Type C. OD and Plastic Timbre

From the vendor

Type C. OD and Plastic Timbre

Jun 30, 2022 6 min read

Updated: Jul 13, 2023

Originally posted on the freestompboxes forum

Type C. OD

The OCD was my first veroboard build and a pedal I've always enjoyed as simple, no-frills distortion. Despite the wild claims of the original maker and the similarities with the Voodoo Lab Overdrive, I like how it sounds.

Despite that, there are some issues with it I decided to tackle, funnily enough mostly caused by the same few changes from the Voodoo circuit.

The main issue is an awful output impedance, of the order that it's not only bad on paper, but giving volume jumps even when switching pedals with decent input impedances after it, or cable capacitance causing a varying amount of treble loss.

At first, I had solved this by adding an output buffer, which I found out is similar to what the OCD v2 did, when traces became available (I had also added a capacitor before it to emulate the treble loss of a normal ~3m cable).

Another issue was the linear volume control, and then in general I thought it could have used a cleanup and standard E6 values for not critical parts. So here it is:

• Op-amp. TL072, pretty much the standard for guitar inputs with the low current noise. I don't know why I should use the slightly noisier 082, but I have tried both.

• Input stage. This is pretty much verbatim, I just made the input impedance a standard 1Meg, but also removed the input resistor because its noise gets amplified by everything, although small in value. It's a judgement call, noise and parts saving against current limiting and protection/reliability: I still haven't hit my face against issues caused by ESD on the input, so until then I choose the former. Gain stopper was removed to make this into an even cleaner boost if wanted: the original can already be very clean, why not go all the way and make the input stage go down to unity gain? Gain control law doesn't suffer, my spreadsheet tells me.

• Clipping. Switched to plain 4148 diodes because the mosfets were used for their body diodes, and there's not any justifiable reason to use them (a previous research I did didn't show differences, at least in the VI curves), I had even made an audio comparison of the two and didn't hear much of a difference. They still go to Vref, not because there's much special about it, but because it saves me two caps and a resistor.

• Tone. Moved between stages so it doesn't affect output impedance. "Hey why are the resistors the same, don't you have to subtract the series 10K?". As a friend pointed out, when the diodes are conducting, they're in parallel with the 10K, effectively dropping the total source impedance to very little, but when the diodes are conducting is when you need the tone control the most. This does mean in theory that the tone control cutoff and attenuation varies between the diodes conducting or not conducting at all, which I don't think it's that bad of a thing. Both simulations and side by side compar