1

LA502T Tube Optical Compressor / LA502T REV1.4 modifications

« Last post by JPK on June 18, 2026, 06:53:57 PM »

D1, D2, D3, D4 diodes connections
These diodes add soft clipping, thus preventing abrupt clipping of the output stage.
The connections for these diodes are missing on the REV1.4 PCB. They can be restored by soldering short wires at the red line positions in the picture.

R16 replacement
It is recommended to replace the resistor R16 by a film capacitor of 100nF.
The leak current of C5 creates a voltage offset of about 20mV in the output stage when the gain pot is at 0. The cap eliminates it.

2

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

« Last post by JPK on June 07, 2026, 04:52:32 PM »

3

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

« Last post by spurton@gmail.com on June 07, 2026, 01:18:00 AM »

Sorry yes R53. I replaced it and it works! I just recorded bass tracks through it and it sounded great. Thanks for the help. 

4

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

« Last post by JPK on June 04, 2026, 05:30:55 PM »

5

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

« Last post by terroirmusic 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!

6

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

« Last post by JPK on June 04, 2026, 03:46:34 PM »

on the heatsink of Q5 (test point I) I get around -0.07V instead of -0.5V.
How much do you get on point F? and on the low side of R22?

7

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

« Last post by terroirmusic 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.

8

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

« Last post by JPK on June 02, 2026, 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.

9

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?

10

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?