3340 Voltage Controlled Oscillator Module for Modular Synthesizers

The Curtis Electromusic Specialities (CEM) 3340 voltage controlled oscillator chip is certainly one of the most famous in the music engineering field. Not only because it made polyphony an "easy" task for synthetizers manufacturers, but also because it was used in some of the most iconic polysynths of the golden era.

This chip was remarkably used in the 80's by Sequential Circuits, but also adopted by Roland, Moog Music, Crumar for some polysynth of theirs: Prophet 5 rev.3 (and the most recent rev.4), Jupiter 6 and Memorymoog among them.

Since 2016 these chips are back in production and readily available not only for big scale synth manufacturers, but also for hobbists and tinkerers.

In this Instructables I will show you how to assemble a full oscillator module based on AS3340 (CEM3340 clone). I will also share with you gerber files to have both printed circuit board and panel board manufactured by professionals at low cost.

Other Instructables in this serie:

4 Channels Mixer

Arduino Wavetable Voltage Controlled Oscillator

Discrete Voltage Controlled Amplifier

Arduino ADSR Digital Envelope Generator

Voltage Controlled Multimode Resonant Filter

Variable Waveshape Low Frequency Oscillator

DIY Linear Regulated Eurorack Power Supply (and Power Bus Bar)

Fiberglass Panels for DIY Modular Systems

The circuit board I have layed down is massively based on previous work by Eddy Bergman, which in turn is based on Digisound 80 design. Take a good reading at Eddy's article on his module because it is a gem, with a lot of details about his project and improvements coming from followers comments.

Another excellent reading is this Electric Druid article about CEM3340 and a disamina about circuits used by various manufacturers in some of their most famous synthesizers. Pure gold.

Main differencies between the design adopted and those suggested on the CEM3340 datasheet are the use of buffers at (already in-chip buffered) waves out, the amplification of the triangle out signal (lower than the others wave outs immediately after the chip output) and some component value.

For most of the circuit we are reiteratively reinventing the weel here (take a look at fig. 4 on page 6 of 3340's datasheet for the "method for sync on rising or falling edge" circuit, in example: it's exactly the one adopted), but having already consolidated solutions makes DIY projects less risky and prone to failure (and in turn more enjoyable).

The PCB I layed down has all the possible inputs and outputs the CEM3340 can handle: frequency control (coarse and fine), three waves outs, soft and hard sync inputs, a CV for V/oct input, a secondary, linear control voltage input, FM and PWM inputs and level control.

Each input/output has a "live" connection and a ground connection. By daisy chaining ground terminals directly on panel components, a single cable to ground will suffice (see the attached picture).

This design is optimized for a +/-12V dual rail power supply, which is the optimal to keep such a chip as cold as possible.

Wave outs are in the 0-8V range (see attached pictures), but I have also placed three optional electrolitic capacitors (C12, C13, C14) in series with the wave out to eliminate the DC offset as per eurorack standard. If you want to leave the three capacitors out, take care of shorting the two terminals directly on the PCB. The important thing to remember is that without a jumper (or a capacitor) you will have no wave at all!

Components values are directly silkscreened on the PCB, which will make the assembly way easier than consulting a reference sheet.

The circuit is protected against polarity invertion between +12V and -12V and has both high frequency and low frequency filter capacitors at the input.

The circuit board is intended to be installed perpendicular to the panel. See next step for details ;)

The latest versions of gerber files for both the oscillator printed circuit board and front plate can be found HERE (Github). You can have them manufactured at THIS link (JLCPCB): press the "quote now" button, upload the gerber file, select a nice color and save to your chart. Repeat the process for all the panels measure you want to have manufactured, then have them shipped to you.

Here is the bill of materials for the 3340 VCO circuit board:

Capacitors

3x 1nF non polarized

1x 470pF non polarized, non ceramic (you need something stable with temperature here)

1x 10nF non polarized

4x 100nF non polarized

2x 10uF electrolitic

3x 4.7uF electrolitic (replace these with a jumper for biased waves)

Diodes

1x 8.2V zener

2x 1N4148

1x 1N4004

Integrated Circuits

1x AS3340 oscillator chip

1x TL074 op-amp

Resistors, Potentiometers, Trimmers

1x 20K ohm resistor

4x 100K ohm resistor

3x 1K ohm resistor

2x 10K ohm resistor

1x 680 ohm resistor

3x 47K ohm resistor

1x 2K ohm resistor

1x 5.6K ohm resistor

1x 24K ohm reistor

1x 1.8K ohm resistor

2x 1M ohm resistor

1x 1.5M ohm resistor

1x 3.3M ohm resistor

2x 470 ohm resistor

1x 270k ohm resistor

1x 300K ohm resistor

1x 100K multi-turn trimmer

2x 10K multi-turn trimmer

5x 100K ohm potentiometer

Others

1x IDC connector, 5x2 pins

1x PCB switch ON-ON

My projects are free and for everybody. You are anyway welcome if you want to donate some change to help me cover components costs and push the development of new projects.

