How much noisy is this preamp?
I've built one channel and I'm testing it (not still with a correct grounding and enclosure).
In my opinion, noise level is good when mic (modified tube condenser APHEX 460) is not connected to input or when mic power supply is off...but noise level increases a lot when mic is on.
What could be the reason of this?
Grounding and EMC?
Transistors matching maybe?
Inappropiate signal capacitors? (I've used ceramic caps instead of polypropylene)