And the other -- whew!!! I just learned some stuff about ghetto circuit board repair that I wish I didn't have to know. I think at least one of the analog outs is still hosed, but I don't use those
Anyway - my findings... all references to circuit board locations are for the Digital MPA, and may not translate to the MPA Gold.
The short story / low hanging fruit: replacing the already socketed OPA2134 chips in U202 and U203 with LME49720NA increased gain some and made the unit sound a little "faster" with more noticeable detail. That could be good or bad (more gain probably isn't ever bad), depending on what you like about the sound.
The long story: Thanks to my stupidity, I'm able to swap out any opamp in the unit - there are a total of 8 5532s and 4 OPA2134s (half of each on each side). I'm sure the circuits are identical, but the board layout is not exactly symmetrical from side to side - no doubt because circuitry that is common to both channels gets in the way - no biggie, it's close enough to follow (heh, as if I could follow a complicated circuit like that
).
There are 3 5532s for each channel on the main board, and 1 on each of the smaller boards to which the gain and impedance pots are attached. These last two are U200 and U201 -- I didn't notice any change when replacing those two - maybe they're related to analog out. I left them as 5532.
I replaced all 6 total 5532s on the main board for LME49720NA, and did notice a small improvement - a little less noise and maybe some more headroom -- but the improvement is DEFINITELY NOT WORTH THE RISK OF BRICKING THE UNIT -- I found out that ART does not sell replacement circuit boards - you bust it, you're on your own.
There are two remaining OP2134s -
U420 and U421 - I tried replacing U421, and got no signal through the channel when I did -- I don't really understand this -- maybe a bad opamp? I don't think so, because I think I used it at another position. I put back the OPA2134 and everything was happy again. I didn't try to do the same thing with U420.
One last tip - when you're putting it back together, there's a long ribbon cable that goes from the left channel side of the main board way over to the output board on the back -- make sure you run this cable around the front side of the tube, instead of the back side, where it gets too close to the power transformer and picks up some hum.
edit: an old quote from Jim Williams on another board on this very subject:
"Audio band parts like OPA2134, AD 8599 are usually safe, the LM4562/LME49720 is a 55 mhz bandwidth part, watch out." -- jeez, it really sucks being ingorant -- anyway, I'm liking the change for now, and if I notice anything wacky, it's easy enough to switch back.