Overdrive Effect Pedal – Schematic and Layout
When I want to build some equipment for my guitar I consider the facts which makes sound distinctive on the way I want. This project also has to be:
…because I haven’t so much time to play with circuitry and it needs to be simple. Simplicity has doesn’t mean to sacrifice the sound and expression quality. In fact the most audio designers have the rule of designing circuits which means the sound path to be less treated and this rule keeps whether or not this circuitry is purposed for instrumental or hi-fi demands. I agree with that and annex that in instrumental circuitry designing device has to fulfil the target properties.
Guitar Overdrive with Top Level of Expression
…and I need expressive overdrive which don’t have to choke the sound and treats strings picking as audiable sound attack differs by the way of guitar playing, so therefore i need soft clipping way of distortion to preserve sound information of source. I have to avoid fuzzy clipping otherwise it would not be overdrive at all.
NOTE: gain potentiometer-rheostat P1 (left snapshot) is substituted by fixed resistor for tone testing purpose which on those circuit means fixed amount of overdrive too.
Overdrive Schematic and Layout on Stripboard (Veroboard)
The device is built on Atarado DIL stripboard-like prototyping board, but with plain stripboard (veroboard) it would be easy breeze to make layout too.
While the recipe for achieving overdrive effect is pretty easy and conventional with those schematic, there’s few main points. The overdrive effect is split in tree phases:
- in first small amount of overdrive is gaining trough transistor BC109, or other small signal transistor, with push it to the edge of amplifying factor. That fairly depends of transistors technology quality and by my experience the same nomenclature of semiconductors behaves pretty same unless it goes into saturation. So I should to test few BC109 by different transistor manufactures to pick those with organic overdrive throat.
- second and third phase are diodes connected in antiparallel of two pairs. One pair are rectifying diodes are 1N4007 (!), which you may consider weird in those topology, but they gives some kind of slow clipping and makes guitar sound a bit darker counter to the second pair of 1N4148 which are fast low signal diode and pull up the tone brightness. The main point are resistors in parallel with every diode pair which ensures the softer signal clipping. Both resistors are with different values to render the sound of guitar overdrive with lusher organic tone and preserve string attack unique in different picking techniques.
Well, on the layout, you may see missing pot P1 which I dropped from showed circuit just to make test so instead of rheostat put fixed resistor. After testing make it with two wires inside coaxial leading to pot to avoid noise (coaxial shield should be connected to ground). Amount of gain is determined by those feedback at BC109 and it is used for drive tweaking.
In general this overdrive fulfils my expectations and shape the guitar sound different from much market products giving the sound bit of vintage bite. Couldn’t imagine the better from one non-germanium transistor. In some further revision could put one more amplifying stage as well as more drive. Cheers!