Tech: Vol level of PA going up and down

Subject: Tech: Vol level of PA going up and down
From: Herge
Date: 27 Jan 07 - 07:14 PM

Hi
Last night during before our gig we were playing background music from an ipod through our PA system - the volume level went up and down gradually of its own accord. Tonight during our gig the overall volume did the same - any ideas? Our amp is about 6 years old and giged nearly every weekend - our desk is only a few weeks old

Subject: RE: Tech: Vol level of PA going up and down
From: DADGBE
Date: 27 Jan 07 - 07:18 PM

Hi Herge,

That same thing happened to me once and it turned out that the room was fitted with big stage dimmers for the lighting system. Every time they were changed, our volume also changed.

Might that be the case with you?

Good luck,
Ray

Subject: RE: Tech: Vol level of PA going up and down
From: Herge
Date: 27 Jan 07 - 07:21 PM

Hi
This has happened in two seperate venues. We have a set of stage lights which sequence and I never thought that may the cause.

Subject: RE: Tech: Vol level of PA going up and down
From: Darowyn
Date: 27 Jan 07 - 07:24 PM

I agree, variation in the supply voltage is the likely cause.
If you complain to the Electricity supplier, they will tell you that they have a statutory duty to maintain the voltage within very tight limits, so there could not have been any variation.
I have in fact seen a readout plot of mains voltage in a building next to a Welding shop. The voltage (UK) varied from 180VAC to 260VAC. Nominal is 240.
Why does the Electrical supply company argument not work when you are stopped for speeding?
The speed limit is 30, officer, so I could not have been doing 45.
Cheers
Dave

Subject: RE: Tech: Vol level of PA going up and down
From: HiHo_Silver
Date: 27 Jan 07 - 08:48 PM

This is a long shot but if everything else fails get a can of contact cleaner and spray the volume and tone pots. If you are a lot in dusty enviroments the wipers in the slides and pots tend to get coated with residue and do not maintain the proper contact. I have run across this on rare occasions. Having said that however, my first guess, like all the rest, would be voltge surges. They are quite common and will cause the problem you describe.

Subject: RE: Tech: Vol level of PA going up and down
From: mandotim
Date: 28 Jan 07 - 02:45 AM

This happened to me. In my case it was quite simple; the tracks recorded on the MP3 player I was using had been downloaded, and some were recorded at different volume settings than others; hence the different volumes through the PA.
Tim

Subject: RE: Tech: Vol level of PA going up and down
From:
Date: 28 Jan 07 - 03:06 AM

If it is the Voltage Spike problem, one way to confirm would be with a UPS used for maintaining power in power problem areas for Computers. Pick one with a capacity roughly twice that of your PA's power rating.

Subject: RE: Tech: Vol level of PA going up and down
From: Richard Bridge
Date: 28 Jan 07 - 03:44 AM

For most PAs that is going to be one HUGE UPS!

Subject: RE: Tech: Vol level of PA going up and down
From: JohnInKansas
Date: 28 Jan 07 - 04:26 AM

The electricity supplier is only concerned that the voltage is uniform up to the front door. Once you open the door and let it in, the internal house wiring, and/or the wiring in your own "distrubution system" can introduce additional voltage drops.

All it takes is one under gage wire, or one bent, broken, abraded, twisted, pinched, or poked cable, and the voltage at the downwind end will change with every fluctuation in the current being drawn through the weak section - especially if your setup draws fairly high current, or if your rig needs more current than the house socket is meant to provide.

Usually, with good cables, it would take rather large variations in current to one load (your dimmable lights?) to affect voltage to your other components (your amps?), but cables are always suspect, since they get dragged around, coiled and twisted, stomped upon, etc. Connectors probably are the second most likely culprits, if the problem is to be found in your own kit, for the kind of variations described.

The venues themselves may have some over-large loads, with "industrial-strength" microwaves or large ice-machines being the usual suspects in my area. If you're in the UK where they use the trad Lucas Electric refrigeration units, I'm told you can discount the ice-maker 'cause they never work anyway, but someone might have patched one up. (?????) It would seem unlikely, though, that two different venues would have similar enough loads to look like "the same thing" in both places.

John

Subject: RE: Tech: Vol level of PA going up and down
From: Herge
Date: 28 Jan 07 - 04:53 AM

Hi Folks
Thanks for the thoughts.

I have ordered an new set of speaker leads as the old ones have had thier day - I'll change the other patch leads on our system while I'm at it and see if that makes any difference. We play at hotels and some seem to use thier own generators at different parts of the day.

Herge

Subject: RE: Tech: Vol level of PA going up and down
From: Richard Bridge
Date: 28 Jan 07 - 05:15 AM

Generators?????

