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The Mute Audio SkeletonDAC

Technical Highlights

The mute audio SkeletonDAC is based on the Texas Instruments PCM2704/5. The PCM2704/5 contains many features imbedded in the chip: USB interface, on-board power regulation for 3.3V digital supply, on-board clock generator, on-board headphone amplifier, and on-board SPDIF output conversion.
  • USB 1.1 specification compliant
  • Supports 12MHz data transfer rate
  • Accepts 16-bit stereo and mono data streams
  • 48KHz, 44.1KHz, and 32KHz sampling rates
  • 98 dB S/N Ratio

(prototype pic)
The PCM2704-7 set of DAC chips is an entire family of DAC chips made by TI that provide economical USB interfaces, Digital Audio Conversion (DAC), SPDIF output, audio line-level output and direct headphone output. The 32TQFP versions, the PCM2706 and PCM2707, have traditionally been used by the audio industry as a primary USB interface. Mute Audio's own GrubDAC uses the PCM2706/7 for the very same purpose. However, the chip family provides a very good audio DAC and even includes a small headphone amplifier. The 28SSOP versions, the PCM2704 and PCM2705, are used in the SkeletonDAC to provide an all-in-one design for line-level or headphone output with a minimum number of supporting parts. This still preserves the specified audio performance of the chip with an outstanding combination of flexibility and economy.
  • Built-in USB 1.1 Interface
  • Built-in Clock Generator (12MHz)
  • Built-in 3.3V regulated digital power source
  • Built-in output amplifier for headphones/earbuds
  • Built-in SPDIF output

(TI PCM2704/5/6/7 block diagram)
The SkeletonDAC design uses the onboard clock generator combined with a 12MHz crystal to provide sampling rates up to the USB 1.1 standard limit of 48 kHz. The internal, regulated 3.3V power supply is used to provide digital power, while the USB bus (typically 5V) is used for the analog sections. The output is powered with a 12mW (into 32 ohms) amplifier, also internal to the chip. This provides either line-level output for use of the SkeletonDAC as a source into high-fidelity audio equipment or as a direct connection to some high-efficiency, low impedance headphones and earbuds. Finally, an option is provided on the SkeletonDAC to utilize the chip's built in SPDIF output, allowing the SkeletonDAC to be used as an economical, USB bus-powered USB-to-SPDIF convertor for connecting higher-end DACs to USB.

While the SkeletonDAC design utilizes a number of 0805 SMD parts, their placement and type are not seriously challenging compared to the larger 1206 parts. Anyone with experience or skills necessary to construct a BantamDAC or GrubDAC should have no issue with the SkeletonDAC. As with any Do-It-Yourself constructed DAC, the DAC chip itself is the most difficult to solder. However, the PCM2704/5 with its 28SSOP planform is the same as the famous PCM2702 chip used all the way back to the AlienDAC and later BantamDAC. All the same soldering techniques apply.
Manufacturer data sheets:
PCM2704/5 SPDIF Transformer
file last changed:Saturday, August 6, 2011 7:00:00 AM
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