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.