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Old 17th March 2012, 11:11 PM   #1
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Default Portable PA Project

Hi everybody,

I've done a fair amount of browsing on here and i can't seem to find a thread that answers my question - so apologies if this sort of thread has been posted before.

In short, i want to make a small PA or sound system that i can run various AUX devices through when i'm out skating. I therefore assume that i'll have to use something 12v?

Just wondering how you guys would go about this, i want it to be pretty loud (don't worry i don't skate in places where my music might annoy members of the public!), and i'm not massively bothered about it being portable. So if the solution involves a big ol' battery i'm fine with that.

Ideas?
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Old 17th March 2012, 11:24 PM   #2
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I guess i should probably clarify on "pretty loud" - i really just want it to be louder than any portable stereo i can buy for 80 or so. It doesn't have to be super powerful, but just... enough.

I was wondering if there are any surround sound set-ups i could just run off a 12v car battery or something?
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Old 18th March 2012, 08:13 AM   #3
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Alternatively i guess any set of 12v speakers that pack a decent punch would do...
My old man has a pair of Akai R7's that work great for portable speakers, just not too good once you're actually outside.
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Old 18th March 2012, 08:34 AM   #4
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use a car stereo. it already has everything it needs to run from 12v. don't use car speakers though, they're generally total **** and grossly inefficient. best bet IMO would be to get an ex-auditorium PA cab with horn+ 12" or 15", install a car stereo into the side and use a 12v battery to power it. if u use a smallish battery you may get away with mounting it inside the speaker cabinet (you don't want to change the volume inside that box).
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Old 20th March 2012, 03:20 AM   #5
dangus is offline dangus  Canada
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Try a TA2024 amplifier; you can get a complete ready-to-use board from eBay for under $10 including shipping. If you want luxuries like a case and volume control, about $20. They do about 10 watts RMS into 4 ohms. That doesn't sound like much, but it happens to be almost exactly the same power as "high power" car head units (read the fine print in the brochure, and that "40 watt" Blaupunkt is really 9 watts RMS per channel.) Power it from a 12V sealed-lead-acid battery ("gel cell") or possibly 10 NiMH cells in series. Because it's a digital (class-D) amp it's extremely efficient, and very small and lightweight.

As for speakers, I've noticed that some car OEM speakers use tiny neodymium magnets and have stiff but light composite frames. That makes for an incredibly light weight speaker. I found a pair at a thrift store, but if you ask nicely at car audio installers, they may have bins full of similar speakers left over after installing aftermarket speakers. If you expect to be carrying them very far, lightness becomes a greater virtue than loudness.

If loudness is important, look for "pro" or musical instrument speakers with high efficiency but modest power handling. Or use an array of less efficient speakers, maybe a line source if you hit the jackpot with free or cheap OEM car speakers.
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Old 22nd March 2012, 06:46 PM   #6
RoboJ1M is offline RoboJ1M  United Kingdom
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Guess it's because summers coming.

My complete n00b thread for a 12V rucksack rig is here

No useful information in it of course!

Although mostly I want to know about how to choose cabinet dimensions.

This speaker efficiency sounds interesting though, what's that all about?

Those amp boards look extremely interesting, especially on the efficiency front.

Knowing that, I then went and found:

2*100W Class-D Audio Amplifier TDA7498
Hifimediy T2 2*100W TRIPATH TK2050 TC2000 10000uf
2 Channel 100Watt Class D Audio Amplifier Board - TK2050

We're going to just use an old left over car amp in the "Mark 1" build.
But high efficiency means you'd turn less precious electricity into heating the bag up!

Anyway, we'll be using a set of these batteries:

12V 7.0Ah Sealed Lead Acid (SLA) Batteries

Also called deep cycle leisure batteries, these are what you have to use, not car batteries. Car batteries are fatally wounded when fully discharged, whereas SLAs are designed to be run flat and recharged!

USB Bluetooth 3.5mm Stereo Audio Music Receiver

Although we'll have a stereo line in, the primary input will be this, Bluetooth!
Also required for this: as 12V-5V step down board, but they're also pennies off of eBay (look for car cigarette lighter usb sockets)

Speakers, we're looking at starting with a cheap set of 6x9s from Vibe
Going to put just one in the bag though, no room for two, or so I think.
Like I said, I know NOTHING about calculating cabinet volume, all I have it this calculator from here:

AJ Software Subwoofer Design Software

But it says Subwoofer on the tin, so that can't be the right one to use can it?

