Simple LFO

Low Frequency Oscillators (LFOs) are fundamental modulation sources for modular synthesis. An LFO module provides cyclical, slowly changing control signals, typically at frequencies below the audible range. The LFO’s control voltage ( CV ) is typically used to modulate the parameters of your sound-making modules, adding tremolo ( amplitude modulation ) or vibrato (pitch modulation ) to oscillators or movement and dynamic changes to aspects of a patch like filter cutoff or an oscillators pulse width.

In this guide, we’ll build a basic DIY Eurorack LFO using the N8 Synth Eurorack breadboard paired with a 4HP Eurorack Control Deck. The breadboard and Control Deck, pre-drilled panel, and pin headers can be purchased together as 4HP 1×6 Eurorack.

This simple LFO module gives us 5v peak-to-peak square and triangle waves up to ~14Hz, a saw wave up to ~24Hz, and a CV input for frequency modulation. As explained below, swapping a few components will allow you to tweak your LFO, making this simple circuit an excellent foundation for experimentation or as the basis for more complex LFO designs.

Photo of the completed synth LFO module

LFO schematic & circuit layout

Schematic diagram of a synthesizer LFO circuit
Eurorack LFO Schematic - click to expand
Diagram of the synth LFO on Eurorack Breadboard
Eurorack LFO Layout - click to expand

We can consider the LFO circuit as input, relaxation oscillator, and output in three blocks. U1B is configured as a summing inverter, which sums the incoming CV signal from J1 with that set by RV1. RV1, R1 and R2 form a voltage divider, which gives us ( around about ) +1.5V when RV1 is fully clockwise and  +100mV when fully anit-clockwise. R2, therefore sets the minimum CV voltage, and thus frequency, of our LFO when no signal is present at J1. Along with summing any control voltage present at J1, UB2 inverts the control voltage. The oscillator produces a symmetrical waveform with an input between -100mV and -1.5V. If these values are exceeded, the waveform will become increasingly asymmetrical. We assume that a control voltage arriving as J1 will be 10V peak to peak, pretty common for sources of modualtion. D1 blocks the negative swing, while R4 scales the positive 5V signal into the range the oscillator expects. Inputs greater than 10Vpp can be trimmed with RV2.

U1A and U2A form a relaxation oscillator. U1A is configured as an integrator. When a negative voltage ( our -100mV and -1.5V CV ) is present at the non-inverting input, U1A’s output will steadily rise until it reaches the positive rail, +12V. However, U2A is configured as a comparator and compares the output of U1A to the reference voltage set by the divider formed by R12 and R13. As the output of U2A feeds R12 and R13, and they are of equal value, the reference voltage will be either +6V or -6V, depending on which rail U2A has swung to. As the output of U1A steadily climbs to +12V, when it reaches +6V, the comparator U2A will swing to the -12V rail opening the PNP transistor Q1. With Q1 open, C1 can discharge, causing the output of UA1 to decrease toward the negative rail, -12V steadily. U2A is now comparing U1A’s output to -6V and will swing to the positive rail when U1A’s output reaches -6V. As U2A’s output swings back to +12V, Q1 is closed, C1 resumes charging, and the output of UA1 once again claims towards the +12V rail. 

The steady climb to +6V and descent to -6V of U1A gives the LFO its triangle wave output, while the swing from +12V to -12V of U2A provides the LFO with its square wave output, which is scaled to +5V / -5V by the divider R14 and R15.

U2B converts the triangle wave to a sawtooth wave. The output of U1A, the triangle wave,  feeds both the inverting and non-inverting inputs of U2B. U2B would also output a triangle wave if it weren’t for the PNP transistor connected to the non-inverting input. The output of U2A feeds the base of Q2. When U2A’s output ( the square wave ) drops below 0V, Q2 starts to conduct, and the voltage at U2B’s non-inverting input drops to 0V. This coincides with the voltage at U2B’s inverting input falling as U1A’s output ( the LFO’s triangle wave ) heads towards -6V. With 0V at its non-inverting input, U2B acts as an inverting amplifier, and the falling input voltage becomes a rising output voltage. This gives the LFO a sawtooth wave output at twice the triangle wave frequency. C10 stabilises U2B; without it, the fast swing of the saw wave would cause the op-amp to overshoot. D2 stops UA2, driving Q2 into reverse active mode and chopping off the point of the saw. 

