Powering a portable speaker externally

[Nazo]

Ok, I recently bought a relatively cheap-ish little portable speaker that only runs off of three AAA (LR03) batteries. The problem is, it manages to use enough power to run the batteries down relatively quickly apparently. I was thinking that it would be nice if I could power it externally. I thought about getting something such as a universal power adapter, but I really don't want to spend a lot on this. After all, if I had a lot to spend on such a thing I wouldn't have bought a relatively cheap speaker for the task. Once I thought about it a bit more it occured to me that it really isn't that far below USB power and being able to power it via USB would really help nicely. Especially since that would go along well with my EeePC.

But I don't really know enough electronics to really be sure how to do this properly. Firstly, USB power can vary and may not be 5V. Given that it can range from 4.75V to 5.25V that makes this a little bit harder. The speaker is designed under the assumption it will be powered via batteries. I'm not sure what the lower limit is, but I think it may be designed to accomodate very little lower than rechargables at 1.2V because I tested the batteries after it basically failed and they registered around 1.0V, but it started sounding fairly bad well before it finally stopped, so there's a chance it requires more than 1.1V per battery even and it may be best to assume the 1.2V typical of rechargables is the best minimum. Since it is designed for batteries though, I really doubt it's designed to handle a higher voltage than 1.5V per battery (or at least I'd rather not push my luck since it still cost enough that I prefer not to break it.) So I guess that puts my range at 1.2*3=3.6V to 1.5*3=4.5V. That's still a pretty good range I guess, but the trick is getting there and keeping it there.

So my main problem is getting the voltage down without going too far down. I was kind of wondering if I could get away with just using resistors. The problem there is that I don't even know how to tell how much resistance to use to get there even if I could use this method. Or do I just need to use something else regardless? I'm not really sure how much current it will draw. I would assume it wouldn't be a really large amount since it's just a little battery powered speaker, but I would prefer that it didn't melt down from too much heat or something, lol. Would it maybe require more voltage regulation? It can handle the voltage changing somewhat due to battery usage obviously, but that's slow and very clean power by comparison. Then again, while I do plan to power it via a USB power plug from time to time, I think most things do regulate USB power at least a little bit -- even those plugs surely do some, though probably not nearly as much as a PC will do (and obviously a laptop running off of a battery will have pretty clean power, though I'm not sure about when it's running from the power plug since the EeePC uses a fairly average AC/DC adapter rather than the tough little PSUs normal laptops use.)

Any thoughts?

 

[gee]

If it runs off 3 AA's, it'll definitely handle 4.5V. And almost any chip that runs off 4.5V will safely run off 5.25V.

If you want to be 100% sure, haul the speaker apart. There's probably a single chip inside which does the amplification. Look up the part # and you can probably find the datasheet for it online, with the maximum voltage ratings.

 

[Nazo]

Hmm. It's kind of buried in there. It didn't occur to me that it might be able to handle more, but I'm not sure if I can get to it right now to see its designation. What about the rest of the circuitry though? Can the rest handle it? It won't really matter if the chip could handle 10,000 volts if the rest of the circuitry goes up in smoke when I provide power to it, lol.

 

[Fenris_Ulf]

You will be fine running it at 5v. The amp inside there is probably designed for 5 volts and is being run under-spec with the batteries. Most of those amps have a minimum voltage of about 2.7 volts, which is why they perform poorly when the batteries run down. It's probably something similar to this:
http://cache.national.com/ds/LM/LM4888.pdf
but it could be any member of that family or a similar one made by TI, Analog Devices, Maxim,or a Chinese knockoff. Performance and specifications are all similar. Most small amp chips are designed to use one of the "standard" operating voltages in small electronics - 3.6v, 5v, or 12v. They also have some safety margin built in. Since you can run yours on 3 AA batteries, it probably has a nominal 5v chip.
Using the above referenced chip, which is the highest powered in its family, maximum power is 2 watts per channel, but you probably won't use that much. USB is 5v at 100ma or 500ma. You should be able to power it off a USB port if the port can provide the full 500ma in the powered USB spec. Unpowered hubs can usually only provide 100ma. The port in your Eee should be able to provide the full amount. All you have to do is build an adaptor which can be as simple as modifying an unused USB cord. You don't want to use both USB power and batteries at the same time - the higher voltage of the USB wil try to charge the batteries and probably end up destroying them through improper charging. If you need details about how to build the cable or how to hook things up, let me know.

 

[Nazo]

You will be fine running it at 5v. The amp inside there is probably designed for 5 volts and is being run under-spec with the batteries. Most of those amps have a minimum voltage of about 2.7 volts, which is why they perform poorly when the batteries run down.

Well, I understood what you were saying about that. My question is, what about the other components. For example, resistors and capacitors. It doesn't use the big kinds that stick out and have clear labels.

