In this series, we build a simple LFO module for Eurorack. After an initial design phase, we’re now ready to test it on the breadboard to see if it actually works. We only need a handful of easy to find components to build a single LFO based on a relaxation oscillator.
First, I changed the initial design of the relaxation oscillator a bit. To prevent too much power loss I changed the 4.7k Ohm hysteresis resistors to 100k.
Second, I swapped the 22k resistor in the RC circuit for a 100k one to decrease the fundamental speed of the LFO. I also added a 500k linear potentiometer in series to vary the rate of the oscillator.
I want this thing to go really slow. Think in minutes per cycle. I’m aiming for an LFO especially handy for slowly evolving patches.
So far so good. Let’s get it on the breadboard.
What components do we need?
We need three 100k resistors. The metal film 1% tolerance type work just fine. Besides that, we need one 100uF bipolar aluminum electrolytic capacitor. I use the Panasonic GA (Bi-polar) but all caps with the same specs will do.
For the op amp I chose the Texas Instruments TL071. This is a low-noise JFET-input operational amplifier in a PDIP 8 pin package. Lots of other op amps with similar specs will do for this purpose though.
The LFO prototype on the breadboard
On the bench, I have a breadboard and a small Eurorack testing case with a Doepfer A-100 DIY 1200 mA power supply.
I use the Moffenzeef Eurorack Power Breakout to bring power to the breadboard. It’s very cheap and easy way to get 12V, -12V, 5V, and ground to your breadboard. Also handy are the four jacks to easily get CV or audio into and out of your circuit.
A dual bench power supply will do as well. Let’s hook things up.
Here’s the TL071 op amp centered on the board with the pin layout. We don’t need the two offset pins 1 and 5. Also, pin 8 is not used by the TL071.
First, we connect the +12V rail from the power breakout to pin 7 Vcc+.
Second, we bring the -12V rail to pin 4 Vcc-.
From the Eurorack power breakout, we connect one of the blue rails on the breadboard. That’s all we need for our circuit to be powered.
The positive feedback loop
With the op amp connected to power we start building the voltage divider to the non-inverting input of the op amp. The voltage divider consists of two 100k Ohm resistors. Place the first one in series.
Place the second 100k resistor in series to ground. The green wire goes from the center of the voltage divider to the non-inverting input at pin 3 of the op amp. Our positive feedback loop is done.
The negative feedback loop
The negative feedback loop of the relaxation oscillator starts at the output pin 6 of the op amp. It has a 100k resistor in series (the 500k potentiometer comes later).
We place the 100uF bipolar capacitor in series between the resistor and ground (not connected yet – see below). Be aware of the polarity. The negative side of the capacitor has a shorter lead and a white stripe on the can.
From the connection of the resistor and capacitor, we take wire back to the inverting input of the op amp.
See the LFO in action on the oscilloscope
I forgot to extend the ground to the blue rail where the capacitor is attached 😱. Fixed that by connecting it to the leftmost ground rail with a grey wire.
Time to hook up the circuit to the oscilloscope and power this thing up.
And there you have it. A nice slow moving triangle LFO.
Make the speed of the LFO adjustable
We set the rate of the oscillator by adjusting the RC network components. The easiest way to do this is with a potentiometer in series with the 100k resistor. I picked a 500k linear pot to vary the resistance from 100k to 600k Ohm.
This gives the LFO a speed range roughly between 20s to 2min.
Next up we will add a dual LED indicator light to the module.