Quad VCO

This is part of my new project (or rather two similar ones) which is Analog Modular Synth. More to come soon. It was inspired by many projects seen on synth-diy mailing list and is one of very few using SSM2164 as a core element (I personally don't know any other). If you have any questions regarding this project, don't hesitate to ask me.

See larger photo of the board

Go and see scope screen shots of waveforms up to 120kHz

Frequency range: 0.003Hz - 120kHz
Waveforms: sawtooth (ramp up and down), triangle and something like sine
Power supply: +/- 9V (or 12 if you like) +1V, and reference voltages.
Board dimensions: 10x12cm

The above diagram shows one of the four identical VCOs. All are built using one SSM2164. This makes them easier to keep in constant intervals over temperature.
3 modulation VCAs for each VCO gives 12 (!) of them - a lot to solder. Each VCO has two V/oct inputs and other two VCA attenuated. One VCA is for linear FM input.
Fortunately those 4 oscillators run independently, i.e. some of them (but not all) will sync only when they are real close tuned - say about 1kHz, beating slower than 0.1 Hz.

VCO core
Nothing unusual. U1A serves as exp. converter and current source. Integrator U2B with 1nF poly-something cap is reset by T1 FET when saw voltage reaches Vref (+5V). Sync pulses affect this voltage, so ramp is reset sooner and we get something like soft-sync.
P2 determines U2D gain to set 1V/oct relation. P1 is for fine tuning.
OP482 may be replaced by TL074. BF245 resets the integrator to about 0.2V. I've tried BS250 instead and it clamped to 0V, but the saw was more distorted.
BEND is current input for pitch modulation (input from VCA)
VREF is temperature compensated reference voltage set to +5V
-VREF is another reference voltage delivered from VREF by inverting amplifier, and is -5.2V.
CV and F are 1V/oct inputs
FM is an input from linear-FM VCA
SAW goes to waveshaping part of the circuit

Rather complicated rectifier, but gives nice triangle. It also gives bipolar (DC removed) sawtooth (up and down) and kinda like sine wave shaped by zener diodes.

Modulation VCA
Frequency may be modulated via VCAs, which are standard discrete circuits. The current mirror is powered from just only +1V to prevent it from self-heating. I didn't select any of these transistors, but they seem to have similar parameters on tape.
LOG is exponential control voltage input
LINx are linear control voltage inputs.
Wave selector
For Modular Classic I need +/-5V output waveforms, and also square ones would be usefull. that's why 'wave selector', which is 8-position rotary switch routing specified waveform to panel output jack

Three VCO's waveforms: sine, triangle and saw are formed in opamp-resistors network to maintain 2.5Vpp amplitude for every selected waveform. Amplifier U2B makes +/-5V otput out of it. Third output is hardwired to saw of oposite direction than from wave selector output (i.e. it's ramp up while out1 and out2 can provide ramp down). U2A attenuates PWM signal down to +/-2.5V, drives pulse comparators, and via R38 resistor changes sine shape (goes to inverting input of U3A in waveshape section) to make wave shaping similar to one used in Synthi.

See also the photo of wave selector


Copyright 1999 Roman Sowa
Last revised: March 9th 1999