I am very happy to convey that I have successfully designed and assembled a 2IN4OUT Digital Audio Processor board based on the ADAU1701 processor.
Following are its features:
1. Two stereo analog inputs
2. One stereo SPDIF (optical/ co-axial) input
The SPDIF section uses WM8805 SPDIF transceiver in HW config and PCM5100 DAC. The stereo analog DAC output is passively added to the other two stereo analog inputs to form a composite analog stereo input to the ADAU1701.
Some of my posts here in this forum exposes my ignorance in digital (SPDIF-SRC) and in the end I decided to keep it simple, yet very practical.
3. 4 Analog outputs
4. 256kb EEPROM
5. PIC 12F1840 controller
6. TSMP1138 IR Learner
7. Front panel (optional) header
The last two features are un-populated in the picture and it is work under progress. Also, don't pixel peep as I have not yet cleaned the board.
The board is 2 layer, star grounded with no ground plane or ground pour. This will raise a lot of eyebrows, I know. I was confidant of my understanding of the system grounding and my layout skills.
The board is very typical of any DIY type but the end result is very clean. The loudspeaker is dead silent with inputs open and the amplifier volume fully opened. This is the only way I can convey its performance, technically, as I have no access to any elaborate instruments.
The above description certainly will not impress any purist, but the results are awesome and most importantly I have all the connectivity and processing power that I need.
Next up is a similar board, 2IN6OUT, based on ADAV4601. I will come back to report about the same, soon.
Thanks to AD and the forum members here, from Jerad Lewis to K J Bob, for all their support and patience.
Do let me know if you need any further details and I shall be only glad to elaborate.
It looks good. Mostly thru-hole is interesting.
You can probably get away with only two layers as long as you are not trying to send digital data out of or into the serial port. The master clock will be the issue in trying to keep reflections down and prevent over and under-shoot but it is not as critical as I2S clock and data lines.
The issue with no ground or power planes is that the impedance of a trace is undefined. It basically has nothing to push off of is one way of thinking about it. So it is difficult to design a good transmission line for digital data. I do not recommend only two layers. Four layer boards are not that expensive anymore so there is not much reason to use only two layers.
I recently had an issue with a high gain discrete analog amplifier that would oscillate at high levels. It turns out that it was a trace on the other side of the board that was causing feedback/crosstalk. Moving the trace fixed the issue. It was an all analog board so I only used two layers. Had there been a ground plane it would not have been a problem. In all my other designs where I used four layers with similar amplifiers I have had no issues.