DAC and Headphone Amplifier

Posted on Apr 23, 2012

The idea behind the portable DAC is to get better sound than from the line out or headphone out of portable CD player, a pc sound card or other digital audio source. Many of us want to listen to music on the go or on PCs, and would like to get better quality than most of audio rigs today can provide. The quality of the headphones is the main contributor to the overall sound quality from a rig. Quite a few higher-end headphones require more powerful amplifiers than what’s supplied with the average portable device. When it comes to audio sources (i.e., PCDP or other), these devices are mass-market, optimized for a set of parameters that doesn't include audio quality.

DAC and Headphone Amplifier
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I set my goal high and wanted to get superior sound quality from both portable and non-portable equipment. This PDAC is my fourth design. It has fewer parts (and fewer SMD parts), is smaller, and is easier to build than the other three prototypes. For a change, this one is both a DAC and a headphone amplifier. It has a 3.5mm input jack with breakout, so that the PDAC can be used as a regular headphone amplifier. The portable DAC is battery-powered, but can run off a good quality wall-wart or better, a separate regulated power supply. The PDAC's compatibility with digital sources is excellent. It works with any optical out I've tried: DVD player (at 24-bit/96kHz, although internally the PDAC is 16-bit/96kHz), portable CD player, Sony CDP-CX300 CD MegaChanger, and the SoundBlaster Audigy Platinum sound card. Beginning with the second prototype, I used the Burr-Brown DIR1703 receiver and Analog Devices AD1866 16-bit R2R DAC. The DIR1703 has very low jitter (75ps) and will handle up to 24 bit and 96kHz but output 16/44 to the AD1866. It needs only 30mA at 3.3V. Setting up the chip is somewhat cumbersome and involved. The biggest issue is its inability to handle S/PDIF directly from the line without any conditioning, and worse yet, an inability to handle both coax and optical in without a switch. Crystal chips allow that, although of course optical and coax cannot be active at the same time. The AD1866 DAC called for different...

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