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Author Topic: instantaneous line voltage drop metering  (Read 11539 times)

TJ (Tom) Cornish

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Re: instantaneous line voltage drop metering
« Reply #30 on: August 06, 2016, 07:27:47 AM »


Constant pink noise is 0db dynamic range, so pink noise isn't an effective current test.  Your real world peak current draw when you see the clip lights come on is somewhere between 8.8 and 32.
A DC signal has a crest factor of 0dB. A sine wave is 3dB. Pink noise typically has a crest factor of 6dB. Pink noise may be similar to some heavy music styles, but you're right that most music has a significantly higher crest factor - 12dB or more.
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TJ (Tom) Cornish

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Re: instantaneous line voltage drop metering
« Reply #31 on: August 06, 2016, 07:39:19 AM »

However, a dynamometer sort of a test might.

Voltage drop is linear. Double the run, double the drop. Is it also double the load, double the drop?

If I hang a 20A load meter on the line, it will give me a loaded voltage number. I'm theorizing that there will be a value for that number above which I can reasonably expect the amp to stay out of protect. I'm making various assumptions about what QSC is doing in there. I expect Bob Lee could clear this up pretty easily.
Ohm's law states Voltage = Current * Resistance. For a given resistance (same cord length), doubling the current draw will double the voltage drop. There really isn't any benefit to multiple measurements other than in fairly esoteric scenarios.

I think your testing methodology is reasonable and about all you can do. 

FWIW, I used to run PLX amps and didn't have trouble with them. It may be that they are slightly less tolerant than other amp designs, but if we are talking about circuits that drop to less than 100v on a 20A load, they really aren't suitable for event power.  If this is normal for you, you need to come up with a different plan - either a generator, working with the venue on their power, or simply turning it down. Remember that you can halve your current draw by lowering your volume 3dB.
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Mark Wilkinson

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Re: instantaneous line voltage drop metering
« Reply #32 on: August 06, 2016, 08:30:02 AM »



FWIW, I used to run PLX amps and didn't have trouble with them. It may be that they are slightly less tolerant than other amp designs, but if we are talking about circuits that drop to less than 100v on a 20A load, they really aren't suitable for event power.  If this is normal for you, you need to come up with a different plan - either a generator, working with the venue on their power, or simply turning it down. Remember that you can halve your current draw by lowering your volume 3dB.

+1

And another FWIW, I think bridge mode is the only time I've seen the PLX's shutdown quicker than other amps (all qsc). 
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Rob Spence

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Re: instantaneous line voltage drop metering
« Reply #33 on: August 06, 2016, 11:20:40 AM »

I run PLX amps.
 My first thoughts when considering a gig is sufficient power.
If I can't get good power (wired or with a generator), then the gig is no-go.

This afternoon I am 200' + from a panel. I will have 150' of 6-4 and 100' of 8-4 to get me 2 good 20a circuits.

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Dan Richardson

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Re: instantaneous line voltage drop metering
« Reply #34 on: August 06, 2016, 12:15:57 PM »

My plan is push pink noise into subs while the amp is on a variac. I'll drive it hard, briefly, to just under the 10A capacity of my variac or clipping, whatever comes first. Then I'll dial the voltage down and note where the amp drops out. I expect that any AC source that will stay above that voltage under a 20A load test is probably good enough.

Another data point. Bridged PLX 3002 into a 4 ohm sub, pink noise, just flickering clipping, analog ammeter wildly swinging around 10A, goes to protect at 87v. Idle protect was 75v.

Quote
A DC signal has a crest factor of 0dB. A sine wave is 3dB. Pink noise typically has a crest factor of 6dB. Pink noise may be similar to some heavy music styles, but you're right that most music has a significantly higher crest factor - 12dB or more.

Do I understand you to be saying that pink is a harder test than most music? Because that's what I want, to provide a margin of error safety factor.

The first time I ran afoul of this, I was at the end of 2 150'+ runs of 12ga, 2 amps per run. That's just too much. I had also split my loads subs on one run, tops on another. That was stupid. Now I put a sub and a top on each, and stage and FOH on a third. Last week, it was 100' of 8ga, 1 amp per run, to a single outdoor receptacle with who knows what length and loads inside the building. I was getting dropouts at fairly moderate drive levels. I strongly suspect occasional loads inside the building were the entire culprit. Going to a generator for that gig this week.
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TJ (Tom) Cornish

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Re: instantaneous line voltage drop metering
« Reply #35 on: August 06, 2016, 03:09:17 PM »

Another data point. Bridged PLX 3002 into a 4 ohm sub, pink noise, just flickering clipping, analog ammeter wildly swinging around 10A, goes to protect at 87v. Idle protect was 75v.

