48V and Pad on a grounded CT input transformer (mic preamp)

[CBAmp]

Hello,
I am currently putting a Lomo YP27 (UP27) preamp in a rack. I'd like to add 48V phantom power and -26dB pads at the inputs, but the center taps of the input transformers are grounded. So I wonder which is the best way to do these mods. CT grounded input trans are not common anymore. I read some threads, and here are some options I have :

1 - simply disconnect the CT ground and add the usual circuits (2x6.8K for 48V, and resistors network for a pad). But @CJ wrote that this could change the capacitance of the coils, and perhaps the overall response ?
2 - disconnect the CT and use it with a 3,4k resistor to feed the 48V. And add a pad somewhere else in the circuit, perhaps on R32 and R33.
3 - another guess ?

Your experience on this would be greatly appreciated, thank you.
Cyril.

 

[emrr]

just disconnect it, and it it seems to work fine then it's fine. Have never seen any appreciable response change from doing so. Only the oddball RCA things with the screen connection tied to that point are a problem, and if it's like that you'll know it.

 

[mjrippe]

Wow, that's my file from over a decade ago! Here are some pics of a pair I racked. And yes, do as Doug said.

 

[CBAmp]

Thanks a lot for your answers, and the pictures mj. Great job. So I did it, it works fine.
Here is a picture of my unit. I made a second output stage to have 2 independant preamps (with NOS soviet caps ! ). But I replaced the pio power supply caps with good electrolytics ones (under the board). The heat sink on the 6.3V is a bit too small, I'll change it.
There are also a lot of ground loops in the original wiring (pots and rear connector), so I simplified all that.
Thanks again for your help.

 

[mjrippe]

Very nice work! I like the added output stage and your power supply is more elegant than my brute force approach