Help needed to understand a hum

[rpjazzguitar]

I just want to make sure I understand this.

I usually play Comins GCS-1 using neck HB pickup > Boss ME70 > JBL Eon One Compact (set to Line input on Ch1, which is not the HiZ input).

The output impedance of the ME70 is listed at 2k ohms, -10dbu.

This works fine.

But, if, by accident, I have the JBL set to Mic instead of Line (with the volume of the speaker adjusted so that the guitar is about as loud as usual), the guitar will hum noticeably Turning the volume knob on the guitar down makes it worse.

I can stop the hum by moving the pickup selector switch to the middle position, which means both humbucking pickups are live.

To make it a little more concise, I can fix the hum either by 1) using both pickups on the guitar or 2) switching the JBL from mic to line.

I'm trying to understand the physics of what is going on.

I understand that the human body can act as an antenna and the pickups can sense that. I can imagine that having two HB on might increase the hum cancellation. But, I don't see how that explains why changing from mic to line also works.

Thoughts and info would be appreciated.

[Rodney Gene]

I just want to make sure I understand this.

I usually play Comins GCS-1 using neck HB pickup > Boss ME70 > JBL Eon One Compact (set to Line input on Ch1, which is not the HiZ input).

The output impedance of the ME70 is listed at 2k ohms, -10dbu.

This works fine.

But, if, by accident, I have the JBL set to Mic instead of Line (with the volume of the speaker adjusted so that the guitar is about as loud as usual), the guitar will hum noticeably Turning the volume knob on the guitar down makes it worse.

I can stop the hum by moving the pickup selector switch to the middle position, which means both humbucking pickups are live.

To make it a little more concise, I can fix the hum either by 1) using both pickups on the guitar or 2) switching the JBL from mic to line.

I'm trying to understand the physics of what is going on.

I understand that the human body can act as an antenna and the pickups can sense that. I can imagine that having two HB on might increase the hum cancellation. But, I don't see how that explains why changing from mic to line also works.

Thoughts and info would be appreciated.

Single coil mode and gain staging.

1. (single coil mode) If the hum goes away with the pickup selector in the middle position, that is because the 2 pickups are now hum-cancelling. The Comins has a tone knob that, when pulled, puts both pickups in single coil mode. Have you checked? Speaking of 'physics' that is a logical and common situation. Any hum or (increased hum) will cancel as the two remaining coils are RW/RP when in single coil mode. TWO humbuckers in parallel will only cut the resistance down to single coil output (1/4" of the two together). Just not that much different than a single humbucker in terms of noise. In other words, that is not the issue.

2. (gain staging) So, if you are already in single coil mode...but the hum is significantly reduced when using the line level, that is likely gain staging noise. Line level is 40+ dB louder than mic level, which means you do not have to turn the JBL gain level up like you do when in MIC input. It's likely the EON gain (is not a great premap) is amplifying the noise floor creating the unpleasant hum. Again very common issue.

Let me know if that solves it for you.

[rpjazzguitar]

Single coil mode and gain staging.

1. (single coil mode) If the hum goes away with the pickup selector in the middle position, that is because the 2 pickups are now hum-cancelling. The Comins has a tone knob that, when pulled, puts both pickups in single coil mode. Have you checked? Speaking of 'physics' that is a logical and common situation. Any hum or (increased hum) will cancel as the two remaining coils are RW/RP when in single coil mode. TWO humbuckers in parallel will only cut the resistance down to single coil output (1/4" of the two together). Just not that much different than a single humbucker in terms of noise. In other words, that is not the issue.

2. (gain staging) So, if you are already in single coil mode...but the hum is significantly reduced when using the line level, that is likely gain staging noise. Line level is 40+ dB louder than mic level, which means you do not have to turn the JBL gain level up like you do when in MIC input. It's likely the EON gain (is not a great premap) is amplifying the noise floor creating the unpleasant hum. Again very common issue.



Let me know if that solves it for you.

Thank you for the reply. The switched pot was in the HB position and I double checked that the switching was working correctly. Pulled out, the guitar got quieter and sounded like a SC guitar.

Apparently, you're right that the mic preamp is involved. My only comment about the quality of the preamp is that it sounds fine to me when I plug a mic in.

