I made a new version of the Zellweger Mic preamp board I posted a couple of weeks ago. turned out that while everything looked good on the bench the preamp got too noisy on the higher stages. the preamp has 80dB of gain split into 4 16dB steps on one switch and 10 1.6dB steps on the second rotary switch. once it was switched to 64dB it just got too noisy.
so I did a new board, removing the relay switched polarity and phantom power and using board mount rotary switches. while I mostly use the auto router in Eagle, this time I was able to hand route the circuit and tried to keep the signal away from the power traces. the new board is much better. at over 64dB I can record an acoustic guitar from 6 feet away with a ribbon mic with plenty of input. don’t think I ever want to do that. turning the gain further up it’s getting hard to tell if it’s not just the noise of the room and the mic itself that I’m hearing, the mic also starts to feedback into the headphones. so I think I’m good.
Still wondering what are some tricks to design this sort of high gain PCBs. would it make sense to run all power traces on the bottom of the board and the signal on top? should I use traces as wide as possible? also I usually ground all the unused copper of the board and am wondering if it would be better to get all the unused copper stripped of the board as it’s done on a lot of old circuit boards?