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.
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.
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.
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.
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
The brass tab on the upper right side is used to ground the enclosure, be careful no to bend or break it.
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.
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
Now following the previous steps in reverse order, put your pedal back together.
Voilà, enjoy !