PAiA 94073 tube colored mic preamps

[velo]

I have a pair of these D.I.Y. PCBs circa 1995-ish mounted in a 1U.

Weirdness.

They have power, 48V, and a tube drive LED. Only the 48V LED was working. Other than that they functioned ok though were irritatingly noisy above 90% output pot setting.

I checked the voltages across the LEDs and got 2V and 9V on the right channel and 14V on one of them on the left--although the LEDs were not lit. I suspected perhaps the LEDs were installed in reverse. I reversed a few of them and then the LEDs worked. Yay?

Initially, I figured that solved the problem but I was unsettled by the graphics printed on the PCB that indicated that the original wiring had the cathode in the correct hole. I had reasoned that perhaps the power supply polarity was reversed and then I remembered the other odd thing about this particular build.

Apparently, and I wish my eyesight was better, this uses two wall wort PSUs connected in series. Also, I am pretty sure the PSUs are wired in series since the unit doesn't work unless both are plugged in at the same time. I see that one lead from PSU1 and one lead from PSU2 are connected to the DPDT power switch and that the other two leads are connected to one of the 9407 PCBs (left channel) and then all leads from the power switch and PSU1/2 are jumpered from there to the 9407 PCB right channel.

PAiA has a forum and so I will likely take this up there to see if this was ever a recommended build procedure but the idea of driving mic preamps with electrolytic caps with reverse polarity at twice the designed input voltage to make up for the loss in gain seems pretty wild. How the PCB would work at all with reverse polarity is an open question. I'm not sure I am on the right track.

Are there other things I should check?

https://www.paia.com/manuals/docs/9407_TubeMicPreAmp_pages_200.pdf
https://paia.com/talk/search.php?keywords=9407

 

[MidnightArrakis]

I have a pair of these D.I.Y. PCBs circa 1995-ish mounted in a 1U.

Weirdness.

They have power, 48V, and a tube drive LED. Only the 48V LED was working. Other than that they functioned ok though were irritatingly noisy above 90% output pot setting.

I checked the voltages across the LEDs and got 2V and 9V on the right channel and 14V on one of them on the left--although the LEDs were not lit. I suspected perhaps the LEDs were installed in reverse. I reversed a few of them and then the LEDs worked. Yay?

Initially, I figured that solved the problem but I was unsettled by the graphics printed on the PCB that indicated that the original wiring had the cathode in the correct hole. I had reasoned that perhaps the power supply polarity was reversed and then I remembered the other odd thing about this particular build.

Apparently, and I wish my eyesight was better, this uses two wall wort PSUs connected in series. Also, I am pretty sure the PSUs are wired in series since the unit doesn't work unless both are plugged in at the same time. I see that one lead from PSU1 and one lead from PSU2 are connected to the DPDT power switch and that the other two leads are connected to one of the 9407 PCBs (left channel) and then all leads from the power switch and PSU1/2 are jumpered from there to the 9407 PCB right channel.

PAiA has a forum and so I will likely take this up there to see if this was ever a recommended build procedure but the idea of driving mic preamps with electrolytic caps with reverse polarity at twice the designed input voltage to make up for the loss in gain seems pretty wild. How the PCB would work at all with reverse polarity is an open question. I'm not sure I am on the right track.

Are there other things I should check?

https://www.paia.com/manuals/docs/9407_TubeMicPreAmp_pages_200.pdf
https://paia.com/talk/search.php?keywords=9407

Seeing that your posting from 2023 have been literally flooded and overwhelmed with responses by dozens and dozens of the members of this GroupDIY forum, I thought I would add-in my 2-cents worth to all of the "noise" in this thread. But, what I gotta say probably won't have any real value for you as far as your problem goes anyway, but.....here goes:

First off.....I personally have -- NEVER -- heard of wiring two transformers of this style in series before!!! That's way too weird!!! Just go onto either Digi-Key or Mouser and buy yourself a -- 12VAC @ 1-Amp similar transformer and "call it a day". I've gone through the manual and even reviewed the schematic and all that I can see is a single transformer used and even shown. So, between what the manual shows and what your reality is do not coincide!!! So, that should be your first "red flag" right there.

Secondly.....even the manual shows the two PCB's as being wired together in -- parallel -- with only a single transformer. This means that there's "something rotten in Denmark" (with no offense to any Danes) about your setup. See the image below:



I have also created and have attached a PDF file of the schematic and BOM/Parts List from the User Manual that you had provided a link to. The schematic has been placed and enlarged onto a sheet of 11" X 17" paper in the "Landscape" mode for your aging eyes. Should you wish to print this PDF file out, it will take 2-sheets of paper to print-out in full-scale, of which you can then Scotch-tape them together for your review and study or to frame and admire in your living-room. With Christmas being just around-the-corner, should you have any guests, family members, neighbors or "throngs of illegal immigrants" over for any Christmas parties.....you could make your schematic the center-piece of your holiday decorations by having it lit-up by some small LED-lamps!!! Wouldn't that be cool???

In any case.....HERE YA GO!!!

/