Cheap USB audio controller |
When plugged into my Linux machine, lsusb shows the following:
$ lsusb | grep -i audio
Bus 005 Device 003: ID 0d8c:013c C-Media Electronics, Inc. CM108 Audio Controller
So, with a little googling for CM108, I found that the datasheet at http://www.halicky.sk/om3cph/sb/CM108_DataSheet_v1.6.pdf and also Kevin Custer's site about modifying it http://www.repeater-builder.com/projects/fob/startech-fob.html
When I opened up the device and compare it with Kevin Custer's photos, the most obvious difference was the missing capacitors.
Looking at the datasheet, it seems that the two 470 uF DC blocking capacitors was missing!
In fact, the PCB traces directly shorts the two pairs pads together. This explains why it works with speakers with built-in amplifier but not headphones. The audio amplifier circuits in the speakers probably has some built-in DC blocking capacitors at it's input or some audio transformer.
Missing DC blocking capacitors |
This means that it will not be possible to fit the caps inside the existing casing. So I had to make 2 holes at the bottom casing to fit the capacitors leads and solder them upside down.
I also had to cut the PCB traces between each pair of pads as mention earlier.
Tip | Ring | Sleeve | |
Mic input jack | Mic In | Gnd | |
Mic plug | Mic In | Gnd |
Cutting the middle terminal connection for the microphone jack |
Finally, everything works properly. The audio input/output quality was ok and comparable to my built-in motherboard audio quality but it costed me extra time and money to fix it.
Final reassembled device with the huge capacitors! |