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d.i.y. USB to S/PDIF converter


This is the project page for a USB to S/PDIF converter. The project was initiated by me, and after a great deal of community discussion, it was ultimatly designed by Head-Fi.org member guzzler. The original discussion thread can be found at http://www6.head-fi.org/forums/showthread.php?t=105088. The group buy thread, located at http://www6.head-fi.org/forums/showthread.php?t=109671, contains some additional information. I am now out of boards to sell, and there are no plans for another round of orders. There may be a few floating around head-fi, however, so ask there if you need one. If you have any other questions, however, you can contact me at usb_boards @ ecp.cc.


Completed project courtesy of Nisbeth.

This page does not offer any information that is not in the discussion threads. Instead, it simply condenses some of the relevant parts.

Please note, this design has NOT been prototyped. As some people get these built and report results, this page will be updated. *** update -- enough of these have been built to know that they work fine ***


The project is based on the T.I. PCM2902. While the chip has the capability of analog output, and both digital and analog input, there are better chips for all of those functions. Thus, if you need that functionality, you should look to other projects. Additionally, this project is designed to be a very compact and simple converter. To that end, it makes extensive use of surface mount (SMD) components. Thus, the usual warnings about soldering skill apply. This is a difficult project to build. For more on SMD soldering, have a look at http://www.infidigm.net/articles/solder/.

Build Notes:

Update 05/20/2005
I changed the LED resistor in the parts list from 4.32K to 1K. 4.32K was too dim for my needs. 1K is reasonable, but you could still go quite a bit lower for more brightness or if you need more current for your LED.

<Update 05/05/2005>
A couple of important things to note ...

1. The suggested USB jack included in the original parts list (DigiKey part WM17109-ND) does not fit the board ... or rather, it fits the board but extends back and touches the pins of the voltage regulator and 3 resistors. As you might imagine, this is not good. It is possible to lean it forward to avoid this, but it has been replaced by a part (AE1085-ND) that fits better (and that is manufactured by Assmann making it funnier to boot.) Here is the technical drawing if you want to convince yourself that this part is the better choice.

2. The parts list suggested using 2 22uF caps while the board says to use 10uF caps. If you already have the 22uF caps, they should not cause a problem, but they have been replaced on the parts list with 10uF panasonic FC's.

3. If you are using tantalum 1uF caps instead of the Wimas, please note that the tantalum caps are polarized. To figure out how to install them, each position has one leg connected to the 2902, and another to the ground plane. Connect the positive (+) leg of the cap to chip side and the negative (unmarked) to the ground plane.

4. The position for the hex inverter (74HC04) seems to be a bit narrow. The best thing to do is to bend the pins in a bit (press the chip on a flat surface to bend all the pins the same amount) which will raise the chip off the board slightly.
</Update>

There is a small bug in the boards. It turns out that pins 8 and 9 should be driven high for stability. Unfortunately, this isn't done on the current board, but is easily fixed. You'll need to carefully create a solder bridge between 8,9, and 10. 10 is the regulated input, and is high enough for logic levels.

If you choose, you can get away without using the voltage regulator (the TPS793 4.75V.) To do this, bridge pins 1 & 5 without connnecting them to the ground above. Of course, the fact that this part is readily available and costs $1.14 at DigiKey suggests that you should only do this if there is a real reason to.

Below, you will find the original project drawing.

and a photo of the completed board (courtesy of Magsy at Head-Fi.)

The recommended parts list is as follows.

1 x PCM2902 (SSOP28) (datasheet)
1 x 74HC04 (SO14) (datasheet)
1 x TPS793 4.75V (SOT23-5) (possibly optional) (datasheet)

1 x 560F 6.3V Panasonic FC
2 x 10F 10V OS-CON SC (or equivalent similar, FCs are fine)
4 x 1F Wima MKS-02 (or similar, tantalum should be ok, 2.54mm pitch)
1 x 100nF 5mm (0.2") pitch capacitor (Wima MKS-2 or similar)
4 x 100nF 0805 ceramic capacitors
1 x 10nF 0805 ceramic capacitor
2 x 33pF 0805 ceramic capacitors

1 x 1M 0805 resistor
1 x 1K5 0805 resistor
2 x 22R 0805 resistor
1 x 91R 7.5mm resistor (Vishay Dale appropriate, or generic 1% metal film)
1 x 360R 7.5mm resistor (Vishay Dale Appropriate, or generic 1% metal film)
1 x generic resistor for LED, V ~ 4.5V

1 x 12MHz HC49 crystal oscillator
1 x 10H 1210 package inductor
1 x USB B-type connector
1 x 3mm low voltage LED
1 x RCA socket
1 x case
1 x USB cable 
If you would like to order all of the parts through DigiKey, below is a convenient way to add all parts to an order. Some of these parts are different, and probably lower quality, from those recommended. It is believed that they will work, but you should double check if you are in doubt.

#PartNeeded For ProjectMin Order NumberQuantityPart NumberCustomer Reference
1 PCM2902 1 1
2 TPS793 4.75V 1 1
3 74HC04 1 1
4 560F 6.3V 1 1
5 10F 25V
changed 05/05/2005
2 1
6 1F Tant 4 1
7 100nF 0805 4 10
8 100nF 5mm 1 1
9 10nF 0805 1 10
10 33pF 0805 2 10
11 1M 0805 1 10
12 1K5 0805 1 10
13 91R 1 10
14 360R 1 10
15 22R 0805 2 10
16 12MHz HC49 1 1
17 10H 1210 1 1
18 USB Type-B
changed 05/05/2005
1 1
19 LED 1 1
20 LED Resistor (1.0K)
changed from 4.32K 05/22/2005
1 5