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Old 5th June 2014, 03:31 PM   #1
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Join Date: Feb 2008
Location: Sheffield
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Default Help - generator is too small!

Hi chaps,

I'm doing an outdoor gig next weekend (14th of June) - acoustic tent for an outdoor festival. Last time they used a pair of Mackie SRM450 active speakers - basically a pair of 400w active 12" two-way cabs.
[Awaiting tent/stage dimensions]

Here's a rundown of what I've got available...

2x Das audio RF12s - 350w 12" two-way cabs
2x 15" subwoofers, loaded with Beyna 15P1200Nd drivers. Different cabs, but both flat to ~40Hz.
All speakers are 8ohm
1x Peavey PV2600, 500w/ch @8ohm, or bridgable into both subs (paralleled) for 2.5kW
1x QSC USA 850, ~250w/ch @8ohm, or will do both subs for 1.1kW, but that's pushing it.
1x Samson Servo-550 - 175w/ch@8ohm, 550w@8ohm bridged.
1x Behringer DCX2496

I had originally planned to bridge all amplifiers, put the subs on the Peavey and then the QSC on one top, and the Samson on the other, then limit the tops to ~400w.

That'd eek the most power out of the amplifiers, but everything will still be running in its comfort zone.

The organiser chap I've been talking to reckons it'd be worth having a subwoofer, so I plan on taking both - if its a wide stage, having a sub off to one side makes very little sense.

Now I'm told that, actually, power is going to be very limited - they've allocated about 1kW of power for me from the generator, as I'm sharing it with some stalls.

So now I'm a bit stuck.
I'm thinking of using the Peavey amp for the tops in stereo, the QSC for the subs, and setting limiters to stop power draw getting silly.
Hopefully the PSU caps will ensure there's not a silly current demand, but I've honestly no idea on this stuff. Could anyone shed some light?
Is it enough to check the amplifier datasheets and find the pink noise current draw?


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Old 5th June 2014, 05:13 PM   #2
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Join Date: Mar 2013
Originally Posted by chris661 View Post
they've allocated about 1kW of power for me from the generator, as I'm sharing it with some stalls.
That's a red flag right there, there is no way they can "allocate" you a certain amount of power, all connected devices will draw whatever they need and if some of those devices are things like ovens or coolers actual current draw will be cyclic and not constant.. meaning you could be fine 1 minute and then see a brownout the next when a big compressor kicks in.
Your audio system requirements aren't that high however, look at 1/8 power draw spec to get an idea of what the system will draw with music playing at light clipping levels. But even then I'd want my own dedicated generator and it wouldn't be a bad idea to put a UPS on the small signal devices like the DCX and mixing console just to keep them from power cycling in a brownout and thumping the speakers.A 1500w-2000w portable generator should be more than adequate for you here.
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Old 5th June 2014, 05:50 PM   #3
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Join Date: Oct 2008
Location: England
Having been a lurker on Speakerplans forums for a while, this question tends to come up a lot, with people always asking if this generator will power that etc.

From reading those threads and the comments made by a few of the more knowledgeable members on there, they always seem to give the same advice:

What type of generator is it? Petrol or diesel? And how many kVA is it?

Generally stay away from small petrol generators as they stuggle to keep output voltage constant with a range of loads, some of which might change as they cut in/out (thinking of fridges, tea urns and the like here). If something cuts out, dependent upon the generator the automatic voltage regulator take some time to reduce the output, until which you could have a nice voltage spike shoved up the equipment. Larger petrol generators and diesels don't have as much of a problem with this as they are able to maintain a more steady speed as load increases/decreases.

What kind of loads are connected?

If the genny is unlikely to have a consistent load, it will struggle to regulate as well, so dependent upon what else is connected (resistive, inductive, transient), it can help to connect a few large purely resistive loads to the genny to aid with the regulation. Normally high-wattage halogen work lamps or lights of some sort will help to calm things down and provide a consistent load for the generator.

Generally you can get away with less power than you think you need, music is incredibly transient in nature and it is highly unlikely that all the amplifiers will draw their maximum current at once, if at all even. So you will probably easily get away with running all that off a 1kW 'allocation' at reasonable volume levels.

If you can, always meter the mains which your kit is connected to. This will help to show any trends such as voltage spikes when a bit of kit attached to the same generator kicks in/out and will let you take preventative action before your kit goes up! It's as much about not drawing too much as it is about protecting your own equipment.

Overall though, you should be alright, especially with it being an acoustic stage, the draw from your kit will be minimal compared to what else is on the generator. Also you are using amps with a linear supply so this will cause fewer problems than a picky SMPS which doesn't like the AC waveform being put into it. The DCX has a universal PSU, so that should be fine, however it is the most likely to be damaged if there is a voltage spike since it has a SMPS.

Hope this helps.
"Near enough is good enough, so good enough is best"
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Old 5th June 2014, 06:16 PM   #4
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Join Date: Jul 2005
Location: North Derbyshire
We did a gig back in the 70's with a generator, feeding the stage (just made from scaffolding) and the beer tent.

No idea what size the generator was, as it was provided by the people running the charity event - but it worked perfectly fine - until it ran out of petrol

No one had brought any extra petrol, we were in a small village with none available (not too far from the OP, 'Stanton In The Peak'), and every one there was too pissed to drive and get some

So, as we were in a field not far from the small school, we broke in to the school and ran a long series of extension mains leads out to the field.

