[LAU] [OT] Building a battery powered PA

[Folderol]

On Fri, 28 May 2010 11:13:46 +0200
Jörn Nettingsmeier <nettings at folkwang-hochschule.de> wrote:

> On 05/27/2010 06:36 PM, Bob van der Poel wrote:
> >> A friend of mine was talking about how he would like to be able to busk with
> >> his keyboard, and I said I would look into the possibility of building him a
> >> battery powered PA for it. Does anyone have any suggestions for good ways of
> >> going about this? We would like to keep the cost as low as possible.
> >>
> >
> > How about an off-the-shelf UPS? Don't know how big you'd need, would
> > depend on the gig time, etc. There is probably a way to calculate that
> > :)
> >
> > The consumer UPS units I've see all have lead-acid (sealed) batteries
> > and are quite reliable.
> 
> not really recommendable for a permanent setup. with an ups and standard 
> mains equipment, you will waste battery (and carry dead weight) twice: 
> in the inverter that generates grid voltage from the battery, and in the 
> PSUs of your gear that will re-transform the precious grid voltage back 
> into low-voltage DC. throw out the middle man.
> 
> the nice thing about car equipment (as a previous poster has suggested) 
> is that all the gear is standardized to work at 12 volts dc. many car 
> radios have line ins these days (for file players etc.). it's easy to 
> hook up a small mixer here - you should be able to find a cheap one by 
> the Brand That Shalt Not Be Named or one of the other manufacturers they 
> steal their designs from - i've seen a few that take 12v dc.

How much sound output power do you actually need? In bridge mode an
amplifier running off 12V can deliver a maximum of 18W RMS into 4ohms.

Here in the UK Maplins supply a suitable power module (they call it 40W
- that's peak power) very cheaply - 15.99ukp
http://www.maplin.co.uk/Module.aspx?ModuleNo=37737

It's actually a Kemco product. I don't know how available that would be
in other countries.

At 500mV sensitivity it will probably be a pretty good match for
direct connection to most keyboards. If not, a small buffer amp could
easily be patched in.

I don't recommend using a car battery. They are not designed for steady
discharge, but for short bursts and long recharge times. Although
initially more expensive you would do a lot better getting a sealed gel
type lead-acid battery with suitable charger.

-- 
Will J Godfrey
http://www.musically.me.uk
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.