Do I understand you to be saying that pink is a harder test than most music? Because that's what I want, to provide a margin of error safety factor.

The first time I ran afoul of this, I was at the end of 2 150'+ runs of 12ga, 2 amps per run. That's just too much. I had also split my loads subs on one run, tops on another. That was stupid. Now I put a sub and a top on each, and stage and FOH on a third. Last week, it was 100' of 8ga, 1 amp per run, to a single outdoor receptacle with who knows what length and loads inside the building. I was getting dropouts at fairly moderate drive levels. I strongly suspect occasional loads inside the building were the entire culprit. Going to a generator for that gig this week.
Yes - pink noise is a much harder test than live music.

Your scenario is WAY, WAY too much cord. If you're more than 100' from a solid circuit it's generator time.
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Stephen Swaffer

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Re: instantaneous line voltage drop metering
« Reply #36 on: August 06, 2016, 07:59:32 PM »

Yes - pink noise is a much harder test than live music.

Your scenario is WAY, WAY too much cord. If you're more than 100' from a solid circuit it's generator time.

How do you define a "solid circuit"?  Its not hard to find receptacles 100 feet (as the wire runs) from a panel.  Running 100 feet from such a receptacle is no different than running 200' of #12 from a receptacle next to the same panel.  A couple of years ago, I pulled some panels fed from a 15 kVA transformer in a building-15 kVA works out to a 40 amp per leg panel-any circuit from that is not as solid as you (apparently) need for this application. Admittedly, this an extreme scenario.

Not asking to be picky-honestly just curious what you look for given your level of practical experience with this type of thing.
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Steve Swaffer

TJ (Tom) Cornish

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Re: instantaneous line voltage drop metering
« Reply #37 on: August 07, 2016, 08:17:16 AM »

How do you define a "solid circuit"?  Its not hard to find receptacles 100 feet (as the wire runs) from a panel.  Running 100 feet from such a receptacle is no different than running 200' of #12 from a receptacle next to the same panel.  A couple of years ago, I pulled some panels fed from a 15 kVA transformer in a building-15 kVA works out to a 40 amp per leg panel-any circuit from that is not as solid as you (apparently) need for this application. Admittedly, this an extreme scenario.

Not asking to be picky-honestly just curious what you look for given your level of practical experience with this type of thing.
A solid circuit by my definition is one that is stiff enough to keep the voltage high enough to keep gear from shutting down under the intended usage. :)

Any circuit that reads less than 105v on the loaded receptacle tester with a 15A load as tested at the venue receptacle is going to be pretty marginal after 100' of #12 cord is added.

Some judgment is required - if I'm running a small wedding with a pair of speakers on a stick, a small sound board, and a wireless mic, I'm not going to be drawing 15 amps and I have some margin.

For a small band situation where my rig takes around 15A to run - If power is stage side and it's another 100' of cable to FOH, I might be OK since my FOH setup draws about 2A; whereas if power comes from The FOH side and I have to put the amps at the end of another 100', it's probably not OK.

I guess my rule of thumb is that any circuit that starts to approach 100v under my intended usage is very marginal, and I'd better come up with a different plan.

Generators are relatively cheap to rent, and if one does this a lot, owning is not that hard. The Honda EU7000 is about $4000 to buy or $125/day to rent (give or take) and if you're smart about how you setup your rig (208v operation where possible), that generator will run a pretty big system.
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David Buckley

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Re: instantaneous line voltage drop metering
« Reply #38 on: August 08, 2016, 07:16:57 AM »

A solid circuit by my definition is one that is stiff enough to keep the voltage high enough to keep gear from shutting down under the intended usage. :)

Oh yes.  The importance of stffness in one's supply is overlooked by all sorts of people who really ought to (or, perhaps - "should") know better, and that is across all industries, not just entertainment technology.
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ProSoundWeb Community

Re: instantaneous line voltage drop metering
« Reply #38 on: August 08, 2016, 07:16:57 AM »


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