A Modular Analogue Sound Synthesizer

This project was begun in October 2009. After completing my Minisonic 2 synthesizer, I slowly came to realise that I would like to build a modular synth that allowed me to experiment with various ideas I had for analogue sound processing. Over several years I accumulated parts and components that I thought might be useful. At some point it came time to actually start building the synth.

Design Goals and Details

  1.  "Cheap and cheerful": no fancy and expensive enclosure, keyboard, minimal cost, maximal use of components on hand
  2. Modular: allowing existing modules to be easily removed and replaced by different modules
  3. Simple PSU: the use of a commercial pre-built PSU (e.g. for a PC) to avoid the boring and frustrating aspects of building a decent one myself. (The PSU is a critical component, and using a commercial unit is therefore preferred)
  4. Components: an eclectic mix of discrete transistors, linear ICs, CMOS, latest generation of linear ICs (e.g. via the free sample programmes of Analog Devices and Texas Instruments),  vacuum tubes and moving coil meter(s)
  5. Module interconnects via banana plugs and sockets
  6. Module face plate graphics designed using PowerPoint

Anticipated Modules

  • Two or three VCOs. These may be based on the Minisonic VCOs, or use purpose ICs such as the Analog Devices VFC32
  • At least two VCFs. Probably Sallen Key or Moog ladder varieties. VCFs are the most important modules IMHO.
  • A 10 step Sequencer allowing variable clock rate stepping, manual stepping, or the input of a clock signal
  • One Ring Modulator based on the Minisonic design (I have one of the SG3002 chips spare)
  • BBD delay, based on the MN3208 chip
  • One dual channel audio amplifier module, also with Line Out, including a single decent size speaker
  • One noise source, using the Minisonic design around the Z5J noise diode (which I have a spare of)

Dual VCO

This is the dual VCO I designed. It has a single power rail and uses a single LM358 twin opamp for each VCO. A third LM358 is used to set the output level for a switch selected triangle/ramp or squarewave. The "Shape" controls vary the ramp from being left handed through triangular through right handed. Unfortunately the Shape control affects the frequency of the VCO.

The "Sync" control allows VCO1 to modify the charge cycle of VCO2's capacitor, and hence to synchronize VCO2 to a multiple or integer dividend of VCO1.

The inputs are linear only. A separate log voltage control will be available in a separate module.

With zero volts at VCin the "Freq" controls can be used to swing each VCO between sub-Hz and around 8kHz.

Here is the front panel of the Dual VCO

Ladder Voltage Controlled Filter

Cabinet

The cabinet is constructed from an old drawer. The base of the drawer, 1/4" particleboard, was removed and cut into a set of module face plates: six of size approximately 5 3/4" x 5" and three of 5 3/4" x 10" (for larger modules e.g. the sequencer).

The frame of the drawer was strengthened by two struts, which will also act as attachment points for the module faceplates. Each module will be attached using a pair of easily removed small screws at one or both vertical edges.

Here is the frame and module faceplates after cutting.

Sequencer

The sequencer allows up to 10 voltage levels to be repeatedly sent to another module. Each voltage level is set by a pot labelled "0" to "9" on the front panel. The sequence length is controlled by a rotary switch that selects between 1 and 10. An internal clock determines the rate at which each of the voltage levels is presented to the output. The clock rate can be controlled by the "Clock Rate" pot on the front panel. The clock can also be turned off (using the "Clock/Stop/Ext" switch) to allow either single stepping through the levels (using the "Step" pushbutton) or an external clock input. When the internal or external clock is operating, an LED next to the "Clock Rate" control flashes in time with it.

As each step is selected, a large Nixie tube display at the top centre of the faceplate indicates the step number.

Power is supplied to the sequences using the Power toggle switch on the faceplate: power is on when the nearby LED is illuminated.

Here is the schematic:

Here is the layout for the Sequencer front panel (Powerpoint file here):

Oscillator

Based on the Intersil 8038. A general purpose oscillator covering the full audio range and offering sine, triangle and square wave adjustable outputs.

Useful Links

HV Nixie supply: http://www.dos4ever.com/flyback/flyback.html

Components: http://www.allelectronics.com/index.php

Schmitz VCO: http://www.uni-bonn.de/~uzs159/4069vco1.png

Documents

ICL8038 Waveform generator

LED Scope http://www.electro-tech-online.com/attachments/electronic-projects-design-ideas-reviews/19395d1211160817-led-oscilloscope-led-oscope.gif