DIY Vintage U47 issue with signal on XLR pin 2

[Henrizzle]

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

 

[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.

 

[Henrizzle]

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