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LFÖ x4 module

More Modulation, Please!

After I got the Doepfer DIY synth up and running I immediately realized I needed more than one low frequency oscillator to modulate things. In the current setup I can modulate oscillator frequency, pulse width, filter frequency, filter resonance, another filter frequency, amplitude of the VCA etc... So one LFO simply isn't enough!

The Schematic

Because of my "quantity over quality" synth building philosophy I started looking for a really simple and cheap LFO circuit that I could easily multiply. I finally ended to the ASM-1 DIY synth page. That's actually one of the pages I spent a lot of time in ten years ago when planning to build a modular synth for the first time. ASM-1 is a complete modular synth but not exactly compatible with the current Eurorack format as it uses -15V/+15V voltage.

After studying the LFO schematics it turned out that the LFO is really simple. The circuit is pretty much as simple as it can get. A Schmitt trigger implemented with an op-amp generating square wave signal followed with a simple op-amp circuit that turns it into triangle wave. Also the lower 12V operating voltage of a eurorack synth is not a problem for an op-amp based circuit this simple. It's only six components plus the op-amp. And because I wanted to build several of these at once I could use quad op-amps shared by two LFOs. So it's only 6.5 components per LFO! Luckily I happened to have pretty much all components available, including several TL074 quad op-amps.

The "Layout"

As there's not that many components I didn't spend much time thinking about the layout. I just drilled another ugly panel for the jacks and pots and started soldering components onto a veroboard. This ad hoc method works just fine with a schematic as small as this.
8HP wide panel for 4 LFO's, each having one potentiometer, square out and triangle out. Components for the first LFO soldered. I had to use two 470nF capacitors in parallel because I had no 1uF capacitors available.

The backside of the circuit board. Ugly but works and costs practically nothing!
After I got the first LFO soldered it was time for a test. It worked OK so I continued building the rest of the LFOs by simply copying the layout from the first one. After the second one I ran out of components and had to visit local electronics store to get some more capacitors and resistors.

I first used 100K linear potentiometers but after trying logarithmic pot with the third one I decided to swap all to log pots.

All LFOs soldered. It was easier to mount the 22Ohm resistor directly to the panel instead of the vero board.

All components in place. I got 1uF capacitors for the 3rd and 4th LFO so it's even more compact than the first ones.

The final connections.
After everything soldered it was time for the final test. Unfortunately I made a small measurement mistake and the panel didn't fit into the eurorack frame any more. Luckily I was able to turn the last potentiometer around and got the problem solved.

I used self adhesive pcb stands to attach the board to the panel. One potentiometer had to be turned upside down because it was touching the eurorack enclosure rail.
Finally some Dymo label stickers and brown knows and we are done!

Should be clear what this module does!

The Result

I'm really happy with the module. Now I have 5 LFO's in the system and lots of new modulation possibilities to make everything wobble wildly! I admit that It's not as good as the LFO in Doepfer DIY synth. It's more difficult to use because there's no range switch and also the range is more limited. I also noticed some weird behaviour in the extreme ends of the range but nothing really problematic. There's also some difference between the LFO's. The first two have completely different pulse width at least with the really slow triangle wave. The signal rises fast but falls much slower. The LFO's 3 and 4 seem to have a more even 50/50 pulse width. It might have something to do with the 2x470nF capacitors in LFO's 1 and 2 versus 1uF capacitors in LFO's 3 and 4. It's anyway such a small annoyance that I'm not planning to debug it any further. And on the other hand it's kind of interesting to have some variation between the units. I might actually utilize this feature somewhere!

It's difficult to calculate the total price for this module because I mostly used old parts I happened to have already. I would estimate that the total cost is less than 15 euros. 15euros, 4 LFO's! Money well spent!

And then the most important part! there's a tongue-in-cheek demo video in my Instagram account. Enjoy! https://www.instagram.com/p/BStwtH7DgsS/
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