Mooer Hustle Drive Volume Mod

I got myself a second-handed Mooer Hustle Drive mini-pedal, a Chinese clone of the Fulltone OCD.
Overall, these little pedals (I also have an Orange Ninety and a Green Mile) sound really good, are really well built and take very little space on the pedalboard. All that for an affordable price.

Mooer Hustle Drive - Top View

One odd thing I noticed with the Hustle Drive was that the Volume control was almost unusable: the pedal has a huge gain when the Volume knob is set fully clockwise, and it becomes attenuated enough to reach unity gain (same level whether the pedal is on or off) at around 2 or 3% of the knob position (on the CCW side), while the rest of the travel has little effect (massive gain throughout).

This is usually a sign of a linear potentiometer being used, instead of a logarithmic. I won’t go into details on why log is better for volume controls, long story short your ears don’t find linear level progressions pleasing.

A quick peek into the pedal’s guts confirmed this assumption. The Volume potentiometer is a 500K linear Alpha 9mm. It seems to be consistent with some revisions of the original OCD schematic.

There are two alternatives to fix this issue:

  • Replace the potentiometer by a 500K log.
  • Use a single resistor to bend the linear progression into a “pseudo-log” curve.

Since replacing the whole pot is more likely to damage the pedal (there is not much space in there, and the pots are well attached to the PCB), I chose to go for the resistor technique. Also, most log potentiometers are actually made of two linear curves, which does not exactly solve the problem, but rather splits it in two.

A Bit of Theory

The fact of connecting a resistor across the legs (center and one of the two sides) of a potentiometer is called tapping or tapering. There is an excellent article on Geofex by R.G. Keen about this explaining all the maths, and I encourage you to read it.

Here is what we aim to achieve:

I experimented a little with some resistor values, getting interesting results between 33k and 3.3k. You may want to try some values for yourself to see which response suits your needs best.

Disclaimer note

I decline all responsibility in any damage you may inflict to your pedal, yourself or others. As written on the back of the pedal, this mod will void your warranty. If you’ve never used a soldering iron before, I’d suggest you practice on easier DIY circuits first, as this one involves hard to reach spots.

The easiest way to try this without tempering with anything is to use crocodile clips, connected on the output jack, between the ground and signal (hot) point.

IMG_0487

When experimenting, just remember that when the effect is disengaged, the resistor will remain in the bypass circuit path so (unlike me at first) don’t be surprised if the bypass sound is dull.
When installed as described, the mod will not affect the bypass sound as it will be connected to the Volume pot.

I chose a 4.7k resistor (approximately 1/10 of the potentiometer value), as it gave a good response to balance extreme Drive settings, between a clean boost and a heavy distorsion.

Effects on Output Impedance

Another side effect of using a linear potentiometer as a simple output voltage divider is that the output impedance depends on the volume setting: the lower the volume, the lower the impedance. That also means that at full volume, the output impedance of the pedal is around 500K, which is quite big.

You can see the effects on the taper shape, the dB response and output impedance on this Desmos graph, and adjust the values for yourself:

Step by Step Guide

Remove the four screws to take out the back plate.Remove the 4 screws and the backpanel.

The brass tab on the upper right side is used to ground the enclosure, be careful no to bend or break it.

IMG_0451

Now detach the power connector by gently pulling on the wires.Detach the power connector by gently pulling on the wires.

Slide the DC socket out of the enclosure, and remove the side jack caps.Pull the DC socket out of the enclosure, and remove the side jack caps.

Unfasten the foot switch and toggle switch that hold the PCB in place.Unfasten the footswitch and toggle switch that hold the PCB in place.

Pull the PCB out of the enclosure.Pull the PCB out of the enclosure.

Mooer Hustle Drive - PCB

Here is where we will solder our resistor, between the Volume pot output pin and the Ground. These two points are quite small and difficult to access, but with a good soldering iron, a steady hand and some patience, it can be done.
Just be careful not to touch any plastic parts (jack or power socket) with the iron, as they could burn/melt.The resistor will be connected between the Volume Pot output and Ground.

Trim the resistor legs, so that it does not take too much space inside the enclosure, and solder it to the two designated points.
You can use the enclosure to hold the PCB while soldering, as it’s symmetrical.
Kudos to the designers at Mooer for this (surely involuntary) nice thought :)Solder the resistor between the two indicated points.

Here is the resistor in place:Volume mod in place.Volume mod in place.

Now following the previous steps in reverse order, put your pedal back together.

Putting it all back together.

Voilà, enjoy !

  • Romain Dejoie

    Great tips, thanks! I might do something similar to my mooer black secret (rat clone) since the volume range is very steep.
    Any mod about cancelling the “pop” noise for other mooer pedals would be appreciated 😉 If I’m brave enough, I will try to sneak in a pulldown resistor somewhere…

  • calansvc

    Awesome. Now, can you tell me how to remove just a hint of the boominess? I’m guessing there is an input or output cap that could be swapped to block some of the low frequencies?

  • Slej Hamer

    Thanks for this great write-up and the accompanying pictures! Easy and very useful mod. I ended up using two 3.3k ohm resistors in series as I didn’t have a 4.7k handy. Might cut one out though, as now I have to set volume between 2 and 3 o’clock with gain at noon (LP setting) for just-above-unity volume with a mild overdrive. It’s definitely more usable than before, though, and the gain settings from noon to full are really sweet. Cheers!