Synth DIY: the LFO (an overview)

Photo of synth LFO controls

Some synth LFOs as they appear on the panel

What is an LFO?

LFO stands for Low Frequency Oscillator. It’s a general purpose modulation source that outputs a slowly cycling waveform. They are commonly used for effects such as vibrato, tremolo, PWM, and filter sweeps.

An LFO’s frequency is usually sub-audio, but in practice many LFOs venture into the low audio range (above 20Hz) or even higher. At the slow end, cycle time is a few seconds, sometimes a lot more – there’s no standard range for an LFO. Sometimes they’re just called ‘modulation oscillator’, which I guess is more accurate.

The output of an LFO will often be a sine or triangle wave, as they give good results for most purposes, but other waveforms are also common – square waves, ramps, and a ‘random’ or ‘sample and hold’ output are also popular (these are good for the ‘burbling’ effect often associated with synth sounds). Technically, a sample and hold circuit is another device, so I won’t cover that here.

Many older synths come with just one LFO, which means all your basic cyclic modulations (as opposed to one-shot modulations like envelopes, or stepped modulations like sequences) have to come from the same source. It would be better to have at least two LFOs, so that pitch modulation and PWM, for example, could use different waveforms at different speeds.

Happily, many new synths have multiple LFOs, and luckily for DIYers, making an LFO is a simple job. The parts are cheap and plentiful, the basic circuit is compact and easy to build, and one or two extra LFOs can enliven a basic synth patch considerably.

There are several ways to make such a circuit but I’ll limit myself here to two basic methods:

I’ll be adding links to designs on this page as I write up a few details, so bear with me!

 

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2 responses to “Synth DIY: the LFO (an overview)”

  1. Anon says :

    This is a good enough post that I was really disappointed to see that the later designs, links, etc never happened.

    • synthnerd says :

      You and me both! Life got in the way and the blog had to take a back seat. I’m trying to make time for it again now though, and am currently working on a complete DIY synth project that I’ll post in several parts here when it’s all tested and working. I’ve already tested four modules, designed another two, and have only one left to go before I can start writing the posts, drawing up the schematics, etc.

      Thanks for the compliment, glad you like the post. And please do keep checking back, because I’m still keeping this updated when I’m able 🙂

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