Speakers Humming

[c0astman]

We have a similar variable buzz in our venue. Dimming is through a number of old Mini-2 dimmers which were on the same power phase and the sound rack. One of the dimmers created most of the interference. We found that moving the sound power supply to a different phase resolved our problem.

[JCC1996]

It was a TRS - Jack cable. I managed to fix the hum by playing around with the gain structure and turning the foldbacks off during unamplified sections such as acted pieces with our mics. Thanks for the help :)

[Brian]

We found that moving the sound power supply to a different phase resolved our problem.

 

No it didn't, it just hid it. It'll surface again 10 minutes before doors.

[paulears]

That's the snag. Brian's quite right. People have been conditioned to ask for 'sound power', as if it's something magical, but many sound panels are just hanging off the same supply as goes to the dimmers, which for many years now have routinely been powered from the building 3 phase supply - often inside one small 6 channel pack. I can produce hum on many visiting sound systems by turning on the dressing room heaters. Nobody has been able to accurately diagnose it, but some PA systems will hum noisily, other remain hum free. I am of the opinion my problem is that the heaters are dragging the neutral away from earth potential, and this is where the hum comes from - the hum coming in this way - and some systems simply less well designed or configured. If you get dimmer buzz, when they are half fade or so, and have more lights on the other phases, then turning on or off equipment on the other phases does change the hum in my venue when we get a problem system in. Our house system is hum free, yet is on the same circuit as an Act 6 pack - which sits near the amps in the orchestra pit - and causes no noise to the system at all.

[Brian]

Serious hum is usually the result of earth to neutral shorts which causes a large current flow in the earth conductor. They are also scarily difficult to track down.

[paulears]

The quite large firm who do the annual inspection gave up trying to sort it out, because the only indication of a problem was the hum - and without a poorly working PA, they couldn't test it. I wondered if calling a hire company and asking to hire a PA likely to hum would work?

[Brian]

...gave up trying to sort it out...

I can quite understand why, it's not a fun job. At least with shorts to Live you can see where it is - just look for the sparks.

[S&L]

Serious hum is usually the result of earth to neutral shorts which causes a large current flow in the earth conductor. They are also scarily difficult to track down.

 

when I work bars there are all manner of electrical problems and frankly I'm glad if I can find sufficient power sources to plug everything and it stays powered on. However, when I work in theatres, the only thing I see regularly is a hum related to operation of lighting dimmers - the last time I had this, it was an active monitor and a guitarist amp. But I want to make sure I understand this in words of one syllable that I can both understand and repeat.

IF I understand the above correctly we appear to be saying:

 

the lighting circuit carrying the dimmer discharges a current (?) to neutral, based on dimmer position. (hence in some positions may discharge more and give more buzzing)

that current to neutral then shorts to earth somewhere further along the neutral path.

this short causes current flow to earth.

some amplifiers with speakers attached are susceptible to the current passing to earth

if the ground in these sound circuits is not lifted (disconnected) it can exhibit that current to earth by buzzing/humming at the speaker

 

have I understood this correctly? Just trying to make completely sure I understand the problem.

[sam.spoons]

Serious hum is usually the result of earth to neutral shorts which causes a large current flow in the earth conductor. They are also scarily difficult to track down.

 

when I work bars there are all manner of electrical problems and frankly I'm glad if I can find sufficient power sources to plug everything and it stays powered on. However, when I work in theatres, the only thing I see regularly is a hum related to operation of lighting dimmers - the last time I had this, it was an active monitor and a guitarist amp. But I want to make sure I understand this in words of one syllable that I can both understand and repeat.

IF I understand the above correctly we appear to be saying:

 

the lighting circuit carrying the dimmer discharges a current (?) to neutral, based on dimmer position. (hence in some positions may discharge more and give more buzzing)

that current to neutral then shorts to earth somewhere further along the neutral path.

this short causes current flow to earth.

some amplifiers with speakers attached are susceptible to the current passing to earth

if the ground in these sound circuits is not lifted (disconnected) it can exhibit that current to earth by buzzing/humming at the speaker

 

have I understood this correctly? Just trying to make completely sure I understand the problem.

 

I'm not sure how relevant this is but my understanding is that in the UK Neutral is always connected to Earth (mains wiring system 'Earth' as opposed to 'planet Earth') at the distribution centre or substation. Earth is connected to some kind of 'earthing' device (a copper stake into the ground in it's simplest form) at the consumer end, leading to 'Earth' paths with differing resistances. I assume this contributes to the hum problems we experience and why they can often be solved by lifting an earth somewhere (obviously not the safety earth). Am I close?

