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downhere 3rd October 2001 09:30 AM

I have no idea which designs are applicable, but I was thinking of building an amp to drive two mini compo speakers I have lying around with leftover computer parts, e.g an old 230W power supply lying around, and mounting it in an old case.

These speakers are 6ohms and probably can't handle or don't need the 50 watts or so. I'm guessing 10W per satellite maximum.

Are there any amp projects out there that are cheap and easy to build?

Secondly, does anyone know the specs of the Cambridge soundworks satellites? I have 2 leftover without a sub. Wondering if it'll work with the cheapo amp.

paulb 3rd October 2001 01:23 PM

"cheap and easy to build"
Check out the LM1875-based power amp at the ESP site: Project 72 at
There are others there that might be suitable.

paulb 3rd October 2001 01:40 PM

Oh, I just noticed you said you want to use the computer power supply - it may not be suitable, its main outputs are probably 3.3 and 5 volts.
See if it has +12 and -12 volt outputs available, and how much current on each. But they're likely to be a pretty noisy source of DC power.

downhere 3rd October 2001 02:07 PM

PS specs
Thanks, I was considering that, but I have no idea how to obtain the IC... maybe I'll check around the electronic parts shops.

The PS does +12V 12A, -12V 1.0A, +5V 30A, -5V 0.3A, +3.3b 15A.

got those numbers off the HowStuffWorks site, suppose my PSU won't be all that different.

paulb 3rd October 2001 02:14 PM

Don't know where you're from but Digi-Key (US, Canada) has the part. It's from National Semiconductor.
You can probably run it off the +12 and -12 supplies (I think you meant +12@1.2A, not 12A).
If you order the ESP circuit boards they come from Australia...

hifiZen 4th October 2001 05:35 AM

hmmm.. computer power supplies usually concentrate all their current capacity on the 3.3V output for the processor core, and also the +5V output. Often, you'll find the -5 and -12V supplies will be very weak...

I'd look for the PSU's specs online somewhere (if you're lucky, you'll find them), or perhaps trying to test their current capability. If you're really stuck on using this surplus PSU because you don't want to spend any $$, then you may have to make the amp a single-supply jobbie, with AC coupled outputs to the speakers.

Personally, i'd say it's not worth the trouble, and just go build a simple linear supply...

paulb 4th October 2001 07:03 PM

I agree. Computer power supplies are not well suited for what you want, you also need a minimum load or they go wonky.
Downhere, how about (say) buying an old amp or receiver at a garage sale or something, ripping the transformer out of it, and use that? Make sure it's old enough that it uses a linear, not switching, power supply (if it's heavy it probably does).
There's lots of old dead equipment out there because it's cheaper to buy new stuff than to fix old stuff.

PassFan 5th October 2001 02:15 AM

Power Supplies
A good place to get free power supplies and parts are electronic repair shops. They are always throwing stuff out that is not repairable and will sometimes let you haul it off for free. Ive gotten a lot of transformersand cases for nothing. Just be sincere and explain that you are a diyselfer just starting out learning and most will generally help you out.

downhere 5th October 2001 10:30 AM

Thanks, in fact I have an old pioneer amp thingy hanging around, and it no longer works... smoke comming out, smells like salted fish or something. It still runs tho, and I have no idea whats wrong with it so maybe I'll just take the transformer out.

I wanted to build the amp because of the speakers I have around, 2 pairs of identical speakers from the Pioneer amp/cd/tape/graphicEQ thingy... hoping to rig up a surround system for my comp.

:) Mebbie when I have time I'll try and hook the graphicEQ up too...

Thanks for the ideas!

BTW, I asked National Semi for samples, hopefully they'll send 4 of those ICs to me in a few days time. Has anyone ever tried their Sample service?

paulb 5th October 2001 12:58 PM

Yes, I got samples quicker from National than the National parts I ordered from a supplier.

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