Just Because You Can Plug It In...
October 18, 2010
...doesn't mean it will work the way you want it to.
On a recent trip to China, I went to a local store and shelled out a few yuan for a power strip that would let me plug my spare laptop into the wall. The laptop's power supply was autoranging, so it could automatically work with the power provided in China, but the plug was the wrong shape; because China sees many different visitors from all over the world, there's no real standard for wall plugs, but perhaps the common one is the angled-spade plugs used in Australia. Power strips with an Aussie plug on one end and several universal sockets on the other are quite common, and the one I bought did its job nicely.
When I got it home, a friend of mine opened it up out of curiosity and discovered that it was so affordable for a reason: there was no circuit breaker inside, no surge suppressor, not even a fuse... just some wires and a switch. It was a glorified plug adapter that looked like a surge suppressor. True enough, I didn't read Chinese and couldn't tell what the product was promising, but it got me thinking about what sorts of suppositions we make when we plug something in.
Recently I had a chance to review some bus-powered USB audio interfaces for an upcoming issue of the magazine. They all worked great, but they all promised to deliver 48V phantom power to condenser mics that needed them, and that was a head scratcher to me. There are lots of ways to step up a 5V USB power rail to a 48V signal, but how much power can you pull from the cable? Enough to give a mic proper phantom power?
The answer is, "Sometimes almost but not quite, other times yes handily if."
I'll be talking about this in more detail in an upcoming issue, but in very simple terms, USB devices come in two flavors: low power and high power. A high power device negotiates with the computer to pull extra power if it needs it. You can't hook them up to unpowered USB hubs, because the hub can't negotiate for or distribute extra power. In an ideal world, the high power device asks for more juice and the computer provides it without any hassles... or you pull more power from a powered hub.
Now, one channel of phantom power takes about seven tenths of a Watt, which is a bit more than a low power USB device can pull. A high power device can pull anywhere from 1 to 2.5 Watts, which is enough to phantom power two mic channels with juice left over. The thing is, not all computers can provide that much juice over their USB ports... they're underspec'd or poorly designed. And when you plug a demanding interface into a wimpy computer, the current draw can actually harm the motherboard.
Whose fault? The laptop designer, for not building in proper reserves? The interface designer, for not designing an interface that shuts down and refuses to work if it can't get the power it needs? The buyer, for not being educated about what happens when you plug device A into computer B and just expect everything to work right?
Compared to SCSI, USB is remarkably foolproof, but in today's world, we seem to be manufacturing better and better fools.