Basic Buchla Bongos
The above patch is based on one Todd Barton made for the Easel in this video.
Buchla bongos are a classic west-coast percussion sound. It has two main components a complex oscillator, and a low-pass gate.
The basic patch is pretty easy. Create an harmonically rich oscillator through audio-rate frequency/amplitude/ring modulation, and wave folding. Then pass the signal through a low-pass gate (LPG) excited by a short, percussive envelope.
What’s a Low-Pass Gate:
The LPG is a cornerstone of the West Coast synthesis philosophy popularized by Don Buchla. An LPG combines amplitude control (VCA) with low-pass filtering (LPF) in a single circuit. The filtering and amplification are tied together. The sound becomes duller as the gate closes. This causes the lower frequencies to ring out longer, creating a natural percussive pitch-bend-like effect.
An LPG mimics the decay qualities of acoustic instruments. This is different from a simple VCA, which lowers the volume without changing the tone. In acoustic instruments, a sound’s decay is tied to its energy. A harder strike produces a louder, brighter sound with more overtones and a longer decay. As the energy dissipates, the sound naturally loses those overtones and becomes darker as it grows quieter. This is why LPGs are often described as ‘organic’ or ‘natural.’
LPGs are built with vactrols. Vactrols combine an LED with a photocell, like a light-sensitive diode or a light-dependent resistor (LDR). When the LED receives a control voltage, it illuminates, stimulating the LDR. The more light that shines on the LDR, the more voltage that passes through the circuit. The LDR has some built-in lag/slew. They don’t turn on and off instantly. If you strike the LED with a trigger signal, the LDR produces a short envelope. This creates natural-sounding plucks out of triggers.
Vactrols have some disadvantages. They have a wide variance in response times. Some are very short and clicky, while others are quite long and sustained. This causes a lot of QA work for manufacturers. Additionally, vactrols are made using heavy metals banned in the EU. As a result, many manufacturers have moved on from producing vactrol-based LPGs.
Pseudo Low-Pass Gate:
Cascadia features an auxiliary low-pass filter (LPF) that can be configured to act similar to an LPG. The principle remains the same. If the LPF switch is in the up position, the LPF dial will simultaneously control both the amplitude and tone of the input signal. You can patch ENV-B to the LPF CV in, and adjust ENV-B’s settings to create a plucky, vactrol-like envelope.
1: Detune VCO-A and B by some inharmonic amount.
2: Patch VCO B triangle to VCO-A FM-2. I’m using the triangle here because I’m trying to recreate a patch from the Easel, but you can experiment with different waveforms.
3: Turn up ENV A decay time. ENV-A is normalled to VCO-A index mod. There is nothing specific about ENV-A’s settings. Just play with it and find something that sounds right to you.
4: Switch the TZFM switch up. Flip the AC/DC switch up and down. Use whichever setting sounds right to you. Then set the VCO A mod and index dials to taste.
VCO-A FM2 has a VCA built in (see fig 2). FM2 goes to the VCA input, and IM (index modulation) in goes to the CV in. The index slider controls the amount of FM2 going through the VCA. The MOD slider controls the amount that IM attenuates FM2.
5: Patch VCO A triangle (below the mixer section) to the LPF audio in.
6: Turn the VCA B/LPF switch up. This puts it into LPG mode (CV controls both amplitude and frequency).
7: Patch ENV B to LPF CV in.
Use a decay only envelope to simulate an LPG. Pull the ENV-B Rise slider all the way down.
Give it a plucky exponential shape. Pull the ENV B shape slider all the way down. Then, push it up just until it stops being a click. It should be fairly percussive, but with a short exponential tail.
8: Patch the LPF to main 1.
9: Play with the octave settings to find different sounds.
10: Add some spring reverb to complete the sound.
Variations:
Try inserting the wave folder between VCO1 triangle and the LPF.
Try sending the ring mod into the LPF.
Try using the VCF as an LPF. Pull the VCF frequency all the way down, and patch ENV-B to VCF-FM1 and VCA-A-MOD.
Further Reading
A basic Buchla bongo patch by Todd Barton.
All hail the 292t Buchla & Tiptop Audio
Stazma making some bongos with the Titptop/Buchla series modules.
An article from Perfect Circuit explaining what low pass gate are and how they work.
A short article from Noise Engineering describing low-pass gates.