Experiments & Tweaks

  • Increasing the size of C1 will decrease the frequency of the LFO; conversely, reducing the size of C1 will allow the LFO to reach higher frequencies.
  • Replacing Q2 with an equivalent NPN transistor and reversing the polarity of D2 will reverse the saw wave.
  • Try modulating the LFO’s frequency with a second LFO for more complex waveforms.

Bill of Materials

We’ll build a Eurorack LFO using a 4HP 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 4HP 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.

CategoryRefValueNotes
4HP 1x6 Eurorack Prototype Kit--Includes Eurorack Breadboard, 4HP Control Deck, 4HP Panel, 40-pin headers (PH1 & PH2), and 2x8 power header - J3.

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PotentiometerRV2B10K9mm vertical trimmer potentiometer

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PotentiometerRV1B10K9mm vertical PCB mounting potentiometer ( Alpha RD901F-40 style )

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HardwareJ1 - J4PJ 3001F3.5mm vertical mounting jack socket

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Hardware--1x Davies 1900H style knob, white

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Hardware--2x 8 pin DIP IC socket for U1 & U2
CapacitorC1Between 10nF and 100nF ( sets the the frequency of the LFO )Ceramic 50V
CapacitorC2, C3, C5, C6, C8, C9100nFCeramic 50V
CapacitorC10, C1110uFElectrolytic. 50V, 5mmx11mm
CapacitorC730pFCeramic 50V
CapacitorC42.2uFCeramic 50V
DiodeD1, D21N4148
ResistorR175K1/8W or 1/4W 1% metal film
ResistorR2, R15, R20, R211K1/8W or 1/4W 1% metal film
ResistorR3, R4,R5,R6,R17,R18,R19100K1/8W or 1/4W 1% metal film
ResistorR7,R9120K1/8W or 1/4W 1% metal film
ResistorR8200K1/8W or 1/4W 1% metal film
ResistorR10, R11, R12, R13, R1647K1/8W or 1/4W 1% metal film
ResistorR141K21/8W or 1/4W 1% metal film
TransistorQ1,Q22N3906General purpose PNP silicon transistor
ICU1 U2TL072TL072 Op amp

Constructing the Eurorack LFO Module

Building the LFO breadboard

Our LFO circuit is built on an N8 Eurorack breadboard. If you’ve used a sprung breadboard before, you should be right at home, as the layout is basically the same. If you need a quick overview of the design and features of the N8 Eurorack breadboard, the product page provides a diagram of the layout and connections.

As with any PCB construction, we want to build low to high. Starting with the wires and then adding components in reverse order of height. Beginning with the lower profile components makes our life easier, giving us more room to manoeuvre without the taller parts getting in the way.

1. First up, we add the wires. Strip around 3mm of insulation from each wire end and feed the conductor through the appropriate holes. It can be helpful to work in sections and use masking tape to hold several wires in place before flipping the board for soldering.

2. Position and solder in place the two diodes

3. Next, we solder in our resistors, starting with those that lay flat on the board – R6, R9, R10, R11, R14, R16, R17 & R18. Again, holding components in place with masking tape can be helpful or bending the pins slightly to stop the component sliding out of the holes.

4. Add the IC sockets, taking care to orient them correctly. Solder pins 1 and 8 of each socket, check they are square to the board, adjust if needed, and then solder the remaining pins.

5. Capacitors. Solder in the small ones and then the big ones! Make sure to check the polarity of the two electrolytic caps, C10 & C11.

6. Add the two transistors ( Q1 & Q2 ) and solder in the 2×8 pin header (J5), our power connector. The power connector can be mounted on either side of the breadboard, but for a 4HP module like this, mount the power connector on the component side. 

7. Solder the single-row, 90-degree, 40-pin male header to the left edge connector. We are using a 4HP Eurorack Control Deck, so the pin header is mounted on the rear of the breadboard.

8. Insert the two TL072 op-amps (U1, U2) in their sockets, taking care to orient them correctly – pin 1 should be on the right.

Photo of the completed breadboard layout of the synth LFO circuit
Eurorack LFO breadboard layout - click to expand

Top Tip: It can be tough to strip the insulation from the short wires, like the ones connecting the power rails to the TL072 op-amp. Instead of stripping 3mm of insulation from each end, remove 6mm from one end of the wire and then cut the wire to length. You should then be able to slide the short piece of insulation along the conductor, leaving 3mm at each end.

