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MP573 Microphone preamplifier / Loud Pop When Changing dB Rotary Switch

« Last post by musicarr on August 25, 2025, 03:11:47 AM »

Hi,

I've just finished building my second MP573. Both of them seem to be working well, from the tests I've done according to the Setup Guide. All tests were good. But on the first one, when changing the dB level on the rotary switch, there are little pops, not terrible, but present. On the second one, the pops are quite pronounced, especially when going from 20 to 30 dB. So much so that it puts the meters into the red within Pro Tools.

So main question: Is this normal, or should there be no pop at all when change dB positions on the rotary switch?

I searched for "pop" in the support forums and found the following two threads:

• Low output and distortion at lower gain levels (https://www.soundskulptor.com/forum/index.php?topic=232.0)
• MP573 Having issues after plugging in (https://www.soundskulptor.com/forum/index.php?topic=260.0)

As I'm quite new to DIY, I didn't fully understand all that was said in the above threads, hence this new thread. Sounds like the pops should not be happing. What would be my starting point to test for what might be wrong?

Thanks

62

MU524 Vari-Mu Tube Compressor / Re: Stuck on step 7

« Last post by soundguy937 on August 25, 2025, 12:23:33 AM »

Unfortunately it does not appear a tube swap made a difference in stabilizing the voltage.

63

MU524 Vari-Mu Tube Compressor / Re: Stuck on step 7

« Last post by JPK on August 24, 2025, 09:18:48 AM »

Can you change the tube with the one from the other unit and check if it stabilizes better?

64

MU524 Vari-Mu Tube Compressor / Re: Stuck on step 7

« Last post by soundguy937 on August 23, 2025, 06:39:50 PM »

okay now with black probe on 0V I'm reading :

V1= 15.68vdc
V2= -15.65vdc

all my tests between TP1 & TP2 have been after at least 15-25min of warming up.

TP1 & TP2 after 30min and U4 still removed varies between +-1 to 2 hundredths of a VDC. 

With U4 replaced shortly after shut down of last test and waiting another 30min I am seeing my DMM with fresh batteries jump around again not stabilizing between 0.00 to +-0.05 VDC. Unfortunately it takes twice as long to mostly stabilize than what I seen in my first kit and the setup recommendation waiting time. Is this to be expected?

65

MU524 Vari-Mu Tube Compressor / Re: Stuck on step 7

« Last post by JPK on August 23, 2025, 04:39:11 PM »

V+= -15.6vdc
V-= 25.5vdc
Do you put the black probe on 0V?
Check your DMM batteries and check your lunchbox voltages.
Voltage variations between TP1 and TP2 are normal. They should stabilize once thermal equilibrium is reached.

66

MU524 Vari-Mu Tube Compressor / Re: Stuck on step 7

« Last post by soundguy937 on August 23, 2025, 01:14:14 AM »

looks like I am stable at the v+ although I am now noticing its reading negative instead of positive 15.6vdc, V- reads 15.5vdc positive vs. the setup saying it should be negative.

V+= -15.6vdc
V-= 25.5vdc

with U4 removed
TP1 to 0V= 32.3vdc
TP2 to 0V= 28.4vdc
TP1 to TP2= maybe slightly more stable, but still jumps around from a few hundredths to a couple tenth of a percent of voltage +-

67

EQP501 Passive Equalizer / Re: Son faible et pas d'EQ

« Last post by Pierro on August 22, 2025, 03:47:10 PM »

D'accord

68

CP4500 Stereo Bus Compressor / Re: CP4500 Cannot level match and incorrect voltages in step 1.6

« Last post by JPK on August 22, 2025, 09:25:46 AM »

You can do the voltage check without U7.

69

MP573 Microphone preamplifier / Re: MP573 weird behavior (no volume after 20)

« Last post by JPK on August 22, 2025, 09:23:31 AM »

The problem is located in the first preamp stage, around Q1, Q2, Q3.
Can you report the voltages A to F shown on the image here:
https://www.soundskulptor.com/forum/index.php?topic=5.msg5#msg5

70

EQP501 Passive Equalizer / Re: Son faible et pas d'EQ

« Last post by JPK on August 22, 2025, 09:19:42 AM »

- R21 est bien connectée a 0V quand SW3 est on, sans court-circuiter la LED.
- la LED s'allume avec le DMM
C'est contradictoire !

- Et de nouveau lorsque je court-circuite Q1 l'EQ s'active
- J'ai déja remplacé 2 fois Q1 avec précautions
C'est aussi contradictoire.

je vous propose un envoi à notre atelier. Il y a peut-être autre chose que je ne peux pas deviner...