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Humming from amp alone

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Humming from amp alone

PostPosted: Thu Oct 10, 2019 7:19 pm
by jeremybozzo
Hi,

I've been struggling to find a solution to the noise coming from my amp when I'm at home. The noise even happens when only my amp is plugged in directly to the outlet with nothing going into the front of the amp (only the amp with standby off). The sound is very similar to 120hz ground loop hum, however my amp is the only component of my rig in such a scenario. I've also tried unplugging all the devices in my room, unplugging all devices from nearby outlets, and turned off the lights and still the hum is happening. In this scenario (and regular use) I've also run a Furman P-1800 PFR power conditioner before my amp but the hum is still there. I've had an electrician by trade quickly inspect by eye the outlets in my room for potential issues with grounding or bad wiring (he was working on another issue at the time and had no electrical tools with him) and I've also checked the outlets myself with a Fluke non-contact voltage tester and Sperry outlet receptacle tester. In both cases the tests confirmed the wiring in the outlets is fine.

I'm running out of ideas of what to do next or where to look for help. My amp is a Fender 68 Deluxe Reverb reissue and it works fine where I rehearse.

Re: Humming from amp alone

PostPosted: Thu Oct 10, 2019 8:51 pm
by blinddrew
Have you tried it in other rooms in the house and what happens?

Re: Humming from amp alone

PostPosted: Thu Oct 10, 2019 9:03 pm
by Wonks
I'd suspect a poor ground connection, despite the circuit test.

Re: Humming from amp alone

PostPosted: Thu Oct 10, 2019 10:52 pm
by Tim Gillett
jeremybozzo wrote:Hi,

I've been struggling to find a solution to the noise coming from my amp when I'm at home.
Do you mean the amp is fine at other venues, or that you've only tested it at your home?

Re: Humming from amp alone

PostPosted: Thu Oct 10, 2019 10:57 pm
by Hugh Robjohns
Tim Gillett wrote:Do you mean the amp is fine at other venues, or that you've only tested it at your home?

jeremybozzo wrote:My amp is a Fender 68 Deluxe Reverb reissue and it works fine where I rehearse.

If it works fine elsewhere, then Wonks' theory is worth investigating properly to make sure that the mains safety earth at the home venue has a suitably low impedance.

But although you say it is fine at the rehearsal venue, the background noise levels and amp volume may well be much higher there, masking any inherent low-level hums that might be more audible at the lower working levels typical in your home space. So it might be worth just checking again whether it really is hum-free at the rehearsal venue.

The other possibility worth checking is that it's not an interference problem from something in an adjacent room (or a room above/below). Power transformers in other mains-powered equipment or low-voltage lighting, perhaps, or fluorescent lights...

H

Re: Humming from amp alone

PostPosted: Thu Oct 10, 2019 11:11 pm
by scw
Further to Hugh's reply above, light dimmer switches in the house generate astonishing levels of noise in tube amps.

Re: Humming from amp alone

PostPosted: Fri Oct 11, 2019 6:46 am
by rggillespie
I had a same/similar problem after a house re-wire and the new wiring was checked and fine. It was traced eventually to the dimmer switches we had added, there is some kind of issue with them and the amp. I had to take out the dimmer in the room with the amp and also others in other areas, even on the floor below. Moving over to normal on off switches sorted the problem out instantly, so it may be worth looking into this.

Re: Humming from amp alone

PostPosted: Fri Oct 11, 2019 6:53 am
by Tim Gillett
A 120 Hz hum is very different sounding to dimmer switching interference.

Could we have an audio sample to clarify please?

Re: Humming from amp alone

PostPosted: Fri Oct 11, 2019 9:32 am
by Hugh Robjohns
To be fair, he did say...
jeremybozzo wrote:The sound is very similar to 120hz ground loop hum...

As we have seen so often in forum questions, one person's 'hum' is often another's 'buzz' and someone else's 'white noise'... so you have to take descriptions like these with a large pinch of salt and not expect technically precise engineering definitions.

While it might be interesting to hear an example of the noise, I'm not sure it would be that informative.

What would be more useful would be to know if there are any dimmer light switches in the vicinity, and whether the noise goes away if the OP switches off the lighting ring main/spurs at the consumer unit/fuse box -- thereby removing all power to the dimmer switches or lighting units and thus positively identifying whether they are implicated as the noise source.

