Studer D19 MicValve Input Transformer

[eddie_ruff_]

I'm trying to build my own version of the Studer MicValve D19. Here's the original schematic for the mic input (duplicated for Left and Right). There are lots of fixed resistor switches for setting the mic and line gain that I would like to simply replace with a potentiometer. However, there's a good chunk of this mic input transformer circuit that I don't understand yet. For example, the 8/9 winding on T5. Is that negative feedback? And when control voltage 2V12 is engaged, what's the purpose of the 453 Ohm resistors (R43, R8 and R128) ? I do have a set of the original mic and line input transformers, so I'd like to try and keep things close to the original but without switching.

 

[zamproject]

Remote control set a combination of passive input pad (R8/43) and NFB gain setting at first stage where as you notice secondary windings are involved.
There is also NFB gain remote at second stage, this is a mux stepped pot, 3bit 8pos ?
Maybe we need a print of the selector(s) logic that set all 2Vxx logic remote ?

 

[gyraf]

Valve-dignified - that's a mission..

 

[zamproject]

Yep, and with -angel zoom- tube circuit !

I just have a look at whole schemo, analog switches are done via encoders, button matrix, MCU and shift register

I used this pre once, in a studio that have it, no complain about sound as far as I remember, but damn what a complex circuit for just two preamp...the schemo is 11pages

@eddie_ruff_ , you really want to clone this ? with the tube section and the -300V switching supply?
I'm not used to discourage DIY initiative but this one is somehow disproportional for what could be expected at the end.

Cheers
Zam

 

[eddie_ruff_]

@zamproject yes, I really want to clone this! I'm in too deep at the moment haha. I've actually finished the SMPS, see this thread SMPS Troubleshooting

I have also finished the Line in, Valve Stage, and Line out and everything is working great so far on a custom PCB. So, this Mic Input is the final piece of the puzzle

The 453 resistors make sense for padding the input now, thanks for that! I'll try to breadboard this circuit today since I have the transformer and figure out a way to use a pot instead of the fixed resistors. Any other advice is welcome!