VCO3340

This project provides tested plans for a 6HP Eurorack-format CEM3340 [CoolAudio 3340] VCO synthesizer module.

Inputs

• V/OCT: Exponential (1V per octave) pitch input
• Lin FM: Linear frequency modulation
• Sync: Hard sync oscillator to external signal; requires audio-rate signal at approx. ±5V.
• Width: Set duty cycle of pulse output from 0-100% with ±2.5V signal; normalled to 0V, i.e. square wave output

Outputs

All oscillator outputs (Sine, Triangle, Sawtooth, and Pulse) range from ±5V. The pulse input width is controlled by the Width input, and by default is a 50% duty cycle, i.e. Square wave.

BOM

Qty	Value	Device	Package	Parts
2	10KΩ	Pot (Alpha 9mm)	6mm	COURSE, FINE
8	3.5mm Jacks		Thonkiconn	LINFM, PULSE, SAW, SIN, SYNC, TRI, V/OCT, WIDTH
4	M06	2.54mm header	Pin	CTRL, KNOB, OUT, POWER
4	M06	2.54mm header	Socket	CTRL, KNOB, OUT, POWER
1	M08X2	2.54mm header	Pin	EXPANSION
3		2.54mm	Jumper
1	M05x2	2.54mm header	Shrouded	POWER
2	100Ω	Resistor	0805	R1, R22
1	390Ω	Resistor	0805	R36
2	470Ω	Resistor	0805	R9, R19
1	1KΩ	Resistor	0805	R32
2	2.2KΩ	Resistor	0805	R34, R40
2	3.6KΩ	Resistor	0805	R23, R25
1	5.6KΩ	Resistor	0805	R2, R21
8	10KΩ	Resistor	0805	R5, R10, R27, R35, R38, R39, R41, R42
1	24KΩ	Resistor	0805	R20
1	47KΩ	Resistor	0805	R3
2	80KΩ	Resistor	0805	R28, R29
5	100KΩ	Resistor	0805	R4, R15, R16, R24, R30
4	200KΩ	Resistor	0805	R14, R18, R26, R31
1	150KΩ	Resistor	0805	R6
3	1MΩ	Resistor	0805	R7, R11, R13
1	1.5MΩ	Resistor	0805	R8
2	1.0nF	Capacitor	0805	C1, C2
1	1.25nF	Capacitor (CG0)	0805	C6
2	10nF	Capacitor	0805	C4, C5
11	100nF	Capacitor	0805	C3, C10, C11, C12, C13, C14, C15, C18, C19, C21, C22
2	1µF	Capacitor	0805	C8, C9
4	47µF	Capacitor (Pol)	PANASONIC_C	C16, C17, C20, C23
1	10KΩ	Trimpot	2.54mm Multiturn	SCALE
1	20KΩ	Trimpot	2.54mm Multiturn	HIGHFREQ
1	50KΩ	Trimpot	2.54mm Multiturn	SYMMETRY
1	100KΩ	Trimpot	2.54mm Multiturn	DRIVE
2		DIODE	SOD-123	D4, D5
3		DIODE	SOD-323	D1, D2, D3
2	3904	Transistor (NPN)	SOT23	Q1, Q2
1		LM79L05A	SOT89	U1
1		MC78L05A	SOT89	U2
1	TL071	OPAMP-SINGLE	SO08	IC1
1		V3340/CEM3340	SO16	IC2
1		LM397	SOT23-5	IC3
2	TL072	OPAMP-DUAL	SO08	IC4, IC5

Notes on assembly:

1. The rear expansion header is not optional; the V/Oct, Pulse Width, and Soft Sync inputs must be connected to ground for proper operation
2. Non-shrouded headers may be used for the Power connector, but this makes it easier to invert the cable polarity, with potentially unpleasant results.

First time setup

The four trim potentiometers must be set up before use. The following procedure is recommended:

1. Confirm that the V/Oct, Pulse Width, and Soft Sync inputs on the expansion header are connected to the neighboring ground pin (with jumpers)
2. Turn HIGHFREQ all the way down. The middle pin should be shorted to ground.
3. Adjust SCALE until V/Oct input tracks at 1V per octave from 40-60Hz to ≅2Khz. This step is the most difficult, but its critical for tuning. If available, compare against a reference VCO with good tracking. Alternative methods include measuring the pulse output with a frequency counter, or measuring with an external tuner.
4. Adjust HIGHFREQ up until higher frequencies track equally well.
5. Measure the SINE output. Adjust DRIVE and SYMMETRY until the output sounds like a Sine Wave. Using an Oscilloscope may make this easiest, but its not critical and your ears are an equally good reference.

Expansion header

While the module is an excellent standalone module, the rear expansion header may be used to add additional controls or further manipulate outputs.

It can provide power rails to a sister module, but the ±5V rails should be limited to <50mA.

The external V/Oct pin duplicates the front-panel input. The Width input sets the default pulse width. The Soft Sync input is ready to use as is, and expects ±5V audio rate signals as input.

All output waveforms are provided for additional waveshaping.