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Author Topic: Amplifier current draw (real world vs paper/lab)  (Read 4224 times)

Steve Litscher

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Amplifier current draw (real world vs paper/lab)
« on: August 08, 2016, 12:42:17 pm »

My little PA rig is growing by the week, and I'm starting to pay serious attention to power when at gigs.

I recently ran an outdoor stage for a smallish festival (about 1200-1500 people). The organizer told me there were at least three 20-amp, 120V circuits available.

Imagine my shock (no pun intended) when we arrived and found a total of two circuits, both of which were at least 150-feet from the stage (each in opposite directions, thus requiring 300+ feet of cord), and... one of the outlets didn't work at all. Turns out the outdoor outlet cover was leaking and had corroded the socket assembly on the building. They got an electrician to fix it within an hour or so.

We ended-up running the entire stage (FOH, monitors, and stage power) on 2, 20-amp circuits, both of which supplied power to the stage area via a total of 300-feet of 10-gauge and 12-gauge extension cord (all cords were rated for 1875-watts).

I couldn't believe it, because on circuit #1, we were running four PLX3602s for the FOH, all bridged, and all just barely touching clipping at points.

Circuit #2 powered the four wedge monitors via 2 stereo GX7s. I also had two QSC KW12s for powered monitors on that circuit.

The biggest act we had on stage brought a total of 7 guitar/bass amplifiers (3 guitarists, 1 bassist), all of which ran off the monitor circuit.

How was this possible??? I'm literally baffled, and slightly impressed.

QSC's specs show the PLX3602 pulls 11.xx amps at 1/8 power, and the GX7 is close to that as well. I know I was probably on the hairy edge of tripping the circuit, but wow. I don't ever want to have that scenario again, but it's nice to know there's some "give" in the power figures.

What's been your experience?

Tim McCulloch

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Re: Amplifier current draw (real world vs paper/lab)
« Reply #1 on: August 08, 2016, 12:58:29 pm »

My little PA rig is growing by the week, and I'm starting to pay serious attention to power when at gigs.

I recently ran an outdoor stage for a smallish festival (about 1200-1500 people). The organizer told me there were at least three 20-amp, 120V circuits available.

Imagine my shock (no pun intended) when we arrived and found a total of two circuits, both of which were at least 150-feet from the stage (each in opposite directions, thus requiring 300+ feet of cord), and... one of the outlets didn't work at all. Turns out the outdoor outlet cover was leaking and had corroded the socket assembly on the building. They got an electrician to fix it within an hour or so.

We ended-up running the entire stage (FOH, monitors, and stage power) on 2, 20-amp circuits, both of which supplied power to the stage area via a total of 300-feet of 10-gauge and 12-gauge extension cord (all cords were rated for 1875-watts).

I couldn't believe it, because on circuit #1, we were running four PLX3602s for the FOH, all bridged, and all just barely touching clipping at points.

Circuit #2 powered the four wedge monitors via 2 stereo GX7s. I also had two QSC KW12s for powered monitors on that circuit.

The biggest act we had on stage brought a total of 7 guitar/bass amplifiers (3 guitarists, 1 bassist), all of which ran off the monitor circuit.

How was this possible??? I'm literally baffled, and slightly impressed.

QSC's specs show the PLX3602 pulls 11.xx amps at 1/8 power, and the GX7 is close to that as well. I know I was probably on the hairy edge of tripping the circuit, but wow. I don't ever want to have that scenario again, but it's nice to know there's some "give" in the power figures.

What's been your experience?

My experience?  "It Depends."

Aside from the things we can easily see, like the gauge of the extension cord conductors there are factors like the corrosion you encountered; the age, design and trip history of breakers supplying your electricity; the length and quality of conductors supplying the outlet your cords are plugged into.

Any of these variables can be the "gotcha" even if you're not fully loading the AC circuits.

Live music audio has a pretty wide dynamic range, and with that there is a commensurate power draw.  The loud peaks (kick drum, for example) are relative short, often around 20ms or so.  Those aren't sufficient to trip a thermal breaker and are unlikely to trip most magnetic breakers (at least initially).

It's when you have sustained draw that stuff gets dicey, fast.  EDM/hip hop/rap gigs or metal shows with continuous 808 drops, double kick drums and bass guitars with octave dividers or lots of signal sent to subs are prime offenders.

Remember, too, that increasing your rig's output SPL by 3dB doubles your current draw... and that turning stuff down a little can mean having a show... or not.
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Jamin Lynch

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Re: Amplifier current draw (real world vs paper/lab)
« Reply #2 on: August 08, 2016, 01:24:46 pm »

I have some friends that run 6 Crown xti4002's on 1 circuit all the time. All the amps are plugged into a Furman power conditioner too.  :o

I've suggested numerous times they separate their amps but haven't tripped any breakers...yet.
Amazing.

They play Texas country and some classic rock.
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Using: JBL VP tops, subs, and monitors, Si Performer

Rusty Stevens

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Re: Amplifier current draw (real world vs paper/lab)
« Reply #3 on: August 08, 2016, 02:00:06 pm »

Only time I trip breakers is when the band insists on plugging a pair of Geysers in. I tried to tell them that for an indoor show the 24 LED pars, 2 scanners, 4 bars, 4 60W movers, 3XLS2500's, a PLD4.3, my mixer, and all the amps on stage pull the same or less power than those stupid Geysers.

If you are running live sound you may hit occasional peaks that come close to max, but the average is much lower. Now run a sine wave at max and you'll pop breakers like candy. It's more about the duty cycle and how long you spend pulling the big number out of the wall, rather than what the peak number actually is.

