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Battery power for street amp....

Started by darwindeathcat, January 07, 2012, 08:11:02 PM

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darwindeathcat

About a month or two ago, there was some discussion (http://www.ssguitar.com/index.php?topic=2305.0) about building 12 volt battery powered amps with more power than the typical 1-5 watts you get with the typical 9volt designs. The discussion mainly centered around battery technology, and which one would give you the most output for the buck, as well as the one that would be the lightest (important for a battery amp). Well, I found the former (best bang for your buck), and bought one for a little combo amp I built for street busking. It's a 12v 7aH Gel-Cell marine battery meant for fish finder: http://www.amazon.com/Humminbird-7700281-Portable-Gel-Cell-Battery/dp/B000N314J0/ref=sr_1_1?ie=UTF8&qid=1325984465&sr=8-1

The battery weighs in at about 6 lbs, so it ain't light! But, for a little less than $30, you get the battery and a charger (not a smart charger, but not a totally dumb charger either. An "average intellegence" charger, if you will). The amp is 20 watts, class AB, discreet transistors (I didn't build the amp, it's a PA head I integrated into a cab). I've played it on battery power at a few busking sessions now, the longest was about 2 hours, with no sign of power loss. My math tells me I ought to get about 4 hours playing on full, but since I don't turn it up past 4, I'm not sure how long could actually go for (maybe 5-6 hours?).

Here's my review that I posted on Amazon:

QuoteThere are a few other 12v 7AH SLA batteries in this price range that also come with chargers. It's hard to tell the difference between them from the info posted here on Amazon, but this one has the most reviews, all positive, even though it's a couple of dollars more expensive than the other ones. So why buy this one instead of those others? Well, partly because this one is sold directly from Amazon.com. That's a big plus because if you live in an Amazon hub city (like Phoenix, where I live), you can get your package very quickly even with the free super saver option. In fact, I ordered this battery at about 4pm on a thursday, and the package was delivered to me at 9am the next day! This is with the free super-saver shipping! Secondly, this one comes with a slightly nicer charger than the others do (at least judging from the pictures of the other chargers). The charger that comes with this one has a nice LED status light that changes color when the battery is charged (from yellow to green), and also has cross polarity protection that will keep the battery from shorting (and light a red warning light) if you hook it up backwards (VERY nice thing to have).

I am using this little 6lb battery to power a small 20watt amplifier I built for busking on the street. I get several hours of play time from it, and it holds a charge very well. Like all SLA batteries, you have to maintain it properly, and be sure that it never drops below 11.5 volts, or sulfation will occur, and the battery will be permanently damaged. But if you keep it charged up, and don't store it for too long, it should last for many years!

In sum, this is a great deal on a very good battery.
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joecool85

Any info on the PA that you are powering with the battery?
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

darwindeathcat

It's a Realistic, MPA 20. That's the RadioShack brand. But it's one of the older ones (maybe from the 70's?) with discreet power transistors rather than a chipamp. It sounds really good, actually!
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joecool85

Quote from: darwindeathcat on January 12, 2012, 07:08:12 PM
It's a Realistic, MPA 20. That's the RadioShack brand. But it's one of the older ones (maybe from the 70's?) with discreet power transistors rather than a chipamp. It sounds really good, actually!

Neat, looks like it was designed to take AC or 12v.  I was thinking you hacked a PA apart and "made" it take battery voltage.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

darwindeathcat

Yeah, I suppose it sounded like a way cooler conversion than it actually was!  ;) One thing I DID do, however, was to add a rotary switch to change which pin of the OT connects to the negative feedback loop. At the expense of some volume I can get it squeeky clean (i.e., as it was supposed to be), or if I want I can get it to have some pretty nice breakup (i.e., so it can be used more like a regular guitar amp).

Believe it or not, I found that MPA20 head in the Goodwill on sale for $8. The only thing wrong with it was a blown fuse. Gotta love those kinds of thrift-store finds!
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