While it worked, the long run of cable produced some nice effects - with the lights in the beer tent going up and down with the music

As others have said, there's no way they can limit your consumption, (or the other users) - so I would suggest trying the complete rig, and if that fails badly, drop the subs off and just use the 2x12's for a much smaller sound, but small sound is better than no sound at all

What style of music are we talking about?.
Nigel Goodwin
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Old 5th June 2014, 08:02 PM   #5
JMFahey is online now JMFahey  Argentina
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Location: Buenos Aires - Argentina
Agree with suggestions above and add:
**insist** on having an extra generator just for you or no deal. Period.
Let them worry on where to get one or worst case rent one, not much money anyway.

It's not a human problem but a Physics one, no amount of willpower will solve that.
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Old 5th June 2014, 09:14 PM   #6
Enzo is offline Enzo  United States
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Location: Lansing, Michigan
You are putting speakers in an "acoustic tent", so what do you need subwoofers for? To me acoustic means non-electric instruments, those generally carried by the performer. Do folk guitars usually have throbbing bottom end yearning to breath free? Not in my experience. Nor a capella groups.

On a similar note, while we all want it to sound great, you mentioned this is a festival. An audiophile festival might have people elbowing to sit at the sweet spot to get all the stereo detail, but I think most people at festivals are not listening that critically. They are more interested in the performance than whether there was a small peak at 1500Hz.

How loud do you expect this to be? Metallica might play at ear shattering levels. But not Peter Paul and Mary.
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Old 5th June 2014, 10:20 PM   #7
tomi is offline tomi  Wales
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Location: The North
At least you should try to get put on a sepperate RCD (and/or MCB) from other loads - that way, when the burger van spills tea in their cash register, it won't trip your power too. Many people will also recommend that you try to get a seperate phase if it's a 3-phase genny; I don't know how muh difference it makes to the overall regulation, but that should isolate you from a lot of higher frequeny noise. Certainly wouldn't do any harm.
Stall holders and caterers have habit of being unreliable when it somes to power - you can almost guarentee thay'll have underestimated their useage ("...but I only pugged in two erns, three fryers, the fridge and 50m of festoon lights... why's it gone dark?").

Further to Nigel's story that kind of stuff still goes on. Only last year I had the diesel run out on our genny mid-afternoon on the last day of a festival. We had to syphon some from a dumper truck that was knocking about on site...
Don't Fear the Repair
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Old 5th June 2014, 11:42 PM   #8
JMFahey is online now JMFahey  Argentina
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To see it in a more realistic way.
1) For starters, FORGET about bridging, which although "doable" is a sure fire way to push amps to the limit .... which is absolutely unneeded by the way.

2) **last year they did very well with 2 small single 12" powered cabinets !!!!**

So to match and beat that, you might use:
a) use those 2x Das audio RF12s - 350w 12" two-way cabs as main PA (you have nothing else by the way) driven by 1x QSC USA 850, ~250w/ch @8ohm , cutting everything below , say, 120Hz, for clean unstrained sound, since they claim response down to 54Hz ... 10 dB down , so cutting one octave above that keeps them happym clean, and efficient.
b) to have Bass (just regular Bass, not Sub Bass which as Enzo noted is not in your program anyway) use the 2x 15" subwoofers, loaded with Beyna 15P1200Nd drivers driven by 1x Peavey PV2600, 500w/ch @8ohm , getting everything below 120Hz.

The system will be cleaner and have wider range than last year´s and everything stays within the comfort zone.

Thanks God those Beyma are not subwoofers bur are real flat up to 1 KHz, only misbehaving above 1200, so you have flexibility in your crossover frequency, it might go as high as 180/200Hz, go figure.
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Old 8th June 2014, 05:35 PM   #9
Jsixis is offline Jsixis  United States
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Join Date: Feb 2011
Location: Columbus Ohio
1 kilowatt of power should be more then enough unless your running sound for a metal band.
I can run my system (3,000 watts) off 1 20 amp service and that includes the band.
Now lights can suck up a lot of power (so glad LED lighting has arrived).
I know nothing, so learning is really awesome.
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Old 8th June 2014, 06:01 PM   #10
dmills is offline dmills  United Kingdom
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Location: High Wycombe
My rule of thumb when I hear I am to be sharing a generator set with **SnackWaggons** is to run away, run far, far away.....

Firstly those guys **ALWAYS** underestimate what they need (By an order of magnitude in some cases), secondly they generally have something, somwhere with a dicky thermostat that does not switch cleanly, thirdly they often have earth leakage from hell, and forthly the fact that the suggestion was made with a straight face tells you that you want nothing whatsoever to do with it.

If the festy was serious about having an acoustic stage it would have its own set or at least be fed from a properly sized real generator (Diesel, silenced, with AVR, generally 50KVA++) via real distro.

Sharing with **CATERING** is right out for anything you care about (I have seen the catering supply get down to well below 40Hz due to undersized prime mover, never good for transformer input loads, like say amplifiers).

A good sanity check (at least in the UK) is to ask to see the test results for the sign off as compliant with BS7909, if you get blank looks it tells you something quite useful about just how paranoid you need to be.

Regards, Dan.
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