Question for those doing "larger" shows

[Richard Rupert]

I have a friend who is in charge of hiring the sound/lighting company for a blues festival. The attendance for the event is somewhere between 2 and 3 thousand people. Unfortunately, I can't give you specifics regarding the PA components, other than that it's basically a "bunch of stacked home made boxes" (no line array or anything exotic), Crown amps and a fairly large analog mixing console.
The issue I am asking about is an excuse offered for the source of a persistent buzz that was in the system this year. Obviously, I can't expect you to trouble shoot the problem, but I'm wondering if the following is even possibly a viable explanation (this came from a sound company employee).
The buzz that was in the PA was 2 different troubles which I will explain in the order they were discovered and responded to.
1) early in the day there was a buzz in parts of the PA on the left side (opposite side from where my console was set up).
this buzz was related to the way the fairgrounds power is (can't be changed without major money and Power Company repairs) but by putting a isolation transformer on each signal connection I was able to end the buzz, and from that point the PA itself had no noise buzz or otherwise) ANYWHERE
2) throughout most of the day there was a loud buzz coming through all the guitar amps and harmonica amps. This buzz (and how loud it was) depended on 1 thing. That 1 thing is... were the lights on!!! The dimmers for the lights, made the amps buzz, because the power came from the same source!!!
And when the guitar amps buzz, if that guitar is in the PA to be
heard, then the buzz sounds like the PA is buzzing... (guess what? Because the guitar amp is in the PA for the playing to be
heard, the buzz is coming out of that amp, and now it's in the PA, but it's not the PA buzzing!!!) This can not be changed without PA and lights being on a different power source. Again, it's not gonna happen without major money!!! The only solution is simple. You run lights from the old fairgrounds power, and you use a Generator (the correct size) for the PA system. If you look into any major shows done there (Garth Brooks , Willie
Nelson, etc) there is absolutely no way possible they used the
fairgrounds power!!! I 100% guarantee, it couldn't have been done!!! A show with the PA required to do that quality of sound could not run on that fairgrounds electric. What is there is a total source to all the poles of that infield!!! It is 1 leg of 100 Amp 220 Volt power it's been that way for 50 years I'm sure
Every Pro Audio Company I know of, would have a power requirement of 100amp 3 Phase 220!!!

First of all, the buzz he claims to have eliminated with isolation transformers persisted through the entire event, and was exhibited on just one side (he refers to it as the "left" side in his explanation). It didn't seem to be related to amp buzz or lighting intensity. But it WAS definitely a buzz and not a hum, like what might be caused by a grounding problem.

Secondly, are you folks always afforded a different power source for your lights? And 3 phase power?
I'd appreciate any input on this; it's way out of my area of expertise and venue size for my PA rental gigs... (I'm in the maximum 800 people department:)).
Thanks!

[Paul Henry]

Dimmers, motors, neon, and signal cables wrapped around AC cables, are all famous for spewing noise into your audio path.

You can also get a really nice hum going if you place your transformer DI box on top of your amp's transformer. :D

[Mattseymour]

Any time you have lots of dimmer packs controlling halogen lamps, you have a risk of noise on the power supply and, the bigger problem, a lot of magnetic field that guitars, mics and some amps can pick up... depending on where the dimmer packs were, and the cables run, you could end up with quite a significant hum induced in the guitar pickups and the amps, which would then come through the PA.

I doubt there's an issue with supply, which is likely either adequate and clean or not. If it isn't and the voltage is sagging you'll see dimming lights and, if it's really bad, amps can shut down and all havoc can be wreaked...

You can have big problems with ground loops, which it sounds like the guy was tackling with isolation transformers

Whether you need multiple phase supplies, or completely independent supplies, depends on the load you're drawing rather than trying to keep noise out of the system. I've done plenty of things with a single three phase supply, using one phase for the lighting and one of the audio equipment. All on the same earth.

[Mattseymour]

it WAS definitely a buzz and not a hum, like what might be caused by a grounding problem.

I'm not sure I understand the distinction you're making.

[Mattseymour]

Out of interest... What did this show look like? How many lights?

90Amps at 220volts is just shy of 20KW... For a decent sized stage, that doesn't buy you many Par64 lamps before you start tripping things out. Probably just one of the reasons Randy Hyde has so many LED fixtures on his rig.

[JLepore]

Having lighting dimmers on the same power as audio is usually a disaster. Newer LED-only rigs can get away with it, but I haven't seen anything using a regular dimmer rack do it.

