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Old 20th September 2008, 07:29 AM   #1
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Default Car Battery Amp for gigs

I've been thinking of making a battery powered pa system for gigs.
I use a Mac (for sequences) which is battery powered, and that can power my Roland SoundCanvas by USB and also my M-Audio keyboard by USB.
I would probably use no mixer or maybe make a passive one.
I recently got two deep cycle marine batteries for free.
A basic pa to power 2 JBL's (12 + horn) is what I'm thinking.

It seems like this would be good as the 12V DC is clean and there's no sense on running a power inverter and plugging regular 120V AC powered amps in - why not eliminate the transformer.

Has anyone tried using something like a Sony XPlod car amp with a 12V battery for gigs?
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Old 23rd September 2008, 02:30 AM   #2
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Default sorry, a bit long....

you will have a transformer in one form or another. look inside a car amp. the +/-65V for the outputs comes from a DC-DC converter with a high frequency transformer in it (a car "plate" amp might make a nice "ready-made" solution, come to think of it. 100 to 300 W/ch amps are commonplace, and they run off of 12V.)

for powering the mixer, of which most of the circuits in there run off of +/-12 or 15V, you could build a simple DC-DC converter. or you could buy switcher modules that do the same (at an efficiency between 70-90%).

also make sure to learn everything you can about the "care and feeding" of lead acid batteries. they've been around a long time, and a lot of engineers and chemists have created a huge amount of knowledge and techniques to make these batteries work reliably and well for a long time.

a few pointers:

REMEMBER, THE BATTERY CONTAINS SULPHURIC ACID, WHICH CAN BURN YOU, AND DISSOLVE YOUR CLOTHING!!!!!!!!! granted it is diluted acid, but it can still do a lot of damage if spilled or splashed. especially keep aluminum objects away from the acid.

when storing them, top off the charge every so often, especially during cold weather. never store a dead battery, the plates will sulphate and eventually dissolve

do not leave them continually on a trickle charge for long periods of time, as they will start bubbling hydrogen and oxygen in the perfect proportions to create an explosive atmosphere, and also will eventually electrolyze all of the water away. during charging, there isn't a whole lot of electrolysis happening, but once it's fully charged, it begins bubbling a lot more. this is why the warnings on the batteries tell you to take the vent caps off during charging.

if you have to add water, add ONLY DISTILLED water. the minerals in tap water will "poison" the battery and quickly make it useless.

if you suspect a battery has begun to sulphate, you can partially reverse it by fully charging and fully discharging the battery a few times.

if at all possible, try to keep the battery at or near room temperature. don't store a battery directly on a concrete floor, as the concrete acts as a heat sink (it's got a very high specific heat) and cools the battery to the point where it sulphates itself. keep a wood block under the battery.

always have a load on the battery when checking it's voltage. a battery with a bad cell can often read 12V with no load, but drop to near zero with a load. use a substantial load, say a 1 ohm 200W resistor or a car headlamp. you want to be able to draw 10+ amps during a voltage test. you only need it connected long enough to get a voltage reading, just a few seconds.

nominal charging voltage for a 12V battery is between 13.8 and 14.2 V. if you have an open frame power supply with foldback limiting, you can usually set it to 13.8V with the voltage adj pot. if the supply doesn't have that much range with the pot, put a 1N4004 diode between the + output and the + sense terminal. this will fool the regulator into responding to a 0.6V drop in "wiring" and bump up the voltage to a more useable range if one 1N4004 is not enough, add a second one. a typical open frame supply, such as 12V/5A should fully charge a battery in about 2-3 days.
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Old 27th September 2008, 12:13 AM   #3
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thanks!
I think my first step will be to get a modern high wattage car amp like a Sony or Alpine and start fooling around with it.
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Old 27th September 2008, 12:23 AM   #4
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I wouldn't bother with standard car amps. They are very inefficient, produce too much heat and run the batteries flat too quick, alright if you have a engine/alternator recharging. Best efficiency you will get from a car amp is around 30%.

Try and get something that is class D (80-90% efficient). If your handy with a soldering iron try one of the 41hz.com amp9-basic kits.

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Old 27th September 2008, 03:53 AM   #5
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there are class D car amps.... saw one come through for repair on thursday..... shorted mosfet in the output stage.... kenwood, because i remember that the manufacturer's part number is the device number, and kenwood is one of 2 manufacturers that use the device number (there may be more, but kenwood and pioneer are the only 2 i know about)
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Old 28th September 2008, 05:19 PM   #6
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just ordered a mosfet for an alpine class D as well.....

back to the original question, it can be done, but you still will need a source of AC for the band's amps. old UPS's can be made to run from 12V batteries as well for this.
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Old 28th September 2008, 09:56 PM   #7
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Iv'e had bad experience trying to run battery powered amps and an inverter of off of the same battery. The noise from the inverter carries over into the amps.

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Old 28th September 2008, 11:03 PM   #8
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As a rule amplifiers are DC powered, even mains powered ones.

Obviously valve amplifiers require high voltages, their power transformers are integral to the design, and running them from batteries is a PITA. You can have an inverter with a battery to itself if necessary tho'.

Solid state amplifiers, however, frequently employ voltages which are readily achievable with a bank of car batteries, or any batteries in sufficient number.

If you have SS PA equipment it may well be possible to bring out the DC power connections to make the equipment dual-use.

If you can use the same equipment it's easier to set up.

Solid-state instrument amps and other equipment can be treated similarly. If you require different voltages you can have a pack for each voltage, although this complicates the charging arrangements. It's a toss-up whether to use this or dc-dc converters which may contribute noise but be more compact and simplify the battery stack.

Most modern PA equipment will not be too inefficient, I wouldn't worry about chasing class D, it's surprising how much sound you get out of a battery.

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Old 29th September 2008, 12:38 AM   #9
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sine wave inverters don't have noise problems. most REAL ups's are sine wave inverters, because computers are susceptible to noise as well. some UPS's would have to be "tricked" into operating without having been hooked up to an actual power line first. others will operate just by turning the power switch on, there's a class of UPS's that operate 100% of the time. the batteries are charged from the power line and the inverter provides power to the load all the time, so there isn't a dropout during switchover from the power line to the inverter. there's no need of a relay circuit, or need of having the UPS connected to an active power line first. most of these are also sine wave inverters. old UPS's can usually be found at computer repair shops for nothing or next to nothing because they're gen.erally considered to be too much bother to fix, and usually all that's wrong with them is that the batteries have gone bad.
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Old 29th September 2008, 09:35 AM   #10
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What Amp hour rating are the batteries? If a 200 watt amplifier pulls approx 15 amps(assuming that it only pulls the full 200watts on peaks), for 2 hours operation you will need at least a 30 amp hour battery.. and thats on a 2hr cycle so you will need even more capacity (some of those batterys ratings are done at the most favorable discharge rate..)
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