Guitar hums with volume pot at center position

[hariossa]

Hello guys

A friend of mine asked me for advice about this issue, I don't have the guitar with me so I can not check it myself. Seems like there is a noise when the volume is at center position, the guitar circuit mixes 2 magnetic pickups and a piezo/preamp. Other than grounding problems I can't think of anything else, any suggestions?

Thanks!

[soundguruman]

Hello guys

A friend of mine asked me for advice about this issue, I don't have the guitar with me so I can not check it myself. Seems like there is a noise when the volume is at center position, the guitar circuit mixes 2 magnetic pickups and a piezo/preamp. Other than grounding problems I can't think of anything else, any suggestions?

Thanks!

This is 100% normal. Any wiring or conductive object connected to the input of the guitar amp will easily induct 60 cycle fields from..
any wiring in the wall, lights, appliances, a transformer on a pole outside your house, a cable tv wire, ANYTHING.
and then the 60 cycle will be amplified. You hear hum.
Closing the circuit, shielding the circuit, using humbucking pickups, grounding the case of the controls, ALL of that reduces the potential BUT NEVER eliminates it.
In fact a lot of the 60 cycle hum is coming from the guitar, picking up the amplifier's power transformer.
You turn the control to "0" this shorts the input to ground, the hum goes away, mostly.
You turn the control to "max" this connects the input of the amp directly to the pickup...without a resistor in between, the hum is minimized...
You turn the control to half, now there is a resistor between the amp input and the pickup. The resistor is inducting 60 cycle, as above.
You hear HUM.
Now how do you minimize the resistors ability to induct 60 cycle (from outside sources), when turned halfway?
OR sometimes the insulator, inside the control, is not really insulating, from the case of the control or the shaft. So we start with a better control, and ground the case of the control.

[hariossa]

Thanks for your reply, I already told him to check if everything was properly grounded, I'll tell him to shield the whole thing then.

Cheers!

[soundguruman]

Thanks for your reply, I already told him to check if everything was properly grounded, I'll tell him to shield the whole thing then.

Cheers!

Shielding, especially with stick-on foil, does not do a heck of a lot, compared to changing the pickups to humbucking...

[hariossa]

...or installing a dummy coil ;D back in the early 90's I remember reading a Buddy Guy interview where he said that he took his Lace Sensor pickups off his Strat because he didn't like the sound and then realized that he even missed the noise. Another friend of mine told me that when he started recording in studios in the 70's he learnt how to "find Mecca" with his strat to avoid hum. If you like single coils then you have to learn to live or deal with the hum.

[David Schwab]

This is 100% normal. Any wiring or conductive object connected to the input of the guitar amp will easily induct 60 cycle fields from..
any wiring in the wall, lights, appliances, a transformer on a pole outside your house, a cable tv wire, ANYTHING.
and then the 60 cycle will be amplified. You hear hum.
Closing the circuit, shielding the circuit, using humbucking pickups, grounding the case of the controls, ALL of that reduces the potential BUT NEVER eliminates it.
In fact a lot of the 60 cycle hum is coming from the guitar, picking up the amplifier's power transformer.
You turn the control to "0" this shorts the input to ground, the hum goes away, mostly.
You turn the control to "max" this connects the input of the amp directly to the pickup...without a resistor in between, the hum is minimized...
You turn the control to half, now there is a resistor between the amp input and the pickup. The resistor is inducting 60 cycle, as above.
You hear HUM.
Now how do you minimize the resistors ability to induct 60 cycle (from outside sources), when turned halfway?
OR sometimes the insulator, inside the control, is not really insulating, from the case of the control or the shaft. So we start with a better control, and ground the case of the control.

This is not exactly correct. You are pointing out the series resistance, but leaving out the other side of the pot which is resistance to ground. A volume control is a voltage divider.

So as you are increasing the series resistance between the pickup and amp, you are decreasing the shunt to ground. Noise will follow that shunt.

I never get any noise when my volume control is half way down on any of my guitars and basses. The only time I have heard this is when there is a ground problem somewhere.

Here's a good article by Craig Anderton on impedance, pots and treble loss:

[kurtdaniel]

This is 100% normal. Any wiring or conductive object connected to the input of the guitar amp will easily induct 60 cycle fields from..
any wiring in the wall, lights, appliances, a transformer on a pole outside your house, a cable tv wire, ANYTHING.
and then the 60 cycle will be amplified. You hear hum.
Closing the circuit, shielding the circuit, using humbucking pickups, grounding the case of the controls, ALL of that reduces the potential BUT NEVER eliminates it.
In fact a lot of the 60 cycle hum is coming from the guitar, picking up the amplifier's power transformer.
You turn the control to "0" this shorts the input to ground, the hum goes away, mostly.
You turn the control to "max" this connects the input of the amp directly to the pickup...without a resistor in between, the hum is minimized...
You turn the control to half, now there is a resistor between the amp input and the pickup. The resistor is inducting 60 cycle, as above.
You hear HUM.
Now how do you minimize the resistors ability to induct 60 cycle (from outside sources), when turned halfway?
OR sometimes the insulator, inside the control, is not really insulating, from the case of the control or the shaft. So we start with a better control, and ground the case of the control.

this answers my question.. tnx.. with this prob it really is annoying to play some guitar chords on my amp..

[pdf64]

Also ensure that a different, high quality / well screened proper guitar lead is tried.
As noted, at a mid volume setting the source impedance feeding the guitar lead will increase significantly, so making the lead far more suseptible to electro magnetic interference.
Re the Craig Anderton article, it puts the problem we have these days of trying to choose from a wide range of excellent audio op amps into perspective!

