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Author Topic: Battery power for small PA  (Read 2079 times)

Stephen Kirby

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Re: Battery power for small PA
« Reply #20 on: June 12, 2017, 12:04:09 pm »

Most of them used buck boost supplies to get 24-40V on the output rails.  But since the development of high speed switching supplies and then class D outputs, now it's crazy time.

They also tend to have a pile of batteries in them along with high amperage RV alternators.  I remember years ago, one guy's subs wouldn't start unless he had the engine running.
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Nathan Riddle

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Re: Battery power for small PA
« Reply #21 on: June 12, 2017, 12:44:58 pm »

If It was me and I wanted to BUILD something that I KNEW would work.

I'd go with something like this:

$240 512Wh @ 15lbs
http://www.batteryspace.com/lifepo4-prismatic-battery-12-8v-40ah-512wh-10c-rate---un38-3-passed-3-2vx4-dgr.aspx?gclid=COKPuszduNQCFQMPaQod8UAM2A

$300 Cotek sp700-112 Pure Sine Wave Inverter
http://invertersrus.com/product/cotek-sp-700-112/

$600 total (with parts/box, etc)
It would easily run 2x K12's moderate music.
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Craig Hauber

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Re: Battery power for small PA
« Reply #22 on: June 12, 2017, 01:01:02 pm »

Yes, exactly. In the olden days before Class D amplifiers and switch-mode power supplies, the most voltage swing output you could get from a car stereo amp was 0 to 12 or 14 volts volts. So that works out to 7 volts peak divided by the square root of two (1.414) which is around 5 volts RMS. To get the watts delivered to the speakers just square the volts RMS and divide by the speaker load. That's 25 volts divided by 4 ohms which is 6 watts max. Now many of the higher powered car stereos of the day used bridge mode amplifiers for more power. So that's 5 volts RMS times 2 = 10 volts RMS. Then 10 squared = 100. And 100 divided by a 4 ohm speaker load = 25 watts. So with a 14 volt car electrical system (the alternator normally puts out 14.4 volts to charge the ~12 volt car battery) you got maybe 6 watts from a cheap 8-track player, or 25 watts from an expensive aftermarket amplifier. Of course with switch-mode supplies and Class D amplifiers it's possible to get thousands of audio watts from a car battery. But of course TNSTAAFL (There's No Such Thing As A Free Lunch), so the amperage draw on the vehicle goes up proportionally.
 

There's a large chunk of those "olden days" taken up by dc-dc conversion power supplies running conventional class-AB amplifiers.  Yes they used a ton of power too.  Class D just allows the same thing without multiple batteries and dual truck-alternators.

Quote
P.S. I believe that each of us has a private hell designed just for them, and mine would be installing car stereos. So please don't make me do this anymore... :-\
Agreed! but have to add that anything behind the dash don't make me work on!
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Craig Hauber
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Tom Provenza

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Re: Battery power for small PA
« Reply #23 on: June 19, 2017, 09:22:39 pm »

This is what I put together for similar events, most recently a graveside ceremony. 80AH AGM and 1K pure sinewave Xantrex inverter:



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Tom Provenza

Scott Hofmann

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Re: Battery power for small PA
« Reply #24 on: June 19, 2017, 10:20:06 pm »

I remember a lot of the old green Altec Lansing commercial gear had terminals for emergency battery operation. 24 volt maybe?
Probably weren't too efficient though.
Also the Shure M67 series mixers could be battery operated.
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Scott Hofmann

Frank DeWitt

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Re: Battery power for small PA
« Reply #25 on: June 20, 2017, 11:01:04 am »

This is what I put together for similar events, most recently a graveside ceremony. 80AH AGM and 1K pure sinewave Xantrex inverter:

I like it.  Neat, just what is needed and nothing else.  Well done.
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Mike Sokol

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Re: Battery power for small PA
« Reply #26 on: June 20, 2017, 03:33:55 pm »

This is what I put together for similar events, most recently a graveside ceremony. 80AH AGM and 1K pure sinewave Xantrex inverter:

I'm curious if that's an inverter/charger? If not, then how are you recharging the battery?

Frank DeWitt

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Re: Battery power for small PA
« Reply #27 on: June 20, 2017, 03:41:59 pm »

I'm curious if that's an inverter/charger? If not, then how are you recharging the battery?
I assume that the charger is though of as the "gas can" and therefor is separate. 
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Mike Sokol

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Re: Battery power for small PA
« Reply #28 on: June 20, 2017, 04:23:07 pm »

Hmmmm..... I've got one of these 1,800 watt Inverter/Chargers laying around that I never installed. Present from Xantrex for some articles I was writing. It's a modified sine-wave wave output with a built-in 40-amp battery charger and a 30-amp transfer relay. I've run audio gear on it before and it's very quiet. So all I have to do is add a battery box like you did and I'm in business. Looks like a good experiment. Thanks for the idea...

Tom Provenza

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Re: Battery power for small PA
« Reply #29 on: June 20, 2017, 10:02:02 pm »

Correct, it is an inverter only. I float a charge on it prior to use with a separate charger.
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Tom Provenza
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