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
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
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