Features

• Four oscillators with independent controls and outputs
  • Saw, Triangle, Ramp variable shape output
  • Pulse output with variable width
  • Rise rate pot and CV input
  • Fall rate pot and CV input
• Switch-selected pairs of oscillators for bounds modulation
• Up Modulation across all four oscillators
• Down Modulation across all four oscillators

My design choices

To simplify the project I decided to omit the panning VCAs and the scroll LFO/Sample and Holds. The angular and pulse outputs are available simultaneously, at full level (3V p-p for angular; 6V p-p for pulse) on banana jacks. In place of the scrolls are CV inputs for rise and fall rates. Perhaps the biggest innovation is the use of toggle switches in the place of the node glitch inputs.

Bounds Modulation

This PCB has direct-coupled inputs for the ‘nodes’. These are applied, through switching transistors, to the B and C selection input of the CD4051 CMOS ship. This chip is an 8-to-1 selector switch. The A selector input is the least significant bit of the three-bit code that determines which of the eight is selected. Peter Blasser used the output of the 4051 as the ‘target’ of the slewing voltage that becomes the angular output (saw, triangle, or ramp). The LSB (A) selects between two of the inputs, depending on whether the oscillator is in rising or falling state. Two inputs selected by default are hard-wired at +3V and +6V. The voltage ramps up to 6V and then down to 3V (symmetrical about the 4.5V mido voltage). This produces a non-modulated output, with variable rise and fall times.

The B and C inputs, logic high or low, constitute a 2-bit binary code that selects between two sets of inputs for establishing the high and low points of the waveform. If either B or C, or both, are active, then pairs chosen from the other three oscillators then control the upper and lower bound dynamically. Unlike the SMD Tetrazzi board, which places a DC-blocking capacitor between the node input and the transistor switch, the circuit here is direct coupled. I decided to use simple SPDT toggle switches to control these bits. On the SMD Tetrazzi (and my Modular Tetrazzi that uses the same board), the capacitor prevents persistence of these states, limiting the bounds modulations to a momentary time period (1 or 2 seconds). In this design I can choose which other two oscillators will set the bounds, dynamically, which results in very cool persistent bounds modulation. This is demonstrated in the video.

Construction

I drilled an 8″ x 10″ piece of clear plexiglass, 1/8″ thick, for the panel. It’s the same size as the PC board and mounts to it with four 1″ nylon standoffs. I drilled out one of the unused pot mounting holds in the PC board on each corner to 1/8″ to fit the standoffs. I put a 7809 regulator on a small piece of perf board, which also holds the DC inlet for a 12V or 15V wall supply. This sits under the panel on 1/2″ standoffs. Each oscillator section has four banana jacks (two outputs and two CV inputs), a pair of bounds modulation toggle switches, and pots for rise and fall rates. There’s a black jack to connect external ground. The gray knobs are for the Up and Down Mod depth common controls.

Video demo