Building a Eurorack 3340 VCO - Part 1 Introduction, Schematic, and Bill of Materials

Introduced in 1979 and popular due to its stable and accurate frequency control, the original CEM3340 featured in many now classic synths, including the Roland SH-101 and MC-202, MemoryMoog, Sequential Prophet 5 and Pro-One, Oberheim OB-8, OB-Xa and OB-SX. 

The 3340 was part of the Curtis CEM33xx series, which included various synthesizer-related components. In recent years, clones and equivalents of the original CEM3340 have been produced, including the AS3340 from Alfa Rpar AS, and both the original and clones are once again widely available.

In this guide, we’ll build a Eurorack 3340 VCO module using N8 Synth’s 10HP Eurorack prototype boards. The 3340 offers triangle, saw and pulse wave outputs which we’ll complement by adding sine wave out. Going all the way back to the circuit presented in the 1980 CEM3340 datasheet, our module will also feature 2 CV inputs, coarse and fine pitch controls, pulse width modulation, hard synchronisation and linear FM.

The Eurorack 3340 VCO schematic presented here will work with either Alfa’s AS3340 or the CEM3340 Rev. G

3340 VCO Build Guide Table of Contents

3340 VCO Schematic & Breadboard Layout

Our Eurorack 3340 VCO module is built using a 10HP Eurorack Control Deck and two Eurorack solderable breadboards, which will be stacked to create the 10HP synth module. The stacked 10HP Eurorack Prototype kit, pots, and jacks are available from the N8 Store

Eurorack CEM3340 / AS3340 VCO schematic
Eurorack 3340 VCO schematic - click to expand
Diagram of the breadboard layout of the Eurorack 3340 VCO
Eurorack 3340 VCO Layout - click to expand

As noted above, the core of the VCO circuit comes from the original 1980 CEM3340 datasheet with some minor adjustments to component values for Eurorack’s ±12V power supply. The following paragraphs detail our design decisions. For a full explanation of the circuit and the 3340’s operation, please refer to the original CEM3340 datasheet or the modern AS3340 datasheet.

RV2 and RV3 provide coarse and fine pitch control, respectively. With the selected values for R53 and R54, RV2 gives us approx. ± three and a half octaves. If you’d like to increase this, reduce the resistance of R54 and R54. Similarly, RV3, R5 and R6 give us approximately ± one semitone. Lowering the resistance of R5 & R6 would widen this range.

The 3340 offers both hard and soft sync input, but we’ve opted for the alternative “Figure 5” ( a reference to the datasheet figure ) hard sync arrangement, which, with the additional protective network of R22-R24, C7 and D2, will sync the VCO to an incoming saw wave.

When operating from a ±12V supply, the 3340’s pulse width can be swept between 0-100% with a control voltage range of 0-4V. As presented, 5V at the PWM input will give a 100% duty cycle, which can be reduced with RV5.

On the output side, we buffer and adjust the levels of the triangle, saw and pulse outputs to give us approx 10V peak to peak. The sine output uses the overdriven differential pair approach to generate an approximation of a sine wave from the triangle wave.

Key Components

We’ve built this VCO using the AS3340 and CEM3340 and are confident they are interchangeable in this circuit. Other clones of the CEM3340 exist (e.g., the Coolaudio V3340 ) and should work, but we haven’t tested them. 

Several components affect the VCO’s performance, particularly 1V/octave tracking and stability, and should be selected with precision in mind.

  • C23 is the 1nF timing capacitor. This should be a good-quality capacitor with low leakage and low-temperature coefficient. A 1nF polystyrene film capacitor has been the traditional choice, but we’ve had solid results with Polypropylene film, either resin-coated or box-type. To fit our layout, pick a capacitor with 5mm lead spacing.
  • R37 is critical in 1V/octave tracking. You can either use a 0.1% tolerance resistor or ( as we do ) use a multimeter to find the best match from a batch of 1% tolerance parts.
  • R1 and R2 scale the CV inputs. These should also be as accurate as possible.
  • We’ve used multi-turn, 3296W style trimmers throughout. However, with the exception of RT4 ( which trims the 1V/octave scaling ), these can be swapped for RM-063 if they are easier to obtain.

