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MP566-II Tube microphone preamplifier with HPF / Signal gets heavy distorted and very quiet.

« Last post by guenter1910 on Today at 07:06:17 PM »

Intermittendly the signal gets distorted and very quiet. Sometimes it disappears and comes back after a while and sounds ok. This happens on and on. It seems to be contact problem (?) - I was thinking of the tube circuit and soldered the connections of the tube pcb again but this does not help. It happens on both inputs line and mic.
Any hints?
Thank you in advance!
Guenter

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MP512 Microphone preamplifier / Re: Clipping/distorting noise on High Z input

« Last post by hanssongt on Today at 10:10:41 AM »

Thank you for getting back so quickly.

Q2 is in the correct position.

R51:
15.77V near C24
15.92V near connector

R48:
131mV near R41
1V near C23

So something seems off when looking at the measurements at R48?

Thanks,
Tobias

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LA502 Optical Compressor / Re: 502 Compressor not working - test results.

« Last post by noeqplease on Today at 06:30:57 AM »

UPDATE:

I spent some time away from testing this compressor. Glad I did.
I saw some new test equipment, so I ordered a few new items to replace my aging oscillator and old as hell multimeter.

So, in the end it turns out that it was my test equipment that was not happy with this compressor.

Today I tested the LA 502 compressor I thought was bad, and it wasn't. At all.

Lesson: Always test your testing equipment, and always test your wires, leads, alligator clips, etc.

Thank you for your help and patience. Time to order a few mic preamps now

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MP512 Microphone preamplifier / Re: Clipping/distorting noise on High Z input

« Last post by JPK on August 18, 2026, 11:04:36 AM »

Check that Q2 (2SK170) is in the correct position A.
Can you measure the DC voltage (against 0V) on both sides of R51: near connector (16V) and near C24 (15V) ?
Can you measure the DC voltage (against 0V) on both sides of R48: near C23 (8V) and near R41 (1V) ?

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MP512 Microphone preamplifier / Clipping/distorting noise on High Z input

« Last post by hanssongt on August 17, 2026, 08:03:05 PM »

Hi!

Recently finished my latest build. Everything works except when connect the DI. I can hear the source (bass in this case), but it sounds like it's clipping/distorting even on low gain. Any idea what could cause this?

I should mention that when I first tried the DI I coulnd't hear anything, but that was because I had missed soldering a couple of the resistors on the DI board. After doing that I got sound but got the problem mentioned above. I hope I didn't mess anything up by testing it without all parts soldered first.

Thanks,
Tobias

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LA502 Optical Compressor / Re: Release time feels bit fast

« Last post by Slembones on August 11, 2026, 08:15:33 PM »

ok. played it in the 10ms mode and the release is better. so I going with it like it is. Thank you

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LA502 Optical Compressor / Re: Release time feels bit fast

« Last post by Slembones on August 11, 2026, 04:49:08 PM »

My construction quality is world class. that being said what happens is with vocals its very good but with bass it swells to fast as the note dies. it feels like with a bit more release time it might compensate for the swell

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LA502 Optical Compressor / Re: Release time feels bit fast

« Last post by JPK on August 11, 2026, 09:27:39 AM »

Do you also find the release time too fast with the 10mS attack?
Check the value and solder joints of R32, C17, C18.
If still too fast try increasing R32 to 4.7M. You could also add an additional cap in parallel with R32. It will also slow down the attack time but you can compensate by decreasing R29.

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LA502 Optical Compressor / Release time feels bit fast

« Last post by Slembones on August 10, 2026, 08:03:50 PM »

Is there a way to slow the release time down a bit. it feels a little non linear. the unit sounds good and is set up per the manual so everything works and is in the right place.

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MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« Last post by guenter1910 on August 07, 2026, 12:59:14 PM »

After reflowing the solder joints it seems to work now. This is a bit of a surprise to me because measuring the connections alway showed that everything is fine. Thank you for your help!