91

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

« Last post by soundguy937 on August 26, 2025, 04:19:53 AM »

Input transformer is in correct orientation.

TR1=202 2k
TR2=102 1k

B+ stable at 45.5VDC

R3= 3k3
R4= 3k3  
R5= 10k 
R6= 10k  
R7= 10k  
R8= 470R 
R9= 10k  
R10= 10k  

input transformer values (input on max)

2-4= 37.2 ohms
3-5= 37 ohms
7-9=100.1 ohms
8-10= 99.1 ohms

input resistance pins 2&3= 4.61 k ohms

everything mostly checks out excepts input transformer 8-10 being 99.1ohms. 

92

MP573 Microphone preamplifier / Re: Loud Pop When Changing dB Rotary Switch

« Last post by musicarr on August 26, 2025, 02:01:52 AM »

I checked all the electrolytic and tantalum caps. There was one tantalum cap that was backwards. I switched it to the correct polarity and the "bib pop" is gone. So that must have been the issue. There still is a slight "pop" going from 20 dB to 30 dB. It registers at about -30 db on the Pro Tools meter. The other positions don't seem to have that large a "pop", as they register about -45 dB on the Pro Tools meter. Is there something else I should look at, or is that "pop" going from 20 dB to 30 dB "normal"?

93

Discussions about the products / Audio Precision plots?

« Last post by det3 on August 25, 2025, 11:13:56 PM »

Hey All,

I'm curious - has anyone performed AP plots of Soundskulptor modules? I just purchased 4 kits and happen to own an older Series 2 AP. I would love to see what comes out after I complete my builds, and compare from any other measurements made if they exist. If not, I plan on sharing my test methods and results if I decide to do it.

Thanks!

94

MP573 Microphone preamplifier / Re: Loud Pop When Changing dB Rotary Switch

« Last post by musicarr on August 25, 2025, 10:31:11 PM »

With the electrolytic caps, it's easier to see where the negative and positive wires are. And on the tantalum caps, the "+" sign to the right of the labeling points to the positive lead. Is it possible, sometimes, that the "longer" lead of might not be the positive lead? I was going off of the longest lead on the tantalum caps. Is that reliable, or should you always look for the "+" marking to know which lead is the positive?

95

MP573 Microphone preamplifier / Re: Loud Pop When Changing dB Rotary Switch

« Last post by musicarr on August 25, 2025, 03:49:53 PM »

I was consciously, and verbally, careful to the direction of all caps set correctly. But it doesn't mean I missed one somewhere. I'll check every cap and let you know what I find. 

96

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

« Last post by JPK on August 25, 2025, 03:32:52 PM »

Check that the input transformer is in the correct orientation.
Check that TR1 and TR2 have the correct value.
Check that R3 to R10 have the correct value.
Check that the voltage on B+ is stable.
Check the input transformer coils resistance (in circuit, input pot on max): 

• 2-4 : 37 ohms
• 3-5 : 37 ohms
• 7-9 : 100 ohms
• 8-10 : 100 ohms

Check the input resistance (between XLR pins 2&3, input pot on max): 4.6k

97

MP573 Microphone preamplifier / Re: Loud Pop When Changing dB Rotary Switch

« Last post by JPK on August 25, 2025, 03:16:16 PM »

Pops are often caused by a reversed electrolytic or tantalum capacitor.
So the first thing to check is the direction of these caps: The + sign on the layout must match the + side of the capacitor. 

98

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

99

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.

100

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?