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Note - You will need basic electronic skills to build the cable in this tutorial. Use this information at your own risk; do not complain if it doesn't work for you. Only use as prescribed.
I really dislike wall-wart power supplies and try to avoid them whenever possible. Many times they block other power outlets, they’re inefficient, and they always use a small amount of power even if the attached device isn’t on.
In many cases it’s easy to get rid of at least one wall-wart, the one which powers your computer’s speakers. This method will only work if the speakers will work off of 5-6 volts and you’ve got a spare powered USB port.
Officially this method violates the USB protocols. Whenever you plug a USB device into your computer it’s supposed to communicate with the computer informing it of what the device is, how much power is required, and whether or not any driver is required. We’re going to ignore all of that and just suck some power off of the USB port. Novelty USB devices like LED lights, heated gloves, fans, humidifiers, and other similar gadgets do the same thing. As long as it’s a relatively small amount of power it won’t cause any harm. Do not plug in more than one novelty device per hub.
There are USB powered speakers from major companies. Using USB for low powered 5 volt devices is a safe method as long as you use it sensibly (don’t draw too much power, don’t try to power a device with the wrong voltage, don’t reverse polarity, etc.).
Step 1: Find a Junk USB Cable
One time I was lucky enough to find a cell phone charger cable with a USB connector on one end and a coax power jack which was the right size for a speaker sitting on the next shelf in a thrift store. I used an AC USB charger to verify that the combo worked, which astonished the store clerk.
If you’re not lucky enough to find an old cell phone cable which already has the correct connector for your speakers, you’ll have to make a suitable cable.
Find a junk USB cable. The best choices are ones that have proprietary connectors for obsolete cell phones. If you don’t already have one you shouldn’t have to pay more than $1 for one in a thrift store or yard sale – and probably can get one for free just by asking politely.
You’ll need a power plug for your speaker. You could reuse the connector from your wall-wart, but my preference is to leave the wall-wart intact in case it’s needed in the future or can be repurposed for another project.
There are many different power connectors. Most are “coaxial connectors”, small cylinders with a hole in the middle. Many different diameters are used. You can probably find the correct connector in a well-stocked electronics shop, if not you can certainly find one online but you’ll have to do some research to make sure you’ve got the correct size and shape connector.
Step 2: Verify That the Wires Are Correct
Cut off the useless cell phone connector on the USB cable. Remove a couple of inches of the outer insulation. There should be anywhere from two to five wires inside. We’re concerned with the +5 volts and ground which are USUALLY colored red and black. The other wires (if any) are used for data communications between your old cell phone and computer. Clip those wires.
Use a voltmeter to verify that the red and black wires do connect to the correct pins on the USB connector. Pin one should connect to the red wire and pin four should be black. If they aren’t – STOP. You may have a cable with non-standard color wiring (it does happen) or something else is wrong. Do not proceed to Go, do not collect $200.
Step 3: Test the Wall-wart to Verify the Correct Voltage and Polarity
Plug the wall-wart which came your speakers in without plugging it into your speakers. Verify that the wall-wart generates about 5-7 volts. If it’s anything outside this range STOP. Some power supplies use the outside of the coax barrel for ground, others use it for power. Make note of which polarity your setup uses.
If all of the above conditions have been met (your speakers are in the 5-6 volt range, you have the correct power connector, you’ve identified the pins correctly in your salvaged USB cable) then you can continue.
Step 4: Solder the Power Connector to the Cable
Slip the plastic cover for the coax connector over the cable, with the threaded side facing out. (Yes, I have forgotten to slip the cover over the cable before soldering on the connector. I’m too embarrassed to mention that it’s happened more than once, but at least it’s still in the single digits.)
Solder the 5 volt and ground wires from the USB cable to the correct lugs on the coax connector, making sure to match the polarity of the original wall-wart plug.
Use your voltmeter to verify the continuity of your cable. Also make sure that the power and ground wires are not shorted.
Step 5: Verify That It Works Before Plugging It Into Your Computer
Before plugging this cable into your computer plug it into a USB wall charger. Use the voltmeter to verify that you’re getting the same voltage and polarity off of your cable as the original wall-wart. It’s okay if it’s off by a volt or so (e.g. 5 vs. 6 volts).
While still using the USB wall charger plug in your speakers and verify that they power up. Assuming you’ve done everything correctly the speaker’s indicator light should come on and you’ve verified that the USB power cable you’ve made will work with your speakers.