The LFO Control Deck

N8 Eurorack Control Decks have logical front and rear sides. The front has screen-printed boxes indicating where pots, jacks, and switches are mounted. The rear doesn’t have these boxes. Components can be mounted on either the front or the rear.

For this module, we mount our jack sockets and pots on the front of the Control Deck, the side with screen-printed boxes.

Check out these guides for further details on mounting components on your Control Deck.

4HP Eurorack Prototype Kit - click to expand

1. On the rear of the Control Deck, position the short wires connecting the “d-bus” to the spare horizontal C pad below each of the jack sockets (J1, J2, J3, & J4) – cells JSP1, JSP4, JSP5, JSP6. Solder them in place from the front.

We are using the d-bus to create a common ground on the control deck so that we don’t need to add a ground connection for each socket on the breadboard. This saves space on the breadboard for our core circuit.

2. On the rear of the Control Deck, connect the tip of J1 to pin 3 of RV2 by connecting a short wire from the horizontal A pad of JSP1 to the vertical C Pad of JSP2. The yellow wire in the diagram and photos.

3. On the rear of the Control Deck,  connect pin 1 of RV2 to ground by connecting a short wire from the horizontal A pad of JSP2 to the “d-bus” of JSP2. The black wire in the diagram and photos

4. Dry-fit the potentiometers on the front side of the Control Deck. If the pots have metal support tabs on their top and bottom edges, they should be tucked under the pot’s body, as illustrated here and shown in the photographs of the control deck on this page.

5. Dry fit the jacks on the front side of the Control Deck. If in doubt, check out this guide for correctly positioning jacks on the Control Deck.

6 Attach the pre-drilled Eurorack panel using the hex nuts and washers provided with the jacks and pots, checking that the jacks are centred in the holes and that each component is seated on the Control Deck. 

7. Leaving the panel attached, flip the Control Deck and solder the pots and jacks into position. The input jack (J1) needs its switch pin connected to ground. We achieve this by soldering its switch pin (B) to its ground pin (C) on the rear of the Control Deck. This is illustrated in the pictures on this page.

8. Position the 40-pin female header on the rear and solder it into place from the front. Hold it in place with masking tape if needed, solder a pin at either end and check it is perpendicular before soldering the other pins.

9. Connect the finished Control Deck to the breadboard using the pin headers.

Testing, Testing, check one two.

Before you power up your LFO for the first time it’s good to do some basic tests. While not extensive, these help keep the magic smoke in the components where it belongs.

Visual inspection

  • Compare your module to the diagrams and schematic on this page. Do all the components and wires look like they are in the right place? Anything missing?
  • Inspect the solder side of the Eurorack Breadboard and Control Deck.
    • Are any of the pads shorted by solder splashes or untrimmed component leads?
    • Are all the component leads soldered? 
    • Have solder bridges indicated on the diagrams been made?

Continuity

We want to ensure there is no continuity between +12v, ground and -12v rails. We’ll do this using a multimeter.

Put your multimeter in continuity test mode, then, with your module unpowered, check the continuity between the following points on the circuit:

  • Connect one of the multimeter’s test leads to the +12V rail and the other to ground. There should be no continuity
  • Connect one of the multimeter’s test leads to the -12V rail and the other to ground. There should be no continuity
  • Connect one of the multimeter’s test leads to the -12V rail and the other to +12V. There should be no continuity

Power Up

If your new module passed the continuity tests and visual inspection, it’s time to power it up. If you have a bench power supply, it is good practice to use this for the first power-up of a DIY synth module so that it is a minimum safe distance from your other modules.

The Eurorack power connector format is sadly a little open to interpretation, and many a module has lost its life to the specification’s vagueries.

N8 Eurorack prototype boards follow the most common convention. A white stripe is printed next to the -12V end of the power connector. Typically, this is where the red stripe of the power cable should be aligned. BUT not every manufacturer follows this convention, and this is a DIY synth tutorial, so chances are you made your cables, right?

Always check that your power supply is supplying -12V at the red stripe before connecting power to your synth module and that the red stripe is connected to the -12V pin on the module.

Making your LFO module look beautiful

The final step is to make your shiny new LFO look the part next to those commercial Eurorack modules. The good news is you can do this with nothing fancier than an inkjet printer and some sticky-back plastic.

We have a complete guide to making labels for Euroack panels here. If bright green is your thing, you can download our finished label, ready for printing.

Download the LFO label template.

We’d love to see your build. Share your pix with us on Facebook and Instagram.

Photo of the completed Eurorack LFO module

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