Carl Martin - Hot Drive'n'Boost MkII & MkIII mods
By vinic on May 2 - 2014, 22:20 - Permalink
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!!!
But as I play in a guitar/drum duo with my baritone telecaster and two amps in stereo, I needed :
- an extra feature : the ability to keep the clean sound (bass loudness) even when the drive is ON
- 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...
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)
Comments
Very nice work! I also love this pedal but would like the ability to bleed some of the clean boost in with the drive while boost is off. This while still being able to increase more with the boost switch. Is there a simple mod for this? I'm struggling with it.
Thanks,
Tim
What do you mean with "clean boost"? The actual Boost is placed at the end of the signal chain so it will amplify both Clean and Drive channels :
(Clean OR Drive) > (Boost OFF/ON)
My mod allows to get some Clean signal mixed with the Drive :
(Clean OR (Drive/Clean mix)) > (Boost OFF/ON)
What I understand from your question, tell me if I'm wrong : if you want to make the Clean part of the (Drive/Clean mix) switchable, then you simply need to add a new (foot)switch to it, between the new Gain=2 OPA and the 22k resistor.
Position 1 : Grounded (clean in drive is OFF)
Common : output to the 22k resistor
Position 2 : From new Gain=2 OPA (clean in drive is ON)
Hope this helps.
Hi!
I got the MkII version. I was thinking if mod the version II to 3 is as easy as change the two diodes???
I canĀ“t find any information, but if i got it probably try it. I needed to know exacty what LEDS do I need to buy
Can you help me?
Thanks in advance
Hi!
Indeed, that's the only difference I noticed. The LEDs are clear blue ones, but that's all I can tell... So it does not help!
You should (temporarily or not) add a switch to easily compare the sound difference, and maybe an easy way to swap between different LEDs/diodes.
Have fun!