sorry for the offtopic.
Originally Posted by NEWTON IN ORBIT
Why are you doing this?
Basically because I don't understand some issues and somethings on the circuit and satisfy my curiosity and learn:
Yes, that I thought.
This is not in the signal path and apparently should not affect, however I did this test and I was surprised by my curiosity:
With a 180ohm resistor and near cranck maximum gain (position 75db, output at 4pm).
With the LME49990, I have not been exhaustive testing because I'm more than a month with all my gear apart and began with acoustic treatment in my bedroom-studio.
I do not have the full test with pictures but in a quick test, observed (listening) noise spikes, but with the same noise floor than opa2134. LME49990 it seemed bit clearer, flat, but showed a strong 20dB peek distortion near 100Hz. I could not put the opamp bypass to ground before dismantle all my gear.
Only a bit: I have a ground Issue on the PSU board to solve (you can see at 50Hz with harmonics).
I explain the pictures:
Pre73 MKI (not new unit) Stock. TL072
Pre73 mk-I Stock (TL072) after little changes on PSU board ground. Not complety solved.
Pre73 MK-I with 2sc3329BL and TL072
Pre73 MK1 with 2sc3329 and OPA2134
Now I am searching the picture with the LME49990 (same noise level, but little better, flat noise but with a 20dB peak at 100Hz).
Opamp not in the signal path ? I don't know, but with some effect on performance. 5dB - 7dB less noise floor.
In the coming weeks, when I have finished the acoustic treatment, I will continue investigating this issue (LME49990 and Opamp effect in pre73) and learn to solve the remaining HUM.
I think that -120dBs are the noise inherit for pre components and card, cables, etc. I received the scope but is still in the box.