D.I.S.T.O.R. Part 1 - A novel Overdrive/Fuzz pedal

Distortion by Internal Saturation Through Optical Resistance

A new kind of overdrive

This design was started many years ago when the world was still in lockdown. It is now in a place where it is complete enough to be shared. To my knowledge no other published designs distort audio using the method demonstrated below.

Keen eyed readers will notice that, among the other many strange looking aspects of this design, there is no AC path through this schematic.

The core of this design is the LDR/LED coupling. Initially the NSL-32 was used as shown, however during early testing it was found that a discrete LDR/LED coupling wrapped with heat shrink resulted in a more desirable tone. For the sake of neatness the integrated schematic symbol was maintained. All clips and recording linked will use the discrete version.

Where the distortion comes from

The LED is biased such that the AC output of U3A causes the brightness to oscillate in a way that approximately correlates to the source audio. The LDR responds to changes in brightness by changing resistance. When arranged as above R6 and the LDR form a voltage divider. As the resistance of the LDR changes, the DC voltage across R6 will also change. Because the LDR is responding to the attached LED, which is in turn responding to the source audio, the voltage across R6 will be correlated to the source audio. Removing the DC bias via C6 results in an AC signal. Due to the imperfect response of any given LDR, and any non linear behaviour of the optical components involved, the signal will be significantly distorted when compared to the input signal.

U3A is a simple opamp gain stage which allows the input signal to the LED to be varied in amplitude.

The "Bias" control allows the user to adjust the default brightness of the LED which will in turn move the oscillation range and can, at extreme settings, result in the LED being off entirely for significant portions of the cycle. R9 and R11 limit the current to the LED to prevent burnout and can be adjusted to taste. Smaller resistors will result in a greater range for any oscillation but can cause the LED to draw an excessive amount of current and burn itself out.

An initial proof of concept schematic featuring just the LDR/LED combination and some buffers

https://www.youtube.com/watch?v=qn-WqSNOO0Y

The schematic above was used as an initial proof that the underlying concept would work as expected. A brief demonstration is available on YouTube. Part of the purpose of this recording is to demonstrate that the majority of the distortion that can be heard is due to the LED/LDR coupling rather than the clipping of any opamps. The output is taken either directly from U3A or from C7 at various points and clearly shows an audible difference.

The harmonic content produced from the proof of concept design was interesting enough to warrant further development. A few tone stacks were trialed until the version above was settled on for a balance of simplicity and useability, and because it is intended to be tonally similar to the EHX big muff. This arrangement was based on work by Dylan159 that can be found here. Dylan has, as usual, included a detailed write up as part of his post.

https://www.youtube.com/watch?v=LHpC6dEUr9s

A wide range of tones are available from the finished design, most of them good. The input and tone stages can be replaced with a functionally infinite number of different designs to suit the tastes of the user. The layout above reflects a sound that was personally pleasing to me.

Future work

This design has been completed to the stage where it stands proud on its own as a distortion pedal. However, there are still many interesting possibilities that can be explored that use this as a starting point. One particular interest of mine is to use multiple antiparallel LED/LDR couplings and some clever biasing to create an octave up type effect. The actual characteristics of the couplings used were never measured empirically and would likely provide some interesting information and pretty graphs.

Update as of 22nd Feb 2026

This design will not work as shown above. There were a few errors made when translating the breadboard to a schematic. The schematic on this page will not be updated as I find some value in preserving my mistakes to remind everyone that I too am capable of making them. The errors are all found in the tone stack part of the schematic. Any competent designer of pedals will be able to find and fix them with minimal difficulty. For the less experienced builders out there (I include myself in this category) an updated version is available in part 2.

Thanks and collaborators

Thanks as always to the members of the Audio Electronics DIY discord server, in particular Dylan, Hayden, chip, EJ and sharpstriker for contributions to the schematic and general encouragement throughout the design process