Berkeley Audio Design Alpha USB Interface

The USB Problem Is Finally Solved

Equipment report
Categories:
USB interfaces, clocks, and soundcards
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Products:
Berkeley Audio Alpha USB
Berkeley Audio Design Alpha USB Interface

Functional and Technical Description
The Alpha USB is housed in an unusual chassis that’s significantly larger than that of most USB converters, but smaller than full-sized components. As with the Alpha DAC, the Alpha USB features a no-cosmetic-frills chassis. The front panel’s only indicator is a single LED that lights up amber when powered on, and then switches to green when connected to an active USB source (the computer). The rear panel is also minimalist, with AES/EBU and S/PDIF outputs (the latter on a BNC jack) selectable via a small toggle switch. The Alpha USB lacks an RCA output because the RCA’s characteristic impedance is 50 ohms, not the 75-ohm standard for S/PDIF. A BNC interface, which has a characteristic impedance of 75 ohms, is vastly superior to RCA for carrying digital audio. AC power is via an IEC jack. There is no power switch; the unit, which draws 4.5W in standby mode, is designed to be left continuously powered. Macintosh computers running Snow Leopard or later will automatically talk to the Alpha USB. Windows users will need to install driver software, which is included on a CD.

The Alpha USB is the epitome of “form follows function.” The unusual chassis size and shape were chosen for sonic performance. In broad terms, the Alpha USB isolates the “dirty” USB circuitry from the “clean” digital-audio output, and provides a high-precision clock for that audio output. The USB input jack is mounted on a plastic insert rather than directly in the chassis to prevent capacitive coupling of noise between the USB input and the digital-audio output. This isolation between the two “halves” of the Alpha USB is improved by powering the circuitry associated with the USB input from the computer via the USB bus, and powering the clocks and digital-audio output drivers from a clean, linear power supply built into the Alpha USB. Berkeley extends this isolation concept by recommending that the computer be powered from one AC outlet and the Alpha USB, DAC and analog components from another, and that the USB cable between the computer and Alpha USB be routed away from the Alpha USB’s chassis and any analog cables or components. It goes without saying that the Alpha USB’s USB interface is “asynchronous,” meaning that the Alpha USB’s output clock is not locked to the computer’s clock. That is, the timing precision of the Alpha’s S/PDIF output is not affected by the computer. A conventional “adaptive” USB interface, in which the computer serves as the master clock, is simply a non-starter for critical applications.

The digital audio signal is clocked with one of two precision oscillators, one for the 44.1kHz family of sampling frequencies (44.1kHz, 88.2kHz, and 176.4kHz) and the other for the 48kHz family (48kHz, 96kHz, 192kHz). To give you an idea of how meticulously designed and executed the Alpha USB is, consider that each oscillator is measured on a $90,000 instrument that creates a spectragraph of the oscillator’s phase noise. Only the best measuring parts go into the Alpha USB; the rest are rejected. This measurement process costs many times what the actual part costs, but Berkeley discovered that this performance parameter was crucial to sound quality.

The Alpha USB’s designer, Michael “Pflash” Pflaumer, comes from a multidisciplinary background that includes writing the DSP code that made HDCD possible, analog design, and RF design. When the first microprocessor became available, Pflash wrote his own operating system for it and built a computer around it. It’s rare for an engineer who can write DSP code to also have an intimate understanding of how electromagnetic fields behave. It’s even rarer when that engineer is also a musically sensitive listener who uses his ears to guide product development.

System Context and Set-up Notes
Berkeley sent to me a bare-bones iMac ($1199) loaded with music, including most of the Reference Recordings HRx 176.4kHz/24-bit titles (which I also have on my PC server). They also shipped a Straightwire Info-Link AES/EBU cable and a Straightwire USB-Link USB cable (both 1.5 meters). The iMac ran iTunes along with the latest version of Pure Music, a $129 piece of software that improves sound quality (see Steven Stone’s review of Pure Music in Issue 209). Berkeley recommends Pure Music, and includes with the Alpha USB a $25 discount coupon for the software. The complete digital front end of an iMac, Alpha USB, Alpha DAC, Pure Music, Straightwire USB and Straightwire AES/EBU costs about $8300.

The iMac was unbelievably easy to set up and use; my Windows PC was a different story. To use the Alpha USB with a Windows machine, you must install the supplied driver. This will let the PC and Alpha USB talk to each other but will not allow you to realize the system’s full sonic potential. You must also install an ASIO driver to avoid data corruption. For Windows XP users, the available ASIO drivers will degrade the sound. Machines running Windows 7 can use a more sophisticated WASAPI driver that reportedly delivers performance as good as that possible from a Macintosh. Installing the ASIO driver on my XP machine was a hassle. Moreover, switching between the Lynx card and the USB output required going into a couple of menu layers in MediaMonkey to manually change a setting.

Considering my experience with both an iMac and a Windows PC, I can unequivocally say that the Macintosh is vastly superior (and I’ve used PCs for all other computing since the late 1990s). The Mac is far more elegant, easier to set up, better sounding, and doesn’t require that you jump through hoops to realize its optimum performance. Even if you own a PC that you are thinking of using as a server, I encourage you to spend the $1199 for an iMac—you’ll be glad you did.

Listening
I began by comparing the sound of the iMac/Alpha USB to my PC-based server equipped with the Lynx AES16 card. The bitstreams from each computer driving the Berkeley Alpha DAC were identical. How do I know this? Every HDCD recording carries a flag in the least significant bit that identifies the recording as HDCD-encoded. An LED on the Berkeley Alpha DAC’s front-panel illuminates when this flag is detected. If the data were corrupted, that LED would not illuminate. In all my tests, the HDCD LED remained lit when playing any Reference Recordings title (CD or high-res file). It follows that the bitstreams were identical for non-HDCD recordings, as well.

Listening to 176.4kHz/24-bit Reference Recordings HRx files through the Constellation Audio electronics ($65k Altair preamplifier and $140k Hercules power amplifiers) driving Sonus faber’s $200,000 flagship loudspeaker is as critical a situation as one could devise. These exquisite recordings contain so much fine information, dense spatial cues, micro-transient detail, and rich timbral colors that any degradation is instantly identifiable. The playback system is truly of reference quality.

Comparing the PC-based server with the Lynx card to the iMac and Alpha USB (by changing which AES/EBU cable was connected to the Alpha DAC) revealed that the PC setup I’d thought was the state-of-the-art was actually a step down from what was possible. Simply put, the Alpha USB took the system to another level of resolution and musicality.