Controlling MRG LFOs frequency range

The MRG LFOs contains two different oscillator cores, each capable of oscillating from slightly more than a second to about 600Hz.

Deciding the frequency range of an LFO is a matter of components selection, arbitrary decisions, physical limits and personal taste.

There are three components that control the timing of each LFO in the circuit:

  1. The resistance at the 1M potentiometer (Rp) in the front PCB.
  2. The resistor (Rt).
  3. The timing capacitor (Ct)

lfo pcb

Location of the components controlling the frequency range on a MRG LFOs PCB. Click Here for full picture.

The mathematical relationship that regulates each LFO frequency is:

lfo freq

The highest frequency is realised when the potentiometer is at its minimum, and can be approximated by removing Rp from the equation:

lfo freq

and the lowest when the potentiometer is at its maximum, approximated by exchanging Rp with the value of the potentiometer:

lfo freq

The MRG LFOs as shipped and designed uses 1K for Rt and 1uF for Ct, creating the following theoretical frequency ranges:

lfo freq lfo freq

In general, these default frequency ranges can be changed by modifying the elements involved. The suggested way of doing that is changing the timing capacitor Ct. A bigger capacitance will make the oscillator slower, a smaller one faster.

As long as the timing capacitor Ct is a non-polarised capacitor with a 5mm pitch, it should be fine. Of course, being a timing capacitor, the suggestion is to use a film capacitor, although 5mm pitch film capacitors at the micro-farad scale are unfortunately rare.

Using a Ct of 4.7 uF as a timing capacitor leads to the following minimum and maximum frequencies:

lfo freq lfo freq

Of course, as everything analog these are approximations, and real life measurement will be subject to some degree of error.

As an example, the picture below shows a MRG LFOs modified with a 4.7 uF capacitor in one of the oscillators (I am using the KEMET R82CC4470Z330K). The measured frequency range was from .126 Hz (about 8 seconds) to 124Hz.

lfo mod