The higher frequency rizz is characteristic of dimmer noise. If he only had one power source, I can believe this caused most of his problems. Being able to get enough iso-transformers in the PA path to get rid of it was pretty good. The fact that he couldn't get it out of your stage amps shows that it was on the AC power line, not in his equipment. Obviously from that point, he was amplifying your noisy amps and re-introducing the buzz into the PA sonicly.

Sounds like there should have definitely been a second power source for the audio and backline.

[Brent Evans]

Single coil guitar pickups are notorious for noise injection. This can be anything from the type used on Strats to the Dean Markley soundhole pickups that are common in bluegrass. This is very plausible for amp noise.


I always specify that all lighting and PA go on separate phases at least, and separate independently grounded panels if possible. I always make it clear beforehand, though, that if I can't have what I ask for in terms of power supply, I make no guarantees as to the quality of sound or the ability of the system to perform under heavy loads.

[RandyHyde]

Out of interest... What did this show look like? How many lights?

90Amps at 220volts is just shy of 20KW... For a decent sized stage, that doesn't buy you many Par64 lamps before you start tripping things out. Probably just one of the reasons Randy Hyde has so many LED fixtures on his rig.

Probably the reason Randy Hyde has *only* LED fixtures on his rig :)

I still use incandescents once in a while, but only for illumination with no dimming capabilities.

And it just isn't the noise -- the power budget for incandescents is too great for my rig. I have 60 KVA generator and can't afford 200-300 amps for incandescent lighting (the generator basically provides 300 amps, 200 of which I dedicate to the sound system).

LEDs are a lot more expensive than incandescents plus dimmer packs, but they are far more flexible and they are much cheaper than another generator.
Cheers,
Randy Hyde

[RandyHyde]

1) early in the day there was a buzz in parts of the PA on the left side (opposite side from where my console was set up).
this buzz was related to the way the fairgrounds power is (can't be changed without major money and Power Company repairs) but by putting a isolation transformer on each signal connection I was able to end the buzz, and from that point the PA itself had no noise buzz or otherwise) ANYWHERE

Sounds like you were given dirty power (versus "clean" or noise-free power).

Dirty power can be produced by having dimmers, fluorescent lights, electric motors, relays, or other inductive loads on the same circuit. Yes, isolation transformers can reduce this noise.



2) throughout most of the day there was a loud buzz coming through all the guitar amps and harmonica amps. This buzz (and how loud it was) depended on 1 thing. That 1 thing is... were the lights on!!! The dimmers for the lights, made the amps buzz, because the power came from the same source!!!

Yes, dimmer packs will produce a buzz in the sound system. Switch to LEDs or use completely separate (clean & dirty) power sources for audio and lighting. Note that a different circuit is not a separate power source; if the lines meet at a common breaker box, the noise is passed along from one circuit to the other.

High-impedance inputs (e.g., guitars and other instruments) are especially prone to picking up dimmer noise.




Yes, as often as I can I run the lights off one circuit and the audio off a different circuit. Genis aren't cheap (my used one cost $8500, new ones cost around $30,000). Another solution is to switch to LED fixtures. I've successfully run 40 LED fixtures off the same circuit as the sound in my rig. However, those LED fixtures are not cheap; I've spent close to $10,000 on LED fixtures this summer and I've still not got any intelligent fixtures.

[quote]
[/INDENT]First of all, the buzz he claims to have eliminated with isolation transformers persisted through the entire event, and was exhibited on just one side (he refers to it as the "left" side in his explanation). It didn't seem to be related to amp buzz or lighting intensity. But it WAS definitely a buzz and not a hum, like what might be caused by a grounding problem.

Secondly, are you folks always afforded a different power source for your lights? And 3 phase power?
I'd appreciate any input on this; it's way out of my area of expertise and venue size for my PA rental gigs... (I'm in the maximum 800 people department:)).
Thanks!

Note that 3-phase won't solve the noise problem; it's still one circuit. 3-phase is great for using smaller feeder cables to carry a larger amount of current, but they won't help much with the noise.

Ultimately, LED fixtures is the way to go.
Cheers,
Randy Hyde

[Richard Rupert]

I'm not sure I understand the distinction you're making.
I think of "buzz" as a higher frequency than "hum". Sorry for not being clearer.

And thanks to all who replied to this post. It is very helpful.

[ssrsound]

Another solution is to keep running the lighting and all the house stuff on hose power, but bring in a gennie to run audio and backline. That would get you the separation you need without re-wiring the whole facility. Whether the re-wire or the gennie rental is cheaper really depends on how often they're running shows there.

