Synth DIY: the LFO (an overview)
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:
- the Relaxation LFO
- the Integration LFO
I’ll be adding links to designs on this page as I write up a few details, so bear with me!