PA pyrotechnics on power dropout-help please

[marvin]

Our church PA system is known to sound a tremendous fire work like report when the power fails during operation. Yesterday the lights flickered for an instant--like a transfer switch was thrown and the system made that very loud bang that scares everyone.

I think this happens because the mixer and other devices in the chain and amps all lose power, but the main amp must still have enough charge in the caps to amplify the shut down thumps of the inline devices. I don't know if there is any solution to this problem. It has happened several times over the years. I'm surprised it doesn't damage anything!!

Any advice appreciated.

Thanks,
Marvin

[Steve Lee]

Ups.

[marvin]

A UPS for that much equipment would cost a bit. Are UPS's commonly installed for PA systems?

[billfitzmaurice]

Use the UPS on the board and the other low current devices, not the amps.

[Steve Lee]

^ exactly.

[marvin]

That seems like the simplest way to address it. The UPS would still need to be quicker than the power drop....unsure that it would actually prevent the BANG!

In our case, would we need two UPS? The mixer is at the rear of the sanctuary, and the main output feeds through an EQ or processor/feedback eliminator...2 or 3 devices anyway between the mixer and remote amp rack.

What causes the pop? The mixer, or could the downstream components also generate the pop when power is lost? If either or all can generate the pop, then we would need at least two UPS. Then the power loss would drop the amps first while the upstream devices remain live.

[billfitzmaurice]

Having the processing remote from the mixer is an odd arrangement. The usual set up is to have them all in one spot, using the same power strip, connected to the mixer inserts. There should be nothing between the mixer and amps other than perhaps an electronic crossover for bi-amping.

[marvin]

Bill,
We have an installed system, not portable, so I don't see it as unusual at all. Our mixer and recording and wireless mic gear are all located within the operator's booth in the sanctuary. The mixer is then connected via snake to an amp rack in another room.

The main out from the mixer goes to >> Shure DFR11EQ >> EV S200 EQ >> Peavey IDL 1000 digital delay >> split to two channels of the main amp which feed staggered main speakers. So there are 3 processing devices between the mixer and amps.

[billfitzmaurice]

Normally all of those peripherals would be on insert loops with the mixer, be it installed or portable. All that distance between the mixer and the rest could be contributing to the problem.

[Steve Lee]

A UPS will supply constant power via batter back-up - there is no switch-over - it keeps the source audio alive - the power amp is gonna sag.

Best bet is to try it and see what happens and go from there.

[Paul O]

That seems like the simplest way to address it. The UPS would still need to be quicker than the power drop....unsure that it would actually prevent the BANG!

It will assuming its something other than the amplifiers that is causing the noise... which is a good assumption because most amplifiers have a start up/shut down mute function.
You won't need anything terribly big or expensive either, a couple of 350-550VA models from an electronics retailer will do the trick. And yes thay are that fast.

[Mike C]

"Having the processing remote from the mixer is an odd arrangement. The usual set up is to have them all in one spot, using the same power strip, connected to the mixer inserts. There should be nothing between the mixer and amps other than perhaps an electronic crossover for bi-amping."


"Normally all of those peripherals would be on insert loops with the mixer, be it installed or portable. All that distance between the mixer and the rest could be contributing to the problem."


Not necessarily.....not uncommon to have a remote amp location in an install with power and audio cabling done correctly. Power sequencing the system can be done with with the master unit in the sound booth.
The EV S200 is a dedicated EV processor for their I'm guessing the mating speakers in the system, having that and the delay lines located with the amps in series with the main outputs is not bad at all.

As for the Shure DFR11 if that is being used as the main system EQ, limiter, that could be at the mixer but I would still put directly on the main output so it could not be "accidentally" pulled out via and insert cable.



What mixer is in the system?
From the equipment you listed I'm guessing your system is a little older.

As mentioned a UPS for the processing equipment, mixer, wireless receivers, CD players, ect. should take care of it.

Do you have a power sequencing system in place? While that in it self would not take care of the system wide dropped power issue it would
be good for normal power up and down operation.

Kind of a long shot here, but maybe there is a wiring issue either power or audio between the equipment that is making the
dropped power noise worse than it needs to be, had a system once with some mis wired audio lines and unbalanced cabling
where it should have balanced make some strange noises when crash powered off.

Could be a single source input to the mixer that is causing the most noise when the system and building power crashes off.

[billfitzmaurice]

Remotely mounting the amps close to the speakers is SOP. That keeps the speaker cables short, preventing resistive, capacitive and inductive losses that result with long speaker cables. Balanced line level signals from the board to the amps don't have those issues, so having those cables 100 feet or more isn't a problem. Amps are pretty much set it and forget it, so there's no advantage to having them close to the board. That's not the case with signal processors.

