While I'm as much a sucker for reinventing (or at least jerry-rigging) the
wheel as the next guy, they do make battery-powered combo amps just for this
purpose. Well maybe not specifically for playing out in a field at a VW
gathering, but you get my drift.

Cya,
Robert

----- Original Message -----
From: "Matt Roberds" <mattroberds@COX.NET>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Tuesday, April 17, 2007 4:49 AM
Subject: Re: Everybus photos

>> From:    The Bus Depot <vanagon@BUSDEPOT.COM>
>> Date:    Mon, 16 Apr 2007 23:04:12 -0400
>>
>> Electronic keyboards are no better, since these are usually acoustic
>> jams in the middle of a field with no electricity; wandering up with
>> a cheesy Yamaha plastic keyboard with 4 inch speakers and D batteries
>> just wouldn't cut it.
>
> Get a guitar amp with a good speaker but thrashed electronics, or an old
> stereo speaker.  Also get a two-channel car stereo amp (50 to 100 watts
> or so) that has bridgeable outputs, and a couple of gel-cell batteries.
> Put the amp and the batteries in the bottom of the cabinet and wire it
> up.  Also wire the batteries to a connector on the outside of the
> cabinet, so you can (Vanagon content) plug it into the van to recharge
> while you're driving home.  If it's an old stereo speaker, mount a
> handle to the top so you can schlep it around.  For a guitar amp or a
> speaker, you could even put some lawnmower wheels on it if you're feeling
> really lazy.  :)
>
> Using the power of math, I will try to show that this isn't an insane
> idea.  50 W is 4.2 A at 12 V; the amplifier won't be 100% efficient but
> you could probably get 66%, making it draw about 6.3 A.  The keyboard
> itself probably doesn't need much - maybe another 1 A @ 12 V, giving you
> a total draw of 7.3 A.  A 12 V, 12 Ah (20h) gel-cell weighs about 8.8 lbs
> (4 kg) and could give you 7.3 A for an hour before being completely dead.
> This is an hour of tunes at 50 W continuous - in other words, turn the
> volume up all the way, tape down the lowest key on the keyboard, and walk
> off.  At less than full volume, and with not playing speed metal on the
> piano, the average draw will probably be less, making the battery last
> longer.
>
> Matt Roberds