808 Rimshot Synthesis
There isn’t much online about reproducing the 808’s rimshot. Fortunately the service manual (linked below) has all the information you need.
The 808 rimshot is built by combining two inharmonic tones. These are mixed into a VCA and shaped by a snappy, percussive envelope. The VCA adds extra harmonics, while a high-pass filter cuts out lower frequencies.
Key Components:
• Two bridged-T network based oscillators (IC20 and IC21) tuned to different frequencies (1667 Hz / G#6 +6 cents, and 455 Hz / A#4 -42 cents).
If you look at the circuit diagram you can see that IC20 is split in 2. When switched to claves, both halves are combined to create a 2500 Hz pulse. When switched to rimshot only the left half is used.
• A VCA (Q62).
• A simple envelope generator (R107 and C24).
• A high-pass filter (Q63).
• A noise gate (Q74).
Breakdown of the 808’s Rimshot Circuit:
1: The two bridged-T oscillators create the body of the sound.
A bridged-T network is a simple circuit often used in older analog drum machines to create a percussive tone at a specific frequency. It’s called bridged-T because the layout on a circuit board resembles a ’T’ with an additional ‘bridge’ component across the middle. It needs to be excited by a triggering pulse that causes a short decaying feedback cycle. This results in a short sine wave burst. If you want to understand more about how this works, I’d recommend watching Moritz Klein’s video explaining his kick drum circuit.
Both oscillators are excited by the same trigger signal.
When reproducing this, you can use either 2 pinged filters, or just 2 normal oscillators. The main difference will be the bridged-T triggering behaviour. When you rapidly fire a bridged-T oscillator, the feedback can build up, causing louder or longer pulse bursts. When pinged, some filters will also exhibit this effect. However, running a standard oscillator through a VCA will not.
2: Both oscillators are summed together and passed to a VCA. This VCA’s design adds saturation and extra high-frequency harmonics, creating a richer, brighter, and more complex sound.
When building the patch for yourself, use a VCA that adds saturation and character, or add some overdrive to the signal.
3: The VCA is modulated by a very short AD envelope. The service manual lists the envelope time as 10 ms. Keep it just at the edge of where it stops being a click. It should be more than just a tick, but not yet a pluck.
4: The signal then goes through a high-pass filter, removing unwanted low frequencies and emphasizing the rimshot’s high-frequency ‘snap.’
5: Finally, the signal goes through a noise gate. This cuts unwanted noise from leaking through when there is no sound.
Further Reading
Pg. 6 has a brief description of the combined clave/rimshot circuit (RS/CL).
Pg. 7 has a block diagram (top right, under the BD).
Pg. 9 the circuit (bottom right).
Pg. 14 has a chart displaying the frequency, and decay time (RS).
Designing a simple analog kick drum from scratch (Youtube).
Moritz Klein, the designer of the Erica Synths EDU DIY Kick Drum, describes how the module works, including the functioning of a bridged-T oscillator.