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Interference in AC power signal ruining my amps


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After three months+ of frustration I turn to HC for help. All guitar amplifiers in my home are now ravaged by an intermittent buzz that is making them useless for recording and unenjoyable for practice. Here is a sample

 

I have eliminated problems with;

Guitars

Leads

IEC Power cables

Amplifiers (I have three)

Speaker cabinets (I have two)

My home's wiring (I ran an extension lead from neighbour's home, no change)

 

The power authority placed a monitoring device on my home's distribution board for a week and said they found nothing out of the ordinary in the signal. My amplifiers all work perfectly at other locations. The problem has to be a signal embedded in the AC power signal, I think.

 

It is intermittent (apparent 90% of the time, no pattern to time of day). It is not due to off peak hot water system. I don't live near high voltage power lines.

 

This all started out of the blue about 3 months ago. Before that all equipment worked fine at home for years. I note my home stereo is not affected, nor is the TV.

 

So as the power authority cannot find anything wrong, I think my only solution is to try and filter the power signal. How can I do this cost effectively?

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First of all, where are you located.

 

Second, have you tried the amps without anything connected to the input jack and with reverb return turned down (if there is a reverb return or master control)?

 

Let's not assume it's a power problem yet. Your troubleshooting does not appear complete from what you wrote abovfe, and it does not sound like a power problem from your clip.

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I am located near Newcastle NSW Australia (240V 50Hz). When you try the amps without an instrument plugged in you can still hear the errant signal. With the master volume at zero you can't hear anything (with no instrument cable plugged in) and as you increase MV the buzz increases. It is not as loud as when you have a guitar connected, but it is there in the same form.

 

It happens on all of my amps and cabs regardless of how I mix and match them. Even my solid state JC60 combo does it and it has never made this sound in 30+ years of ownership. My Mesa DR does it now too and it never did in the past.

 

I initially thought it was a problem with the used 1980 JMP100 I bought about 4 months ago. It was my new toy and I used it exclusively for a while, then the buzz started. I thought it had to be the amp as I had never heard this horrible buzz. I took this amp back to various repairers a few times and they never found anything wrong. They all thought I was mad I think. Then one day out of curiosity I got my Mesa DR and my JC60 (amps that have worked fine at home for years) back out of the storeroom and found that it wasn't the Marshall after all. Every amp made exactly the same noise.

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I was thinking along the same lines as Don actually. To short the input, you need a plug with the tip connected to the sleeve (with a short jumper wire). Generally, most amps short the input with a switch inside the jack, but to rule this out I recommend a shorting plug.

 

When you say the noise gets worse with a guitar plugged in, that is independant of the power system and points even more strongly towards outside interference entering through the wiring and circuitry rather than the power cable. Does the noise change if you move the amp (with input unplugged) around? Does the noise change when you move the instrument around (pluuged into the amp)?

 

Have you double-checked your earth connection, specifically from your neutral to earth connection and bonding? (I am assuming that you have similar earthing rules as most other countries, just that you operate 240/480V or ~240/400V (3 phase). The buzz sounds higher than 50Hz, but not full wave rectified 100Hz so it really points to some form of interference (though we haven't ruled out a power problem, it's just getting less likely).

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Thanks agedhorse. I'll try the experiments with moving the gear around and I will try and build a shorting jack (I have a soldering iron). I had previously set the rigs up in another room and the buzz was there. The JC60 is in another room and the buzz is mirrored in that amp.

 

Note that I did run a power lead from my next door neighbour's house to bypass my home's electrical system entirely. This had zero effect. Not sure if that is helpful.

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I switched all light circuits off at distribution board and the buzz was still there. Then a couple of hours later the buzz was gone. I can't think of anything that changed in the house in terms of appliance use. I've tested turning if fridge, TV etc etc all with no effect.

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Done some research on shorting jack and haven't got specific instructions on how to build and use it yet. Is it simply a matter of soldering one wire inside a jack then plugging it into the amp? I plugged my LP in and waved it around in the air. The buzz intensity varied slightly. I did the same with a Strat and the effect was a little more noticeable. I wheeled the amp into another room and it had no effect on the buzz.

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Shot in the dark: We discovered a ground loop hum caused by connecting our cable TV to our music stuff. (TV audio out into the mixer, etc. for play-along practice). The cable TV feed is earthed independently... Anyway, a CATV ground loop isolator on the coax between feed and TV sorted it...

 

-D44

 

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Shot in the dark: We discovered a ground loop hum caused by connecting our cable TV to our music stuff. (TV audio out into the mixer, etc. for play-along practice). The cable TV feed is earthed independently... Anyway, a CATV ground loop isolator on the coax between feed and TV sorted it...

 

-D44

 

With a separate ground rod? All grounding electrodes (ground rods) within a system should be bonded. Meaning there should be a solid large-gauge conductor from one rod to the other, preferably with few or no splices. A #6 copper conductor would be the minimum gauge to do this.

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If the noise is gone with your mains off, next thing is to shut off all branch breakers, then turn on the main, and then one branch breaker at a time until the noise reappears. It may take a bunch more experimenting with combinations of breakers to get the complete picture. In other words, if you turn on 5 branch breakers and the noise appears when you turn on the 6th, shut off all of the other 5 breakers and see if the noise remains.

 

Be sure to allow time for any bootable devices in the house to boot up before you proceed from one breaker to the next.

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With a separate ground rod? All grounding electrodes (ground rods) within a system should be bonded. Meaning there should be a solid large-gauge conductor from one rod to the other, preferably with few or no splices. A #6 copper conductor would be the minimum gauge to do this.

 

 

Hmmm... I'll check further, if it ever stops raining around here....

 

-D44

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With a separate ground rod? All grounding electrodes (ground rods) within a system should be bonded. Meaning there should be a solid large-gauge conductor from one rod to the other, preferably with few or no splices. A #6 copper conductor would be the minimum gauge to do this.

 

 

 

Hmmm.... OK, I checked, and the CATV ground does lead to the mains ground. (Although it looks a little sketchy. Could be #6, though.) Anyway, I think that means I can't explain why the CATV isolator eliminated the hum we experienced we first connected the TV audio out to the mixer.

 

Unless by odd coincidence, doesn't seem remotely related to OP's issue, in any case...

 

-D44

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Your solution was "lifting" the ground at the CATV cable (the isolators are capacitors in both the center lead and the shield). There was a ground loop caused by the cable system having one path to ground, and the PA having another.

Check that "could be #6". It needs to be a low impedance path to ground, otherwise transients from nearby lightning strikes will find a better path...hopefully not your gear.

 

I'd also recommend you measure the voltage difference between the CATV ground and the house power ground. A few volts is okay, and would be expected (that's your hum), but anything over say 4 volts indicates a problem that could be a safety issue.

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Thanks, Craig, I appreciate the clarification... and the tip.

 

I had been pretty sure I was lifting the CATV ground...(or at least trying to)... but after your first note, wasn't sure you meant the same thing. And after checking outside, the only "ground" on the CATV feed (at least right here at the house) is that "could be #6 wire that connects to something at the electrical service box. (And not directly to a grounding rod.) In any case, I have a multi-meter, and I'm not afraid to use it! :)

 

-D44

 

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You could always get a AC isolation transformer.

 

Well, my problem's gone... which may or may not mean "fixed"... even if I did maybe just luck out after some serious trial-and-error. without really knowing what I was doing or why. (Even a blind squirrel...)

 

What would an AC isolation transformer have done, and how would it have done it?

 

-D44

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