Total Harmonic Destruction: Distortion FX

This project needs reviving as the initial idea wasn't exciting enough for me to continue with the development. There is new art in the form of John Martin's Sodom and Gomorrah, which provides some angsty inspiration.

For the new project I am thinking of trying a minimal component design, with a single transistor or transistor->diode clipper centre, and tone circuitry surrounding it. If I eliminate buffer circuitry I can reduce the component count down even further and the pedal will produce all sorts of crazy effects when used with different equipment.

More to come, hopefully.

Some quick and dirty AC analysis using LTSpice shows some interesting problems:

[Figure 1: Three AC passes using LTspice showing both phase and amplitude. The blend control has been set at low, mid, and high illustrating the change between the diode saturation and hi-pass filter.]

The hi-pass filter has an approximate cutoff frequency of which I'm worried is both ineffectual as a desirable effect, and will always cut some bass so if I wanted to use this to get a really grimy bassline out of a synthesiser it would come out somewhat weak.

To fix this I think the blend control needs more options. I think by adding a three pole switch to the hi-pass filter side that allows selection of either [hi-pass; neutral; lo-pass] settings would be much more user-friendly. This would also be great for the other side so that the user could switch between a couple of sets of diodes.

Back to Figure 1: the blue line shows the characteristic when the blend potentiometer is set mid-way. It shows a drop of around 0.9dB in comparison to when the potentiometer is set to one of the extremes. This is around a Just Noticeable Difference, but for the purpose of this circuit I think it is probably forgivable. It'll have to wait until the breadboard stage to find out if I'm right about this.

The initial schematic is finished and hosted as an LTSPICE IV file on github. The schematic is currently quite a typical distortion effect and so will require further adaptation in future iterations, but the aim of this iteration is to get a range of operation that sounds musically pleasing. This will require a secondary breadboarding stage to the work done with SPICE.

The current layout is:

• Input Buffer
• Transistor saturation stage
• Blend control between a hipass filter and a diode saturator.
• Output buffer

The blend control idea comes from Jack Orman @ muzique.com where there are a large number of takes on using diodes for saturation. Definitely worth a read!