Skate park eh? I wonder if it's in Southsea...

J.

Last edited by RoboJ1M; 22nd March 2012 at 07:03 PM. Reason: Fixed duff link
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Old 23rd March 2012, 09:39 PM   #7
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How low do you want to go? Speaker efficiency makes a vast difference, and generally the lower the frequency, the bigger (and heavier) the speaker you need to need to get that effieciency. If you need it to go thump, thump down in the feely regions, horns get distinctly non-portable, and power requirements grow rapidly.
How hi-fi does it have to be? You can increase apparent loudness (and decrease audio quality) without increasing power requirements by limiting the peaks off. Satisfactory if all you need is to be able to recognise rhythm and instrumentation, local radios do it all the time. Less so if you actually want to listen to the music.

Ice and snow are good for keeping amplifiers cool, less good for bettery life, and quite difficult for some loudspeaker cabints. are you planning on building your own?
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Old 25th March 2012, 01:32 AM   #8
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Has anyone had any luck running bluetooth from the same battery as the amp without noise? I can't seem to do it.

Tor
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Old 25th March 2012, 03:35 AM   #9
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don't think we're all talking about the same thing here.

torclausen, just add simple cheap filters. The power amp probably has a switch-mode power supply or inverter to get more voltage, and that may inject lots of noise on the battery. A cheap electrolytic cap across the power line of each (amp and bluetooth) usually do the job, maybe put a snap-on iron ring on the amp's power cord too, or you could make a ladder filter with inductors and capacitors if necessary. Or use an odyssey-type battery which behaves more like a capacitor to begin with. Usually just one small cheap electrolytic at each device cleans it all up nicely. Get the polarity right if you use electrolytics. It will also sound better while charging the battery. But for best results, filter both the device which injects the noise on the power system (the amp or charger) and also filter the supply to the noise-sensitive device (the bluetooth adapter). I'm talking here about two cheap little 20-volt electrolytics. Maybe a mica disc in parallel too...what does the noise sound like (high frequency or low).

Vivica,
Out skating as in ice skating or ice dancing, or more like rap for skateboarding at the local skate park, or do you need the sound system on your back as you strafe or during in-line skating to exercise & lose weight?

I'm assuming you want some stationary sound system that you can move in your car, not carry on your back? For use at some skating venue?

Consider an Odyssey brand of automotive or deepcycle marine battery if you don't have to carry it with you. It supplies surge current demands wonderfully, is clean, sealed, spillproof, has good charge density, long-lasting and can be run down nearly dead without sulfating and dropping lead to the bottom and shorting cells. If battery life becomes an issue, buy more and add them in parallel. Normal automotive batteries don't take well to major shocks or being discharged.

Stick with 12 volt automotive amplifiers, so you can power the system from a boat or car or charger plugged in, or power supply, or you can charge the PA batteries from your car. And automotive stuff can always be re-purposed.

Avoid a passive crossover if possible; they often waste a lot of power and require larger amplifiers. Consider a 1-way system for a very small system (Bose used to make some "rommmate" passive 1-way), otherwise use an automotive active bi-amp crossover. That will save you a lot of battery power. It also lets you dedicate one power amp channel directly to each woofer. Using several cheaper mass-produced right-sized power amps instead of really expensive higher-powered ones saves a lot of money in the long run. You might find automotive amps with built-in input active crossover swtches, so you don't need to buy an additional crossover device.

Use a class-D or "T" power amp. They are FAR more efficient, lighter, cooler. Saves battery power. Puts out more watts to the speaker.

Use efficient horns whenever possible.

Consider mono if coverage and clarity are more important than stereo soundstage image. Just mix the two channels into one. One good loud mono bi-amped system will be far better than a crummy inefficient 2-channel. Then you can add channels as you please later.

Would need to know more about the exact intended use to make better recommendations.

Last edited by cyclecamper; 25th March 2012 at 03:39 AM.
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Old 28th March 2012, 03:24 AM   #10
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A 3' x 10" chunk of sonotube in a backpack with a decent 10" driver or two would sound good, check with the boominator thread for tips on battery/amp/electronics. Anything smaller can be classified as a run of the mill boombox and the commercial units are going to be heavy.
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