Rusty

I have some friends that run 6 Crown xti4002's on 1 circuit all the time. All the amps are plugged into a Furman power conditioner too.  :o

I've suggested numerous times they separate their amps but haven't tripped any breakers...yet.
Amazing.

They play Texas country and some classic rock.
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Doug Fowler

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Re: Amplifier current draw (real world vs paper/lab)
« Reply #4 on: August 08, 2016, 04:28:53 pm »

My experience?  "It Depends."

Aside from the things we can easily see, like the gauge of the extension cord conductors there are factors like the corrosion you encountered; the age, design and trip history of breakers supplying your electricity; the length and quality of conductors supplying the outlet your cords are plugged into.

Any of these variables can be the "gotcha" even if you're not fully loading the AC circuits.

Live music audio has a pretty wide dynamic range, and with that there is a commensurate power draw.  The loud peaks (kick drum, for example) are relative short, often around 20ms or so.  Those aren't sufficient to trip a thermal breaker and are unlikely to trip most magnetic breakers (at least initially).

It's when you have sustained draw that stuff gets dicey, fast.  EDM/hip hop/rap gigs or metal shows with continuous 808 drops, double kick drums and bass guitars with octave dividers or lots of signal sent to subs are prime offenders.

Remember, too, that increasing your rig's output SPL by 3dB doubles your current draw... and that turning stuff down a little can mean having a show... or not.

I remember Dave metering  a Carlson 850 rig for a decent sized show, maybe 16x per side.  It ended up metering 60ish amp peaks per leg.  This was with Macrotechs IIRC.

fwiw....
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Debbie Dunkley

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Re: Amplifier current draw (real world vs paper/lab)
« Reply #5 on: August 08, 2016, 04:34:25 pm »

My experience?  "It Depends."

Remember, too, that increasing your rig's output SPL by 3dB doubles your current draw... and that turning stuff down a little can mean having a show... or not.

And turning down was what got me through my last outdoor show where you might remember the organizer forgot to order the generators  and I was only working off maybe 10 amps total!
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Mac Kerr

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Re: Amplifier current draw (real world vs paper/lab)
« Reply #6 on: August 08, 2016, 05:02:09 pm »

I remember Dave metering  a Carlson 850 rig for a decent sized show, maybe 16x per side.  It ended up metering 60ish amp peaks per leg.  This was with Macrotechs IIRC.

fwiw....

I have metered a good sized V-DOSC rig with 40 Macrotech 5000VZs runnin' really loud for John Mellencamp at 50A per leg. Because of the expected heavy load I had spec'd a 250A/leg service for sound.

Mac
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David Buckley

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Re: Amplifier current draw (real world vs paper/lab)
« Reply #7 on: August 08, 2016, 05:21:47 pm »

Circuit breakers are slow things, so if your average current is 10A and the on-beat peaks are 60A, a 20A breaker won't even notice.

You need a quite fast measuring device to accurately measure peak current.
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Mike Monte

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Re: Amplifier current draw (real world vs paper/lab)
« Reply #8 on: August 08, 2016, 05:23:32 pm »

My little PA rig is growing by the week, and I'm starting to pay serious attention to power when at gigs.

I recently ran an outdoor stage for a smallish festival (about 1200-1500 people). The organizer told me there were at least three 20-amp, 120V circuits available.

Imagine my shock (no pun intended) when we arrived and found a total of two circuits, both of which were at least 150-feet from the stage (each in opposite directions, thus requiring 300+ feet of cord), and... one of the outlets didn't work at all. Turns out the outdoor outlet cover was leaking and had corroded the socket assembly on the building. They got an electrician to fix it within an hour or so.

We ended-up running the entire stage (FOH, monitors, and stage power) on 2, 20-amp circuits, both of which supplied power to the stage area via a total of 300-feet of 10-gauge and 12-gauge extension cord (all cords were rated for 1875-watts).

I couldn't believe it, because on circuit #1, we were running four PLX3602s for the FOH, all bridged, and all just barely touching clipping at points.

Circuit #2 powered the four wedge monitors via 2 stereo GX7s. I also had two QSC KW12s for powered monitors on that circuit.

The biggest act we had on stage brought a total of 7 guitar/bass amplifiers (3 guitarists, 1 bassist), all of which ran off the monitor circuit.

How was this possible??? I'm literally baffled, and slightly impressed.

QSC's specs show the PLX3602 pulls 11.xx amps at 1/8 power, and the GX7 is close to that as well. I know I was probably on the hairy edge of tripping the circuit, but wow. I don't ever want to have that scenario again, but it's nice to know there's some "give" in the power figures.

What's been your experience?

FWIW This past June I ran a yorkville TX4/9s rig with an amp rack consisting of one crown IT6000, one XTI4000, one XS4300 at an outdoor festival on one 13amp circuit from a generator. Bass-heavy DJ music. 
If I crunched the numbers on paper I would have had my doubts but that is all that I had to work with.
but....it worked fine....
DJ / House music

BTW: on my contract I had listed: two dedicated 20 amp hard-wired circuits within 20 feet of the performance area.   

Sometimes you have have to work with what is given to you...
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Robert Piascik

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Re: Amplifier current draw (real world vs paper/lab)
« Reply #9 on: August 08, 2016, 06:07:34 pm »

Just did a show with:
8x Danley TH-118 subs powered by
4x Crown iTech 8Ks
4x Danley SH-50 powered tops
plus lights, backline, etc.

We were at 110db at 100' out

Powered it all with my Honda eu6500 generator (2x 20 amp circuits)

Occasionally hit 5000w on the generator read out

I agree with Tim that the quality of those extension cords would have a lot to do with your ability to be 150' in either direction from the power. I wouldn't advise it, but I'm not surprised that you made it work.
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