[marvin]

Mike, I can tell you have seen a few systems before. ;-)

This system layout was designed to minimize sound booth size so I suspect that is why it is laid out as it is.

The S200 serves two FR 12? unsure of that model number which are distributed--one in front, one near the rear and balcony with a time delay between them.

Yes, the DFR11 serves as EQ, limiter, and I think feedback eliminator, too.

You are also correct about the mixer, it may be 20+ years old. It is a Mackie VLZ pro 24 channel 4 bus? I would love to have a modern digital mixer just to take advantage of more individual channel EQ features, but we don't need to spend the money at this time. The system performs remarkably well for our needs.

We have Furman power sequencer suppressors at the booth and amp rack.

As far as mis-wired equipment... If I recall, the booth may be on a ground lift outlet. That was the only way we could stop a ground loop hum when everything was first installed way back when. The booth and amp rack are on different circuits from separate panels. I don't know if that could cause it.

Marv

[billfitzmaurice]

If the connection from the board to the amps is properly balanced you'll never get a ground loop. I've worked with hundreds of systems in major concert venues with the FOH a hundred feet or more from the stage, using both FOH and separate backstage monitor consoles, and never had a problem. However, not all devices, especially signal processors, are properly balanced. Another reason for having all signal processors next to the board is that allows everything to use the same breakout box, and if there are improperly balanced devices or even some that are unbalanced the interconnects are short enough to keep ground loops from arising.

[Mike C]

"As far as mis-wired equipment... If I recall, the booth may be on a ground lift outlet. That was the only way we could stop a ground loop hum when everything was first installed way back when. The booth and amp rack are on different circuits from separate panels. I don't know if that could cause it."

You do not want to lift the ground pin on an electrical outlet........

I hope that audio connection between the sound booth and the amp room is a balanced line.

Just to double check what equipment is located in the remote amp room?

To get rid of the ground loop hum (with all the power outlet ground pins connected) first try lifting the AUDIO ground at the input to
the first piece of equipment in the amp room, on an xlr connector that is pin 1.
If that does not take care of it you will need an audio isolation transformer, something like what I linked to.

Are the Furman units linked together so the master sequencer in the booth controls the remote sequencer in the amp room?

[Mike C]

Here is another isolation transformer link and a link to an XLR inline pin1 lifter. If lifting pin1 takes care of the hum you could remove the ground lift barrel and open
up the connector and clip the lead off of pin 1.

[Mike C]

And now the ground lift

[marvin]

"As far as mis-wired equipment... If I recall, the booth may be on a ground lift outlet. That was the only way we could stop a ground loop hum when everything was first installed way back when. The booth and amp rack are on different circuits from separate panels. I don't know if that could cause it."

You do not want to lift the ground pin on an electrical outlet........

I know, Mike, but it's been fine for 20 years LOL



I hope that audio connection between the sound booth and the amp room is a balanced line.
All balanced lines between booth and rack.



Just to double check what equipment is located in the remote amp room?
3 Crest stereo amps
2 stereo graphic EQs
1 peavey time delay
Shure frd11
1 getner hearing assist xmitter
By memory that's most of it. I'm out of town this week



To get rid of the ground loop hum (with all the power outlet ground pins connected) first try lifting the AUDIO ground at the input to
the first piece of equipment in the amp room, on an xlr connector that is pin 1.
If that does not take care of it you will need an audio isolation transformer, something like what I linked to.

I will try that after I return home.
If lifting the ground at the first amp rack device eliminates the hum, are you suggesting the solution is to leave it lifted? Is that safe? No risk of line noise without the ground?



Are the Furman units linked together so the master sequencer in the booth controls the remote sequencer in the amp room?

Yes

Thanks, Mike.

[Mike C]

You really do want to reconnect the ground on the power outlet under the right circumstances it could be dangerous!

Lifting pin 1 audio ground is safe, sometimes lifting the pin1 ground is all it takes other times it may not make any difference or could make the
noise worse, in that case you need to go with a transformer.

A few post back you mentioned a digital mixer, they offer so much more than just more EQ on an input channel.
Being able to step out of the booth and mix with an iPad is very handy in itself.
The Allen Heath QU series is worth taking a look at, easy to get around on, lots of routing options without getting too over complicated and it would sound better than the Mackie.
Also you could pull at least the EQ's out of the system as the QU has EQ on all outputs.

[marvin]

Thanks for all the advice everyone.

Mike, I plan to reconnect the power ground and experiment with the xlr ground pin soon.