'You turn the control to "max" this connects the input of the amp directly to the pickup...without a resistor in between, the hum is minimized'
I don't follow the rationale there; if the guitar pickups are the source of the hum, then reducing the guitar vol control would surely attenuate the hum?

Pete.

[J M Fahey]

Dear Pete, the confusion comes because soundguruman mixes two different problems (with different solutions) into one, and, once derailed, the discussion continued off track.

1) electromagnetic noise , which is picked up by the microphone coil.
Single coils show this in all its glory ; humbuckers put two out of phase coils in series to cancel it by balancing it out (duh !)
This kind of noise, whatever remains of it, follows the volume control setting: highest when on "10" ; less and less while you turn it down, nothing on "0".
You will have this noise even if you use a guitar buffer, simply because it's being magnetically picked up at the source, and treated the same as the legitimate guitar signal.
This noise will also vary in intensity with the guitar orientation.
Fine, but THIS IS NOT WHAT WORRIES THE O.P.
No need to add that a Piezo Pickup does not suffer from this kind of problem ... because it has no coil !!!!!!

2) electrostatic noise.
This is picked up by the wiring, most of it by the guitar cable but also by the unshielded guitar wiring and even poor wiring at the amp itself.
Most of it is blocked and shorted to ground by shielding, but guitar cable shielding is (almost) never 100% effective, far from it.
If you read a "serious" cable manufacturer data, such as Belden, in their catalog they state the "% shielding" of their wires.
Regular braided guitar and mic cable shows only around 85% !!!
Easy to check: take some braid in a dark room, put a small lamp inside and turn it on; you will see dots of light where braid wires cross, the "metal surface" is not continuous. Simple as that.
Some of their better shielded cables, specifically for Recording Studio use, *also* have a grounded metallized polyester wrapping.
These can go *up to* 98% shielding.
I once helped Recording "Estudios Ión" in Buenos Aires solve an unstoppable interference problem they had: a subway line with the thousands of amperes its uses and immense sparking run less than half a block from them.
The red line shows the subway route; the green square shows the actual studio , in the middle of the block.
I suggested they instal a 6" copper pipe from the studio to the control room, and run all sensitive wiring through it.
100% shielding, end of the problem.


BACK TO THE OP GUITAR NOISE PROBLEM:

Since guitar shielding is never 100%; it always picks *some* interference, this is unrelated to the pickup used.
The situation should be desperate, because, after all, 85% shielding means 15% of the noise (a LOT) still gets through.
Thanks God such interference is VERY high impedance and most of it gets shunted to ground through the circuit's much lower impedance.

Let's see:
Pot on "0": zero impedance, zero electrostatic noise.
Pot on "10": circuit impedance= pickup impedance = a few kiloohms DC resistance plus coil inductance.
Some noise, bearable unless very close to a motor or very dirty power line.
Pot halfway in resistive value(5 on a linear one, around 7 on a log one)= maximum resistance= 1/4 the pot value (125K on a 500 K one; 62K on a 250K one)= max resistance, maximum noise.
This is what bothers the OP.
In this case, using a buffer inside the guitar, driving the output jack, will mean negligible cable noise.

[David Schwab]

Re the Craig Anderton article, it puts the problem we have these days of trying to choose from a wide range of excellent audio op amps into perspective!

Yeah, he used a lot of 741s back in the day, as did everyone else. I had an old MXR Phase 90 filled with 741s! And of course the infamous Distortion +. At unity gain they weren't too noisy.

I think the problem here is not noise from the pickup, but a type of grounding noise you get. I have heard this on a few instruments, but never figure out the source. This was on instruments with humbuckers.

[pdf64]

Maybe this type of electrostatic noise seems worse with humbuckers because
a/ humbucker guitars tend to have 500k pots, SCs 250k, so the 'halfway' impedance is twice as high.
b/ in comparison to SCs, there's negligible PU electromagnetic hum to mask it.
Pete.

[hariossa]

Thanks everybody! finally he said that swapping the amp solved the problem...

[philbrooksjazz]

I had the same issue after installing a piezo pickup on an archtop guitar. The solution is to lower the pot value, 100k worked well for me (after trying 1 Meg, 500k, and 250k) I believe the impedance of the piezo is higher, so the higher pot value tends to 'float' the circuit and makes it noisier. The trade off is a bit lower overall gain, and possibly a slight difference in tone. I can compensate for those things, but it's very hard to eliminate 60 hz hum. Also, make sure the wiper (center) terminal of the pot goes to the output jack. The 3rd terminal of the pot should be grounded, and shorted to the pot case.

[Chuck H]

The solution is to lower the pot value, 100k worked well for me (after trying 1 Meg, 500k, and 250k) I believe the impedance of the piezo is higher, so the higher pot value tends to 'float' the circuit and makes it noisier.

I don't agree with this for standard SC and HB pickups because it destroys the signature tone. No one with a strat or an LP would like this solution. If you're strictly referring to the piezo then, as noted, there's no coil picking up noise so it's more likely a result of the on board preamp circuit. This isn't to say that the lower value pot didn't help with hum. But if the hum you had was from the piezo preamp then a better solution would be to modify that pickups preamp. Maybe adding a load to the output. This is especially the case if it's a guitar that uses both a piezo and a standard inductive type pickup. This way a normal value guitar volume pot could be used without detriment to the guitars standard pickups tone or any signature sound intended from the instrument.

JM2C

EDIT: Not to mention this is a twelve year old thread!?!