Below you can find some information about the modifications I did in the first unit. For the main project page, go here.    

    It is rather simple to make the first and most important mods.

    To bypass the input mic preamps, I cut the first two wires from each channel that go from the front panel to the main board, and wired the part that go to the main board with a resistor directly to the input jack. You can see it in the two next pictures. It preserves all circuit at the main board, with its differential input op-amp, accepting balanced and unbalanced sources with full input swing. Make sure you preserve the polarity, or at least wire it consistently across all channels.
The resistors are needed to set the gain of the first input op-amp. For unity gain, you could use 3K9 as the feedback and ground resistors (the big blue metal film ones). However, these first op-amps are set to a +-10V swing, and the AD chip input takes approximately only 1/5th of the signal (check the series 1K1 470R 1K1). So you will probably need some gain here, what will make you need low resistance. When I tried it with 500R (as I needed a gain of 8x) I did not like the sound, and suspected that the feedback started to affect my source signal. To solve this, I had to replace the 3K9 resistors at the main board for higher values.



    The second main modification was to replace the op-amps. I used LT1359 at the inputs and OPA4134 at the outputs. If you never worked with SMDs, ask for help. You need an air-flow soldering/desoldering station, also called reworking station, to do it right. In terms of sound, to be honest, I think the inputs are very good, but I am not so sure yet about the outputs, and perhaps it has little to do with the op-amps choice, just that DACs sound very different from each other and when interfaced to different circuits. I will test this subject again in the near future.



    The third main modification is the hardest one, but you will probably not need it so much. I decided to uncross the outputs. The DAC´s differential output is reversed at the first output op-amp, and feeds the negative output pin, and later inverted again to generate the positive signal. As in most of the cases we use and prefer to use only the positive, unbalanced signal, we are getting it through a series of two op-amps instead of only one. And we need to uncross it twice as to preserve the correct XLR pin assignment. The first uncrossing is easy, just unsolder and uncross the large electrolytic capacitors. But to correct the output pin assignment, if you do care for polarity, you need to uncross the two small 0805 SMD 220R resistors right after the XLR. It is good they are there and there are pads for soldering. But I had to make some very tiny ´3D´ circuit boards that put these resistors upwards and cross their connections by using two (handmade) vias.


    Last, but not least, take care of your power supply. To avoid heating, cut the +-15V supply to the front panel, that serves only the now unused input preamps, put some heatsinks on the 7805 regulators, and also replace if possible the small ceramic capacitors, as a friend of mine reported that his interface failed because of one of these.


    Good luck! Remember, this is for your information only, and I take no responsibility whatsoever for any misuse or damage caused to you or your equipment. Be safe, take all precautions, and learn first. remember also that this is only possible with the help from many others that put their suggestions and reports in many forums around. Google it.

Porto Alegre, Brazil. johann at inf dot ufrgs dot br 
    Created on May 13, 2010.