41

MP573 Microphone preamplifier / Re: MP573 LED not behaving correctly

« Last post by davidfloegel on August 27, 2025, 11:23:36 AM »

Hmm I'm not sure how to best measure this.

With an input signal of 1kHz, Gain=10dB, Trim=-10, Z=1200.
What AC input voltage do you need to light up the green LED?
- Normally about 140mVAC (measured with the preamp connected)

I can't actually get the signal loud enough through my speakers to trigger the LED at all.

With an input signal of 1kHz, Gain=10dB, Trim=+10, Z=1200.
What AC input voltage do you need to light up the red LED?
- Normally about 1.4VAC (measured with the preamp connected)

I can get it to go green, but I can't get the signal loud enough through my speakers for it to go red.


Are there other ways I can feed in a signal?

42

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

« Last post by JPK on August 27, 2025, 09:52:54 AM »

Contact me here: https://www.soundskulptor.com/en/content/6-contact

43

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

« Last post by JPK on August 27, 2025, 09:50:24 AM »

Here are the voltages for the REV 1.3
The voltages are measured against TP3.

Pins 3 and 12 are both with -9v and i dont think this is right.
I think this is not measured against TP3?

44

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

« Last post by JPK on August 27, 2025, 09:34:38 AM »

I don't know this particular brand but the specs are the right ones: 22uF - 16V.

45

MP573 Microphone preamplifier / Re: MP573 LED not behaving correctly

« Last post by JPK on August 27, 2025, 09:28:58 AM »

With an input signal of 1kHz, Gain=10dB, Trim=-10, Z=1200.
What AC input voltage do you need to light up the green LED?
- Normally about 140mVAC (measured with the preamp connected)

With an input signal of 1kHz, Gain=10dB, Trim=+10, Z=1200.
What AC input voltage do you need to light up the red LED?
- Normally about 1.4VAC (measured with the preamp connected)


It is normal that Q6 gets warm. It dissipates around 1.25W.

46

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

« Last post by soundguy937 on August 27, 2025, 03:45:16 AM »

Pretty much spot on. Goes to about 2.VDC in about 5 seconds & within 10 seconds goes to 6.43VDC and is stable.

At this point I think I am ready to send it to your lab , just let me know the best way to do so. I'm bummed but this is obviously beyond me. Either a faulty part or a faulty installation of one by me somewhere.

47

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.

48

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

49

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).

50

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?