So, if I understand your point, the mic preamp is amplifying noise which is already in the cable between the ME70 and the JBL. If the noise was being created within the mic preamp, why would changing the pu selector switch matter?

Anyway, I'm not clear on how the noise got there in the first place or why the pu selector switch makes a difference.

Also, it might be useful to note that the noise is there with either pickup, but not both together.

[Rodney Gene]

Thank you for the reply. The switched pot was in the HB position and I double checked that the switching was working correctly. Pulled out, the guitar got quieter and sounded like a SC guitar.

Apparently, you're right that the mic preamp is involved. My only comment about the quality of the preamp is that it sounds fine to me when I plug a mic in.

So, if I understand your point, the mic preamp is amplifying noise which is already in the cable between the ME70 and the JBL. If the noise was being created within the mic preamp, why would changing the pu selector switch matter?

Anyway, I'm not clear on how the noise got there in the first place or why the pu selector switch makes a difference.

Also, it might be useful to note that the noise is there with either pickup, but not both together.

The only comment I can offer on the middle position reduced noise (if both humbuckers are indeed working) is that the reduced output of the pickups running parallel is having a greater effect than I would expect. But from an electrical point of view, there isn't much it can be, so I would consider it.

As to the other question, yep. The mic preamp is massively amplifying the guitars 'signal' / pickup/electromagnetic noise. It will always be the case. The MIC input wants a low-impedance balanced signal such as your microphones (XLR cables cancel noise). The guitar is a high-impedance unbalanced signal and a mismatch.

[rpjazzguitar]

I may have figured it out, or maybe complicated it further.

I had the problem last night in a big band rehearsal.

That room has a number of those lights that are recessed into the ceiling. Not sure what they're called. I don't know what else might be generating electrical fields.

The problem remained even when I ran the JBL on battery. I ran the ME70 on a 9v wallwart.

But, just now, at home, I couldn't reproduce the problem with the exact same gear. The guitar, btw, was recently shielded as completely as the luthier could do it.

So, is it possible that the electrical fields in the rehearsal room were so substantial that they overwhelmed the HB pu's (not sure if that's a sensible thought given how hum cancellation works) and only running them in parallel was able to cancel the hum? Does that make any sense to somebody that actually understands EMF or whatever initials apply? Or is some unshielded part of the wiring creating this issue?

[Rodney Gene]

I may have figured it out, or maybe complicated it further.

I had the problem last night in a big band rehearsal.

That room has a number of those lights that are recessed into the ceiling. Not sure what they're called. I don't know what else might be generating electrical fields.

The problem remained even when I ran the JBL on battery. I ran the ME70 on a 9v wallwart.

But, just now, at home, I couldn't reproduce the problem with the exact same gear. The guitar, btw, was recently shielded as completely as the luthier could do it.

So, is it possible that the electrical fields in the rehearsal room were so substantial that they overwhelmed the HB pu's (not sure if that's a sensible thought given how hum cancellation works) and only running them in parallel was able to cancel the hum? Does that make any sense to somebody that actually understands EMF or whatever initials apply? Or is some unshielded part of the wiring creating this issue?

Possibly dirty power? Of course.
Fluorescent lights? Of course.

Those are both relevant factors, though it doesn't change the fundamentals, but it certain can affect them.
Here in Texas, when you gig at an old dance hall for instance, out in the country or older parts of Texas, you have no idea how your gear is going to respond. It's a genuine thing here.
Glad to hear it's getting somewhere.

[nevershouldhavesoldit]

Fluorescent lights? Of course.

LEDs can be much noisier than fluorescents. LEDs rapidly switch on and off, which generates transient burst and spikes in the line. The good ones have optimized drivers for minimal emission of spurious energy, and they cycle far faster than the eye can detect. Even so, that switching circuit can generate significant electrical noise. Many LEDs (especially the inexpensive and/or poorly made ones) emit a lot of EMI (electromagnetic interference). If those ceiling lights are LEDs, they’re a big potential noise source. Dimmers are also electrically noisy, and using the wrong dimmers on LEDs (or using a dimmer on LEDs that are not “dimmable”) adds to the noise.