>>HERE<< is my paypal donation page, just in case you would like to contribute ;)

In a previous Instructable I showed you how easy and convenient it is to use fiberglass panels in place of the most common metals panels.

In that article I shared with you gerber files for 3U tall, 4HP, 6HP, 8HP and 12HP blank panels. For this project I was in the need for 16HP panels, so I layed down both a blank panel and a pre-drilled panel with adeguate silkscreened text.

Being that the panel dimensions are limited, I had to take some decision on what inputs/outputs to include and what to leave out. Using 1/4" jacks in place of the most used 1/8" has it's pros and cons. The main cons is for sure the amount of space required for every single female jack. In order to avoid clumping too many connectors toghether, the pre-drilled front panel includes:

- 3 waves out (triangle, square and sawtooth)

- 2 frequency control potentiometers (coarse and fine)

- FM and PWM potentiometers

- FM and PWM control voltages inputs

- V/oct input

- a secondary CV input

I basically had to leave out all the sync inputs. At some point I thought to use a three poles rotary selector to switch between waveforms and save a "slot" to include at least one of the three sync options. Then I realized that having all three wave outs in conjunction with a mixer could lead to an interesting variable waveshape (like Arturia did with the Minibrute) so I aborted the idea.

Holes are spaced 1.05" in the vertical direction, which is the exact spacing between PCB mounted potentiometers. Holes for potentiometers have a diameter of 0.32 inches; holes for 1/4" female jack have a diameter of 0.38".

As said, I use 1/4" jacks in my synthesizer, so panel's holes are too big if you use 1/8" jacks. You should use the blank panel file and drill it the layout that works best for you, or modify gerber files directly with the layout that best fit for you. You can download 4HP, 6HP, 8HP, 12HP and 16HP blank panels gerber files at THIS link (Github).

You can have them manufactured in multiple of 5 very easily: press the "quote now" button, upload the gerber file, select the quantity and a nice color and save to your chart. Repeat the process for all the panels measure you want to have manufactured, then have them shipped to you (but read step #4 of this instructables before proceeding!).

Having a bunch of 5 pre-drilled panels for every module could not be cost effective (well: it actually is if you have more than one copy of the same module in your synthesizer), but what if we can adapt the same layout for two different modules at once?!

This is not always possible, but sometimes we could be lucky enought for this to be applicable. One of the projects I have developed for my synthesizer is a simple-but-active mixer. I will use it for both blending more than one AS3340 waveforms toghether and/or control more than one oscillator at once. When I layed the mixer's main PCB down and started to think at the panel, I immediately noticed that the VCO panel simmetry was such that it could be used for the mixer simply by rotating it on the other side!

You can see the panel B-side in the attached "rotating" gif animation :)

A dedicated Instructables on the mixer circuit and whole module can be found HERE (Instructables).

In a previous Instructables we have seen that standard 1.6 mm thick, fiberglass panels/paceplates are an actual option for synth modules because they are easy to work on, cheap and sturdy enought for the application. But what if we still want them in aluminum?

Well, aluminum PCBs are actually another already available option!

Just to test them in my synthesizer I had a batch of 10, blank and pre-drilled and silkscreened panels manufactured by JLCPCB... and they look and work great!

Take a look at the pictures in this step: those are my VCO panels made of aluminum!

Then, if you have your panel layed down on a CAD software like EasyEDA, with all elements placed (holes, silkscreens, but also LEDs or other components you want in the front panel) having them manufactured could be an option to speed up the assembling process.

Just in case you want to speed your custom panel drawing: at THIS link (Github) you can download gerber source files for 3U tall, 4HP, 6HP, 8HP, 12HP and 16HP wide panels respecting eurorack dimensions.

Please notice that, at today, aluminum PCBs are limited to one layer, so it's not possible to have the B-side adapted/silkscreened for a second module like we did with VCO/Mixer fiberglass panels: the B-side is exposed aluminum here.

