Building A "Volume Pedal De-Scratcher"
Last updated 09/23/04
After
being frustrated with my scratchy volume pedal, I decided to
build Craig Anderton's "Volume Pedal De-Scratcher". I
tried to avoid building this circuit by taking apart my sealed
pot (which showed no real signs of wear) and spraying contact
cleaner inside and cleaning it up very well. I also lubricated
all of the moving parts, too. (except the pot) Believe it or not,
just lubricating all of the moving parts does wonders. This stuff
all worked, for a while. To my dismay, it started being scratchy
again within a couple of months. I like to use volume swells a
lot, so this is a problem. So I figured that if a replacement pot
is around $25 + shipping, it would be advantageous to build this
circuit for around the same amount of money, and permanently get
rid of the problem. It's really not that much more of an effort to
build this circuit than it is to take your volume pedal apart and
clean or replace the pot.
The impetus for this project came about because I recently bought Craig Anderton's book, "DIY Projects For
Guitarists", and saw this schematic , which is exactly the
same one published in the original Guitar Player article
years ago. So I decided to "just do it".
The
circuit is pretty simple, just using a single opamp and a CA3080
OTA (Operational Transconductance Amplifier) opamp. I designed a PCB layout because I hate using perfboard. It's just
too messy and tedious for me, just as making a PCB can be
tedious. But, I like the finished product a lot better with a PCB.
And I find the assembly of a PCB is just a lot easier, as I don't
have to worry about getting all the connections right. It's
already done on my PCB layout, worked out beforehand. But you can
do the same thing with perfboard, too. There is also
a PCB layout which uses a TL072 at www.tonepad.com which has been
verified as functional.
Here is how I modified my Ernie Ball passive volume pedal:
 |
Here is the basic assembly. I used a 1/4" piece of plywood to mount everything on, cut and shaped to fit snugly inside the volume pedal body using no screws or adhesive. Height is a limitation, so I kept the PCB as small as possible, 1.9"x1.6". I painted the wood flat black. |
 |
Here's a closer view. I drilled three holes on the left side for wires to pass through to the jacks, pot and battery clip/switch. I elected to use a SPST switch to turn off the power. I just don't like unplugging and plugging in cords all the time... I had to create a rectangular opening for the switch body, not as hard as it sounds - this is wood! Easy to work with. A piece of plexiglass would look cool, too. |
 |
To avoid possible RFI or EMI problems, I decided to build a shielded enclosure for the PCB. It's simply just a piece of thin sheet aluminum cut to the appropriate size and bent as required. I used two small wood screws to fasten it to the plywood. I lined the inside of it with 1/16" self-adhesive neoprene sheet foam to avoid shorts. You can get an 8.5"x11" sheet of it from the crafts section at Walmart for 50 cents, and it comes in various colors. |
 |
This is how it looks with the shield installed. The shield is about 2.25" wide, 1.625" high, and 0.75" deep. I also applied a protective coating on the back of the PCB to inhibit oxidation of the copper traces. Not really all that necessary, but this circuit is the most exposed to the environment than all of my other circuits, so it seemed like a good idea. |
 |
This is how it looks with the whole assembly installed in the volume pedal. A 9V battery sits in a battery clip behind the power switch, fastened to the body of the volume pedal with double sided adhesive tape. Connections to the pot and jacks are made with quick disconnect terminals. The whole assembly is very easily removed, and this mod is completely and very easily reversible, too. Getting your fingers in there is a bit of a pain, though. And, the jack nearest the block that holds the pot is a very tight fit... |
So, how about the performance? Well... now the scratchiness is gone! Noise level of circuit is pretty low, mostly dependent on what opamps are used. Even with a standard 741 opamp, it's a fairly quiet circuit, although there is a fair amount of white noise with a 741. A TL071 would be better. And better still is the TL051, which is twice as quiet as a TL071, but has higher THD (Total Harmonic Distortion) than the TL071. An OPA134 will work in this circuit with no tweaking required, but it does use a little more current than the aforementioned ICs. The NE5534 has virtually no THD, and is a pretty good all-around performer. I used the NE5534 in my circuit. If you find that your volume pedal has too much gain and is overdriving your amp, then try decreasing the value of R4 until the distortion is not present any more. As Anderton suggests, the easiest method is to install a 100K trimpot to dial it in exactly where you want it. This also gives you more flexibility to use the circuit with instruments of varying output levels.