The OP’s noise is probably being picked up by the guitar’s pickup(s). One reason it’s louder when driving the mic preamp than the line input is that (as has been pointed out by others) the mic preamp has far more input sensitivity. Another is that the impedance mismatch between the high impedance guitar signal and the low impedance mic input causes a lot of distortion and nonlinear frequency response that rolls off highs. So hum becomes relatively louder in the signal that reaches the preamp.

The fact that it’s reduced when both pickups are on is a bit odd. Curiously, no mention is made of the noise when only the bridge pickup is on. I’m wondering if the guitar’s wiring is correct. Coil splits can be tricky, especially with pickups that have 5 separate wires. If there are taps on both pickups, there are several wiring combinations and permutations that will produce an output but cause various noise and level anomalies.

[Rodney Gene]

LEDs can be much noisier than fluorescents. LEDs rapidly switch on and off, which generates transient burst and spikes in the line. The good ones have optimized drivers for minimal emission of spurious energy, and they cycle far faster than the eye can detect. Even so, that switching circuit can generate significant electrical noise. Many LEDs (especially the inexpensive and/or poorly made ones) emit a lot of EMI (electromagnetic interference). If those ceiling lights are LEDs, they’re a big potential noise source. Dimmers are also electrically noisy, and using the wrong dimmers on LEDs (or using a dimmer on LEDs that are not “dimmable”) adds to the noise.

True, and It's nealy impossible to find new lights that aren't LED, particularly recessed lights. Good culprit.

The OP’s noise is probably being picked up by the guitar’s pickup(s). One reason it’s louder when driving the mic preamp than the line input is that (as has been pointed out by others) the mic preamp has far more input sensitivity. Another is that the impedance mismatch between the high impedance guitar signal and the low impedance mic input causes a lot of distortion and nonlinear frequency response that rolls off highs. So hum becomes relatively louder in the signal that reaches the preamp.

The fact that it’s reduced when both pickups are on is a bit odd. Curiously, no mention is made of the noise when only the bridge pickup is on. I’m wondering if the guitar’s wiring is correct. Coil splits can be tricky, especially with pickups that have 5 separate wires. If there are taps on both pickups, there are several wiring combinations and permutations that will produce an output but cause various noise and level anomalies.

This covers everything I mentioned, though these pickups don't have coil taps, just coil splitting. And, yep agreed, there is not enough info or detail to truly manage the issue from a distance, but it really isn't complicated. A simple method is to start with just the guitar and cable and work your way forward toward the noise.

[rpjazzguitar]

LEDs can be much noisier than fluorescents. LEDs rapidly switch on and off, which generates transient burst and spikes in the line. The good ones have optimized drivers for minimal emission of spurious energy, and they cycle far faster than the eye can detect. Even so, that switching circuit can generate significant electrical noise. Many LEDs (especially the inexpensive and/or poorly made ones) emit a lot of EMI (electromagnetic interference). If those ceiling lights are LEDs, they’re a big potential noise source. Dimmers are also electrically noisy, and using the wrong dimmers on LEDs (or using a dimmer on LEDs that are not “dimmable”) adds to the noise.

The OP’s noise is probably being picked up by the guitar’s pickup(s). One reason it’s louder when driving the mic preamp than the line input is that (as has been pointed out by others) the mic preamp has far more input sensitivity. Another is that the impedance mismatch between the high impedance guitar signal and the low impedance mic input causes a lot of distortion and nonlinear frequency response that rolls off highs. So hum becomes relatively louder in the signal that reaches the preamp.

The fact that it’s reduced when both pickups are on is a bit odd. Curiously, no mention is made of the noise when only the bridge pickup is on. I’m wondering if the guitar’s wiring is correct. Coil splits can be tricky, especially with pickups that have 5 separate wires. If there are taps on both pickups, there are several wiring combinations and permutations that will produce an output but cause various noise and level anomalies.

At the site with the ceiling-inset lights, the noise was there with either pickup, but not both. There are taps on both pickups, so the switched pot splits both of them. It seems to work as advertised.

I was feeding the JBL from the ME70 which has an output impedance of 2k ohms. As I understand it, this is a middling level of impedance (not 600 and not 10k). Might that be responsible for the noise? Next time I'm there, I'll try going into the hi Z input (Channel 3 with and without the ME70 in the chain) and see if that changes it.

