Rhythm Wolf Kick Drum

Overview

Original schematic

Its interesting to see the level they went to copy the 808: on the regeneration amp (U26A) there is a resistor from the noninverting input to ground. This is typically done to counter the offset voltage generatred by bias currents on the opamps, and this done throught the 808. But, this is the only opamp in the rhythym wolf that uses this technique. Also, the output pot goes to an inverting amplifier stage rather than a buffer, which is the only instance of this as well; not sure why. There is the addition of the "attack", which is a small pulse right at the beginning of the drum hit (through D12). The frequency of the drum voice does not shift down throughout the note, as it does on the 808 due to R205 and R188 being apparently swapped, making the decay too quick to be noticeable. Overall, this is the best designed part of the RW.

Mods

Modified schematic

i liked how the kick sounded, so i didnt make many mods, only adjusting the tone sweep. After doing the mod, i decided i liked it without the tone sweep for a lot of the range, but that the low end sounded better with the tone sweep, so i did a compromise (100nF C98). i would reccomend trying a few different values, as the sound changes pretty drastically depending upon depth and decay time. Ideally, you could put a pot on this parameter. i was thinking of repurposing the attack knob for this, but i wanted that as well, so maybe later i'll put one in, or at least a switch.

A word of caution with this mod: this part of the circuit is entirely transistor beta dependent, which means it will change with temperature and between synths. For this reason, i've picked some conservative, midrange values, so that when beta changes, the circuit is still in a reasonable operating point. This is particularly important if you pick large value resistors, as beta could shift and the transistors won't turn on anymore.

1. Change R181 to 100k. This adjusts the pitch bend amount. The low and high tones are 40Hz and 62Hz, respectively. With the pitch bend these can go up to 64Hz/83Hz at the beginning of a note. 10k is the lowest i would use here, and that gives the full bend and a sharp transition between the hi/lo end of the note. 47k also gives the full bend, but has a smoother transition. 100k only pitches up to 58Hz on the low end, and gives a longer transition. 220k goes up to 55Hz and gives the longest transition. going above 220k makes the effect pretty subtle. i decided to go with 100k as the really high sweeps seemed a little comical.
2. Change R188 to 1.5M. anything between 500k and 3.3M would probably be fine. The larger the value, the longer the sweep becomes for a given capacitor value. i went with a middle range value to ensure operation across temperature.
3. Change R205 to 100k. This can be anything from 10k to 1M, with larger values giving longer decay times. But, when R205 becomes greater than half the value of R188, the max sweep goes down, so there are diminishing returns. 100k to 500k is probably ideal. This can be replaced with a pot to make the sweep time adjustable. The 50k pot could be fitted here, with a 2.2k resistor in series to limit the current at max value. Ideally, a 500k pot would be used, as the current is still pretty high with the 50k/2.2k.
4. Change C98 to 330nF. This changes the sweep time with R205. for a 100k R205, values of 100nF to 2.2uF are reasonable. 330nF this will give a 40ms sweep time. The way i measure sweep time, is the time it takes for the collector of Q29 to go from 10V to ground. Around 5ms is when the effect is no longer noticeable, at 10ms it is subtle, and 50ms - 200ms are very noticeable. After that, its hard to hear the effect anymore, as the tone has decayed out. This voltage drops linearly, by increasing R188 it can be made more exponential.

Comparison to the 808

With the level of detail they paid in trying to replicate the 808, its interesting to note that they picked a value of the TUNE pot to be 1/100 that of the 808. This has some severe consequences for the behavior, and if your goal is to replicate the 808, the only way to do so is to change P7, and R60,114 and C14,15. Due to the low impedance at this juncture of these components, the voltage level never gets large enough to pass enough current through R39 to pitch the tone of the drum. D1 never turns on. This pitch shifting can be accomodated for with the above mods, setting the decay time to 150ms. But, the pitch shifting will be slightly different, as the 808 mostly shifts pitch when the equivalent of D1 turns on, which only happens on positive excursions, so on the half cycle. So there are two different mechanisms for pitch shift, one at the beginning of the tone for ~10ms, and one over the duration of the tone, which is a much more subtle effect.