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

« Last post by terroirmusic 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.

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TS500 Tape Simulator / Re: Tape head coil signal audible

« Last post by JPK on May 23, 2026, 03:29:43 PM »

the varnishing of the coil is probably insufficient.
Contact me for a replacement.

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MP566-II Tube microphone preamplifier with HPF / Re: Possible ICs fried MP566-II

« Last post by JPK on May 23, 2026, 03:24:35 PM »

You mean R53, right? Replace it.
Before applying power again, check that you don't have a short circuit between V- and 0V.

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TS500 Tape Simulator / Tape head coil signal audible

« Last post by giveaway on May 21, 2026, 10:11:52 PM »

Hey,
I can actually hear a slightly modulated/distorted version of my signal physically coming from the small "tape head" coil on the circuit board. Is this normal? For moderate level musical signals it's pretty inaudible unless I really crank the drive, but for sinusoidal test signals I can hear it from a meter away. The level does need to be fairly hot. Anyone else noticed it?

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MP566-II Tube microphone preamplifier with HPF / Re: Possible ICs fried MP566-II

« Last post by spurton@gmail.com on May 21, 2026, 10:10:19 PM »

It looks like R3 is scorched. I measured it and it’s open loop. I Noticed the left lead looks burned. 

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TS500 Tape Simulator / Re: TS-500 VU meter setup

« Last post by JPK on May 21, 2026, 04:36:25 PM »

TAPE SAT to the left (-).
The other switches have no effect on this setting.

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MP566 Tube microphone preamplifier / Re: 5.66 Problems

« Last post by JPK on May 21, 2026, 04:29:03 PM »

what do you mean by sequenced?
When you turn on power, the high voltage comes up after 2-3 seconds and the heater voltage after 5-6 seconds.

Check that you don't have any DC voltage on TP4.
Try to follow the signal: https://www.soundskulptor.com/forum/index.php?topic=209.msg981#msg981

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TS500 Tape Simulator / TS-500 VU meter setup

« Last post by giveaway on May 21, 2026, 12:54:12 PM »

Hi!
The setup guide does not seem to mention the state the saturation and bias switches should be in when calibrating the VU meter. Does anyone know?
Sorry if this info is available somewhere, but I have searched quite a bit without finding it.

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MP566 Tube microphone preamplifier / Re: 5.66 Problems

« Last post by MRMNW on May 20, 2026, 05:57:34 PM »

Thanks, what do you mean by sequenced?

It's one of your 501 Chassis, but does the same in my API lunchbox too.

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MP566 Tube microphone preamplifier / Re: 5.66 Problems

« Last post by JPK on May 20, 2026, 05:47:34 PM »

The voltages are OK.
Check that the 2 voltages are sequenced and come up after a few seconds.
Check that it is not the chassis that is loaded too much. What brand is it? Did you add more modules recently?