I understand that the Mic input has greater input sensitivity. But does it change the ratio of signal to noise? If the signal coming in has a certain ratio of hum to guitar, wouldn't that ratio be preserved by the mic preamp? But, OTOH, if there was that much noise coming in, of a certain ratio, the line input should reflect it as well. What might I be missing? And, thanks for the assistance thinking this through.

[nevershouldhavesoldit]

At the site with the ceiling-inset lights, the noise was there with either pickup, but not both. There are taps on both pickups, so the switched pot splits both of them. It seems to work as advertised.

I was feeding the JBL from the ME70 which has an output impedance of 2k ohms. As I understand it, this is a middling level of impedance (not 600 and not 10k). Might that be responsible for the noise? Next time I'm there, I'll try going into the hi Z input (Channel 3 with and without the ME70 in the chain) and see if that changes it.

I understand that the Mic input has greater input sensitivity. But does it change the ratio of signal to noise? If the signal coming in has a certain ratio of hum to guitar, wouldn't that ratio be preserved by the mic preamp? But, OTOH, if there was that much noise coming in, of a certain ratio, the line input should reflect it as well. What might I be missing? And, thanks for the assistance thinking this through.

I guess I shouldn't have assumed that you know this and pointed it out earlier. If you have the phantom power switched on and you run an unbalanced line into that mic input, it can cause some nasty noises. Make sure the phantom power is switched off on any inputs for which it's not needed. That 48V power can actually damage mics and other signal sourcs that don't use it.

You're saying that the same switch grounds the center tap on both pickups? I hope this is done with a double pole switch, rather than connecting the two tap leads together and grounding them through a single pole switch. Connecting both taps together would provide an additional path to ground for the signal from each pickup through the other one. This would add one of the coils in each to the other's load when the wires are not grounded through the switch, making it a single coil in parallel. That would be another way for EMI to get into the signal coming out of the guitar. If your humbuckers are wired and installed with reverse winding and reverse polarity, they will be hum canceling when used together. This would reduce the noise picked up by the parasitic coil, which would explain why the hum gets quieter when you use both pickups. If your guitar is still factory wired, it's unikely to be wired incorrectly.

The impedance mismatch wouldn't cause the noise. It causes a dropoff of high frequencies in the signal being passed through, which would make the low frequency hum louder relative to the guitar signal. Also, remember that the mic preamp input is designed for a balanced low impedance line. Connecting an unbalanced source to a balanced input can cause noise and/or allow more noise in the signal to enter the preamp. True balanced inputs reject common mode noise because the same noise appears in both input leads and is canceled. An unbalanced input lead has only one hot wire, so the noise is only reaching one side of the input stage - there's no signal 180 degrees out of phase to cancel it.

From what you describe, using the hi z unbalanced input should reduce the noise quite a bit.

[Rodney Gene]

There are taps on both pickups, so the switched pot splits both of them....

Just for a reference here - there are no coil taps on a humbucker. Coil taps are only available on single coil pickups. In this case, one coil (of two coils) is being shorted to ground. Coil splitting. While a common mistake in guitar semantics, it is a useful and relevant distinction.

[rpjazzguitar]

Thanks. Coil split, not coil tap. I didn't think about Phantom Power, but it's off. I don't think it was on when I had the problem.

I'll be there again on Monday. I'll try to reproduce the problem and then try different things. First one will be turning off the lights. Next, going into the hi Z input. Then taking the ME70 out of the chain. Moving the guitar around. Unplugging everything. And whatever else I can think of.

I appreciate the help. Thank you.

[rpjazzguitar]

Thanks to all who replied.

Turning off the circular ceiling-inset lights (LED?) stopped the hum.

Running two HB pickups simultaneously seemed to reject hum better than either HB pickup alone. This was the Comins GCS-1. It has a coil split option, but it was not engaged. Hum was there even without the mic preamp, but maybe not quite as much -- I'm not sure about that. I ended up going into the hi Z input. Had to turn up the volume on the speaker pretty high and take some bass out with the EQ. I ended up with pretty good tone and no concerning hum.

[339 in june]

[QUOTE=rpjazzguitar;1446424].

. ...........The guitar, btw, was recently shielded as completely as the luthier could do it.