Bill of Materials

This module is built using the stackable version of our 10HP Eurorack Prototype kit. The kit contains a Eurorack Solderable Breadboard and Eurorack Control Deck, a pre-drilled Eurorack panel, and the pin headers used to connect them. The Euroack breadboard provides power to our module, and the Eurorack Control Deck makes mounting our controls and I/O a breeze.

The 10HP Eurorack Prototype kit, jacks, and pots required for this module are available from the N8 Synth store. The remaining components are widely available and relatively inexpensive. If you are just getting started building modular synths, stock up on these components, as they are ubiquitous in the schematics you’ll find online.

10HP Eurorack Prototype Kit--1Our 10HP, stackable, prototype kit includes the prototype boards, pre-drilled panel, and pin headers required for this project

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PotentiometerRV1, RV5, RV610K39mm vertical PCB trimmer pot ( Alpha RV09AF-40 style )

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PotentiometerRV2, RV3, RV410K39mm vertical PCB mounting potentiometer ( Alpha RD901F-40 style )

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Hardware -3x Davies 1900H style knob, blue3Davies 1900H style knob, blue

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HardwareJ1, J2, J3, J4, J5, J6, J7, J8, J9PJ-3001F93.5mm PCB mounting jack socket

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Multi Turn TrimmerRT1100K13296W or RM-063 style trimmer

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Multi Turn TrimmerRT210K13296W or RM-063 style trimmer

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Multi Turn TrimmerRT320K13296W or RM-063 style trimmer

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Multi Turn TrimmerRT5, RT650K23296W or RM-063 style trimmer

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Multi Turn TrimmerRT45K13296W style trimmer

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CapacitorC1, C2, C3, C4, C6, C11, C12, C13 ,C14 , C15, C16, C17, C18, C19100nF14Ceramic 50V
CapacitorC9, C1010nF1Ceramic 50V
CapacitorC21, C2210uF2Electrolytic 50V, 5mm x 11mm
CapacitorC231nF11000pF 2.5% Polypropylene Film/Foil Capacitor with 5mm lead spacing
CapacitorC71nF1Ceramic 50V
CapacitorC8300pF1Ceramic 50V
CapacitorC530pF1Ceramic 50V
CapacitorC2047uF1Electrolytic 50V, 5mm x 11mm
DiodeD1, D21N41482
Hardware -16 pin DIP IC socket for U21
Hardware-3 x 8 pin DIP IC socket for U1, U3, U43
ICU2AS33401AS3340 or CEM3340
ICU1, U3, U4TL0723TL072 Op amp
ResistorR1, R2100K21/4W 0.1% tolerance
ResistorR371K811/4W 0.1% tolerance
ResistorR1, R2, R3, R4 , R7, R10, R22, R24, R29100K91/4W 1% tolerance
ResistorR9, R11, R14 ,R16, R18 ,R25, R39, R40, R53, R5410K101/4W 1% tolerance
ResistorR13120K11/4W 1% tolerance
ResistorR47, R50, R5115K31/4W 1% tolerance
ResistorR34, R4618K21/4W 1% tolerance
ResistorR33, R49, R521K31/4W 1% tolerance
ResistorR15, R191K221/4W 1% tolerance
ResistorR28, R31, R351M31/4W 1% tolerance
ResistorR8, R271M221/4W 1% tolerance
ResistorR20, R4420K21/4W 1% tolerance
ResistorR5, R6, R12330K31/4W 1% tolerance
ResistorR42390R11/4W 1% tolerance
ResistorR23, R4839K21/4W 1% tolerance
ResistorR41,R433K21/4W 1% tolerance
ResistorR26, R30470R21/4W 1% tolerance
ResistorR4547K11/4W 1% tolerance
ResistorR3651K11/4W 1% tolerance
ResistorR32, R385K621/4W 1% tolerance
ResistorR176K211/4W 1% tolerance
ResistorR217K511/4W 1% tolerance
TransistorQ2, Q32N39042General purpose NPN silicon transistor
TransistorQ12N39061General purpose PNP silicon transistor


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