808 Bass Drum with Falistri and Polaris
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.
Modules:
Falistri (or any 2 function generators) The yellow function will be used to create the trigger. The green function will be used to create the pitch sweep.
Falistri 4QM (Or a VCA to create the trigger/accent combo)
Intellijel Polaris (a bandpass filter capable of self resonating with a 6/12/18 db slope.
SVF 1U (any lowpass filter)
Duatt (an attenuator to control decay time)
Tuning:
Tune Polaris:
1: Set Polaris to bandpass mode A (6db/oct).
2: Turn the filter in knob fully counterclockwise to 0% so that there is no input signal.
3: Patch a tuner or oscilloscope to the MULTI out, and make the filter self oscillate by turning the Q knob fully clockwise. Adjust the cutoff knob 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 the green Falistri’s time scale to slow.
2: Set the green Falistri’s play mode to hold. Patch the green unipolar output to Polaris’ FM1 input. Holding down the green gate button should now cause Polaris’ pitch to rise.
3: Adjust Polaris’ FM1 amount knob to tune what pitch you want the sweep to start from. To match the 808 tune it to 130 Hz (C3 minus 11 cents).
4: After the FM amount it is tuned, set the green Falistri’s play mode to transient (AD). Temporarily unplug the cable from Polaris’ FM1 input.
5: Turn the Q knob down until the filter stops self oscillating.
Trigger Timing:
You can patch your trigger directly into the filter in, but filters will react differently to being pinged by triggers of different lengths. Using a function generator will allow you to set just how clicky or boomy you want the pinged filter to sound.
1: Set the yellow Falistri’s time scale to slow.
2: Set the yellow Falistri’s play mode to transient (AD).
3: Function shape (the small trimpots): Set the yellow Falistri’s rise shape fully clockwise. Set the fall shape fully counterclockwise.
4: Patch a trigger signal to the yellow trigger in.
5: Patch the yellow unipolar out to Polaris’ audio in.
6: Set the yellow Falistri’s rise time fully counterclockwise to create an immediate attack.
7: As you fire the yellow Falistri’s manual gate, raise the yellow decay time knob. 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.
Mix the trigger with velocity:
Use Falistri’s 4 quadrant multiplier as a VCA to sum the trigger with a velocity/accent signal.
1: The yellow unipolar out is normalled to the 4QM’s top input.
2: Patch a velocity signal to the 4QM’s bottom input.
3: Turn the 4QM level knob fully clockwise.
4: Patch the 4QM to Polaris’ audio in, and turn Polaris’ level knob about half way. You’ll have to adjust the level knob so that a low and high velocity signal gets the sound you want.
Pitch Sweep:
Adjust the timing of the pitch sweep:
1: Patch the green unipolar out back into Polaris’ FM1 input.
2: Function shape (the small trimpots): Set Falistri’s green rise shape half way up (linear). Set the fall shape fully counterclockwise.
3: Set the Falistri’s green rise time fully counterclockwise to create an immediate attack.
4: Adjust the green fall time knob. If you have an oscilloscope, patch it into the green unipolar 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 in the audio samples below with and without pitch sweep:
Decay Knob:
I’m using the Duatt as a decay time control.
1: Set the x2 switch down to x1.
2: Set the polarity to unipolar.
3: Turn the knob fully clockwise to 100%.
5: Set Polaris’ Q attenuator about 50% up.
4: Patch from Duatt out to Polaris’ Q in. This should push the filter back into self oscillation.
5: Adjust the Q and Q Attenuator knobs to get the desired maximum decay length. When the Duatt knob is fully counterclockwise you should get a click with the sound getting more boomy as you turn it up.
Tone:
I’m using the SVF 1U for tone control, but any lowpass filter will work..
1: Patch the MULTI out from Polaris to the SVF 1U input.
2: Patch the SVF 1U to your output module.
No Filtering
VCF @ 50%
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.