Run a mile. Do not connect anything to them without a written indemnity!

Back in the 70s I was booked to run discos twice in relatively short succession at open air venues - runing off generators that I was ASSURED were large enough and stable.

The first one my working lights were dimming in time to the music!

The second one the damned generators turned out to be nearer to 60Hz than to 50, and christ knows what voltage, 'cos they wrecked the output transformers in two fine old valve amps (a Geloso and a Linear). Did I have a written indemnity? I did not.

However, if you are running music from an Ipod the most likely fault is different level MP3s. I think Audacity may let you normalise them.

Subject: RE: Tech: Vol level of PA going up and down
From: Herge
Date: 28 Jan 07 - 06:05 AM

Hi
I'm not sure the generators were running at the time - as for the ipos all tracks run normally in the car / through headphones - the vol increases and deceases mid track

Herge

Subject: RE: Tech: Vol level of PA going up and down
From: Richard Bridge
Date: 28 Jan 07 - 07:56 AM

Hmm.

If the Ipods are OK elsewhere (sure now? it's at high levels you'll notice changes most)...

I'm suspecting something in the input signal path. In modern equipment all the electrickery takes place at pretty low voltages and most of the supply voltages to the clever bits (the op amps in the input stages, and the Mosfets or whatever in the output stages) OUGHT to be stabilised. Either it works or it blows up.

If its a minijack out of the Ipod they are the invention of the devil.   Are you running a DI box to run the Ipod into a balanced mic input, or what?

Have you checked the lead from the iPod to the input socket?
Have you tried other things into that input socket?
Is there a sensitivity switch or gain pot that might not be reliable?
What about the fader for that channel?

Have you tried the ipod into a different input socket?

Does the same happen with your old mixer now (or did you trade it in?)

How does it behave at home?

If it's sudden, rather than gradual, and if you are using balanced line connectors anywhere, have you checked that both sides of teh balanced line are robust (don't pfaff about with meters, get a cable tester and give the cable some good tuggage while it is on test): If you lost half a balanced line the volume will drop 3db and when it makes again, up it comes again.

Does it happen with someone else's power amp? Has the power amp got input gain controls (intrinsically unreliable) or switches?

Subject: RE: Tech: Vol level of PA going up and down
From: GUEST,Graham Bradshaw
Date: 28 Jan 07 - 09:59 AM

Yes, Richard. All those things are worth checking. But my money's still on the voltage fluctuation. It wouldn't be too difficult to clip on a meter and see what is actually happening.

As some of you may know, I 'do' sound at a number of festivals, and this problem occurs from time to time. I remember one particular occasion,in a field of course!, when you could definitely hear the fluctuation. And when everybody decided to use the showers at the same time (heated by electricity of course), there was a massive voltage drop. A lot of modern digital equipment doesn't like this at all, and one band 'lost' all their keyboard settings when the voltage droped down to 183 volts. (For our American readers, it should be 230/240 volts in the UK, and was consistently hovering around the 200/210 mark at this gig).
The local electrician wouldn't believe it until we strapped a meter across the supply and produced the evidence.
It is caused by a combination of excessive load, and too long supply cabling (usually running from an adjacent building). This is why a lot of festivals now use generators, which, I don't care what anybody says, are considerably more stable and consistent.
Graham

Subject: RE: Tech: Vol level of PA going up and down
From: Richard Bridge
Date: 28 Jan 07 - 06:25 PM

Hi Graham

In one sense you confirm what I say - keyboard settings fine until the voltage goes below the threshold, then vanish....

But I guess we'd have to go over and measure the OP's gear. the $64,000 question is whether it does it at home. If not, then it HAS to be something peculiar to the location (ie voltage or local RF).

LOCAL RF - I think I just got it!!!! Is there a compressor in the signal path? If so all you need is RF interference outside the audio band and bingo, the signal ducks.



Back to your festival....

Your mixer had a stabilised power supply, yes? So it wasn't that.
Your FX (and controllers) ditto.

That leaves power amps.

What power amps were you running?

Subject: RE: Tech: Vol level of PA going up and down
From: The Fooles Troupe
Date: 29 Jan 07 - 05:08 AM

"If so all you need is RF interference outside the audio band and bingo, the signal ducks."

Very good Richard!

depending on how much 'RF blocking' the amp has, that could play merry hell...

Subject: RE: Tech: Vol level of PA going up and down
From: GUEST
Date: 29 Jan 07 - 07:28 AM

Under voltage would cause terrible distortion so it's not that. Work back from the speakers testing hadware connectors etc.

Most likely you have an impedence mis-match
at input - try DI'ing your ipod into another amplifier to see if it works ok- if it does then you will need to directly input your ipod through the (pa) power amp in order to stop any (pre amp) automatic clip kicking in.