About bending: 1.6 mm aluminum panels are sturdy, at least up to 3U panel heights, and they have no tendency to bend.

What about costs?

Single layer aluminum PCBs have more or less the same cost of two layers fiberglass PCBs.

I made some simulations and, in this very moment (august 2021), 10-20 pieces of aluminum panels shipped to my place (Italy) with the slowest method available (which depends on package weight) would cost me (customs dutyes and taxes included):

10 pieces, 3U 6HP - 9,13 euro (Standard special Air Mail, 13-22 business days delivery) -> 0.9 euro/piece
15 pieces, 3U 6HP - 11,80 euro (Standard special Air Mail, 13-22 business days delivery) -> 0.8 euro/piece
20 pieces, 3U 6HP - 16,52 euro (Standard special Air Mail, 13-22 business days delivery) -> 0.8 euro/piece

10 pieces, 3U 12HP - 16.62 euro (Standard special Air Mail, 13-22 business days delivery) -> 1.7 euro/piece
15 pieces, 3U 12HP - 21.05 euro (Standard special Air Mail, 13-22 business days delivery) -> 1.4 euro/piece
20 pieces, 3U 12 HP - 27.73 euro (Global Direct Line Saver, 12-15 business days delivery) -> 1.4 euro/piece

10 pieces, 3U 16HP - 19.53 euro (Standard special Air Mail, 13-22 business days delivery) -> 2.0 euro/piece
15 pieces, 3U 16HP - 27.73 euro (Global Direct Line Saver, 12-15 business days delivery) -> 1.9 euro/piece
20 pieces, 3U 16HP - 33.82 euro (Global Direct Line Saver, 12-15 business days delivery) -> 1.7 euro/piece

Definitely good for a custom, color-masked, pre-drilled and silk-screened (or blank if you prefer), 1.6 mm thick aluminum panel/face plate!

A fundamental task with analog oscillators is having them tuned.

I have spent some good time on it, and succeeded in determining a simple and effective way (even if not perfect). I collected some hints along the way and drafted a working procedure for your (and my) future reference.

We are in the need for a variable voltage source and a guitar tuner for this procedure. I used a laboratory voltage regulator, but a keyboard with CV out will be perfect.

The board has three multi-turn trimmers for tuning: trimpot A (TA), trimpot B (TB) and trimpot C (TC).

TC has little to no effect on tuning, so forget it.

Now we can start:

1) Disable the Coarse tuning control with the dedicated switch (SW1);

2) Set the Fine tuning potentiometer to half way;

3) Turn TA fully counterclockwise and TB fully clockwise to maximise the oscillator frequency range.

From now on, our goal will be trimming that frequency range to someting usable.

4) Apply a 1V signal (or press C2 on your CV keyboard) to the V/oct input, center the closest note/pitch by rotating the Fine tuning potentiometer. Which note doesn't matter at this time!

5) Starting from 1V, apply gradually tension ending to 3V (or press C4 on your keyboard);

6) If the pitch is higher than two octaves with respect to the previous one (and this will be the case untill the end of the tuning procedure), turn TA clockwise and TB counter clockwise by one turn each;

7) Go back to point #4 and repeat untill your scaling is ok over the whole octaves range.

When the scaling procedure has ended, re-enable the coarse tuning control and move the potentiometer to the base note of your choice, also with the help of the fine tuning potentiometer.

Essentially, in this tuning procedure it's not important which note you end with at every integer voltage: the only thing that matters is that it's always the same (pitched) note. The coarse and fine tune potentiometers will transpose to the base pitch of choice afterwards.

The procedure is "imperfect" because having "Coarse" and "Fine" potentiometers perfectly centered with a C note would be the optimum (especially if you have more than one of these oscillators in the same synth), but you will get used at that as far as your oscillators are actually tuned!

All the printed circuit boards, fiberglass and aluminum front plates pictured in this Instructables were sponsored by JLCPCB, a high-tech manufacturer specializing in the production of high-reliable and cost-effective PCBs. Their customer service is very good and PCBs a great value for the money!

Please, let me stress out that this type of sponsorship is essential to experiment new solutions and share good quality and full working circuits and layouts. It allows testing a first prototype and then share a second (or third, or fourth...), improved and corrected revision of the board. This mechanism has a very positive impact in the quality and reliability of shared projects.

Many thanks!