Low Frequency Oscillator Module for Synthesizers V2

After more than 3 years it was time to give my DIY low frequency oscillator a refresh, with a complete redesign and the addition of new features (more details in the followings).

In this Instructable I will show you how to realize a feature-loaded, simple-but-effective, Low Frequency Oscillator Module for modular synthesizers.

Features:

- two waveforms: triangle and square

- control over triangle waveform skew and square duty-cycle

- three frequency ranges of operation (0-10Hz, 0-100Hz and 0-1000Hz)

- control over gain (from total attenuation to 8Vpp)

- control over bias level

- wave polarity live visual indication

I will also share with you PCB board and face plate Gerber files to help you have it ready in the shortest time possible (we are all out of free time, isn't it?).

Let's go! :)

In the following the bill of materials needed to assemble the two PCBs (front board and main board). Panel board has no traces.

All components used are easily available and cheap, dont worry :)

Front board

4x 100K ohm linear potentiometer, WH148

2x LED, 3mm (your favourite color. These are the polarity indicaror LEDs)

1x mono jack connector 3.5 mm, female, PJ301M

1x single-pole double-throw (SPDT) switch, ON-ON

1x single-pole double-throw (SPDT) lever switch, ON-OFF-ON

24x (total) pinheaders, 0.1" spacing, male

Main board

2x TL074 op-amp

2x 1N4148 diode

2x LED, 3mm, YELLOW (the color must be respected here)

2x 100nF non polarized capacitor

2x 100nF non polarized capacitor (non ceramic is better)

1x 10nF non polarized capacitor

2x 10uF electrolitic capacitor

1x 100K ohm resistor

5x 1K ohm resistor

1x 2.2K ohm resistor

8x 47K ohm resistor

1x 4.7K ohm resistor

1x IDC connector, 2x5

24x (total) pinheaders, 0.1" spacing, female

You are also in the need for a soldering station, some solder wire and some spare time to put everything toghether :)

Low frequency oscillators are very common modulation sources in eurorack synthesizers mainly because of their effectivness and ease of use.

Despite this, they often require some external module to reach destination module's modulation soft-spot.

Just to cite an example, as far as the destination module accepts AC coupled input voltages eveything is ok, but what if some DC biasing is necessary? Take a look at the modulation of CEM3340 voltage controlled oscillators PWM frequency (ok, ok: not the most eurorack-ready IC, to begin with). For musical results, you want a DC biased source of modulation, or the square wave output will be silenced on negative bias (0% duty cycle).

Another example: what if you want to (and most of the time you do) subtly modulate your destination instead of applying a full range modulation? An attenuator stage comes to the rescue.

All these scenarios can be easily faced adopting extra modules that apply due voltage processing to the "dry" LFO signal, with the drawback of an increased use of "vital" rack space.

Since I already made some good-working external module for signal processing, I knew I could add some of them with little-to-no waste of space to my previous LFO project. In particular, two extra circuits found their way inside the module: an active attenuator/amplification stage and a level shifter (DC offsetter).

Even with this increase in features (which also means "increased components count") I could keep the module in the 6HP ballpark. This made the faceplate a little more crowded, but still more than decent operation-wise.

In this new iteration of the LFO module I have adopted a stacked board design instead of the previous "flag-like" design. It's more complex to draw (more work on my side), but more "professional" looking, if you know what I mean.

It is also more compact, then "less prone to damage", especially if one moves modules often.

The oscillators circuit here adopted was first introduced by Nicolas Whoolaston in 2008 at Electro-Music forum. Main components are a generic quad op-amp, a non polarized capacitor, two clamping LEDs and only a few others surrounding elements.

The wave generation is based on the use of an op-amp in relaxation oscillator configuration. Relaxation oscillator means that the opamp is configured to run “open loop” in which no feedback is used to limit the gain. Because the amplifier is essentially unstable, it will move as quickly as possible to either its largest possible positive or negative output. The output is automatically toggled between these two states by connecting the output to the inverting input with an RC time constant. This enables square wave oscillations. Wave actual amplitude limits are set by by two LEDs used as voltage limiters (positive and negative). The clamped relaxation oscillator output is then buffered before going to the outer-world as square wave. The triangular wave is generated in an adiacent section of the circuit, by charging a capacitor with a constant current. A simmetry modification circuitry makes the variable skew possibile for both waves.

The result is a nice oscillator with two different waveforms (triangular and square) and control over waveshape and frequency.

