Hi,

I just finished building the AB528 preamp and the noise floor seems higher than it should be. There's a constant white noise/hiss that doesn't change level when changing gain, although it's a bit louder at 300R/1200R than at Line Level. This is when the the polarity switch is engaged (either normal or reverse). When in the mute position the noise goes away.
I checked the DC voltages and most are correct (within tolerance) except the top of R25 (test point H) I get -4V instead of -5V (even after replacing R25 with a new resistor), and on the heatsink of Q5 (test point I) I get around -0.07V instead of -0.5V.
The preamp is otherwise working correctly (mic, line, and DI) and sounds fine. It's just that the noise is higher at minimum gain than my other pres at max, and it's audible even at moderate listening volumes.
Any ideas where I should look? Thanks.