808 Bass Drum
The 808 bass drum is built around a bridged-T bandpass filter that is temporarily excited into oscillation by a trigger pulse. The drum’s decay time is controlled by adjusting the amount of feedback going into the bridged-T filter. A fast envelope is used to create a subtle pitch sweep. And a lowpass filter is used for tone control. The user has control over output level, tone, and decay time. I have a more detailed description here. Or, you can read the referenced notes linked below.
In this patch:
The VCF is pinged to create the sound source.
ENV A is used as a tigger/accent combo.
ENV B is used to create the initial pitch sweep.
The LPF is used for tone control.
The mixuverter is used to control decay time.
Tuning:
Tune the filter:
1: Set the filter mode to BP2 (12db bandpass filter).
2: Turn the filter level knob fully counterclockwise to 0%.
3: Patch a tuner or oscilloscope to the VCF out, and make the filter self oscillate by raising the Q slider all the way up. Adjust the frequency slider to tune the filter to 49.4 Hz (G1 plus 14 cents). People on the internet have reported measured values between 48 and 55 Hz (G1 to A1).
Tune the pitch sweep:
1: Set ENV B’s mode to AHR. Patch ENV B to one of the FM inputs. Holding the manual gate button should now cause the pitch to rise.
2: Adjust the FM amount slider so that when ENV B is high, the filter’s pitch is 130 Hz (C3 minus 11 cents).
3: Set ENV B to AD mode and temporarily unplug ENV B from the FM input.
4: Pull the Q slider down until the filter stops self oscillating.
Tone:
The LPF will be used to tone control.
1: Patch the VCF to the VCA B/LPF in.
2: Set the control switch down to lowpass filter only.
3: Patch the LPF output to MAIN 1, and turn the LPF know fully clockwise to start.
4: Turn the drive knob fully counterclockwise. You can boost it a touch, but don’t let it sound driven (unless that’s what you’re going for).
Trigger:
The 808 has a 1 ms trigger, but Cascadia’s trigger has a minimum of 5 ms. When you ping the filter from the trigger you get a slight ‘tchch’ on the transient. You want a straight tick/snap. On top of that, the 808 sums its trigger with the accent signal. So we need an alternative that will produce a faster click than the trigger and has velocity applied to it. ENV A does both of these.
1: Set ENV A’s control source to level, and the speed to fast.
2: Set all sliders to 0.
3: Patch ENV A to the VCF audio in, and turn the VCF level knob fully clockwise to 100%.
4: Fire the manual gate as you raise the decay slider. Stop when you get a nice bass drum sound. Too low and it’ll be too clicky. Too high and the transient will lose its snap.
Pitch Sweep:
Adjust the timing of the pitch sweep:
1: Patch ENV B back to the VCF FM in.
2: Turn the ENV B shape slider up a little. You want the envelope to have an exponential shape with some tail.
3: Adjust the fall slider. If you have an oscilloscope, patch it into ENV B out and adjust the envelope’s time to be a little under 10 ms. The ideal is 6 ms. If you don’t have an oscilloscope, just do it by ear. It needs to be long enough that the sweep is affecting more than just the transient, but it shouldn’t sound like a tom. It shouldn’t be a big dramatic thing. The pitch sweep should feel subtle, but when you remove ENV B from the FM input it should be obvious that it is missing.
Compare the difference with and without pitch sweep below:
Decay Knob:
The mixuverter is used as a decay time control knob.
1: Set the x2 switch down to x1.
2: Set the polarity to unipolar.
3: Turn the mixuverter knob fully clockwise to 100%.
4: Patch one of the mixuverter’s outs to the VCF QM (resonance modulation) in.
5: Push the QM slider up just a touch. This should push the filter back into self oscillation. Adjust the Q and QM sliders to get the desired maximum decay length. When the mixuverter knob is fully counterclockwise you should get a click with the sound getting more boomy as you turn it up.
6: Alternatively, you could try using the mod wheel to control decay time.
LPF @ 100%
LPF @ 75%
Download PDF
Further Reading
A Physically-Informed, Circuit-Bendable, Digital Model of the Roland TR-808 Bass Drum Circuit
A paper by Kurt James Werner, Jonathan S. Abel, and Julius O. Smith III. They analyze the 808 BD for the purpose of building a model of it in gen~.This is an excellent resource.
Page 5 (bottom right): A brief description of how the BD circuit works.
Page 7 (top right): A block diagram of the BD circuit.
Page 9 (top right): Schematics of the bass drum circuit.
Page 14: A chart with frequency and decay times.
Designing a simple analog kick drum from scratch
A YouTube video where Moritz Klein describes the Erica Synths x Moritz Klein EDU DIY Kick Drum. While not a direct clone of the 808 BD, the circuit he builds is inspired by it, and the information in the video is highly valuable to understanding how the 808 BD works.
Scroll down to download the manual for the EDU DIY Kick Drum. The manual contains much of the information from Moritz Klein’s video above.
A webpage by some of the original designers of the 808. They describe some of the history and circumstances of its development.