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No Hands...and No Hum!

by Dan Berkowitz

That annoying hum! If you're playing a Jazz or Precision bass, or one of the many copies out there, you've probably noticed some annoying humming and buzzing when your fingers come off the strings. For the Jazz, that can be controlled a bit by keeping both pickups set to approximately the same level, but either bass will still be a somewhat noisy…unless you shield your bass.

This article describes how to shield those kinds of passive instruments to get rid of the hum…those without a preamp. Disconnecting and reconnecting a preamp is more complicated and requires more confidence in your soldering and wiring abilities.

If you have a bit of comfort with soldering…nothing difficult really…and a bit of patience, you can do the job and make your axe much quieter. That's especially nice for the studio, but you'll enjoy having buzz-free practice sessions as well. The idea here is to surround the innards of the bass in metal to ward off the interference that causes the buzz and hum. This should be done with either aluminum or copper foil, so that the pickups' magnetic fields aren't affected.

To shield your bass, you'll need a few things: Phillips screwdriver (to remove pickups and pickguard screws), small flat blade screwdriver (to remove knobs), soldering iron, solder, shielding material, heavy-duty aluminum foil (like from your kitchen), and spray glue. A basic volt-ohm meter is also helpful to test your connections. A small socket wrench or a nut driver will help remove the pots easily and cleanly, but you can carefully remove them with pliers or an end wrench, too.

For shielding material, some people use a special conductive paint, but it's expensive, messy and smelly. I've either used adhesive backed copper foil or aluminum foil. The copper foil can be ordered online from Stewart-MacDonald, Carvin, or other guitar supply places…or you can try a local hobby shop that sells stained-glass supplies. The adhesive-backed aluminum foil is easier to find…it's at your local hardware store. The drawback to using the aluminum foil is that you can't solder to it, so you end up making little tape "bridges" to connect sections of foil in the control cavity and behind the pickups. You'd also need to either tape wires down or screw in a soldering terminal.

To get started with your shielding project:

1. Remove strings, remove pickguard, disconnect pickup leads and ground wires from pots…just cut them close to their connections. If there's a brass shield plates on the back of the cavity like Fender uses, you can remove it....they only do a partial job.

2. With the guts of the bass removed, line the pickup and control cavities with copper tape. Put a spot of solder at each joint on the tape to make the connection continuous...you don't need to solder the full seam (If you're using aluminum tape, make little tape bridges by sticking a small face-down strip onto a larger piece and burnish it down for good contact). Run the tape slightly onto the top in at least one spot...this will serve as a contact for the next step. You could also solder a wire to the pickup cavity and run it through the hole for the pickup leads into the control cavity, soldering it to ground. Some purists would insist on star grounding where you only use one point for ground, but I haven't found this to be a problem.

3. Remove the pots and jack from the pickguard. If your bass has press-on knobs, you can fashion a puller from a piece of thin metal that can slide under two sides of the knob at once and pull it straight up. You might find some foil around the controls area, but unless foil covers the whole guard, you'll want to do the next step. Lay the pickguard down on newspaper and cover with a thin layer of spray glue. Cover with a sheet of heavy-duty aluminum foil. Press down the foil with a rolling pin or other roller. Trim out the foil with a box cutter or other sharp knife, being careful not to cut the guard. You can use your fingernail to remove any last pieces that show.

4. Reinstall pots and jack. Run pickup leads into control cavity and reconnect to pots. Reconnect ground wire from bridge.

5. I find that it's helpful to use an ohm meter and check for ground continuity at this point in the job. Sometimes a connection is not solid but it can't be seen. Look for very low reading on the meter, close to what the meter reads when the leads are touched together...somehow even that isn't zero, but if you see more than a couple of ohms, it's not right somewhere.

6. Screw down the pickguard, with the pickups outside the guard. They need to be fit after the guard is in place. Check continuity between the guard, the bridge, and the control knobs with your meter. You can use the guard screws as contacts, since they've been pushed through the foil. Note that until the guard is screwed down tight, you will hear some crackling or some hum if you plug it in and listen through an amp.

7. Put back the foam behind the pickup. If you think the pads have lost their sping, new ones can be cut from an old mousepad...be sure to use dense closed-cell foam…like mousepad foam...open-cell foam will squish and add little springiness.

That's it...you're ready for the big test. Plug in your bass. The bass should now be eerily quiet, hands on the strings or hands off. You can even use those black tapewound strings again and listen to the sounds of silence!