12v portable soundsystem

[tRasH]

Hi!
I'm about to build a portable mono soundsystem using a car amp and car speakers conected to a 12v battery. The whole thing will be built in a wooden carriable case with speakers in the lid. The thing will be used with a linelevel input and taken on the streets in the sole purpose of musical entertainment.
So to my worries...

I've been looking at 12v batteries with up to 17Ah wondering if
there's an amplifier that would run on such at a decent outputlevel (I will play in the streets so it can't be too loud) for at
least 3 hours. As I said, I'd like it all in a carriable case so a car
battery would be too heavy.

Thanks

/tRasH

[Stocker]

You will be surprised how far and loud 17AH will take you.

[oPossum]

A 50W RMS car amp should work.

Typical current draw for such an amp would be....

Less than 1A at idle.
4 to 7 A average playing loudly.
15 A peak on heavy bass.

Should last 3+ hrs on a 17 AH battery.

[oPossum]

An amp with a regulated power supply would be best. I will be a bit more efficient than an unregulated supply. If an amp has higher wattage rating at 14V than at 12V it has an unregulated supply. As a general rule amps rated at 14V or 14V/12V are unregulated, and those rated at 12V are regulated.

[Stocker]

That's placing an AWFUL lot of faith in the claims of some potentially less-than-wonderful amplifier manufacturers.

How is it more efficient, exactly, to wast heat in the regulators instead of the output devices?

Besides, this is likely to all be PWM stuff... high efficiency anyhow.

[oPossum]

Amps with regulated supplies typically use PWM to provide regulation of the primary supply.

Amps with unregulated supplies typically do not use PWM. They are often simple self oscillating circuits.

The amp itself is linear (if not class D) and will produce the least heat if the supply is just enough to prevent clipping and no more. A regulated supply offers the opportunity to operate at peak efficiency and performance over a range of supply voltages such a would be seen in this application.

This is not the claim of any amp manufacturer. It is theory.

[magicrobot]

i have put together a 3 way p.a. system for that very purpose using car audio. the crossovers on car amps make them very versatile, and the system grow-able. a new gr24 deep cycle gel or glass mat battery will run a nice sized car amp for about 8 hours. some higher quality car amps have internal regulation to cover pwr slumps or surges. this would be better as car audio sould get no lower than 11v if it has no regulation. i use my batt from 12.8 full charge down to 11 . my friend has a very high end amp that he can work down to 8v batt voltage with the regulator . a cheap inverter and one more batt for decks and we're on! Teknival this weekend and we are going out to represent again.

[Dr. Photon]

yeah, it'd definitely work, and work well. You aren't going to be using that much power unless you are running high powered bass. a regular 50 watts x 2 car amp should be fine. that 17 AH battery will last for quite some time.

For speakers, a pair of car 6X9's or 6.5's would do ya. A better solution would be to look into pro audio equipment. most car speakers are 4 ohms. the generic 2 channel car amp is good for a single 4 hom load bridged or 2 stereo 2ohm loads (remember, when you bridge, it looks like hooking up aspeaker of half the impeance to each output). Most pro audio speaker cabinets are also 4 ohms. If you get a pair of 4 ohm car speakers and build them into your mobile sound system box and connect them to each channel of the amplifie (not bridged, running in regular stereop mode), the amp will be loafing along and running happily. You could them also add some external 4 ohm (or 8 ohms) PA speakers if you need more volume - this would still be safe for the amp as it is now seing a stereo 2 ohm load.

I would recommend adding in a battery meter - a panel voltmeter would be ideal. I also recommend adding in a cigarette lighter socket if you can spare the space. that way you can use your box to power anything, such as charging up cell phones if you need to.

for a source of sound, you can use a car radio/CD player, or a walkman with a car power adapter, or grab a cheap inverter and run whatever you want.

the only things I would wory about would be fuses and the battery charger. For the charger, i recommend getting an "intelligent" charger that can charge at severl amps (so you can fill the battery overnight or quicker) and then switch into a safe "trickle" charge that will maintain the battery but not overcharge it. cheaper "dumb" chargers just put out a high charge current untill you unplug them. they will eventually overcharge your battery while will greatly reduce it's life. Investing in a smart charger will mean that you don't have to replace the battery as often, and you can have more confidance in your battery to give its entire capacity. I would also make sure that you use fuses in your box. Even though the batteries look small with respect to "real" car batteries, they can still provide enough current to melt wires and start fires. I would either build the charger into the box if there is available room and weight so you can plug the box in anywhere to charge it up. However, to save weight and room in the box, I would just put a connector on the back of it that connects to the battery with a fuse and put a mating connector on the charger.