As many of you will know many USB DACs or SPDIF converters come powered by the same USB cable that’s used to access files from your computer. There's no doubt in my mind that most, if not all, of such USB DACs and converters will sound better if they have a better power supply than that supplied by the host computer. To add an external power we need a USB cable that will connect to the host computer for the signal, and to the external PSU for the power. I've already reviewed three such cables from Elijah Audio, KingRex, and iFi, and they all do the job very well. But if you don't want to go out and buy such a cable, here's how to make your own. It doesn't involve too much work, but you do need to be able to use a soldering iron.

First the list of components that is quite short. You will need a *USB cable that has an A-type USB connector on one end, and a B-type on the other (I'll refer to this as the signal cable). Then another USB cable that has a type-A plug on one end (the other end doesn't matter as we will be cutting that off). I'll refer to this as the power cable. I'll leave it to you whether you want to use something that you have lying around, or buy something more exotic. We won't bring any snake-oil into this article and I'll suggest that a screened cable should be sufficient.

The length of the cables should be such that the finished cable will be suitable for your set up, ie the distance between the computer and the USB DAC/converter. Next you will need some solder, and lastly (I said that it was a short list) some insulating tape. Of course you could probably make this cable look 'better' with some braided sleeving, or some heat-shrink, but I'll leave that up to you.

* if you are using an in-line DAC/converter such as the hiFace DAC, then use a type-A USB extension cable for the signal cable. The power supply should be connected close to the female connector.

Tools required are a soldering iron, a pair of wire cutters, a sharp knife, and something like a very small screw driver.

Before we start, let's be clear about what we are actually doing.

The diagram to the right shows representations of a typical USB cable wiring, where we cut those wires, and how we join in the power wires.

There are four wires in a typical USB cable, two for power, and two for the data (top). To make the 221 cable we need to cut the red wire, and expose the conductor in the black (middle) so that we can join in the two new wires for the power supply (bottom).

Take the power cable and cut it such that the length is suitable to run from your external power supply to the USB DAC/converter.

On the piece of cable that has the type-A plug on carefully remove about 25 mm of the sleeving, and the screening material from the cut end.

Now separate the four wires and cut off the green and white ones flush with the insulation. This should leave you with the red and black wires.

Strip about 6 mm of insulation from the red wire, and about 15mm from the black wire.

Now take the signal cable from which you want to cut off about 25 mm of the plastic sleeving very close to the type-B plug.

Start by making cuts as shown in the picture on the right. Be very careful not to cut through the screening, or any of the four internal wires.

Be very careful not to cut yourself. Applying pressure to that round cable will cause it to try and roll over, so I suggest holding it with some pliers while you make the cut parallel to the sides.

Having cut the plastic sleeve, peel the section off the cable as shown in the picture. You should now see the braided screen if you have used a screened cable.

Once you have removed the section of plastic sleeving, use the small screwdriver (or something similar) to tease the red and black wires outside the screening.

If there is foil as well as wire braid, you will probably have to cut or tear some of that away.

Cut the red wire so that about 10mm of it remains on the plug end of the cable, and then strip about 6 mm of the insulation from it.

Cut the rest of the exposed red wire away.

Cut the black wire about half-way along the exposed section of the cable, and then remove about 6 mm of insulation from both of the cut ends.

Note carefully, that although we cut the red wire to isolate the 5 volt supply from the computer, we must retain the connection through the black wire to maintain ground, or the lead will not work. So although we cut the black wire to expose the conductors, we reconnect it by soldering the black conductor from the power cable across BOTH ends of the black wire in the signal cable.

Now bring up the cut end of the power cable beside the signal cable such that you can solder both the red and black wires to their counterparts. Before soldering it is best to tape the two cables side by side.

Solder the two red wires together first, then fold back the black wires, and put a layer of insulation tape around the cable covering the soldered joint on the red wires.

Solder the three black wires together, and then cover that soldered joint with more insulating tape.

I found that taping the two cables together provided enough strength to stop them coming apart and straining the soldered joints.

And that's it, one TNT 221 USB cable completed and ready to use. The only thing to watch is that you don't inadvertently make any shorts while you are joining everything together. Do stagger the joints for the red and black wires so that they could not touch even if you didn't use the insulation tape.

I have assumed that you will be using a USB power supply that has a type-A socket. If you are not then you could cut the USB plug off the other end of that cable and solder on a suitable connector to match your PSU. Or you could use a twisted pair of wires instead of the second USB cable.

If you are using a USB plug on the power lead, then I suggest that you mark your plugs as 'signal' and 'power' so that you don't get them mixed up and plug them into the wrong socket.

I would like to tell you how good these cables sound, but our esteemed editor has said no more cable reviews! But seriously, the real sonic gain is not from the cable but the quality of the external power supply.

You may also like to add some RFI noise filters to your cable, perhaps one for the signal cable, and one for the power. A quick search on Ebay doesn't show much choice, or say which bands of frequency are attenuated, so if anybody can tell us which are the optimum items to use, please let me know, and I will modify the article accordingly.

OK, you've read this article, you've got a USB DAC/converter powered from your computer, and you want to try the 221 but you don't have an external power supply. About the cheapest option (other than a wallwart) would be 4 rechargeable AA batteries (I recommend Energizer 2300 mah), a charger, and a suitable battery holder, all of which are obtainable on Ebay for under 25 UK pounds. There are ready made battery supplies such as the KingRex U-Power, and of course there are any number of 'off the shelf' mains PSU's ranging from cheapo wallwarts, to items like the Paul Hynes SR3-5 that I actually prefer to a battery supply. For the hard-core DIYer, it's not too difficult to make a 5 volt linear PSU. Whatever you use, it should be an improvement on running the DAC/converter from the computer's power supply.