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Old 6th May 2012, 05:07 PM   #1
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Join Date: May 2012
Default mobile PA placed in van battery powered

Hello,
i am currently planning a diy Pa system that will be built into a Volkswagen bus transporter. It will be used for spontanious street gigs for a band playing music in the field of Disco/House (Voc, Bas, Guit, Keys, Drums).
It must be possible to start immediately after arriving at a spot, so it should be:

runnning on 12V Batteries (currently 2 150A boat batteries)

At least the woofer should be working from inside the bus.
the audience in a circle of about 15meters should be able to get a club/live feel with enough low end power.

The drummer and keyboarder will stay in the bus, the singer, bassplayer and guitarist will stand infront of the open sidedoor. only the singer will use a microphone all other instruments are not feedback critical, so it will be possible to play with the pa behind the musicians.

i am thinking to use a hornloaded design, custom fitted to the available space inside the bus, leaving enough room for the two musicians inside the car. The mouth of the bass-horn should look outside the side door. the tops will be mounted on a rig that will swing out of the side door over the roof of the bus.

i hope this gives a rough expression what i am planning.

I will start with calculating/building the bass system.
would you agree the best solution will be a high efficiency bass horn?
Do you know similar systems from which i could learn?
My biggest question is about the size of the speakers because i have only small experience in outdoor pa systems.

any help/tips are welcome.
thanks in advance,
Joa
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Old 7th May 2012, 08:48 PM   #2
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Join Date: May 2010
Location: was Chicago IL, now Long Beach CA
You've got a real problem with power, space, and efficiency. I've designed a few 'stage trucks' and I prefer to start with a flat-bed with two boxes on hydraulic arms, so the boxes lift above and to the sides of the flatbed stage. And then it 'plugs in' or plugs into a big rental generator trailer.

You need really efficient power amps as you have no power to waste. You could get a big high-efficiency inverter and standard high-efficiency switching-mode PA amps, or you might look for high-efficiency automotive amps with fewer voltage conversions.

You really need efficiency. So all my favorite bass designs are disqualified. You really only have one practical choice for real bass, and that would be a tapped horn, since there's nothing else that's as efficient. A tapped horn can be twice as efficient as a conventional horn. Probably on the roof, or built as a new roof. You will need to bi-amp or tri-amp for efficiency too; passive crossovers waste power and then burn off more to balance the bands. Think horns all-around and tapped horns for the bass.

You will need some lights, and should consider HID spots and LED washes, because again you need efficiency. HID automotive spots. Disco LED banks run from an inverter.

Let's say you get your power requirements down to only 1000 watts. That's 200 a side for bass, 200 a side for lights, and 100 a side for everything else. Odyssey batteries, Optima batteries...you really need a hundred thousand dollars for lithium-ion. What you need is a hydrogen energy cell. So consider just one tapped horn sub.

One $800 Odyssey PC1800-FT might have only 100 amp hours draining that fast for a 2-hour show. You have to de-rate when you drain fast; they work best for a 3-hour show or with some simultaneous charging. That's about 33 amps or 396 watts for 3 hours. 2 would not do for any more than a few songs; you'd need 3 or 4, figure about $3000 for batteries.

Consider adding 2 Wrangler dual alternators to the van engine. That will keep 4 batteries going, but you'll want a long long hose on the van's exhaust. Even then you might need some recovery time between tunes. Or to tow a quiet generator in a trailer you can park 100' away.

Now if you want to hit-and-run and just do two 3-minute tunes, no lights, you might get away with just 2 good marine batteries.

Any amplified instruments? Consider 4 more marine batteries and a big inverter, unless you want them to plug into your PA.
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Old 7th May 2012, 10:30 PM   #3
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Hey Cyclecamper,
many thanks for your answer.
One point is that the max. duration of a show will be 30minutes. the gigs will be announced online and the band will play a few songs before moving on.
Here in berlin there is a real street band scene and the audience is aware of the "not optimal" sound quality of these improvised spontanious gigs. they often use simple battery powered guitar amps as a amp for vocs/keys and guit but even these manage to get a small crowd moving if they are good...

I will go a little further but it should not loose the character of such hippie-style concerts.

all instruments will go directly into the mixer. I will use a custom fitted Dc/Dc converter for powering the mixer .

The hint with the passive crossovers was very helpful.

My motivation is to get the max out of a limited budget, with a good combination of diy and uncommon solutions.

I am very fascinated by the horn systems and i learned already a lot about the vintage pa speakers which had similar problems as the battery powered ones because the amp technology was not able to deliver the power todays amps do. so high efficiency was also a main design aim.

I think a good way will be to use a well known design and "refold" the horn to fit in the boot of the volkswagen. the space there is 140cm*130cm*70cm and another 50cm*50cm*150cm under the back seat , that is about 1500 litre of usable volume. i think this should be enough to build a suitable system. I do not want the speakers to be mounted outside (like on the roof) everything should fit inside, but it would be cool if i will find a solution that the tops swing out of the sidedoor above the car, and the mouth of the bass speaker leads to one of the door openings. perhaps also as a movable part that swings in the right direction after the doors are opened.

a small petrol driven generator would be possible as a backup but not as smart as a complete battery solution. fuel cells (i know them from boats) are cool but perhaps too high-tech for my taste ...

could you suggest any common horn design for the bass section to start from?


and many thanks again for your long answer,
joa
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Old 7th May 2012, 11:55 PM   #4
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Join Date: May 2010
Location: was Chicago IL, now Long Beach CA
One more bit of advice: forget stereo and go mono. You'll get a lot more sound for your power, space, and money and a lot less to set up or go wrong.