In addition to Nicolas circuit, I have included the possibility to change the frequency range of the oscillator, a very welcome 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, as far as you don't use it to set a pitch. Changing their values will change the frequency range of the oscillator (bigger -> lower frequencies, smaller -> higher frequencies).

Another addition is a voltage shifter circuit to bias waveforms to positive or negative voltages and a final amplification stage to boost up waves amplitude to Eurorack levels.

Voltage shifting is obtained through the most basic (inverting) mixer, where one of the two LFO waves and DC offset voltage are added toghether. The signal is then inverted (again) to restore it's original polarity.

User has control over the gain of this final op-amp stage, where the waveform amplitude can span a "zero" to "2X" range.

Waves peak-to-peak voltage depends on clamping LED's color and final stage amplification. I have used white LEDs to have 7Vpp circa when the gain potentiometer is turned fully clockwise (2X amplification).

The circuit works on +/-12V dual voltage, but should also work on +/-15V.

I have layed down a Falstad's CircuitJS simulation of the whole LFO circuit. Click >>HERE<< if you want to toy with it :)

Assemblying this module is not that complex. All components are THT (no SMD components here) and easy to source.

As a general rule:

1. solder smaller components first (resistors and diodes);
2. proceed then with IC sockets;
3. capacitors, LEDs and pinheaders are next;
4. pinheaders (pay attention to male and female)
5. finish with bigger components (potentiometers, LEDs that pop up in the front panel, switches, IDC socket, etc).

About pinheaders: I use male and female pinheaders for connections between boards. Female type should be used in the main board to limit the risk of short circuits in those rare cases where the main board is powered while not connected to the front board. Use male pinheaders on the frontboard PCB since there's not the risk of having it powered and pins not connected.

Start by assemblying the front board (the one with potentiometers and switches). The pinheader between the two single pole, double throw (SPDT) switches should be soldered first, or you will have a hard time soldering it after. The two SPDT and output jack are next. Please notice that the two SPDT are different, one being ON-ON (the waveform selector), one being ON-OFF-ON (the frequency range selector).

Every time you add a component, check that the panel board and front board are aligned, with no residual stress on PCBs.

The 4 potentiometers are of the same type and value (linerar, 100K ohm, single turn, 6 mm shaft). I use common WH148 potentiometers that are intended to be installed perpendicuarly to the PCB. I bend the pins and use wires cut from resistors or other components to elongate the pins and reach the PCB. First lock the potentiometer to the panel board, check that boards are parallel and then solder it in place.

To have perfecly aligned LEDs, place them in position first, firmly connect panel board and front board, align LEDs to the panel then solder them in place. This way they will be perfectly aligned to the panel, with no residual stress on their solder pads.

The module is straightforward to use.

First, select the waveform with the ON-ON switch located in the bottom of the module. Available waveforms are triangle and square.

The "Shape" potentiometer gives full control over the skew of the waveshape. On the square wave it affects the duty cycle, while on triangle it shapes continuously from backward saw, to triangle, to forward saw (ramp).

The second potentiometer ("Rate") modify the oscillation frequency of the wave of choice.

The frequency range can be set through a dedicated, 3-positions switch located on module's bottom. Frequency ranges are: 0 - 10 Hz, 0 - 100Hz and 0 - 1000 Hz. You can hit higher frequencies by using smaller caps than those I adopted.

"Offset" potentiometer is used to shift up to positive or down to negative your oscillating voltage. This has practical applications such as in modulation of some external module parameter expecting voltages with a well-defined polarity.

The fourth potentiometer is used to increase/decrease the gain of the output signal. Gain goes from zero (completely attenuated wave, full counter clock-wise potentiometer) to 2X gain (8Vpp circa, full clock-wise pot).

Many thanks to the nice girls and guys at JLCPCB for sponsoring the manufacturing of PCBs for this module. Without their contribution this project would have never reached it's actual level of developent.

JLCPCB is a high-tech manufacturer specialized in the production of high-reliable and cost-effective PCBs. They offer a flexible PCB assembly service with a huge library of more than 350.000 components in stock.

3D printing is part of their portfolio of services so one could create a full finished product, all in one place!

By registering at JLCPCB site via THIS LINK (affiliated link) you will receive a series of coupons for your orders. Registering costs nothing, so it could be the right opportunity to give their service a due try ;)

All Gerber files and sketches I realized for this project are stored >>HERE<< (Github).

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 ;)