Biasing a Microtech Gefell UM705

[adamasnan]

Hi everyone! Seeking some advice

I had to replace the FET in my UM705 and it's unclear to me which would be the biasing resistor? The source of the FET is connected to a transistor.. The new FET is a little noisier than one of the other mics i have, so I want to see if it's something I can adjust.

Schematic attached

Thanks!

 

[adamasnan]

Can anyone please help?

Thanks!

 

[Voyager10]

R8, R14, R15 and VT3 are a constant-current source, which will set the FET's bias current 'automatically' to the value intended by the designers.

You can change the current by changing R14 (smaller R = higher current) but that might upset other parameters (e.g. headroom).

What FET are you using? It would also be helpful to know what the bias current is at the minute- can you measure the voltage across either R14 or R3?