DIY Vintage U47 issue with signal on XLR pin 2

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Henrizzle

Member
Joined
Jan 22, 2017
Messages
11
Hey there! I'm new here to the group and hoping there is someone here that can help me trouble shoot an issue with a U47 build.  The symptoms are:

- Mic has low gain and sounds tinny
- If I disconnect pin 2 on the XLR connector, it sounds great, I compared with my U87ai, about the same amount of gain
- If i have pin 2 connected the signal seems to be very low and out of phase
- With pin 2 disconnected, the mic seems very sensitive

I suppose it is somehow working with just negative and ground. The PSU seems to work good except I have to use a plug adapter to do a ground lift or there is loud grounding noise.

Anyone with good knowledge of these circuits here that can point to things to check?

Thanks!

Henrizzle

 
Okay got it sorted out. I think:

- Tube had to burn in
- Was out of phase against my U87ai so I reversed the pins 2 and 3 on the XLR connector
- Works nicely now

Seems to be lower gain than the U87ai, but this allowed me to turn up the nice tube preamp more, very smooth sounding now :)

I might put the original RM47 capsule back in to see how it performs as now the U87ai seems a little more sibilant.
 
Okay so I should build in a ground lift switch on the power supply :) Can anyone suggest a 3 way toggle switch that can be used to do say:

- up power on with ground
- middle off
- down power on without ground
 

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