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|4th September 2007, 05:38 PM||#1|
Modding the x-mod
The last week I came across this little thing that claims it increases mp3 quality beyond the original cd. I plugged it into my laptop and realized it does not, instead it made the music more metallic and harsh.
Turning the "enhacements" off showed the real face of that card, dull, low-fi sound that reminded me of some < $50 mp3 players. So it was the screwdriver time.
Total project cost is less than $100 ( 1 x-mod, 6 resistors, 6 caps and 1 opamp).
( 1 ) I assume no responsibility for any damage to anyone/anything derived from doing that mod.
( 2 ) Portable and computer equipment use small pads and smd devices that are both very sensitive and very hard to solder, so as the x-mod is both portable and computer-intended it's very easy to damage it while changing parts.
( 3 ) That something sounds better to me does not mean that it will sound better to you, as some people like tube distortion, you may like some capacitor/opamp distortion.
( 4 ) This is not a begginer's project, since there are no pcb's, no bom, and the schematic suggested is only a working starting point for you to experiment.
Step 1: Opening the case:
Opening the x-mod is quite easy, simply remove the black plastics that prevent it to move over the desk and unturn the four screws. There is another screw that fixes the volume control. Unturn it and pull the volume control.
On the board you should see the TMS320DA255 DSP and the AIC23 I2S codec. This codec offers -85 dB ( 0.0056%) THD+N, is quite good for a $70 pc-based solution.
The next things you should see are four yellow SMD's labeled 105C. These are 1u caps. I'm not sure but they seem to be tantalum ones. Who at creative has used a tantalum cap for audio coupling? No idea, but we must get that cap far away from our music's path.
Step 2: Deciding what to do:
Since that thing is very sensitive, I will apply the principle of least manipulation: Taking away the signal from the dac and removing the caps. The signal will then be redirected to an external buffer. If you want to keep it portable you sould think of a way to make the connection with the external buffer removable. Since I don't need to keep it portable, I havn't conceived a way to do that, but shorting the signal path directly to the output may be a nice way to go.
Step 3: Identifing the correct caps:
There are two pairs of caps that bypass signal between the codec and a chip labeled "AAU NAD". This chip seems to be an external buffer/amplifier since it's between the DAC and the headphone/line output. If people at creative have put tantalums in the signal path, I don't belive they have worried very much about the quality of the buffer.
It may seem that you can use any of the two outputs for the bypass, but looking through the AIC23 DS you can see that the headphone output has a thd between 0.1% and 1%, which isn't good at all, while the line output keeps the aforementioned 0.0056%.
The caps to change are those marked in red.
Step 4: Removing the caps:
Simply desolder them and solder a small wire where it was the orange band of each.
Turn the thing on and, while listening, carefully touch the end of the two wires. Each wire should produce a pop in one of the channels. If one of them does not produce the pop, or it produces a pop affecting both channels then something has gone wrong.
Then it's time to solder the ground. Take it as far from the usb input as you can, the line output ground may be a good point.
Finally glue the three wires to release the soldering from any mechanical effort.
Step 5: The external circuit:
I've experimented for some time and have found that this circuit is extremely sensitive to both noise pickup and parastic oscillations. I've used the LM4562 as operational amplifier, but you should go for the one you like the most. Of course you can also use a discrete buffer, but I personally prefer to go the opamp way.
The schematic I suggest is simply a noninverting buffer AC-coupled with a 1u polyester cap + 47K resistor. You are also advised to put a 1K to 10K resistor at the input of the LM4562 because it helps preventing oscillations. Proper source decoupling caps ( 10u || 220n ) should also be included, because these opamps have gain-bandwith products up to 50 MHz and are prone to oscillations.
This circuit must be symmetricaly powered. That's ok if you don't need a portable device, because it prevents noise pickup from the computer power supply. If you want a portable device, then the way to go will be DC-coupling from DAC to the buffer and AC-coupling from the buffer to the driven device. If you choose to go that way you won't be able to use the neither LM4562 nor most burr-brown high fidelity opamps since they require a minimum of 5V to work.
Step 6: The sound:
Personally, I've found the resulting sound to be much better, clear and detailed, but also quite harsh. The boredom from the original x-mod (with both crystalizer and CMSS 3D turned off) was away, but the x-mod the kind of thing that would take the most benefit from a tubed preamplifier/headphone amp.
I'll soon test it through the millet headphone amplifier and comment the results.
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