Raducu

Hello.please excuse my gramar mistakes..i'm asking for your help bicause i want to build a power suply for a 2x50w(rms) audio power..and i have 5 (200w each)computer power suply's did anybody now;s how to make them give me 2x40vDC(never mind if curent is smaller)or how to build one smps power suply(a project :?: ) i want them bicause ar smaller,i have a friend wich build 200w smps power suply but he don't want to to show me too,so i decied to learn and found how to build one..

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Downhill maniac

Nigel Goodwin

You would have to rewind the transformer in a PC PSU to give +/-40V, it's more usual to use linear PSU's for audio amplifiers - SMPS's are probably too complicated and a source of problems?.

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Dr.EM

They are sometimes used now for amplifiers, this is where i've seen them:

http://www.bwspeakers.com/index.cfm/...Model%20ASW825

But still quire a rare i'd think. The switchng is likely to intruduce problems on higher bandwidth audio systems. Linear is probably still best, it won't need any regulation for most power amplifiers.

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Angry!? I'm absolutely electrolytic!

Will have to make do with myspace now I guess...

Nigel Goodwin

Yes, I've seen occasional amplifiers with SMPS's, in fact my 400+400W PA amp has one, but it's specifically designed as such, not a modified PC supply.

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Dr.EM

Ah, of course, the Behringer powered mixer, needs to be compact/lighter so I guess thats why they choose that type of power supply. Yeah, modifying a PC supply isn't likely to work well, I wouldn't like to try it myself :lol:

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Angry!? I'm absolutely electrolytic!

Will have to make do with myspace now I guess...

evandude

PC PSU's don't use transformers, otherwise they'd weigh a ton... they rectify the AC input and then it's all switching power supply from there.

It makes no sense to try and modify one to give some significantly higher output voltage; at best, you could probably get some useful components out of one to build what you need.

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Someone Electro

They do use transformers! But not runing on mains.They filter it to DC and then make it AC agen but at 100 to 1000 Khz And then drive that in a small and light ferite core transformer(A cell phone is heavyer).This is the trick of them being so light.Cell phone chargers are also all gone to switch mode.

But switchmodes also have an problem.They make lots of noise on the output .

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evandude

Ah yes, well that's true in many cases (although DC-DC's can be done with just inductors instead of transformers, at the cost of isolation)

I should have specified that I was talking about a transformer that dropped down mains. Whatever the case, rewinding any transformer used wouldn't necessarily help. The idea of a SMPS is that is uses feedback on the PWM driving the switching transistor to regulate the output to a certain voltage, so even if you changed the transformer ratio, thanks to the feedback it would still attempt to regulate to its desired voltage (although probably less effectively). To really get it to do what you wanted, you would really need access to (and knowledge of the circuit of) the feedback path and controller chip, as well as the transformer. reverse-engineering that could be a nightmare, especially if they have scraped the part numbers off all the chips as is commonly done in production lines.

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EEgeek.net

Nigel Goodwin

A SMPS does just that, and it's usually refered to as a 'mains' transformer, the only difference is it operates at a far higher frequency - but it's still dropping down 'mains' just the same.

Switch-mode power supplies only use one output voltage for the feedback (for obvious reasons), on a PC PSU it's probably the 5V rail?, or perhaps these days the 3.3V rail?.

On TV's they use the main HT rail for the feedback, normally 150V or so, the lower voltage rails are either unregulated, or regulated by 78xx type regulators (although keeping the main rail regulated helps the lower ones anyway!).

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Someone Electro

There would also be no point monitoring the other rails because they are all winded on the same core and so if one is regulated all other are.

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i_build_stuff

Normally, the rule is to take the feedback from the highest-current output.

Not necessarily. Some SMPS have extremely clean outputs.

They are worse for line noise and radiated noise, though.

Someone Electro

Yea the highest is the 5V rail.

But switcmode are no where as clean as linear(Stupid heat).

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cjk2

I just finished moding a SMPS and it was a great success. To get 40v you probably need to rewind the output transformer to give about 3 times as much out as usual. You will need to mess with the voltage devider on the feedback also. The 16v caps will need to be replaced too. The highest I have taken my modifyed PSU to is 22v but there is no reason it cant go higher with a little rework.