41

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

« Last post by JPK on August 21, 2026, 09:57:31 AM »

Check you didn't swap Q1 & Q2.
Check the voltage on R46 top pin (7.4V).
Check the solder joints on R45, R46, R47.
Check the direction of C23.

42

LA502 Optical Compressor / V- Rail returns low

« Last post by Namy on August 21, 2026, 02:04:05 AM »

Hi!

So, relatively new DIYer. I have just built my first LA502! Yay!

In testing, the v+ rail looks great (+15.7v)

Second test of the v- rail returns… .787 mv.

I have checked all polarized pieces (caps, diodes, etc), and they’re fine. I’ve looked for bad solder joints across the board, and particularly between pins 13, 14, 5, —> OV / V-, but I don’t see anything of concern. I also double checked connectivity and resistance on those joints, and everything checks out.

I’m sure it’s something silly, but I’m still learning how to fully understand the schematics and I’m not sure where to turn my attention. Any help would be appreciated!


(PS: I am using an XT500 with an API Lunchbox 6B. The XT500 seems to fully check out, though there might be something I missed)

43

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

« Last post by guenter1910 on August 20, 2026, 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

44

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

« Last post by hanssongt on August 20, 2026, 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

45

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

« Last post by noeqplease on August 20, 2026, 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

46

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) ?

47

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

48

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

49

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

50

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.