Cube speakers/amp

[Steffen Barabasch]

Guys, stop the speculation and do your homework. Really. This 
guessing doesn't help anyone.

Here are the facts:

Every (standard) USB port is specced to deliver 500 mA of power.

Low power devices like mouse/keyboard etc. can share this power 
supply by using an unpowered hub. Usually this hub takes it share for 
its own needs, so if you've got an unpowered 4-port hub that claims 
100mA itself, you can use 4 100 mA devices with that hub, resulting 
in a rated power use of 500mA total.

The Cube amp is a high power device, rated 500mA. So you can only use 
it plugged directly into a computer or on a powered hub.

But now make your calculations, 5V x 0.5A, that's a measly 2.5 Watts 
input, resulting in a max. audio wattage of max 1.5W. That's not 
quite much, and since Steve Jobs is a bit deaf Apple engineers 
decided to "expand" the USB specs on the Cube a good bit.

If plugged in into the Cube, the amp gets switched into "booster 
mode" and draws a LOT more power than the actual USB specs allow. 
That's why the Cube speakers are a lot louder if plugged directly 
into the Cube.

That doesn't mean that the speakers won't work on a powered hub, it 
just won't be as loud since it uses just the rated 500mA max. to 
behave like a nice citizen in USB land.

The amp will not work on a unpowered hub, since the rated 500 mA plus 
whatever the unpowered hub claims is more than the max 500 mA.

If it DOES work (=the computer doesn't complain about too much power 
usage) then you've got one of those weird critters like I did. I've 
got an *unpowered* hub that claims (to the computer) to be a 
*powered* hub. That means, you could use 4 500 mA devices plus the 
hub drawing all their power (thats more than 2A in the worst case!) 
from one 500 mA port. Even with the Cubes "high-power-ports" this 
could be very dangerous. Be careful if you stumble across an 
unpowered hub that behaves like that!

And if you still don't believe me, I made actual measurements on some 
of my USB devices to see how much power they draw in real life:


Low power devices (rated from 50 to 100 mA):

Apple Pro Keyboard (not using the built-in hub)
15 mA

LeadData Kartenleser (not using the built-in hub)
60 mA

Keyspan TwinSerial
70 mA

iMate + Apple Extended KB plugged in (LEDs on KB dark)
30 mA

unpowered Hub + Logitech Mouse + Logitech Trackball + PowerMate (total)
120 mA

This shows that the ratings here are really conservative. The last 
combination is rated to a total of 314 mA, actual use is far less 
than half as high.


Now some high-power devices, rated 500 mA:

Roland UA-30
320 mA

Cube USB-Amp plugged into Cube port
800 mA

The Roland USB audio interface is once again well below the rated 
value, the Cube amp on the other hand is far over the actual USB 
specs, if allowed (=plugged into Cube). If the Cube amp would have to 
be rated, and if you'd use the same conservative rating as on the 
other devices, this would have to be called a 1500 mA 
ultra-high-power device...

BTW, to get the rated power usage, don't use the System Profiler, it 
doesn't have a clue. Use USB Prober (part of the Developer 
Tools/Xcode) instead.

Steffen