Richard Brewster's Synthesizer

Making Tetrazzi Drone

Richard — Sun, 29 Jan 2012 20:30:17 +0000

I made a video demonstrating how I made the previous post.

http://www.youtube.com/watch?v=4G8YPeT7r_U

Tetrazzi Dark Drone

Richard — Sun, 29 Jan 2012 17:18:57 +0000

I’ve had the Tetrazzi Organ for about a week. I have a pretty good understanding how it works now, and I’ve been practicing to come up with playing techniques. It’s oddly named as an organ, since it has no sustain (the salient characteristic of an organ). To get sustain I used the Cocoquantus as a sampler. This recording uses the dolby features of the Cocoquantus, set up to duck the loop mix whenever I play the Tetrazzi. The loops are a sample of the Tetrazzi, pitch-shifted to a very deep bass region, and these provide the slow drone cadence. Over the top of that I improvised on the Tetrazzi. The CGS dual tube VCA adds overtones and sub-octaves, and a pair of Blacet Time Machines follow those.

This is kind of scary sounding. Don’t listen if you’re easily frightened by scary sounds.

Dual Resonant Gate

Richard — Sun, 08 Jan 2012 20:14:41 +0000

Quantity: 2 panels (4 total)

This is the Resonant Gate designed by Craig Lee, based on the original low pass gate Don Buchla circuit. The complete discussion is on Muff’s message board.

There have been many variations on the low pass gate. All share the central concept of using a Vactrol in the signal path as an amplitude controller. Various options provide for different levels of feedback, or resonance. Most low pass gates make us of a manual switch to select the mode of operation. This one is different in that it features a voltage-controlled feedback, which is labelled Mode. The mode is variable from no feedback at all to feedback large enough to make the circuit self-oscillate. The Mode pot sets the initial level of resonance, which can be added to or subtracted from by an external Mode CV through a reversible attenuator. The initial frequency is set by a pot I labelled ‘FR’ for frequency. (This is labelled ‘Offset’ on the schematic diagram.) External CV of frequency also uses a reversible attenuator.

My panel uses the ‘small knob’ MOTM grid format, which allows me to fit two identical resonant gates into a single 1U wide panel. This choice required me to omit the second CV input that the PC board provides.

My Modification

A problem was reported about the resonance circuit. It turned out that due to a characteristic of the Vactrol response, the resonance level could fall off over time. (See the full discussion, linked above.) I solved this for myself with a simple modification that set the minimum resonance to greater than zero. This means that the plain VCA mode is no longer available, and there is still some resonance at the lowest setting of the Mode pot. That’s OK with me. I like some resonance in the module at all times. Here’s the backside of the completed module:

My modification consisted of determining a parallel resistor for the resonance Vactrol. I described the procedure for finding the value in the post listed above. The end result is a resistor soldered across the Vactrol leads on the top of the board. Detail:

Finally, here’s a short demo of the resonant gate. It is filtering a triangle wave. A MOTM-800 envelope generator is hitting the CV. The demo starts out with the lowest resonance with the gate all the way open. Then I bring up the CV and play around with the controls a little.

Party Horns

Richard — Mon, 02 Jan 2012 01:38:32 +0000

It was a small party. Cocoquantus + Morphing Terrarium, some envelopes and VCAs, S&H with glide for pitch. The coco quantussy petal controls the … —Read more

Sensitive Dependence

Richard — Sun, 25 Dec 2011 23:19:17 +0000

This is the cocoquantus patched back to itself and left to run. I used the configuration in which four of the five oscillators are patched in a circle. The name refers to the concept of sensitive dependence on initial conditions in chaos theory. The cocoquantus is a chaotic pattern generator. Set the initial conditions and it generates patterns. A tiny move of the chaos pot changes those conditions. I didn’t touch it during this recording. There is a lot of bass in this one. Use of tiny earphones may lose some of it.

Late Nite TV

Richard — Sat, 17 Dec 2011 23:23:18 +0000

I’m trying out different combinations of equipment with the cocoquantus. In this patch the quantussy oscillator cluster is patched into the MOTM 510 Wavewarper, back through the COCO delays, and then through the Lexicon MPX-1 to add echos (way overdone, of course).

Petal On Its Own

Richard — Sun, 11 Dec 2011 17:46:09 +0000

This week’s cocoquantus patch again utilizes the CGS Tube VCA as a timbre enhancer. This time not only do the coco outputs each pass through one of these, in addition I’ve taken quantussy triangle outputs over to the CGS VCA CV inputs. This makes for interesting timbres. I finally understood the workings of the cocoquantus’s chaos circuit. For a decade I’ve been trying out different aleatoric techniques. The cocoquantus exceeds my expectations for an aleatoric controller. To duplicate the quantussy section would require five VCOs and ten sample and holds. For this recording I used a chain of four out of the five oscillators and let it run on its own for the entire recording without intervention.

Could It Get Some?

Richard — Sun, 04 Dec 2011 22:53:40 +0000

The cocoquantus with two of its oscillators patched back into the coco inputs, patched through the Uhbik phaser VST plugin in Audio Mulch. I … —Read more

Aleatoric Croaks

Richard — Sun, 27 Nov 2011 21:08:46 +0000

The Cocoquantus integrates well with the main synthesizer. All I need is cables with a phone plug on one end and an alligator clip on the other. In the patch for this recording the audio and CV all originate in the Quantussy, the cluster of five oscillators. Two of them provide the audio, which is patched first into a CGS Tube VCA to generate more harmonics, and then through a MOTM-480 VCF, then through an Oakley Cycling ADSR/VCA and back into one of the COCO delays. There are two of these routes, making for a stereo patch. Two more of the cocoquantus ‘oscirators’ are patched over to the frequency CV inputs of the MOTM-480. For this recording I didn’t use the feedback on the COCO delays, so that the result would be simple and clean. In essence, I used the cocoquantus as a cluster of oscillators. The aleatoric (random) quality comes by means of the chaotic interaction within the cocoquantus among the oscillators.

Buchla Dual 281 FG

Richard — Sun, 27 Nov 2011 20:54:22 +0000

This is a DIY Buchla 281 Function Generator, built on the printed circuit board by Toppobrillo. The discussion on Muff’s message board has the details. It’s actually one half of the circuit of the original 281, which had four generators. As usual, my design wanted to fit into a 1U panel. This required some feature compromise in CV. Although each of the two function generators has CV inputs to control the rise and fall rates independently, I provided only one CV input that controls both. I also omitted the peak output feature.

Be sure to look at the Toppobrillo page that describes the module. Here’s a photo of my build.