H

Re: Humming from amp alone

PostPosted: Fri Oct 11, 2019 10:40 am
by mick.n
Last time this happened to my amp (Marshall vs100r) it was a crugged up plug that goes into the acutronics spring reverb tank. A quick clean up & spray of Servisol into the socket stopped the problem.

Re: Humming from amp alone

PostPosted: Sun Oct 13, 2019 8:24 pm
by jeremybozzo
I have tried my amp in other rooms and other outlets at home and the noise is the same but does vary in intensity from outlet to outlet.

If it is a poor ground connection how could I go about checking it?

The amp is fine at other venues, and I've tested my amp before anyone else arrived at rehearsal. I agree my house is quieter than the place where I rehearse, but the noise is significantly worse at home. How do I check if it's an interference problem?

In my house there are no dimmer switches, but is it possible that if the neighbors upstairs have dimmer switches it could affect the circuit(s) in my room?

What is the best way to upload an audio sample for this purpose if needed?

What does this "switches off the lighting ring main/spurs at the consumer unit/fuse box" mean? I'm know very little when it comes to electrical panels.

I'll see if my amp needs to be cleaned but it works fine in other locations/venues.

Re: Humming from amp alone

PostPosted: Sun Oct 13, 2019 11:00 pm
by CS70
jeremybozzo wrote:What does this "switches off the lighting ring main/spurs at the consumer unit/fuse box" mean? I'm know very little when it comes to electrical panels.

I think Hugh was just trying to tell you to turn electricity completely off, not by using the normal light switches, but powering down entire subsections of electric circuit in your house which may create the issue.

Typically (here at least) houses have several separate circuits for different room/zones (kitchen, bathroom, main room, bedrooms) and normally the house panel allows you to activate/deactivate them separately (so that for example you can make some electrical work in the bedroom without having the fridge lose power).

Here's a good summary of possible "normal" noise causes: https://www.techhive.com/article/306359 ... stems.html

But you seem to have gone thru all of these...

You say you don't hear noise in the studio. Are you using the same cable and pedalboard as home?

Re: Humming from amp alone

PostPosted: Mon Oct 14, 2019 3:33 am
by jeremybozzo
I'll do this "turn electricity completely off, not by using the normal light switches, but powering down entire subsections of electric circuit in your house". Any tips going about doing this would be awesome.

It's the amp alone, no pedals or cables.

Re: Humming from amp alone

PostPosted: Mon Oct 14, 2019 6:17 am
by Tim Gillett
jeremybozzo wrote:...What is the best way to upload an audio sample for this purpose if needed?...

Often, people just upload a short sample to YT and share the link here. You can decide how public or limited is access. A short hum sample is hardly confidential or sensitive material anyway.

Re: Humming from amp alone

PostPosted: Mon Oct 14, 2019 6:59 am
by jeremybozzo
Okay makes sense for the sample, and yeah definitely I was unsure if there would have been a preferable way to do it.

Re: Humming from amp alone

PostPosted: Mon Oct 14, 2019 9:10 am
by Wonks
A nearby dimmer upstairs could cause enough interference to be picked up by the amp if it's an older dimmer switch, though it's probably less likely as a source of hum than a high ground impedance is.

Older dimmer switches were fairly basic and normally chopped up the sine wave at any point along the waveform, creating sharp edges to the waveform and loads of harmonics as a result, which got carried through the lighting wiring. This acted as an aerial which radiated the electrical noise.

Modern dimmers should cut the power waveform at the 'zero point crossings', where the sine wave crosses zero volts, so that the waveform continues to be sinusoidal with no extra harmonics.

However, you are far more likely to notice dimmer noise with a guitar plugged in, as the lead and the pickups acts as an aerial to pick up the noise. With no jack plugged in, the amp signal inputs should be grounded, so any noise pickup by signal cables within the amp will be after the first valve gain stage, so a lot quieter than if picked up by a guitar lead etc.

The other main forms of hum production in a house are transformers/PSUs and florescent lights.

But if all lights are off and the amp isn't near any electrical device like a PC or a fridge or another amp or next to a pedal PSU, then it's more likely that it's internal hum pickup that isn't being taken to ground efficiently.

All you can really do is get an electrician in to check the ground impedance value from your sockets back to the house ground point.

I believe there is one fairly basic socket tester available by Martindale that shows ground loop impedance in a band value, but I don't think it's particularly cheap and getting an electrician in to test the ground impedance of your sockets is probably better value for money, and you get a more accurate reading, plus someone who should be able to remedy the situation if the readings are high. If they are low, then at least you've got peace of mind over your electrical safety.