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

« Last post by JPK on Today at 03:38:24 PM »

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.

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

« Last post by terroirmusic 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 / Re: AB528 build has an abnormally high noise floor

« Last post by JPK on June 01, 2026, 03:29:29 PM »

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

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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