[terroirmusic]

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.

[JPK]

Does the noise disappear when the OUT potentiometer is at 0?

[terroirmusic]

Hi, thanks for the response.

No, the noise stays the same level no matter the setting on either gain control. When adjusting the Out Level there's some crackling like there's DC on the pot.

I also get an increase in the noise level if I touch the outside of C18, or either of the heatsinks. And if I touch the outside of C10 I get even more noise that sounds almost like radio interference. The other caps don't behave that way. There's no change in the noise if I touch the outside.

Should I look at replacing those parts?

[JPK]

You may have an HF oscillation in the output stage.
Check the value and solder joints on C14 and C16.

When adjusting the Out Level there's some crackling like there's DC on the pot.
Measure the DC level on TP2 and TP3. It should be 0 mVDC.
Check the direction of the lytic caps C12, C13, C18.