[airickess]

If he had a buzz on only one side of the P.A. that tells me it isn't really dirty power but some issue with his signal path from his console to the left side of the P.A. There could have been a bad cable in that signal path. If it did sound like a higher-pitched frequency than 60Hz hum then one possibility (out of many) is that he had a broken or frayed pin 2 or pin 3 in the signal path. An isolation transformer would not help that. You stated that the noise was present throughout the event, which contradicts his statement that the issue was fixed.

It could have also been broken shielding, RF or electromagnetic interference, or, as mentioned by a previous poster, noise induced by an instrument or amplifier. Still, my guess is that there was an issue in the signal path to that side of the P.A.
Take my opinion for what it's worth on the internet.

[RandyHyde]

I think of "buzz" as a higher frequency than "hum". Sorry for not being clearer.

And thanks to all who replied to this post. It is very helpful.

Buzz is generally a high-frequency component modulated with a low-frequency signal. For example, when a dimmer turns on at 50%, you get a big spike half-way through a 60-hz waveform. That spike has a lot of HF material, but occurs every 120th of a second (1/2 of a 60Hz waveform).
Cheers,
Randy Hyde

[Soundguy]

If he had a buzz on only one side of the P.A. that tells me it isn't really dirty power but some issue with his signal path from his console to the left side of the P.A..

Makes perfect sense. No obvious reason why one side would be clean.

But getting back to the power requirement issue ... no, don't run the sound and lights on the same service at a concert. I'm not an electrician, but I reviewed a couple of riders I have handy, and this is what is asked for:

Separate electric services on separate transformers for sound and lighting.

400 amps for each service, 3 phase, five wire system with cold water pipe earth ground and separate neutral. (This was for a mid to large size lighting and sound system)

And more importantly ... an electrician on duty from load in until end of load out.

Soundguy

[Richard Rupert]

If he had a buzz on only one side of the P.A. that tells me it isn't really dirty power but some issue with his signal path from his console to the left side of the P.A. There could have been a bad cable in that signal path. If it did sound like a higher-pitched frequency than 60Hz hum then one possibility (out of many) is that he had a broken or frayed pin 2 or pin 3 in the signal path. An isolation transformer would not help that. You stated that the noise was present throughout the event, which contradicts his statement that the issue was fixed.

It could have also been broken shielding, RF or electromagnetic interference, or, as mentioned by a previous poster, noise induced by an instrument or amplifier. Still, my guess is that there was an issue in the signal path to that side of the P.A.
Take my opinion for what it's worth on the internet.
Excellent point-- if it were power supply related, why was it not on both sides? Also, if it's caused by an amplifier buzz, why wouldn't it stop between acts? They certainly didn't (or at least, shouldn't) leave the guitar mics open between acts... and I'm quite certain they did not. But the buzz persisted.
And I know I disagree with his statement, but I definitely heard the buzz through the entire concert.

Again, thanks to all who've chimed in on this subject. VERY informative!

[gdougherty]

If he had a buzz on only one side of the P.A. that tells me it isn't really dirty power but some issue with his signal path from his console to the left side of the P.A. There could have been a bad cable in that signal path. If it did sound like a higher-pitched frequency than 60Hz hum then one possibility (out of many) is that he had a broken or frayed pin 2 or pin 3 in the signal path. An isolation transformer would not help that. You stated that the noise was present throughout the event, which contradicts his statement that the issue was fixed.

It could have also been broken shielding, RF or electromagnetic interference, or, as mentioned by a previous poster, noise induced by an instrument or amplifier. Still, my guess is that there was an issue in the signal path to that side of the P.A.
Take my opinion for what it's worth on the internet.

+1, and you've described two separate issues for sure. I had a drive snake connection go bad and would intermittently get buzz on one side of my PA depending on how the xlr connection was plugged in. That was my first thought on the unfixable issue. With so much buzz going on, it can be even harder to conclusively troubleshoot some of those issues. I could also see it being an issue of a cable with some of the pins reversed or shorted. I've found those on "repaired" cables before.

The other issue is definitely power related and short of separate power, as others have noted, that can't really be fixed. Noise injected back on power, whether lighting or kitchen equipment, is not going away unless maybe you've got some expensive filtering in place.

[Jim]

As often as I can, I provide a large computer UPS to supply all the FX pedals, guitar amps and keyboards to avoid voltage sags and noise. I've had bands remark what a difference it made on stage. A lot of the small pedals with wall worts for power do not respond well to power sags. Some tube amps get flakey as well.

