I recently discovered that it's possible to add a mini TOSLINK digital audio output to RCA's RD900W Lyra Wireless product, a little gadget on the market from about 2003-2007 for streaming audio from a computer to a home stereo system. It sold for as little as $20 and can still be found on eBay occasionally. This product has pretty decent audio quality to begin with (supports 44.1 and 48 KHz 16/24-bit digital transmission over a dedicated 900 MHz link) but it's equipped with only a 3.5mm output jack providing line-level analog output.
I suspected that since the signal coming into the receiver is digital, there might be some place to pick up a S/PDIF signal somewhere, and sure enough, there is.
The receiver unit of the RD900W decodes the stream using a Philips SAA7325H CD decoder ASIC. I learned from the datasheet that pin 51 of this chip provides a 3.3V EBU biphase mark digital audio stream in IEC60958 format, basically S/PDIF.
While it would be possible to use this to add a coaxial digital output, getting the electrical characteristics correct would take a little work, so it's easier to go optical and simply convert the existing 3.5mm audio jack to a 3.5mm analog + mini TOSLINK jack by adding an LED. This eliminates the need to modify the case and allows you to use a standard TOSLINK-to-mini-TOSLINK cable to connect the RCA receiver to your stereo system.
To do this modification, you'll need:
- A 3mm/T1 red LED (Radio Shack 276-026)
- Solid, insulated 30AWG rework wire (Radio Shack 278-503)
- Two small squares of double-sided foam tape
- A low-wattage soldering iron and a steady hand
The first step is to relocate the surface mount component that is directly behind the audio output jack. Just carefully desolder the two leads on the left and right sides, being careful not to heat them too much, then stick the component to the board upside down, some distance away, using a small piece of double-sided foam tape (or epoxy, if you're more patient). Connect both of its leads to the original pads; be sure to get the polarity right.
Then tack-solder a length of wire to pin 51 of the SAA7325H and a second wire to the 5V ground, which is easily accessible as the ring contact of the audio output jack. When soldering to pin 51, be careful not to let solder bridge the tiny gaps to adjacent pins.
Connect the wire from pin 51 to the LONG lead (anode) of the LED near the LED housing, then connect the ground to the short lead (cathode). Trim the leads and mount the LED behind the round hole in the back of the audio output jack using another square of double-sided foam tape. Insert the mini TOSLINK cable into the jack and tweak the optical alignment between the LED and the tip of the cable. Eyeballing it is good enough.
Check your connections and give it a try. This modification works perfectly with my Sony 777ES receiver. I've been using the Lyra Wireless to stream my library of FLAC files from a Linux server running mpd (music player daemon), and this little hack noticeably improved the sound quality. It's pretty cool that we can add a $100 feature to a $20 product using a single inexpensive component.
Technically, there should probably be a 68 Ohm 1/8 watt resistor in series with the LED to limit the current and properly bias the LED for the 3.3V supply. But I haven't run into any trouble with the resistor omitted so far.
One other tip: The RCA RD900W Lyra Wireless works fine with Linux, OS X, and Windows 98-XP, but there's a flaw in the transmitter's firmware that makes it incompatible with Windows Vista and Windows 7. I wrote a device driver that hides the flaw and allows the Lyra Wireless to function properly on Windows 7 and Vista (both 32 and 64-bit editions). You can get the driver here.