[Brian]

...my understanding is that in the UK Neutral is always connected to Earth (mains wiring system 'Earth' as opposed to 'planet Earth') at the distribution centre or substation.

For public supplies, at the substation/transformer, Neutral is connected to planet earth.

 

Earth is connected to some kind of 'earthing' device (a copper stake into the ground in it's simplest form) at the consumer end, leading to 'Earth' paths with differing resistances.

Only in some, mostly rural, locations.

 

Have a read up on TN-C, TN-S, TN-C-S, TT and IT power systems.

[sam.spoons]

...my understanding is that in the UK Neutral is always connected to Earth (mains wiring system 'Earth' as opposed to 'planet Earth') at the distribution centre or substation.

For public supplies, at the substation/transformer, Neutral is connected to planet earth.

 

Sorry, that's what I meant.

 

Earth is connected to some kind of 'earthing' device (a copper stake into the ground in it's simplest form) at the consumer end, leading to 'Earth' paths with differing resistances.

Only in some, mostly rural, locations.

 

In my house my water supply and gas supply are bonded to 'Earth' and are still on lead or copper to the property boundary.

 

Have a read up on TN-C, TN-S, TN-C-S, TT and IT power systems.

 

Will do, cheers

[jonathanhill]

 

In my house my water supply and gas supply are bonded to 'Earth' and are still on lead or copper to the property boundary.

 

 

This is PME (Protective Multiple Earthing) as part of a TN-C-S earthing arrangement.

[S&L]

Serious hum is usually the result of earth to neutral shorts which causes a large current flow in the earth conductor. They are also scarily difficult to track down.

 

when I work bars there are all manner of electrical problems and frankly I'm glad if I can find sufficient power sources to plug everything and it stays powered on. However, when I work in theatres, the only thing I see regularly is a hum related to operation of lighting dimmers - the last time I had this, it was an active monitor and a guitarist amp. But I want to make sure I understand this in words of one syllable that I can both understand and repeat.

IF I understand the above correctly we appear to be saying:

 

the lighting circuit carrying the dimmer discharges a current (?) to neutral, based on dimmer position. (hence in some positions may discharge more and give more buzzing)

that current to neutral then shorts to earth somewhere further along the neutral path.

this short causes current flow to earth.

some amplifiers with speakers attached are susceptible to the current passing to earth

if the ground in these sound circuits is not lifted (disconnected) it can exhibit that current to earth by buzzing/humming at the speaker

 

have I understood this correctly? Just trying to make completely sure I understand the problem.

 

umm..could someone give me a yes or no. I just want to make sure I understand the theory.

[Brian]

umm..could someone give me a yes or no. I just want to make sure I understand the theory.

 

In general, with a live band, there are twoe source of hum and mains noise...

 

1) Guitars. They have a pickup which is, unless magnetically screened, very susceptible to picking up any electromagnetic signal that happens to be floating around. This could be the local taxi service, noise from cables nearby or the venue's induction loop. That last one is a favourite if you get feedback in the guitars that you can't seem to get rid of. Turn off the loop amp to check.

 

2) Unbalanced audio connections. ie ones that use the earth/screen to complete the audio circuit. Here the problem is usually that there is a difference in voltage between the source and destination kit's earth/screen. Connect your audio lead between the two and a current flows along your audio lead's screen which, because that screen is also in the signal path, adds to the wanted audio causing hum/buzz/etc.

 

See this picture produced in PaperCAD...

 

 

The dimmer has filtering in it which works by filtering noise into the earth connection. This appears as a current flowing down to the main earth. But the connections have resistance. So this noise current causes voltages to appear at various points. The noise flowing through R1 causes a noise voltage to appear at point VA. But this is also connected to the destination audio kit via it's earth wiring R2. So the noise appears at VB. But the source kit has a clean earth connection via R4. So there is now a difference in the earth voltages between the source and destination kit. They are connected by the audio cable and any voltage difference will cause a current to flow in the screen. This current/voltage superimposes itself on the audio signal. Result is hum/noise.

 

This is HUGELY simplified and the moment you have more than a couple of bits of kit involved the number of noises paths goes up dramatically.

 

The solution is balanced audio.

 

With balanced audio you can still have noise currents flowing through cable screens, which still ought to be tracked down, but because your wanted signal doesn't rely on using the screen the noise doesn't appear.

 

 

One day I will sit down and write 'Real World Entertainment Systems' because there is a scary amount of rubbish on the 'net.

[dbuckley]

Just to be sure: the problem is with the sound system, not the lighting or the electrics. A correctly configured sound system will reject hum from earth loops. (Yes, even Paul's dressing room heater problem).