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Old 11th January 2005, 11:41 PM   #1
robmonk is offline robmonk  United States
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Question ATX (computer) power supplies for power amp

I'm researching schematics and inspiration for building a stereo 2-way bi-amp component (all in one chassis and, I hope, on one power supply) in the 20w/40w range for each of the four (L+R, hi/low frequency) channels. My biggest obstacle right now is power supplies: they're expensive!

So, I'm also wanting to combine this project with designs that take advantage of the low cost of computer power supply units. 550W models meeting ATX specifications go for under $50. 300W models go for under $10, and they are available with and without fans.

Since I don't know much about what is acceptable ripple current (or voltage noise) from a regulated power supply, I wonder if anyone can comment on the suitability of the ATX specification ( for audio applications (I'm also intriqued by the idea of using high quality tower cases (with their multiple, standardized bays and sleds for various ad-in devices) as chassis for multi-channel audio amplifier and control/filter equipment.

SO: I'm looking for bona fide "hi-fi" designs, suitable for a beginner, that run on +12v single-pole power. There are many high-power, great-sounding automotive amplifiers, so there must be some good designs out there, somewhere.

AND: I'm looking for suggestions on phase-linear active crossovers that would match well to drive such amps (ideally, from the same +12v power supply). I've looked at the Marchand X-1 ( and like just about everything about it, except that it requires a +15, 0, -15v psu. Anyone know of a +12v
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Old 12th January 2005, 12:46 AM   #2
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I don't think you can get 80 watts from the 12v portion of a computer power supply, certainly not the ATX anyway. Someone correct me if I'm wrong.

Car audio active crossovers will work just fine from 12v. I'm using one for a home system right now with a little 12v outboard supply.
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Old 12th January 2005, 02:04 AM   #3
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Car audio amps utilize 12VDC, but use it to power a switching power supply to crank that up to a higher voltage. Bacically you would be dropping 120VAC to 12VDC, then bringing it back up to 40-50(+)VDC (or whatever). Not very efficient. The other drawback is the the amount of current required to to this.

A few years ago, I tried using the 12V from a 300watt power supply to power a "25x2" Sony head unit (really more like 12Wx2 when measured properly). Anything over 1/4 volume, and the power supply would **** out.

Decent computer power supplies are actualy not much cheaper than a toroid/cap/bridge setup. Much like car audio, the power ratings on cheaper power supplies are typically "peak" or "surge" ratings, and not good for continuous use.

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Old 12th January 2005, 02:45 AM   #4
quasi is offline quasi  Australia
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Default Cheap power supplies

I have seen on the web ideas for converting AT / ATX power supplies to +/- supplies for audio.

There is extensive work required however and involves dismantling and rewinding the transformer and making changes to the feed back circuitry as well. In addition extra filtering capacitors are required on the output to ensure a clean and fast supply.

You can also series connect unmodified supplies (ok if you get them for no or little cost) and achieve +/- 24v with 4 supplies using the +12v outputs. Do not use the -12 or other outputs.

But have you considered rewinding an old toroid or variac. You can find these at scrap metal recyclers inside discarded equipment (big power supplies, medical equipment, photocopiers, old mini-computers etc.) Buy a couple of these for say $10 each and you may find everything you need (transformer, capacitors, power transistors, heatsinks, bridges etc.) plus a whole lot of extra goodies.

If you look at my thread "power amp under development" you will find details of my stereo amp project with a total cash cost of around $60 US.

If you need any help rewinding or using appropriate components drop me a line.

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Old 12th January 2005, 02:59 AM   #5
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quasi wrote:
If you need any help rewinding or using appropriate components drop me a line
I would like mail..
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Old 12th January 2005, 05:01 AM   #6
quasi is offline quasi  Australia
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Default Forgot this bit;

If you use 6 300 watt ATX supplies you can get +15/0/-15 at a theoretical 1800 watts (probably more like 1000)

Cheers again.
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Old 12th January 2005, 07:09 AM   #7
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If I remember correctly, ATX supplies must have a certain sequence of 'activation' for the supply to turn on. Once this occurs, I believe there is a minimum current draw on certain circuits.

Also, computer power supplies are of a switching design instead of being linear like most of the designs in 'high' end audio, so you may have issues with noise (I don't know what freq. they switch at but would suspect around 40 to 50kHz). There is no reserve capacity to the power supply either, as the type of product it is designed to drive doesn't have transient power demands.

Or, I might be wrong.
I want mine to go to 11.
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Old 12th January 2005, 07:09 AM   #8
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ATX power isn't anyways good power source for poweramps because of poor pulse current ability, unless added a few capasitors. Most modern ATX high-power powers can do happily about 140w @ +12v. But again, pulse-like current output is still an issue conserning poweramps without extra capasitors.
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Old 12th January 2005, 10:11 AM   #9
Eva is offline Eva  Spain
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A low EMI power supply with +V and -V outputs rated at up to 500W continuous may be made from almost any 250W AT/ATX power supply, but this requires extensive redesign and definitely a new PCB and new components, mostly storage capacitors, output filter inductors and output diodes
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Old 12th January 2005, 11:19 AM   #10
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Bottom line... Is it worth it?

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