The UPS is cheap, easy and reliable. And, when the Circuit breaker blows, all the signal sources stay hot. I rarely plug in the bass amp but you could.
Here's a great unit. I use a lot of the small cyberpower units. My experience has been really, really good with the CyberPower but other brands can be OK.
http://www.amazon.com/CyberPower-PR1500LCDRTXL2U-Smart-Sinewave-Green/dp/B001RJEF7M/ref=sr_1_10?s=electronics&ie=UTF8&qid=1346805248&sr=1-10&keywords=ups+cyberpower

When you buy a UPS for any reason make sure it has
1. Automatic Voltage Regulation (AVR) to avoid power sags and noise.
2. Pure sine wave output. No "simulated" or "approximate" sine waves. These can kill small devices or create its own noise.

[RandyHyde]

Makes perfect sense. No obvious reason why one side would be clean.



OTOH, there is the clear indication of "the buzz occurred when the lights were on..." (I'm assuming, here, that the buzz went away when the lights were off?)

Note that buzz doesn't magically flow through the amp's power supply and then gets fed into the output section. It usually gets picked up by a transducer (typically pickups) which is then amplified through the system. Guitars' high-impedance pickups, cables, and pedals are the worst for this. It could very well be that something panned to the left side, or some circuit that was part of the left channel, was responsible for picking up and amplifying the buzz. I've certainly had my fair share of "buzz-off" sessions to find the offending piece of gear that was picking up the buzz and putting it through my system.
Cheers,
Randy Hyde

[gdougherty]

OTOH, there is the clear indication of "the buzz occurred when the lights were on..." (I'm assuming, here, that the buzz went away when the lights were off?)

Note that buzz doesn't magically flow through the amp's power supply and then gets fed into the output section. It usually gets picked up by a transducer (typically pickups) which is then amplified through the system. Guitars' high-impedance pickups, cables, and pedals are the worst for this. It could very well be that something panned to the left side, or some circuit that was part of the left channel, was responsible for picking up and amplifying the buzz. I've certainly had my fair share of "buzz-off" sessions to find the offending piece of gear that was picking up the buzz and putting it through my system.
Cheers,
Randy Hyde

Unless I read it wrong, it sounds like there were two issues. A buzz that carried with the dimmer and a buzz in only one stack.

[Butch Bos]

Went through hell one time when public tv recorded a jazz festival with two 12 ch Carvin boards and NON isolated splitters that they supplied
This went on for 2 of the 3 day festival public tv would NOT let me fix it
Turned out their audio recording was useless
since that time I send them a board mix

Butch

[Richard Rupert]

<SNIP>OTOH, there is the clear indication of "the buzz occurred when the lights were on..." (I'm assuming, here, that the buzz went away when the lights were off?)


No, the buzz was constant, and apparent only in the right set of speakers.

[RandyHyde]

No, the buzz was constant, and apparent only in the right set of speakers.

Then that probably had nothing to do with lights.
Cheers,
Randy Hyde

[JLepore]

It depends where the signal cables and amps were, where they were sourcing power, etc. Too many unknowns.

If some bozo ran the lighting feeder over the cross-stage signal cable, or decided to run DMX through a multipair to that side, all kinds of things could happen. They should have been able to trace and fix it however.

If it was pure power, there is nothing they probably could have done (with what they had to work with anyway)

[glass50]

Remember common power source for audio. Have several times plugged up my shure wireless racks to a different circuit than what my ada8k are using. Yep here comes the ground loop buzz. Laying out a 3 wire extension from ada8k plugs to wireless rack cures problem quick and easy.

[Russell Landwehr]

Yup, lighting dimmers... I found the noise getting into my stuff through unbalanced audio signals... and single coil pickups.

rl

[keithsplace]

Remember common power source for audio. Have several times plugged up my shure wireless racks to a different circuit than what my ada8k are using. Yep here comes the ground loop buzz. Laying out a 3 wire extension from ada8k plugs to wireless rack cures problem quick and easy.
Yep had that at a festival I do every 4th of july.
I run the stage and FOH for two days and another sound crew comes in for comes in for a few hours on one day for a radio show. He just puts a breakout box between my snake and SAC to break out to his recording gear. This being first year I use SAC at the feast, we had a ground loop problem when the break out was inserted.
We had to run his breakout and gear from the same cuicuit to iliminate it.
Weird part was it was only on a channel or two, probably ones with DI's but didn't think to check at the time, in the heat of trouble shooting,