Ti Kan's Alien DAC

April 20, 2007

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Also see the official Alien DAC website.

Small and simple

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).


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