........../QUOTE]

Sorry to derail your thread, but can you elaborate about that
How has been shielded your Comins GCS1 ?
Thanks

[nevershouldhavesoldit]

Turning off the circular ceiling-inset lights (LED?) stopped the hum.

Yep! LEDs (especially cheaply made ones) can be very, very noisy. Throw in a dimmer, and you've got a recipe for sonic disaster. I'm glad you got to the bottom of it.

[Rickco]

Yep! LEDs (especially cheaply made ones) can be very, very noisy. Throw in a dimmer, and you've got a recipe for sonic disaster. I'm glad you got to the bottom of it.

"Turning off the circular ceiling-inset lights (LED?) stopped the hum."
So the hum is coming from the outlet caused by the "power" to the light or the hum is coming from the fixture and being picked up by the pickup? or...I thought hum was 60hz ground noise coming from power transformers? My house has all LED ceiling lights as described by OP can this be the cause of humming amps (hi fi etc) throughout the house?

[nevershouldhavesoldit]

"Turning off the circular ceiling-inset lights (LED?) stopped the hum."
So the hum is coming from the outlet caused by the "power" to the light or the hum is coming from the fixture and being picked up by the pickup? or...I thought hum was 60hz ground noise coming from power transformers? My house has all LED ceiling lights as described by OP can this be the cause of humming amps (hi fi etc) throughout the house?

LEDs cause electrical noise in at least 2 ways. The light itself is powered by a high frequency switched power supply (which is inside the bulb housing) - LEDs flicker, but almost all do so more rapidly than we can detect with our eyes. The power supply is switching on and off very rapidly, which also causes AC ripple in the power line. It also radiates electromagnetic noise directly. So guitars and amplifiers can pick up the EMI directly through their pickups and wiring, and there's added noise in the power line itself. The power line issue is usually only significant if the amp is plugged into the same branch line to which the lights are connected. But the EMI from LEDs radiates through the air.

The actual induced hum and noise varies in frequency depending on the design of the LED, e.g. what kind of ballast it uses. You'd think the noise would be inaudible because the switching power supply is running at 20 kHz or above (some as high as 1 mHz). But many of these defintiely cause buzzing or humming in the audible range. It's not as low as the 60 Hz hum of a power supply cap failure, but it's definitely audible. I don't kow what accounts for the frequency modulation.

Dimmers can cause hum at low or high frequencies. Dimmers switch on and off during each AC cycle, limiting the portion of the wave that reaches the bulb. Good ones properly designed for LEDs also have circuitry to keep the light on at a lower level during the cyclic power outage, so they don't flicker or strobe. Incandescent lights are less prone to strobing because the filament keeps glowing enough during the cycle cuts to prevent it. But if you use a dimmer designed for incandescent lights with LEDs (which reach full light output almost instantaneously during each cycle), they will either not work or will flicker badly and emit lots of EMI.

[Mick-7]

The "power conditioners" that are being sold, do they not lessen RFI/EMI noise as they claim? - "AC noise filtering reduces RFI/EMI"

Furman Power Conditioner and Surge Protector - Amazon.com

[rpjazzguitar]

[QUOTE=339 in june;1447339]

.

. ...........The guitar, btw, was recently shielded as completely as the luthier could do it.

........../QUOTE]

Sorry to derail your thread, but can you elaborate about that
How has been shielded your Comins GCS1 ?
Thanks

He had the wiring harness out to replace the push pull pot. The guitar was humming already and the pot needed to be changed anyway. So, I asked him to shield whatever he could.

He mentioned that there were unnecessarily long runs of unshielded wire. He either shortened them or replaced them with shielded wire. He mentioned that the shielding operation on a semi has to be different than a guitar like a LP with a small, paintable cavity with most of the electronics. I don't know what else he did. This was done by John Jordan in Concord Ca who is a very capable luthier with decades of experience.

All that said, I don't think the extra shielding made much difference.

[rpjazzguitar]

The "power conditioners" that are being sold, do they not lessen RFI/EMI noise as they claim? - "AC noise filtering reduces RFI/EMI"

Furman Power Conditioner and Surge Protector - Amazon.com

Short answer is, Idk. But, I might add that I was using the JBL Eon One Compact as my amp -- so the first thing I tried was unplugging it from power and running it on battery. Made no difference.