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AB528 Microphone preamplifier / Re: AB528 build has an abnormally high noise floor

« on: June 04, 2026, 04:36:02 PM »

It turns out I am, in fact, a dunce, and was misreading the value of C14 thinking it was correct. I got it and C5 mixed up. Swapped them and it is functioning correctly now.
Apologies for any hassle this may have caused, and thanks for your patience!

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AB528 Microphone preamplifier / Re: AB528 build has an abnormally high noise floor

« on: June 02, 2026, 09:54:22 PM »

All the caps are correct (value and orientation). I reflowed the solder joints, too.
I get 0.2mV of DC on both TP2 and TP3.

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AB528 Microphone preamplifier / Re: AB528 build has an abnormally high noise floor

« on: June 01, 2026, 07:42:50 PM »

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?

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AB528 Microphone preamplifier / AB528 build has an abnormally high noise floor

« on: May 28, 2026, 07:09:12 PM »

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.