A Not-So-Universal Serial Buss Audio System
A Not-So-Universal Serial Buss Audio System
USB Audio for Apple’s Flagship
Monday, May 14, 2012
The Cube’s audio system is entirely USB based. There is a small box which plugs into a USB port attached to two 4” diameter clear acrylic balls fitted with 1.5” aluminum cone Genesis drivers made by Harman/Kardon.
So what’s in that little box? It contains a USB DAC, a Tripath class-D “digital” amplifier, and a DC-DC converter to step the 5V from USB up to 12V for the amp chip. Here’s the teardown:
Splitting the case reveals an inner smoked plastic modesty shield and a small circuit card. There are two major chips on the card, one on each side, and a switching DC-DC power supply. The power converter is 5V to 12V step-up.
On the bottom of the board is a Micronas UAC 3552a USB DAC. It takes audio packets from USB, reclocks them, and outputs analog audio. It also has a headphone amplifier which drives the headphone jack on the box. There is an internal DSP unit, which Apple doesn’t appear to use, but could be configured for EQing or amplifying the signal in the digital domain.
The top side has lots of coils and capacitors and a Tripath TA1101B class-D amplifier chip, popularly known to audiophiles as a T-Amp. Advertised as a “digital” amplifier, class-D is not really digital. It is a PCM switching analog circuit, like a switch-mode power supply whose reference voltage is an audio signal.
What makes the setup NOT so Universal is the power requirement. The amplifier is capable of 10 W/Ch @ 4 ohms, though here it’s driving eight ohm speakers, so power is halved. Still, it needs at least 10 W of DC power, and USB provides 1/2 W at a standard connector. The cube provides high power at its USB ports, and the acrylic Studio and Cinema LCDs also provide high current. Many Apples have at least one high power USB port for charging iPads and iPhones as well as driving USB speakers and the like.
I modified my amplifier so that the speakers and the headphone jack are active simultaneously for driving a subwoofer. Normally, plugging in ‘phones mutes the speakers. I also cut the speaker wires and installed RCA plugs and jacks so I can experiment with different speakers, and try the speakers on different amplifiers.
Other articles here cover the speakers and a subwoofer for the Cube.
When I bought an Apple G4 Cube computer system a few years ago, I was intrigued by the clear round ball speakers and digital amplifier which came with it. Nice sound, but how does it work?