battery powered p.a. system

[konnektor]

i want to build/locate a battery powered stereo p.a. system for street music performance.

it's the power amp i need guidance with most.

i need a preamp which takes line and mic level signals, preferably with 2 x phantom power on the mic pres, no more than 4 inputs total.

(at times, i'll also use a usb audio interface with my macbook which will give me all the inputs and phantom power i need, but it would be good to be able to play without a computer involved).

i'd use efficient 1x10" speakers, and i'd only need 20W or so per channel.

i'd need the batteries to last 4 hours minimum, assuming room temperature and fairly constant usage over that time.

my guess is that the technology is available, and a lot cheaper than is commercially available as a finished product...

any pointers?

 

[JMFahey]

I have made tons of such amps with TDA2005 : 16W rms each driving a 4 ohm speaker.

You can use multiple amps fed from the same 12V battery, each driving its own speaker.

One of them will last 7 or 8 hours from a 12V 7A "alarm/gel cell" battery, 2 of them will last 4 hours.

Now there are some bridged class D amps for car radio with a little more power but specially more efficient.

 

[konnektor]

cool - thanks. if you have any links for suppliers of the batteries and those bridged class D amps, that would be helpful.

 

[mr. doom]

I use a 2050 chip @24v with two or six 10" drivers.
ClassD audio and 41hz have nice boards. The Sure boards are cheap but I have one that I run pretty hard and it is still going strong. 2020 chips will use less power @12v.

 

[csom]

I have made tons of such amps with TDA2005 : 16W rms each driving a 4 ohm speaker.
......................................
.

Dear Sir,
Could you give me some idea/ share of the preamplifier section of this cute amp?

 

[dangus]

Searching ebay for TA2020 and TA2024 will turn up a bunch of amp boards.

If you have a source for old DVD players, some players had a karaoke mic input board that ran off a single 12V supply and included an echo/reverb chip.

Craig Anderton's classic book, "Electronic Projects For Musicians" has various circuits that could be useful. The book can be found as a PDF online, and people have created updated board layouts since Craig used some opamps with unpopular pin arrangements.

 

[cyclecamper]

The key is going for efficiency. That generally means class-d switching-mode amps, and starting from 12volts you can find efficient car audio power amps that are plenty powerful. But car speakers are not efficient; go for horns if you really want coverage. And you need an electronic active crossover before the power amps, not a passive one at the speakers. That will do the trick.

 

[cyclecamper]

You also might check out Optima batteries or if you can afford it the new lithium motorcycle batteries are really light.

 

[Paule133]

Hi. I have also tried building a simple PA system with 3 mike I/P's.
Seems simple ... Mikes -> preamp -> amplifier -> speaker.
However with preamp and amplifier running off same 12v power supply I get severe oscillations with the volume at less than halfway.
I've tried this with several different amps/preamps (even radio mike receivers) and always the same problem.
If I run the preamp off a separate battery then no problem.
I need to run it all off one battery so how do I get rid of these oscillations.
Warning! I am not technical with electronics so please be very gentle with me.

 

[Gnobuddy]

I need to run it all off one battery so how do I get rid of these oscillations.

The problem you described occurs because the power amp draws a fluctuating current from the battery. In response, the battery voltage fluctuates slightly. When you feed the preamp from the same battery, those tiny voltage fluctuations get into the preamp, where they are amplified, and fed back to the power amp.

Now you have positive feedback (same concept as a microphone too close to the P.A. speakers), and you get oscillations.

The solution is to keep those tiny battery voltage fluctuations from getting back into the preamp. In most cases, this is quite easy to do - insert a resistor (R1 in the attached figure) in series with the positive battery rail. Connect a capacitor (C1 in the figure) directly from positive power supply rail to negative power supply rail in the preamp.

Together, R1 and C1 form a low-pass filter. They let DC electricity flow through from the battery to the preamp. But those small AC fluctuations in battery voltage are filtered out by R1 and C1, and never make it back to the preamp.

-Gnobuddy

 

[Paule133]

Oscillations on 12v PA sytem

The problem you described occurs because the power amp draws a fluctuating current from the battery. In response, the battery voltage fluctuates slightly. When you feed the preamp from the same battery, those tiny voltage fluctuations get into the preamp, where they are amplified, and fed back to the power amp.

Hi Gnobuddy
Thanks so much for this clear reply. I had in fact found this solution on the web and was astonished that just a simple resistor solved the problem completely. However I did not understand why it worked. Now I do. Thanks again, Paul

 

[Gnobuddy]

...just a simple resistor solved the problem completely.

It does require a capacitor too, probably there was already a capacitor in the preamp, which worked in conjunction with the resistor you added.

And I agree, after all these years, I still find many things about electronics quite amazing. A century ago, we humans figured out how to accurately control streams of trillions of millions of electrons to do our bidding. How amazing is that?

Thanks again, Paul

You're very welcome!

-Gnobuddy

 

[Sawdust123]

Finding phantom power on battery powered speakers is going to be tough. The only solution I am aware of that provides this is the Carvin S600b. If you can forego the phantom power by using an inline power supply, the choices go up dramatically. Samson and American Audio have a few models that will fit the bill for a much lower price.