Low Frequency Oscillator Module(s)

In a previous Instructable I showed you the first version of a simple-but-effective Low Frequency Oscillator PCB based on a design from Nicolas Whoolaston.

Now it was the time to give that circuit an upgrade and develop a whole LFO module, face plate included.

In this Instructable I will show you how to realize a simple-but-effective Low Frequency Oscillator Module for modular synthesizers. I will also share with you the main PCB board files and two different versions of face plate to have all of them manufactured at the best price.

Let's go! :)

Selected Instructables in this series:

4 Channels MIDI to CV Interface Module
3340 Voltage Controlled Oscillator Module
Arduino Wavetable Voltage Controlled Oscillator
4 Channels Mixer
Band Pass Resonant Filter and Amplifier Module
Arduino ADSR Digital Envelope Generator
DIY Linear Regulated Eurorack Power Supply (and Power Bus Bar)
Fiberglass Panels for DIY Modular Synthesizers

The PCB I layed down is built around a circuit first introduced by Nicolas Whoolaston in 2008 at Electro-Music forum. It's main components are a common LM324 quad op-amp, a non polarized capacitor and only a few others elements. The result is a nice oscillator with two different wave forms (triangular and square) and control over waveshape and frequency. HERE is the link to the original post (electro-music.com).

The project is probably the one with few components between those I have realized so far for my modular synthesizer, but nonetheless it is very effective in doing it's job.

The "Shape" potentiometer acts, bet it, on the oscillator waveshape. On the square wave it affects the duty cycle, but it's the triangle wave that deserves more attention. It infact can be shaped continuously from backward saw, to triangle, to forward saw.

Both waves can be used at the same time and eventually mixed with an external mixer.

The opamp I used is a TL074 quad opamp, which is pin-to-pin compatible with LM324. You can use the easiest to source (or cheaper) in your area.

With respect to the first version of this LFO, I have included the possibility to change the frequency range of the oscillator, a feature present in most commercial LFO modules. The maximum frequency is determined by the non polarized capacitor (C1 in THESE schematics) and it was a simple matter of adding other two caps in parallel and a 3 positions switch to include this welcome feature.

Capacitors must be non-polarized and Best Practice suggests using non ceramic mainly because of the tendency to drift with temperature. Ceramic caps will work, anyway. Changing their values will change the frequency range of the oscillator (bigger -> lower frequencies, smaller -> bigger frequencies).

The circuit works on +/-12V dual voltage, but should also work on +/-15V. I placed a protection diode between the +12V and -12V lines just in case of an accidental inverse polarization of the circuit.

You can also vary waves peak-to-peak voltage value depending on the LEDs color: use blue or white LEDs to have +/- 2.6V voltage range circa.

All main PCB components values are silkscreened on the PCB to make assembly easier (a reference sheet makes the assembly tedious).

The board is intended to be mounted perpendicular to the front panel.

Bill of Materials (BOM):

Capacitors

- 2x 10uF electrolitic

- 2x 100nF non polarized

- 1x 100nF non polarized (non ceramic if possible)

- 1x 1uF non polarized (non ceramic if possible)

- 1x 10nF non polarized (non ceramic if possible)

Resistors and Potentiometers

- 2x 100K ohm single turn, mono, linear potentiometer

- 3x 1K ohm resistor

- 2x 100K ohm resistor

- 1x 47K ohm resistor

Diodes, LEDs and Op-Amps

- 2x 1N4148

- 1x 1N4004

- 2x LED (white or blue)

- 1x TL074 Op-Amp

Others

- 1x (5x2) IDC connector

- 1x SPDT (Single Pole, Double Throw) ON-OFF-ON switch

- 2x 1/4" female jack connectors

Designing face plates dedicated to cutom PCBs and have them manufactured at low cost is a joke today. It's a lot of fun too!

For this module I designed two different panel boards: a full featured panel board with oscillator frequency range selector (8HP, black in pictures) and a smaller one, missing the frequency range selector but 2HP narrower (white in pictures).

Triangular wave can be shaped from a falling saw to rising saw with continuity. Square wave changes it's duty cycle by turning the shape potentiometer. I have used white and blue LEDs: in both cases the voltage amplitude is close to +/-2.6V.

Waveshapes are not DC biases at any frequency and range spans from a tenth of an herz up to 960 Hz in the "high" range. In "med" position you can go up to 96 Hz. 10Hz circa is the maximum frequency at "low" position.

The LFO's slim version faceplate misses the frequency selector, so you are forced to choose a single, fixed frequency range. If you "blindly" fully populate the main board following the silk-screened values the frequency range will be the higher one. I personally find the lower frequency range the most usable in an LFO, so my suggestion is to solder a 1uF (105) non polarized capacitor in place of the 10pF (103) one and leave the other two capacitors out of the circuit. It should be mentioned, anyway, that for some applications (like the carrier signal in a ring modulator) high frequencies are what you would need, so... the selection of the capacitor value is up to you and your module environment. Those three capacitors are in parallel, so the circuit will work even without those two "additional" caps, but a (non polarized) capacitor in position "H" is mandatory.

Gerber files for both main PCB and faceplates are hosted HERE (Github). Please notice that faceplate holes are dimensioned to host 1/4" jack connectors (bigger), not eurorack standard 1/8" connectors (smaller).

When you order your face plates, be careful to select "aluminum" as material, not "FR4" (fiberglass). Faceplates files are two layer files, but only top layer is needed. Put a note in your order like "file is two layers: consider top layer only, thanks!" or the manufacturer will probably write you for a confirmation.

Add an LFO to your syntesizer and you will not regret it: the "L", in it's original meaning, probably stood for (patch) "LIFE" :)

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 ;)

Low frequency oscillator main PCB 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!

Without their contribution this module would likely not have been realized sooo... many thanks!