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Old 15th May 2006, 05:44 PM   #1
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Default How to use a computer (PC) power supply for Class T amp?

I'm not sure how poor or well these things "sound" in audio applications, but I have seen photos or heard of a couple of people using this type of power supply for their Class T amp, but cant find info on how they did it. I have some old PC power supplies I don't use, and am thinking of modifying one for use with my Sonic Impact T. I have no electrical engineering background, so can anyone give me a clue on what exactly do I do to interface the PC PSU with the Sonic Impact? And is it worth doing, can it potentially sound better than the 12v 1a walwart I'm using?
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Old 15th May 2006, 06:41 PM   #2
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The net's full of exact details with pictures, but essentially you need to[list=1][*]Connect Power_on (green wire) to ground (any black wire) - this can be used as a secondary on/off switch.[*]Fit a 1A minimum load across the 5V line (red & a black) to ensure accurate regulation - use a 10W 4.7R resistor, or two 10R 5W in parallel, and they'll get hot so keep away from plastic.[*]Yellow wires are +12V[/list=1]
One guide here.

One addition I'd make with an audio amp is an extra stiffener capacitor as recommended in several threads. The internal cap on the 12V line is fairly low value.

Note there's a (usually) noisy fan inside, though you can calm it down by reducing the fan voltage and/or a swap to a quieter model. And remember a modern ATX supply is live whilever it's plugged into the mains.
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Old 15th May 2006, 11:39 PM   #3
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Quote:
Originally posted by cpemma
The net's full of exact details with pictures, but essentially you need to[list=1][*]Connect Power_on (green wire) to ground (any black wire) - this can be used as a secondary on/off switch.

Thanks, I tried researching all kinds of links on people customizing PC PSU's for general electronics use, and while I learned a bit, I ultimately found them a little too complicated for my grasp, as I lack the technical background. But if I know what steps to take (and I dont have transformers to wind...), I'm sure that the project is not beyond my ability. The power cable in my PC PSU has 4 wires, black/green/brown/white. When you say "any black wire", do you mean the green wire in this bunch, tied along with the black wire?


Quote:
[*]Fit a 1A minimum load across the 5V line (red & a black) to ensure accurate regulation - use a 10W 4.7R resistor, or two 10R 5W in parallel, and they'll get hot so keep away from plastic.

[/B]
Okay, I'm going to go to RS and get a 10w 4.7r "sandbar" resistor and adhere it to the casing to vent off heat. I'm not exactly sure how its connected:

a) does it have a +/- leg or it doesn't matter which way round it goes?

b) are you referring to the black & red wires at the end of one of the PSU's white molex connectors?

Quote:
[*]Yellow wires are +12V

[/B]
Very helpful, thanks! What I'd like to do is cut the wire off a walwart and connect the PC PSU to this wire, so the PC PSU can be easily plugged into the SI amp. But if I'm not mistaken, a typical walwart power wire has two leads (+/-), whereas the yellow +12v wire at the end of the molex connector is only one wire. So is it necessary connect the black wire adjacent to the yellow wire on the molex connector?


Quote:

One addition I'd make with an audio amp is an extra stiffener capacitor as recommended in several threads. The internal cap on the 12V line is fairly low value.

[/B]
I couldn't find much info on the use of this in PC PSUs. Why would this be needed, and how and where is it connected? Is there a certain value necessary, and is it something readily available at, say, Radio Shack?

Quote:

Note there's a (usually) noisy fan inside, though you can calm it down by reducing the fan voltage and/or a swap to a quieter model. And remember a modern ATX supply is live whilever it's plugged into the mains. [/B]
I read somewhere that a fan may not be needed if using the PC PSU with a chip amp because of the reduced consumption compared to a pc. So do you think its needed at all, or do I risk blowing up the SI amp if I dont have a fan in the power supply?
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Old 16th May 2006, 12:18 AM   #4
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Quote:
The power cable in my PC PSU has 4 wires, black/green/brown/white. When you say "any black wire", do you mean the green wire in this bunch, tied along with the black wire?
LORDY NO!!!! Those are the mains wires, you dont wanna mess with those if you don't know what your doing. He meant the black wires coming off the board around the same place as the yellow and red ones.
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Old 16th May 2006, 04:55 AM   #5
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I think you will be MUCH better off with a ready made supply.
Check All Electronics and BG Micro for what you need. You risk killing the amp or yourself trying to adapt that PC PSU. Plus it's a pain in the a$$.
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Old 16th May 2006, 03:52 PM   #6
RtV is offline RtV  United States
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Quote:
Originally posted by panomaniac
I think you will be MUCH better off with a ready made supply.
Check All Electronics and BG Micro for what you need. You risk killing the amp or yourself trying to adapt that PC PSU. Plus it's a pain in the a$$.
Definitely in the case where the experimenter doesn't know which wires are AC mains, but...

There's a huge suppy of these things around. I can walk into our town recycling center and salvage 3-4 every week. Once you figure out how to use them, it's almost criminal not to do it.

So far I've been getting away with disabling the fans when powering tripath amps. They use so little power that the most of the heat comes from the load on the 5V circuit, which I ventilate exterior to the ps case.

Randy
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Old 16th May 2006, 08:52 PM   #7
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Quote:
Originally posted by theAnonymous1


LORDY NO!!!! Those are the mains wires, you dont wanna mess with those if you don't know what your doing. He meant the black wires coming off the board around the same place as the yellow and red ones.

