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

« Last post by camarada78 on August 26, 2025, 06:15:10 PM »

I think the TL074 is weird.

Pins 3 and 12 are both with -9v and i dont think this is right.

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

« Last post by soundguy937 on August 26, 2025, 04:50:52 PM »

I will try to test the Tube heater test tonight or early tomorrow. At this point I am strongly considering sending it in as I cannot see what I may have done wrong. I have built many diy kits from microphoneparts.com and diyre.com and have had little to no issues. Your kits seems pretty strait forward and I was able to do the builds with very little frustrations lol. 

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

« Last post by musicarr on August 26, 2025, 03:57:57 PM »

Go it. Thanks for the clarification. Just to see if that might be the case, I'll replace it. As a reference, it was C20 that I had backwards. Just to be sure I'm ordering the correct replacement, I'm looking for a 22uf 16v tantalum capacitor to replace C20. Is this what I'm looking for (amazon)?

https://www.amazon.com/Uxcell-a14042900ux0341-Yellow-Tantalum-Capacitors/dp/B00PZLX93O/

Thanks

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

« Last post by JPK on August 26, 2025, 03:29:39 PM »

is that "pop" going from 20 dB to 30 dB "normal"?
Some pop at this position is normal because the 30 position adds a preamplifier stage to the audio path.
If it is feels too loud it could be that the reversed tantalum as suffered from the reversed polarity (not unlikely).

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

« Last post by JPK on August 26, 2025, 03:22:56 PM »

All OK.
Check again the tube heater (step 4 of the setup guide).
I am sorry, I am short of ideas. Do you want to sent it to us for a check at the lab?

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

« Last post by camarada78 on August 26, 2025, 03:21:35 PM »

I measured the three transistors with a component tester and they are working fine (q1, q2 and q3)

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

« Last post by camarada78 on August 26, 2025, 03:11:56 PM »

Thanks for the reply.

My version of the preamp is a little different than the one of the picture.

its the rev 1.3

But, if i measured correct, the voltages are

F = 12v9
A = -9v

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MP573 Microphone preamplifier / Re: MP573 LED not behaving correctly

« Last post by davidfloegel on August 26, 2025, 10:29:15 AM »

Ok I've just tried testing it..

I've played the sine wave on a loop and put an Audix I5 in front of it. I can see the LED light up green and I'm getting:

~0.1 on the left side (towards the controls)
~0.4 on the right side (towards the back side)


Side note: 
My Q6 is getting pretty warm right away, I rested my finger on it while measuring. It's not so hot I can't touch it, just very warm.

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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. 

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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"?