Using the above referenced chip, which is the highest powered in its family, maximum power is 2 watts per channel, but you probably won't use that much. USB is 5v at 100ma or 500ma. You should be able to power it off a USB port if the port can provide the full 500ma in the powered USB spec. Unpowered hubs can usually only provide 100ma. The port in your Eee should be able to provide the full amount.

The Eee can provide the full 500mA, but I'm kind of hoping it won't have to. For one thing, you're supposed to be able to power this thing with AAA batteries -- the second smallest of the common household betteries (the smallest being the AAAA rarely seen outside of a 9V battery which in turn isn't seen often anymore) -- so if it were using that much power I'd be worried. I'd really rather it didn't use more than 100mA though as USB specs call for what they call gadgets to use no more than that since they don't negotiate properly. It's probably ok, but I'd rather not test that too thoroughly. The main problem is, if it doesn't negotiate, the bus is going to reserve only 100mA and should it use more it won't be available to other devices on the same bus and can cause some problems besides. (In particular, I'm still not sure which USB port is sharing the same bus as the SDHC card reader.) Luckily its startup power isn't much, which is probably the biggest requirement in USB specs. BTW, USB devices can use any amount in increments of 100mA from 100mA to 500mA, so it's entirely possible for something to reserve, say, 400mA. Admitedly you don't see this much since most USB devices are either really low powered or really high powered.

If you need details about how to build the cable or how to hook things up, let me know.

No thanks. On that subject though, if you don't already know about it, here's a REALLY useful site: http://www.pinouts.ru/ It has saved me from a world of trouble on more than one occasion.


BTW, that quote in your signature: "We've already got enough youth. How about a fountain of smart?" Who said that? Lol, I think I may have to quote that elsewhere...

 

[Fenris_Ulf]

Well, I understood what you were saying about that. My question is, what about the other components. For example, resistors and capacitors. It doesn't use the big kinds that stick out and have clear labels.

Resistors (depending on type) should probably be good to over 100v, but I've never seen a resistor that wasn't good for at least 10v. Capacitors usually have one of several standard voltage ratings, the lowest one generally seen is 6.3v. You should be fine with 5v from the USB port.

The Eee can provide the full 500mA, but I'm kind of hoping it won't have to. For one thing, you're supposed to be able to power this thing with AAA batteries -- the second smallest of the common household betteries (the smallest being the AAAA rarely seen outside of a 9V battery which in turn isn't seen often anymore) -- so if it were using that much power I'd be worried. I'd really rather it didn't use more than 100mA though as USB specs call for what they call gadgets to use no more than that since they don't negotiate properly. It's probably ok, but I'd rather not test that too thoroughly. The main problem is, if it doesn't negotiate, the bus is going to reserve only 100mA and should it use more it won't be available to other devices on the same bus and can cause some problems besides. (In particular, I'm still not sure which USB port is sharing the same bus as the SDHC card reader.) Luckily its startup power isn't much, which is probably the biggest requirement in USB specs. BTW, USB devices can use any amount in increments of 100mA from 100mA to 500mA, so it's entirely possible for something to reserve, say, 400mA. Admitedly you don't see this much since most USB devices are either really low powered or really high powered.

No doubt. Some USB ports won't provide the extra current without the negotiation, some will. Personally, I think you'll be fine - I was just giving a worst case scenario and showing that even with max current draw it will probably be sufficient to power the speakers.

BTW, that quote in your signature: "We've already got enough youth. How about a fountain of smart?" Who said that? Lol, I think I may have to quote that elsewhere...

I saw it on a sign of a local lighting store and it always stuck with me, I don't know the entimology of that particular quote unfortuneately.

 

[Nazo]

Resistors (depending on type) should probably be good to over 100v, but I've never seen a resistor that wasn't good for at least 10v. Capacitors usually have one of several standard voltage ratings, the lowest one generally seen is 6.3v. You should be fine with 5v from the USB port.

Gotcha. I figured the capacitors would be fine as it's probably more likely they'd be 10V or something rather than lower than 5V, but I must admit I still don't know a lot about resistors either (though not as bad as the way I still can't get any sort of feel for how capacitors ACTUALLY work, lol.) Anyway, I guess I'll give this a shot as soon as I get the chance.

No doubt. Some USB ports won't provide the extra current without the negotiation, some will. Personally, I think you'll be fine - I was just giving a worst case scenario and showing that even with max current draw it will probably be sufficient to power the speakers.

Yeah, I'm hoping that 100mA is enough. I just remembered though. When I used my external harddrive with a two connector USB cord, I usually plugged in the power cord first so it would be less likely to have troubles starting up (I still worry that when they start up with insufficient power and click repeatedly that it can't be good for them) and that would of course pull very nearly the full 500mA without negotiation (though it does allow a split second for the bus to adapt I think.)

I saw it on a sign of a local lighting store and it always stuck with me, I don't know the entimology of that particular quote unfortuneately.

Darn. I may still quote it even if I can't give proper credit where it is due.