I've finally decided to do away with the old version of this writeup because it was really poorly done. The design of the device has changed so much since the original write that it is now irrelevant to the actual hardware. Anyway, let the (new, possibly worse) documentation commence. All aboard the fail train, woot woot!

I thought I'd start with the DAC first as it is the core of the project. I chose a Burr-Brown PCM1793 because it's Texas Instrument's best voltage output DAC in terms of SNR at 113dB. Most of the silicon in this project is Texas Instruments due to their awesome sampling program. I don't have time to fully finish this documentation; I'm just throwing this out quickly so that whomever gets my two randomly-distributed PCBs from ITEAD Studio's OpenPCB will have a chance to find at least a little documentation. Unfortunately, I did not think to add any URLs to the board. The $.10 random PCB thing was an impulse buy on my part, and I'm sorry for that. For the sake of easy googling, the most prominent feature on the front silkscreen layer is Vinyl Scratch, a pony from Hasbro's series My Little Pony: Friendship is Magic. The silkscreen also contains the Open Hardware gear/lock logo and the texts "PCM1793 DAC" and "Open Hardware". All the pins are labeled with their functions, and the components are labeled with their values. The physical size of the board is about 10cm by 4.6-4.7cm. Here are the relevant KiCAD files:

On to component selection. The large 1800pF capacitor footprints in the center are sized for 1800pF 500V Silver Mica capacitors and the four footprints surrounding the resistors near "OP_DUAL" are sized appropriately for 560pF 300V Silver Mica capacitors. "OP_DUAL" can be populated with any 8-pin DIP operational amplifier as long as the analog supply on "A+" and "A-" is high enough. Of all the amps I've tried the OPA2228 seems to sound best; I speculate that this is due to its low bias current (due to its JFET input) an d low voltage gain noise.

The jumper block near the center of the board marked "F0 F1 F2" map to FMT0, FMT1, and FMT2 on the PCM1793, respectively, with 22k pulldown resistors. To set the DAC to receive 24bit I2S, for example, you would add .1" jumpers to F0 and F2. To supply the board with data, I used a TI DIR9001 IEC-60958 receiver wired to a Toslink optical package for initial testing. The Toslink connector can be replaced with a TI PCM2705 USB DAC, which has a LVTTL S/PDIF output. The DE pin, when jumpered to 33 (3.3V) sets the PCM1793 to 44.1khz de-emphasis mode. This is usually not necessary unless your source is mid-80s Japanese-produced CDs. ZD is an active-high pin that shows 3.3V wh. For input power, A+ is regulated to +5V onboard to supply power to the analog parts of the PCM1793 and unregulated to "OP_DUAL". D+ is the digital power supply pin, which is regulated onboard to 3.3V. If you don't have a seperate power supply, this can be jumpered to the same supply as A+. A- MUST be the negative equivalent of A+. A+ and A- may be any dual rail supply that is less than the maximum voltage that the opamp you use can accept, less than the voltage ratings of the capacitors, and less than the max voltage the LM7805 can withstand. I also recommend populating the LM7805 space with the tightest-regulated 7805 grade available. D+ can be any voltage over 4.3V up to the max rating of the LD33V. the power setup that I use is a Traco Power TMP30212C +-12V 1300mA switching power supply module with 22uF and .1uF low-ESR caps on both rails. D+ is isolated from +12V by a LM7805 regulator with another set of 22uF and .1uF caps on its output. The mounting holes are sized appropriately for 4-40 machine screws. The fourth hole needed for mounting can be made by drilling a hole in the PCB through the hole in the 7805's tab, which is grounded.

I also feel obligated to say a little about the license. This board and design is licensed under Creative Commons Unported 3.0 with Attribution. This means that you can do whatever the hell you want: change it, sell it, incorporate it into your products. I don't care. I only hope you will find building this as rewarding and intellectually stimulating as I did designing it. Happy hacking!

If you happened to get my boards or have any questions, please shoot me an email at sightunseen1337@gmail.com; I'd love to hear from you!