Subject: RE: Tech: Vol level of PA going up and down
From: Richard Bridge
Date: 29 Jan 07 - 09:18 AM

Impedance mismatch wouldn't go up and down of its own accord.

Plenty of op-amps will run on anything between 3v to about 21v - and the latter is needed for full power output - but all modern equipment is likely to stabilise the votage source to anything in the preamp stages.

Hence my question about what power amps are in use.

Until I thought of a copmressor....

Subject: RE: Tech: Vol level of PA going up and down
From: Bernard
Date: 29 Jan 07 - 10:47 AM

A common fault on equipment, even on new stuff, is dirty 'break springs' on effects loop jacks... connecting the two sockets with a spare jack lead would test that possibility.

For the non-technical, effects loops (and the like) usually have'break springs', which are connections on jack sockets which disconnect the 'input' ('return') socket from the 'output' ('send') socket when an effects unit is inserted.

The connections are so small that the slightest bit of muck will lead you a merry dance...

It's worth a try...

Subject: RE: Tech: Vol level of PA going up and down
From: Leadfingers
Date: 29 Jan 07 - 12:00 PM

GUEST 07.28 - Low voltage does NOT cause distortion ! Just knocks your Power Output down to Damn All !

Subject: RE: Tech: Vol level of PA going up and down
From: Richard Bridge
Date: 29 Jan 07 - 01:59 PM

If it was a break spring, then the addition of percussive persuasion would cause a sudden cure - but yes, I have heard levels fade gradually down because of a break spring fault. I've never heard them fade down and then up again or vice versa, without a brisk tap in the right place...

THis is killing me - has the owner done the tests and found the asnwer yet????

Subject: RE: Tech: Vol level of PA going up and down
From: Bernard
Date: 29 Jan 07 - 02:06 PM

Not always, Richard... giving it a bash sometimes reveals the fault, but by no means always. It depends a lot on the equipment!

Our friend hasn't mentioned the 'gentle persuasion' technique, yet...

Have to concur with your compressor theory, though.

Subject: RE: Tech: Vol level of PA going up and down
From: Herge
Date: 29 Jan 07 - 02:21 PM

Hi Folks - some of this advice is leaving me well behind as I am not tech minded! 'dirty break springs' 'op amps in the input stages, and the Mosfets' 'stabilised power supply'!!!!!

The set up is as follows - ipod into Behringer - Xenyx 2442FX desk. From desk to behringer feedback destroyer. From that to Yamaha CP200 power amp - speakers.

Havent got round to testing at home yet, but will soon.

Subject: RE: Tech: Vol level of PA going up and down
From: Bernard
Date: 29 Jan 07 - 02:25 PM

I'd guess the feedback destroyer is the culprit... or rather, the human who set it up!

;o)

Subject: RE: Tech: Vol level of PA going up and down
From: Bernard
Date: 29 Jan 07 - 06:07 PM

Hmmmm... really you shouldn't put the feedback destroyer between the mixer and the amplifier, as it will try to control everything.

Instead, you need to use insert points, and chapter 5 of the 2442 user manual explains how.

Basically, you only want to be processing the signals that can affect feedback - the iPod and any non-acoustic instruments will not feed back no matter how high the volume.

Subject: RE: Tech: Vol level of PA going up and down
From: Richard Bridge
Date: 29 Jan 07 - 06:17 PM

I too find the feedback destroyer a likely suspect in principle - its entire function in life is to turn things up and down (at least within particular frequency bands) - but what's likely to be affecting it when only the ipod is running? Is it likely to be mistaking thyristor or triac buzz for oscillatory feedback?

Subject: RE: Tech: Vol level of PA going up and down
From: Bernard
Date: 29 Jan 07 - 06:31 PM

Any transients (such as the RF you suggested, Richard) could potentially be doing it. It does depend a lot on how the thing is set up.

Many's the time I've been asked to look at a system where 'there isn't enough volume' and it turns out to be a badly set up feedback eliminator of some sort.

They often cause more problmes than they solve - especially when set up between the mixer and amplifier. Often there is no other way to do it, but the Xenyx has insert points, auxiliaries, subgroups and all the other toys...

Subject: RE: Tech: Vol level of PA going up and down
From: Grab
Date: 30 Jan 07 - 05:10 AM

Why is the feedback destroyer controlling front of house sound anyway? If you have to have it controlling an entire output, surely this should be on the foldback output? Turning down FOH won't do a damn thing if you've got the monitors cranked up.

Graham.

Subject: RE: Tech: Vol level of PA going up and down
From: Herge
Date: 30 Jan 07 - 01:23 PM

I'll try reconfiguring the feedback destroyer to just put the radio headset through it.

Thanks

Herge