This is a drive pedal that I like a lot. It is very versatile out of the box with its Clean Out and the Boost switch. And above all, it has a great sound!!!

cm_hotdriveboost_mk3.jpg

But as I play in a guitar/drum duo with my baritone telecaster and two amps in stereo, I needed :

  1. an extra feature : the ability to keep the clean sound (bass loudness) even when the drive is ON
  2. a correction : the "clean" path to the Output makes the signal out of phase with the input signal!

Here we go, schematic! With mods, of course...

Click to enlarge... CARL_MARTIN_HOT_DRIVE__N_BOOST_Mk_II___III_MODs.jpg

NB : This is the MkII schematic, mine is the MkIII, but the only difference I noticed is the clipping diodes. MkIII as 2x 2 LEDS instead of the 2x 1 diodes shown on the schematic... More headroom, I like that! Anyway, mods work for both!

As you can see, the last stage of the pedal, the Boost stage, is a inverter OP-amp. While the Drive path has a second inverter with the Wave stage (low pass filter), the Clean path doesn't. Here I used the first half of a NE5532A OP-amp.

Then I added a 500k-A (log) potentiometer to pick the clean sound and mix it with the drive sound. Both signals are being summed in the Boost OP-amp with the 22k resistors. But before, we need to amplify the clean signal. Why?... OK, I'll answer. Imagine that no sound is coming from the drive path because Level (of drive) potentiometer is fully counter clockwise, then voltage coming from the drive is 0V i.e. GND!!! So you get a resistive divider for the clean sound flowing to the next stage! And division is simply 22k/(22k+22k) = 1/2. So we need to have a x2 amplification to get a constant volume switching from clean to drive (if the 500k pot is fully clockwise).

As I added those 22k resistors for the summing amp, there was no need to keep the initial 22k resistor BUT for the clean sound if you want to keep the original designer's intentions. So I simply moved it before the switch.

And while the pedal was opened (it is pretty hard to remove the board from it because of the jack sockets on both sides), I took the opportunity to add another extra feature with the Drive filter, adding a potentiometer to select the amount of distorted bass in the drive sound.

Now I have a VERY versatile overdrive pedal !!!

(More pictures and sounds later)