Night Train OD

Design:

I designed an adaptation of the circuit back in 2007, the project evolved and finally got optimized down to a very basic 4 transistor circuit, which in my opinion delivers the most interesting sound.

Circuit is very simple and built with four common available silicon transistors. Starting from the input, the signal goes into high input impedance buffer, which drives the main distorting transistor pair. The gain control is achieved with changing the amount of negative feedback (1kC potentiometer in series with the bypass capacitor C12). Distorted signal is fed into a simple tunable lowpass filter (Tone) and into the main output volume control. The last transistor serves as an output buffer keeping the output impedance at low and constant level. Please note, although the schematic shows the battery power option, i didn't include it in the design presented below. If you need a battery powered version you will have to design your own enclosure.

The choice of transistors is not critical. Use anything relatively low noise for the input and output buffers, but watch out the pinout. The PCB is designed for a typical CBE pinout of the BCxxx series transistors. The main NPN+PNP pair is a place where i'd encourage to do a few experiments with different types (i installed a simple sockets to be able to change the parts). Definitely worth trying are pairs 2N3906+2N3904, BC550C and BC560C will deliver more gain. I also tried the pedal with higher power but smaller beta BD139+BD140 pair, delivering much softer and less distorted sound.

The PCB uses a classic through hole components only to make it easy to build even for beginners. After finishing the pedal the main pair needs to be properly biased. The best and simplest way to do that is to tweak the trimpot while playing and set in position where the sound will be smoothly overdriven and not gated. Pluck the strings and check if the sound does not abruptly decays.

I have incorporated two bypass options on the PCB. One is a standard 3PDT footswitch with two sections switching the signal and one for the status LED. The other options is the Millennium Bypass ( (c) by R.G. Keen). It uses a few parts extra and a DPDT footswitch.

Schematic: