Alien DAC is a simple digital-to-analog converter with a USB
interface, intended to be connected to a computer as an upgrade
to the embedded sound card. Based on the TI/Burr-Brown
PCM2702 chip, it is an 8x oversampling DAC supporting
16-bit PCM digital audio with 44.1KHz or 48KHz sampling rate.
Highly integrated, this DAC chip provides line-level audio output
and internal digital and analog filters.
Designed with community input in the
head-fi.org
DIY forum, the Alien DAC is a small circuit board utilizing
virtually all SMD (surface-mount device) parts. Two onboard
voltage regulator chips provide the 5V and 3.3V DC needed by
the DAC, and an optional chip (either a BUF634 buffer or OPA551 opamp
operating as a unity-gain voltage follower) could be added if
the Alien DAC was to share a PSU with a single-supply virtual-ground
amplifier.
Alien DAC works on any operating system platform that supports
USB streaming audio (Linux, MacOS, Windows XP).
Ti's build
The first eight pictures above shows Ti's original Alien DAC,
which was housed in a shortened silver Hammond 1455C801
extruded aluminum case. A USB port, power-on indicator LED
and external DC power input jack are located on one end.
The other end has the left and right RCA phono line-output jacks
and a toggle switch to select the DC power source. One position
selects USB bus power, while the other position selects an external
power source, if connected.
Inside, most small electrolytic capacitors
are Rubycon Black Gate NX type. All other parts are basically
what is specified on the BOM. The DC power input (whether from
the USB bus or external source) is further regulated by onboard
REG101UA and REG101A-3.3 IC regulators, which supplies the DAC's
analog and digital sections, respectively.
The optional buffer chip is not populated since this DAC is
standalone.
This Alien DAC has since been re-cased into a black Hammond 1455C801BK
enclosure, with custom fabricated front and rear panels (made by
Front Panel Express) to match Ti's
Mini³ portable headphone amplifiers.
The external DC power jack and switch was eliminated, and the
left and right output RCA jacks have been supplanted by a single
3.5mm mini phone jack.
A second Alien DAC was also built, and it is identical to the
first one except for different output coupling capacitors (ERO 2.2µF
film type).
The nineth through sixteenth picture above shows Ti's current
pair of Alien DACs, along with the matching Mini³ portable
headphone amplifiers.
RMAA Test Results
RightMark Audio Analyzer
software, running on a Toshiba 2.8GHz Celeron laptop computer via an
M-Audio Firewire Audiophile
mobile interface running in 16-bit, 44.1KHz and 48KHz modes as the
loopback device. Similar measurements were also made using an
M-Audio Transit USB mobile interface.
This test provides data and graphs of frequency response, noise, dynamic
range, total harmonic distortion, intermodulation distortion and
stereo crosstalk performance. Measurements were made on Ti's first
Alien DAC. The second one produced virtually identical results.
The measured results were pretty good, but the noise floor spectrum
shows broadband "hash".
The measurement environment is known to be free of RFI so the noise
is probably from within the DAC chip. This is not unexpected because
the analog output stage is on the same chip as the DAC and there is
no isolation between them. Nevertheless, the amplitude of the noise floor
is very low, mostly within the datasheet specifications and not
likely to be audible.
Curiously, the M-Audio Transit noise floor results were better than
the M-Audio Firewire Audiophile's, and this is atypical of other devices
I've done measurements on using both interfaces.
The noise floor does not appear to change significantly whether the
Alien DAC power comes from the USB or from a 9V battery (both regulated
via the onboard REG101UA chip).