Rhythm Wolf Kick Drum

Overview

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

i just finished the kick drum. i liked how it sounded, so i didnt make many mods. i adjusted the tone sweep, which pretty much wasnt doing anything before. after doing all the mods, 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. 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 ill put one in, or at least a switch.

1. change R181 to 100k. this adjusts the pitch bend amount. the min/max tones are 40Hz/62Hz. with the pitch bend these can go up to 64Hz/83Hz at the beginning of a note. 10k is the lowest i would go here, and that gives the full bend, and gives 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. and 220k goes up to 55Hz and gives the longest transition. going above that 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 for reasons explained at the bottom of this post.

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 here. 100k to 500k are 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.

4. change C98 to 100nF. this changes the sweep time with R205. the values given here gives a 10ms sweep time, which is very subtle. if you actually want the sweep effect, i would reccomend upping this to 330nF or 470nF. the way ive been measuring sweep time, is the time it takes for the collector of Q29 to go from 10V to ground. i set mine to 10ms, which is really short. 50ms is a pretty good setting, and times up to 200ms are reasonable. 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, but this is risky for the reason noted below.

a word of caution with all of this: this part of the circuit is entirely beta dependent, which means it will change with temperature and between synths. for this reason, ive picked some conservative, midrange values, so that when beta changes, the circuit is still in a reasonable operating range. this is particularly important if you pick large value resistors, as beta could shift and the transistors dont turn on anymore.