Troubleshooting sound issues with battery-powered outdoor PA - diyAudio
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Old 17th June 2012, 11:33 AM   #1
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Default Troubleshooting sound issues with battery-powered outdoor PA

We have a small setup consisting of two Mackie SRM-450s, two deep cycle 100Ah 12V batteries in series and a 24V 300W pure sinewave inverter. We tested the Mackies at home at very high volumes going through a watt meter thingy in the wall socket, and found that they never really pulled more than 150W or so. Therefore I picked a 300W inverter.

Last weekend we did our first test run, about a 6-hour session, with an added laptop/controller combo for DJing also powered from the inverter. It worked reasonably well, although as we went a bit louder, especially towards the end, we had frequent drop-outs where the sound would just scrounge up horribly. Quickly lowering the volume and slowly pulling it up again fixed it, but soon it would happen again, especially if pulling in something bass-heavy.

I had the wattmeter plugged in at the output of the inverter all the time and never really noticed it going above 200W. Nevertheless, could the problems be due to the inverter being too lowly rated for handling temporary spikes, or was it just a matter of the batteries running out of juice?

Another strange thing we noticed was that one laptop was not happy about being powered from the inverter at all. It just started flashing and beeping as soon as its charger cable was plugged in. A second laptop worked absolutely fine.
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Old 17th June 2012, 04:20 PM   #2
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Lots of battery chargers have labels warning to not use them with inverters.

Why not use automotive type power amplifiers that are optimized to operate on a 12V DC supply?
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Old 17th June 2012, 04:40 PM   #3
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Probably the batteries running low, and the inverter not so good at handling peaks from bass, which your watt meter will be too slow to pick up.

You had 1200 watt-hours of juice available. If you pulled 200W at the meter this would be about 240W from the batteries due to the converter not being totally efficient. 240W for 6 hours is 1440 watt-hours.

Maybe you got lucky by not having it so loud for the first few hours!
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Old 17th June 2012, 04:51 PM   #4
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Quote:
Originally Posted by Speedskater View Post
Why not use automotive type power amplifiers that are optimized to operate on a 12V DC supply?
That would be an option but my friend already had the Mackies from a while back and we wanted a way of powering them outdoors.

In fact, I've already bought one of the HifimeDIY Class T 24V amps but no suitable passive speakers yet.
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Old 17th June 2012, 04:58 PM   #5
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Quote:
Originally Posted by richie00boy View Post
You had 1200 watt-hours of juice available.
Wouldn't that be 2400 Wh? 2x12V in series gives 100Ah at 24V. 200W/24V = 8.3A. 50Ah (half a discharge) / 8.3A = roughly 6 hours (ignoring inverter losses).
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