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# replace power supply with battery question? Answered

hi all,
my son and i are getting in to electronics so go easy on us, so we have an old set of computer speakers, we have extracted the amp out and want to use it for a battery powered amp for other speakers we are building. (only small speakers for his treehouse),
the power supply was 6v 400ma, so if i wanted to use a 9v battery, do i need a 6v regulator?
it would be great if someone could give us some info on how to do this.

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I humbly suggest using 4 AA cells connected in series.  This gives you a nominal voltage of 4*1.5 = 6.0 V for the 4-cell stack, and this arrangement will give you more battery life (current*time) than a "9-volt" battery, at the about same monetary cost, or less, since the AA cells are so ubiquitous.

There exist inexpensive battery holders designed to hold exactly this arrangement (4 in a series) of batteries.  If you live in the former US, there is a retailer called "Radio Shack", and they probably have a brick-and-mortar store in a town near you, and they probably have this battery holder in stock.  Here's a link to the product page:

Actually, I think they've got a few different versions of 4*AA battery holders.  One of them is enclosed box.  One has a USB receptacle as its output. Etc.

Also I suspect this particular circuit, your amplified speakers, will be pretty robust with respect to input voltage; i.e. it will work over a range of supply voltage from, and this is a *guess*, like maybe 4 to 10 volts.  Also guessing the actual current draw is much less than 400 mA.  The number printed on the adapter indicates the maximum current the adapter can supply, and moreover that number somewhat approximate, in some cases not even truthful.  I am guessing the actual current draw will be less than 100 mA.

To make a long story short, I suspect this device actually could be supplied by a single "9-volt" battery, if you want to try that.  However, I think using a 4xAA battery holder is a better,and cheaper, way to power this thing with batteries.

So thanks all for the help, I did try to upload a little movie of it in action, but can't seem to upload it. We put the amp in an old power supply case, and a bit of hacksawing and hey presto 4 AA batteries and an Amp, sounded quite good to (for the purpose intended).

I wholeheartedly agree with Jack A Lopez in that a standard 9 volt (rectangular or PP3 style [carbon-zinc]) or even alkaline 9 volt batteries are not very useful for long term medium to high drain use (which is what an amplifier is classified). They just don't have the energy density required for that sort of job. Definitely AA over AAA, or better yet, C or D size batteries for even longer run time. I would stay away from the 6V lantern batteries, just because of their cost.

I looked up the JRC 2073D shown in your photo. It is a low voltage, low power, stereo amplifier. From what I can discern from the DataSheet, it will operate on 1.8 to 15 volts. At 6 volts, you get about 1/4 watt per channel or about 1 watt bridged. Since there are 2 of them, I'd say they are running in bridged mode, so 2 watts total. A little math: 2 watts / 6 volts = 0.3333 amps = 333 mA. A good 9 volt only has about 270 mAh, so you can see where it wont do. Now cheap AA's run from 600 mAh on up, so you'll get about 2 hours at full power, more if you turn it down somewhat. D cells can range around 2 Ah (2000 mAh) or more so you can get about 6 hours from them at full blast.

If you really want to do it up right, go with a SLA (sealed lead acid) or GelCell. They come in much bigger capacities and are rechargeable. Here's a cheap way to get a 6V, 4Ah SLA - go to Home Depot or Lowes and look for their cheap 500,000 candlepower rechargeable spotlight. Last I saw them, they were under \$20. Tear it apart, save the circuit board (take a picture of it as it is woefully under engineered as to parts ratings - you'll thank yourself when you have to replace the resistors that will burn out with higher powered ones) and replace the light with the amp. Turn it on and presto! It'll run for about 12 hours before it needs charging (it comes with a cigarette lighter plug). A full charge only takes about 15 minutes. That'll save money in the long run.

Whatever you decide, have fun with it. And I hope he enjoys the tree house as much as you enjoy helping him trick it out. I'm sure he'll remember this for a long time.

Rock on!
Qa

Again thanks for the information, very much appreciated, good idea to about the torch, might try that next..

Whilst the speakers might work at a very low volume, a little 9V battery really won't hack it. 9V rectangular batteries are nasty, Expensive, and very low capacity.

If the PSU isn't marked as "regulated", its output could easily be more than 9V anyway, so you are unlikely to have a problem.

I just noticed the PSU in the picture is marked as regulated.  It has the word "regulated" written on it, although I had to turn my head sideways to read it.
;-)

Yeah, picture wasn't there this morning when I posted my answer.