How to Power 12V Amplifiers from an ATX Power Supply

Car Audio: Without a Car

In an earlier decade, I accumulated an inane number of car audio subs and amplifiers. I’ve finally come to my senses (thank you, tinnitus :-/). But, I have yet to find a more pleasing sound than that which comes from an oldschool pair of Infinity Perfect 10.1‘s in a custom sealed box, powered by a JBL BP600.1. So, the pair now resides in my office, rather than my hatchback. This initially posed one problem:

How does one power a 12V amplifier indoors?

Obviously, we need a way to convert 120VAC mains to 12VDC at 600W+. High output DC power supplies are not cheap. It makes more sense to find a 4 ohm capable home audio subwoofer amp, but those are not cheap either.

There’s an easy solution, and that is to sacrifice an old ATX power supply (PSU). Audiophiles are now cringing; switching power supplies are a no-no in the audio realm. But so are Class D amplifiers, like the JBL BP600.1, which has an internally limited frequency response of just 10-320Hz. And again, it’s a car audio amplifier. This means its power supply stage was designed with excellent noise rejection, and thus, a switching power supply is a fine match.

Numerous tutorials explain How to Convert a Computer ATX Power Supply to a standalone DC supply, but most are targeted towards low power projects. In this instance, we need more than a few watts. We need every last electron the PSU can muster.

I pulled an old Thermaltake 430W PSU out of the closet. Before putting it to work, two modifications were needed:

  1. Combine all 12V wires and all ground wires to create two heavier-gauge wires, for adequate ampacity
  2. Force the power supply to turn on
430W ATX PSU Specifications

ATX PSU’s manufactured after ~2005 sometimes feature multiple +12V rails. While it should be safe to merge multiple 12V rails, doing so could be a bad idea for this project (possibly more noise from multiple switching supplies). Multiple rails are usually labeled on the side of the supply as “+12V1” and “+12V2”, or similar. Luckily, the supply I butchered has only a single +12V rail rated for 18A, meaning 216W of 12V goodness. This isn’t enough power to push the 600W RMS amp to its limits, but that’s okay. I don’t need 600W of bass in my office.

Safety

Note that this project involves potentially lethal mains voltages (120VAC). A PSU should never be opened or modified by an unqualified individual. Perform these modifications at your own risk! I am not responsible for any damages that may occur as a result of this article.

Capacitors in a PSU store a large amount of energy. With the PSU running, turn its switch off (or pull the power cord) and wait for the connected device(s) to shut off. Only open the PSU after you are sure it is drained of energy.

Wire Pruning

A modern ATX 2.x PSU contains a 24 pin motherboard header, at least one 4- or 6-pin +12V connector, and a multitude of +5/+12V molex, floppy, SATA, and GPU connectors. We only need the black wires (ground) and +12V wires (yellow). Ignoring the safety warning stickers, I removed the cover and proceeded to extract all extra wires. All useless wires from the ATX motherboard header and various connectors landed straight on the circuit board. These were easily removed with wire cutters.

A few inches of the green “power on” wire (ATX pin 20/24 as explained here) was preserved and soldered to a ground wire. I temporarily used hot glue, but this isn’t safe or recommended. Use twist wire caps or heat shrink wrap for a permanent installation.

Top view of PSU with cover removed and extra wires pulled
Some leftover ATX wires
Green ATX wire tied to ground

With the PSU cover back on, I untangled the remaining yellow & black wires, trimmed them to equal lengths, and wired the modified PSU to the amplifier. The final product, with tidy wire-ties in place:

Yellow to +12V and Remote, Black to GND

Results

The BP600.1 and ATX PSU deliver deep, clean, ridiculous house-shaking bass. The setup absolutely blows away my Boston Acoustics CSSUB10B (and especially the lowly Logitech Z-2300’s). I’ve only cranked the volume too high a few times in the past year. When the amp draws too much current, the PSU struggles to keeps up and the amp’s LEDs dim during bass hits. It’s rare that I need/want ~135dB bass in my office, though.

There’s one downside: Activating the subwoofer is slightly inconvenient since the PSU must be manually switched on. I’ve tried going back to my home audio subs, but they don’t stack up to the quality of the Infinity Perfects.

12 thoughts on “How to Power 12V Amplifiers from an ATX Power Supply”

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    1. Yep, this is addressed in the post–don’t need that much power in my house! Neighbors already glare at my office as they walk by. Any more power and I’d have the police at my doorstep.

  1. Awesome stuff. Awesome explanation. Thank you.
    I am attempting this tomorrow 🙂
    @%)W PSU, 200W RMS DHD Amp + 350W RMS Infinity Reference Sub.
    Its only for my home theatre and do not require much power. I did not want to spend extra $$$ as i have these things at my disposal already

  2. Hey guys iv had this setup for awhile but its been put on the metaphorical shelf. I have a shitty levono PSU thats 18A on the +12V lane. Anywho It works. Problem is I get a crazy hum. Im plugging an Xbox into as the source. Not sure if the source is the issue or the shitty PSU. Input would be very appreciated.

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