A tapped horn is similar to a normal folded bass horn, but with an extra feature.

Like normal horns, one side of the driver works into a small space with a slot that acts as an acoustical low-pass filter and also is the start of the horn taper. Then the horn goes along its way expanding as it goes, folds once or three times or whatever, and when it gets about 1/2 wavelength down the horn and the sound is about 180 degrees changed in phase it encounters...THE OTHER SIDE OF THE SAME DRIVER STICKING THRU THE WALL. Normally the sound from the two sides of the driver are out of phase, but now one side has changed phase by 180 degress so both are pushing together in-phase there and down the rest of the horn. Neat trick uses the energy from both sides of the driver. If you use multiple drivers that enter along a length of the horn it also adds bandwidth.

Maybe one Danley Sound Labs DTS-20
DANLEY | TAPPED HORN SUBWOOFER
and a used Altec Lansing 311-B horn for mids and highs
and some mid-bass cabinets with 2 12" drivers in a direct (non-folded) horn.
or maybe a Danley SH MINI if you can come up with a bit more power:
DANLEY | SYNERGY HORN
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Old 9th May 2012, 11:31 AM   #5
dangus is offline dangus  Canada
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That sounds like fun. I agree with the bass horn, tapped or some other proven design (see Speakerplans - there's plans (not surprisingly) and a community of DIY sound system folks). Use a mono class-D car amp for the bass, even if you build two horns. For the main speakers, a car amp in the 100 to 200 watt range (maybe a 4 channel amp, bridged) should be enough if the speakers are in the 100 dB/1w/1m efficiency range.

I suspect power consumption won't be an issue: music has dynamics, so the average power could be 1/10 of the peaks (a mere 40 watts if you have a 400W RMS amp). 30 minute sets off a couple of boat batteries should be no problem.

Last edited by dangus; 9th May 2012 at 11:36 AM.
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Old 9th May 2012, 12:18 PM   #6
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For efficiency you are almost certainly going to have to go for an SMPS power supply to lift the 12V up to Power Amp voltages. Then you would be better with one of the digital classes of amplifier to give you the power, punch and efficiency that you are looking for.

High Power car amps are OK but they're not renowned for their reliability. OK some are MUCH BETTER than others.

A DIY SMPS power supply and proven amps would be better for maintainability in the event of something going wrong.

2-5Kw generators crop up in local pawn shops occasionally.
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Old 9th May 2012, 06:28 PM   #7
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I tend to agree with cyclecamper; you're not going to get loud enough long enough. Last time I did something like this (and it was long enough ago that I used class AB amps) I had plenty of speakers from the epoche when power was expensive, so they built efficient (some Vitavox fifteen watt light cone, low excursion twelve inchers, excellent for horn drive, among others), used two batteries in series to get twenty four volts, and put a step-up autotransformer on the output, and still had difficulty pushing the vocals through the (unamplified) drummer. I put a voltage doubler on the alternator to charge the batteries, and frequently had to run the engine during the performance to maintain power.

We still invested in a Honda generator after a short period touring, and built it a somewhat soundproofed box. It removed lots of problems, and I had a quick changeover system in case there was ever any line AC available (which there quite often was), and a long extension lead (so I could climb up 32 kV pylons with a pair of crocodile clips? Actually, no, but I did once plug into a street light).
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Old 9th May 2012, 08:49 PM   #8
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this forum is great! Many thanks for all your response. This weekend the band will run the first testgigs with equipment that lay around in my workshop. 2 car audio amps with integrated active crossovers. a 2*12" reflex subwoofer i built many years ago with jbl pa speakers (do not remember the exact type ) 4 2-way speakers (8" low mid and tweeter) which i normally use as dj or keyboard monitors on stage. this system could improved in many ways but we just want to see if the basic idea works before i invest more time and money for further steps. it sounded quite ok in a short test session. i will post how it worked...
joa
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Old 9th May 2012, 10:08 PM   #9
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In theory there are reasons to go with automotive amps. All good automotive amps have built-in circuits to increase the supply rail voltage nowadays. The trick will be to find amps with a really efficient switching mode amplifier section. Alternativley, if you use a big inverter, it ups the voltage, then a conventional amp plugged into the inverter lowers the voltage again, with losses at each conversion. Then again, some big inverters start with 24 volts from the batteries, which can represent some improvement in efficiency. And like someone else said, it lets you plug into a streetlight etc. if necessary. If you are 12-votl based you need a really big charger for each battery in order to recover quickly.

Street performers often face similar issues for battery-powered guitar amps and/or PA. Are the guitars fussy about tone? If so, you may find only an inverter and real tube amp can keep them happy.

Using marine batteries, you should consider adding a box full of cheap electrolytic capacitors right at each amp. And if you're sticking with 12 volts, remember that the power must all be in current when the voltage is that low, so use hefty 12v wire; there's no power to waste making small wire get warm, so use large wire. If you use Optima type batteries with the round cells of rolled foil and acid/gauze they act somewhat like a capacitor already. Hefty speaker wire will avoid losses there also if the speaker impedance is low. Speaking of which, you will want the speaker impedance as low as the amp will tolerate. Consder carefully where you stash the amps, as they will need to keep cool. Also, make sure the marine batteries are vented to the outdoors while charging, just in case they vent hydrogen.
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