Oh. But now I'm more confused, because the wires attached to the white molex connectors are yelle/black/black/red. So where's the green wire coming from if not the mains wires, and which of the two black wires is the green connected to, the one closest to the yellow wire (-12v?) or the one closest to the red wire (-5v?)? The only other wires have the motherboard connectors attached, and none of them are green (there is a green ground wire connected to the casing).
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Old 16th May 2006, 09:11 PM   #8
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Quote:
Originally posted by panomaniac
I think you will be MUCH better off with a ready made supply.
Check All Electronics and BG Micro for what you need. You risk killing the amp or yourself trying to adapt that PC PSU. Plus it's a pain in the a$$.
Come on, it can't POSSIBLY be more of a pain in the a$$ than working on the SI amp board to fit it into a larger case with better connectors!
So far, all it seems you need to do with the PC PSU is attach a 10w resistor to the 5v red & black wires of one of the supplies power connectors so that there's a load and the PSU will be tricked into operating. I don't see whyI couldn't make use of what I have if I were sure of the answers to these questions:

1) Does the 10w 4.7 sandbar resistor have a +/- leg or it doesn't matter which way round it goes?

2) I'd like to connect a plug from a Walwart to the PC PSU so I can attach it easily to the SI amp. If I'm not mistaken, a typical walwart power wire has two leads (hot/neutral?), whereas the yellow +12v wire at the end of the molex connector is only one wire. So is it necessary connect the black wire adjacent to the yellow wire on the molex connector?
I know from experience that it's *very* important that polarity be correct when connecting a PSU to the SI amp, so I want to be absolutely sure how the power connector wire is to be wired up to the PSU!

3) Is a "stiffener cap" necessary here, and if so, which type is used and how and where is it connected within the PC PSU?

4) Is the PC PSU fan necessary or not, or is there a risk blowing up the SI amp if I dont have a fan on the power supply?

5) Is adding a 10w resistor, the appropriate power connector and perhaps a stiffener cap all I need to do to modify a 200w PC PSU to work with the SI amp or was there anything else not mentioned?
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Old 16th May 2006, 10:56 PM   #9
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Quote:
The only other wires have the motherboard connectors attached, and none of them are green (there is a green ground wire connected to the casing).
There should definitely be a green wire connected to the 20 pin ATX connector. If there isn't then the manufacturer of the supply must have used a different color, which I somehow doubt, so check again. There should be a black ground(read -) wire right next to it on the connector that you can connect it to so the supply powers up.

And to answer some of your questions.....

1) Resistors don't have a + or -, connect it any way you want.

2) Yes, connect a yellow wire and black wire to the plug and then triple check for correct polarity before you attemp to plug it into your amp.

3) A cap is not "necessary" but will help, just don't use something too large, it can interfere with the output filter of the supply and cause some undesirable effects. I would say 5-10,000uf is a good start. I would take a few of each the black wires and the yellow wires coming off the board and cut them as short as possible and then connect the cap to those. Again, make sure the polarity is correct.

4) If you remove the fan, keep a close eye on the temp of any heatsinks on the supply, if they get too warm to hold a finger on I would say keep the fan running. If you do want the fan on you can always connect it to one of the 5v red wires so it spins slower. And no, you don't really risk blowing up the amp, you risk blowing up the supply if it overheats.

5) Nope, thats pretty much all it takes, although I dont completely agree on the need of a load on the 5v section for the supply to work properly. I have never put a load on the 5v or 3v section while using the 12v section and regulation has never been a problem. You can try it with and without the resistor to see for yourself. I prefer not to turn the supply into a space heater.

The only other thing I can think of worth noting is the green ground wire that comes in from the mains usually gets coupled to the supplies negative section like "AC ground->case->mounting legs->pcb groundplane". So in effect any black - wires on the board are now connected to the AC mains ground. I mention this because it can often cause groundloop issues. If you hear a hum from your speakers when using the supply you can try disconnecting the AC ground from the case or put a coupling cap between the two.
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Old 16th May 2006, 11:05 PM   #10
RtV is offline RtV  United States
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The sandbar resister does not care about polarity.

Yes, use one of the two black wires adjacent to the yellow (on the 4-wire connectors that are designed to power peripherals, not on the connector to the motherboard) for - on your new supply cable for the tripath amp. These connectors each provide a +12, a +5 and two grounds (0).

Stiffener caps are not necessary, but they are recommended to improve the sound of the amp.

I have been getting away with disabling the fan, but I'm no EE. It's quite possible that I'm taking a risk here. Maybe someone else can provide more info. But I detect no excess heat as long as the PS is only providing the measley wattage needed by a tripath amp. It would be nice if someone could tell us how many watts is safe to use without a fan.

About switching it on:

There are various power supplies conforming to several standards. The most common are versions of ATX power supplies, and these are designed to be switched on by the computer's motherboard. After the mains switch is on, the PS supplies limited power to the motherboard, then after doing some diagnostics the motherboard switches the PS on fully. It does this by connecting the green power-on wire to ground.

Locate the 20-pin connector to the computer motherboard. A full diagram of the connector can be found here:
http://www.bluemax.net/techtips/ATXP...pplyWiring.htm

To power on the supply fully, tie the green power-on to one of the two blacks on the other side of the connector. Be sure the load is already attached to the 5 volt circuit. A good diagram of this process can be found here:
http://web2.murraystate.edu/andy.bat...OWERSUPPLY.HTM

Not that Dell power supplies are not standard, and I would double check your information if your PS is a Dell.

Randy
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