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Old 12th January 2015, 08:56 AM   #1
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Question Help with understanding electrical output/demand when using a computer PSU to 12v amp

Hi everyone,

I have an old car sub (300w RMS 4ohm) with 12v 280w amp (has a 20a fuse in it)

These are in perfect working order and seem to match each other well as both can play full volume for long periods without either item getting too hot or failing or clipping etc when used in a car

In the car the amp is connected right to the 12v battery.


I no longer need these items in a car environment, but would like to use them in my home as they are perfect for my requirements (sound response, freq range etc)
Rather than go out to buy a new 240v amp, I would like to try and use the 12v amp that works so well with it

So I borrowed a computer power supply unit from work and wired it all up:

240v mains to PSU
PSU advises that it's output is 250w - 20amp - 12v
I assumed that this would be perfect

I bridged the motherboard on/off pin, and connected one of the PSU rail yellow wires to my amp positive and a PSU black negative wire to the amp negative
The amp powered up and the sub driver worked, but only very quietly (talking volume) and would clip very hard when turned up (so I promptly turned it back down)
After a short whike of testing a very low frequency (also at low (non clipping)) volume, the PSU turned itself off
Turning it back on returned the system to its previous, quiet state.


My question boils down to:

Do I need to connect more yellow and black wires from the PSU to the amp? (I thought this was only necessary to spread heat loading across more wire, rather than allow more current through...?)

Do I need a more powerful PSU? I can get hold of a 750w unit... But.. I don't know if the PSU 'pushes' the current (and all 750w) into the amp (and might be too much)
Or if the amp 'sucks' the current and therefore would only ever 'suck' out its maximum requirement...


My electrical knowledge is average at best, but I'm a fast learner with detailed explanation.


I Thankyou hugely for reading all of this, I hope it makes sense, and I really would appreciate any opinions or knowledge offered in response


Thankyou!
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Old 12th January 2015, 10:21 AM   #2
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The answer depends very much on the age and type of computer PSU. Recent good quality ATX power supplies give lots of current at 12V and would be ideal for powering an amp.

A larger power supply is no problem, the amp will only take what current it needs. But be aware that some of the more powerful ATX supplies actually have dual 12V outputs, and I don't know if it is OK to parallel them. If it is not OK then half of the power will be unavailable (and therefore half of the money spent on the PSU wasted)

Some power supplies may not work properly without a minimum load on the 5V rail. I think this is most probably the issue with yours. The article below recommends simply stuffing a 12V halogen bulb into the 5V and ground prongs of one of the disk drive power connectors.

See here for everything you never wanted to know about the PC power supply. http://reprap.org/wiki/PC_Power_Supply
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Last edited by scopeboy; 12th January 2015 at 10:26 AM.
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Old 12th January 2015, 12:12 PM   #3
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As scopeboy mentioned there can be problems with the 5V load on the PSU. Plus normally 1 yellow wire shouldn't take more than 3-6A depending on the type. VGA and CPU yellow wire can take 6A. Otherwise it may get hot. So use as many yellow+black wire as you can.
It is not a problem to connect the split sides of a PSU together. It is also done in PC, if one side is too weak for a strong VGA, then from another side of it is connected there.
There is no too strong PSU, PC also only "sucks" as much juice as it needs. Only if there is not much load, the efficiency gets lower (goes downhill below 10% load,).

One more thing. Low quality PSU may not be able to give out the Amp it is written on it.
Low quality PSU sometimes can only give out about 60-70% of the power. If you try to suck out more then it can handle it may catch fire. There can be seen some freaky videos from PSU testers. Brand PSU will only shut down and may even handle bit more then it should, but not considered healthy. Crappy PSU can also give out a big spike, usually killing a few PC components before it "lets out the steam". What brand is that PSU?
I wouldn't use the 250W PSU with the 300W sub. You can test with it, but don't crank it up.

Last edited by mrWagner; 12th January 2015 at 12:22 PM.
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Old 14th January 2015, 07:27 PM   #4
jvhb is offline jvhb  Denmark
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Above posts make good points. Also, a standard (cheap) atx PSU with total rating of 250w, usually has only no more than 5-6a on 12v rail, so maybe around 70w available. In other words, not any where near 20a! But the consumption on the car amp is properly not actually that high. It should run fine a lower volumes at least. Try the 750w if you can, there is no risk of overloading the amp - it will just take the power it needs. And check the small print regarding 12v ampere on either PSU..
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Old 16th January 2015, 11:47 AM   #5
AndrewT is offline AndrewT  Scotland
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There are some sites that describe modifying the computer PSU to provide a dedicated 12Vdc output and move all the VA capability of the 3.3V and 5V windings to the 12V output.

BUT!
this involves working with direct to Mains circuits, which are usually banned on this Forum, except when the Moderators turn a blind eye.
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Old 16th January 2015, 12:35 PM   #6
infinia is offline infinia  United States
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like the others mentioned a more modern ATX supply has most of the rated power taken from the +12V. You are correct about the wires (#connector pins actually driving the specs) for splitting the rails. 90% of ATX supplies internally have a single rails, of those I reckon maybe 50% have working dual current limited channels in either case you simply connect all the yellow wires at the load. Take the power rating on the box as lies. Look closer at the label pasted on the supply they usually tell you max rating of combined loads. better if you can get rid of the molex connectors and switch to 2 heavy lugs. shorter and heavy wires are key. Banana plugs aren't all that good > 20A or so.

BTW If you have an older ATX most require some minimum loads be on the +5V to have regulation on the 12 >
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Last edited by infinia; 16th January 2015 at 01:00 PM.
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Old 16th January 2015, 01:52 PM   #7
infinia is offline infinia  United States
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Quote:
Do I need to connect more yellow and black wires from the PSU to the amp? (I thought this was only necessary to spread heat loading across more wire, rather than allow more current through...?)
yes connect many 18 AWG wires to match the fuse rating. you need to be sure fuses blow and PS current limits before wires melt. I reckon each yellow/black (as a pairs) wire pin is rated for 6A or so.
so 20A/6A use 4 pairs minimum if not adding any wire length

Quote:
Do I need a more powerful PSU? I can get hold of a 750w unit... But.. I don't know if the PSU 'pushes' the current (and all 750w) into the amp (and might be too much)
Or if the amp 'sucks' the current and therefore would only ever 'suck' out its maximum requirement...
if the ATX 12V current matches the amps fuse ratings yer probably close to good enough, although more PS watts will help reliability a bit if you tend to push volume and all channels to max. The amp takes what it needs. The power is related to the volume control and # of speakers connected.
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Last edited by infinia; 16th January 2015 at 02:11 PM.
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