If you want to take my “Simple Dog” Metal Zone mods further, here are more ideas. In keeping with the naming convention I’m calling these the “God of Cake” mods; you’ll feel like a young Allie Brosh playing through your sugared-up MT-2 once you’ve done them.
D3 or D4 – Add a 1K resistor in series with one of these
De-solder one side of D3 or D4 and put a 1K resistor (or less, if you want less of the effect) in series between the de-soldered end of the diode, and the point on the circuit board it had gone to. This will make the clipping asymmetric and soften the edges of the waveform on one side of the cycle for a sound I’d describe as smoother, more dynamic, and… not really like adding a clean blend, but moving just 7% in that direction, if that makes sense.
I’ve tried a lot of diodes here, but this is still my favorite mod. If you experiment, I recommend starting with germanium and Schottky diodes rather than LEDs or other diodes with higher forward voltages than the stock silicon diodes. (LEDs will barely clip at this position in the circuit, leaving you with mostly opamp clipping, almost like lifting the diodes entirely.)
D3 or D4 – Add a .01uF capacitor in parallel
Solder the capacitor to D3 or D4’s leads, one capacitor leg to each side of the diode. This will filter out some harsh high frequencies; if you want to customize it, larger capacitors will have a lower cutoff frequency (i.e. filter out more treble) and smaller capacitors, vice versa.
Final gain stage
R26 – 68K
Increasing the value of R26 turns this opamp buffer stage into a gain stage, making up some of the gain reduced in my other mods and smoothing out the decay of sustained notes a bit (as the different stages exit saturation at different times in the decay). Experiment with the value.
R26 – Add 3mm red LEDs in parallel
Diodes added in parallel with R26 turn this gain stage into a soft-clipping stage (via negative feedback), which I really like. They also decrease output level relative to the noise floor a bit, so I like to have a toggle switch to bring them into or out of the circuit. This is a great place to experiment with all kinds of diodes, particularly various colors of LEDs (which have different forward voltages, very approximately, from red (low) to yellow, green, blue, and violet (high)).