, but will also attenuate the hat more on subsequent retriggers when the closed is still sounding. An alternate version of this involves a transistor and a few resistors off the trigger signal. This latter method is far more abrupt, and mutes the open hat whenever the closed is activated. i didn't like it in comparison to the former version, so i didnt explore it completely. R214 would need to increase in order to keep Q32 from over-powering the pulldown resistor on the mute. In theory, it should be >10x the value of the pulldown resistor.
Other mod options
There is a lot of design space here, and i did not explore it fully. i tried adjusting one of the oscillators on the 74AHC14, as there were two set to 1kHz, but after changing it, it didn't sound as good, so i put it back. The main area that could use some work, is the settings on the comparators. i removed R386 and R410, which eliminated the hysteresis. This raised the threshold for detecting note off, which worked well in keeping the pitch from always hanging low. For pitch shifting purposes, it depends upon which note is being shifted down. I found that triggering off the closed hat made the most sense, as it was the shorter of the two envelopes. But, this sends some voltage to the open hat envelope if you have the open-hat closure mod. Instead, cut the signal from the open hat to U36A, and run the output of U36B to the U36A input. This basically gives you an inverter with a buffered output that can be patched to the pitch mod. As it also inverts which hat is pitched down, you can also send the signal to the non-inverting input of U36A, remove R412, and tie the inverting input to something around 1V. i found that the pitch shift had a bit of slew, and also turned off halfway through note at times, which was rather cheesey, so i didn't use this mod. For open hat muting, you might not want to hold the mute too long, so a higher threshold would be preferred, which can be achieved by increasing R398. If the mute comes off too early, it won't matter as much as the envelope will have probably decayed to a very low level by that point anyways.