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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!

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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?

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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. 

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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

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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. 

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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

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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.

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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?

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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?

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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.