Question about hum

[rpjazzguitar]

The rig is the Comins GCS-1 right into a Roland JC55.

So, I needed to control my volume with the pot on the guitar (using neck pickup only).

When I rolled off the volume, I got hum. Touching the strings improved it, but didn't eliminate it.

No hum if the knob was rotated all the way on, or off.

I never noticed this before, but I almost always play with a volume pedal -- and the guitar knob up full. Until yesterday I never played this venue without the volume pedal.

Then, at home: Same issue with a different amp and different cable. But, at home, touching the strings eliminated the hum completely. So less EMI at home?

So, a couple of questions (and thanks in advance for any help).

1. Can someone explain the physics? Why is it humming only with the volume control rolled partway down?

I have read that the problem is that your body picks up EMI and the pickup, nearby, can sense it. Then, when you touch the strings, you're grounding your body and thereby shunting all the EMI to ground. So, according to this, the guitar is grounding you, not the other way around. Is that correct?

2. What is the fix?

3. If it's shielding -- did Bill Comins really produce a guitar that isn't properly shielded? If not that, I'm back to trying to understand the physics.

[jads57]

Try Google, and the technical questions over at Thegesrpage.com

It sounds like a loose ground wire or maybe a bat pot?

[skiboyny]

You're at home experiment sounds normal. Hum disappears when you touch the strings. Tells me the guitar ground system is working and connected to the strings. I would try this again with the cable you were using when you first noticed the problem. The location could have had more EM and the cable poorly shielded. Amp as well. You should be able to touch any metal part on the guitar and make the hum go away. Strings, pickup covers, reach in the f hole touch the back of a pot. They should all be connected to ground. If you have an ohm meter you can check to see if they are all properly connected. The reducing of the volume control changes the impedance of the circuit so it's more susceptible to the EM. This has to do with the tone controls and capacitors. This is normal.

[AllanAllen]

Any neon signs at the venue? 20 feet from the stage? There’s a place by me and the sign just buzzes like crazy though my guitar.

I figured it out by turning it off, but for some reason they really wanted the NEPO sign on.

[John A.]

The rig is the Comins GCS-1 right into a Roland JC55.

So, I needed to control my volume with the pot on the guitar (using neck pickup only).

When I rolled off the volume, I got hum. Touching the strings improved it, but didn't eliminate it.

No hum if the knob was rotated all the way on, or off.

I never noticed this before, but I almost always play with a volume pedal -- and the guitar knob up full. Until yesterday I never played this venue without the volume pedal.

Then, at home: Same issue with a different amp and different cable. But, at home, touching the strings eliminated the hum completely. So less EMI at home?

So, a couple of questions (and thanks in advance for any help).

1. Can someone explain the physics? Why is it humming only with the volume control rolled partway down?

With the pot all the way off, there's no hum (for the obvious reason); with it all the way on the innards of the pot are bypassed; with it on partly, the resistor in the pot is in the signal path. My guess is that there's something broken inside the pot that is messing with the guitar's grounding (that's as scientific as I can get, alas). I don't know why touching the strings only partly eliminates the hum in the club but completely eliminates it at home, but I suspect my "something's broken" analysis is applicable here as well , along with there being more sources of EMI at the club.

2. What is the fix?

Swap in another pot. If that fixes it, it's fixed. If not, check the rest of the harness for bad solder joints [Note: I don't do this myself; I pay others to do, but I have watched a tech go through the process of troubleshooting the electronics in two of my guitars].

3. If it's shielding -- did Bill Comins really produce a guitar that isn't properly shielded? If not that, I'm back to trying to understand the physics.

I doubt it's due to lack of shielding. In general, hollow guitars are poorly shielded because it's usually not possible to fully enclose the circuitry the way you can with a solid body, but they're usually not dramatically more buzzy than guitars with better shielding.

[KirkP]

I suspect it’s a problem with the ground wiring in the guitar. Set an ohmmeter to it’s lowest range (or use the audible continuity test function if it has one), plug in an instrument cable, touch one probe to the sleeve of the cable jack, then check each piece of metal that should be grounded. This would at minimum include the shield side of the guitar’s jack, the strings, the back of all pots, and the ground connection of the pickup switch (if applicable).

The bridge and string ground became disconnected on a couple of my guitars, and correcting that easily fixed it.

Since your EMI is only present when the guitar’s volume pot is mid-range, that suggests that the EMI is being picked up by the length of wire between your guitar’s jack and the wiper of the volume pot. I do think your body helps couple the EMI to that section of wire, and if you ground your body that reduces the coupling.

Other possibilities:
A bad cable shield (try another)
A poor or missing ground in the electrical outlet powering the amp
Other electronics connected to your amp, creating a ground loop

[aquin43]

The rig is the Comins GCS-1 right into a Roland JC55.

So, I needed to control my volume with the pot on the guitar (using neck pickup only).

When I rolled off the volume, I got hum. Touching the strings improved it, but didn't eliminate it.

No hum if the knob was rotated all the way on, or off.

I never noticed this before, but I almost always play with a volume pedal -- and the guitar knob up full. Until yesterday I never played this venue without the volume pedal.

Then, at home: Same issue with a different amp and different cable. But, at home, touching the strings eliminated the hum completely. So less EMI at home?

So, a couple of questions (and thanks in advance for any help).

1. Can someone explain the physics? Why is it humming only with the volume control rolled partway down?

I have read that the problem is that your body picks up EMI and the pickup, nearby, can sense it. Then, when you touch the strings, you're grounding your body and thereby shunting all the EMI to ground. So, according to this, the guitar is grounding you, not the other way around. Is that correct?

2. What is the fix?

3. If it's shielding -- did Bill Comins really produce a guitar that isn't properly shielded? If not that, I'm back to trying to understand the physics.

You are correct that the main source of hum is the voltage carried by the player. This tends to be quite large and a small fraction of it is coupled by capacitance into the parts of the guitar circuitry that are not shielded. One of the worst unshielded parts of most guitars is the output jack. This is usually a Switchcraft style which has a large metal tongue which contacts the jack tip, runs the length of the jack and is completely uncovered. Another offender can be the pickup switch. Some types are a telephone type blade switch with large exposed flat metal spring contacts.

I have found that most times the jack can be taken out through the f-hole without unsoldering the wires. I wrap it in insulating tape and then in copper foil that is connected to the shield at the jack. This has always considerably improved the hum performance.

The reason that the hum is worse with the volume turned down is as follows. The hum comes from a large voltage through a small capacitance so it is almost like a hum current that will produce a hum voltage that will be proportional to the resistance that it passes through on its way to ground. The resistance at the jack will be low when the volume is up full - it will just be the 7 to 8k of the pickup. When the volume is turned down, however, the much larger resistance of the pot comes into play. The worst case is at -6dB when you have half the 500k pot resistance to ground via the pickup and half directly to ground. When you do the sums this comes to 125k between the socket and ground - over 15 times larger, so roughly 15 times more hum but with half the signal.