U87 without imput cap

[LevinGuitar]

What is the use of C4 and R6 in the classic U87 circuit? Would not be better to have one capacitor less (jumper over C4) and leave only one 1G resistor? Since the polarization goes to the backplates there is no voltage on the diaphragm, right?

http://cdn.recordinghacks.com/images//mic_extras/neumann/U87-schematic.png
Same question for C2 and R4 of the MXL 2001 and similar circuits

http://cdn.recordinghacks.com/images/mic_extras/mxl/MXL2001-schematic.gif
Merry xmas for all

 

[kingkorg]

C4 protects the fet in case of arcing. The voltage across the capsule is relatively high, the distance tiny, and diaphragm vibrate. People have reported this happen.

Also in the period when u87 was designed they didn't care about absolute minimal bom. Reliability in broadcasting was imperative.

 

[LevinGuitar]

Thank you. But it sound better, less compressed, deeper low end response... :/

 

[RuudNL]

If you want to take the risk...
In the case of the MXL, it will lower the (already low) polarisation voltage, because the gate of the FET isn't at ground potential.

 

[kingkorg]

Fwiw i did a null test with and without the cap, recorded same exact source with no change in position or level. Pink noise and audio canceled when adjusted for volume. I couldn't hear any difference either.

 

[LevinGuitar]

I removed it from several modded mics with tests before/after and for my ears there was a difference, same with tube mics. Will tell if have any problem with the FET someday.

Thank you again!

 

[rogs]

Interesting to note that Schoeps didn't bother with the extra cap and 1G resistor to protect the FET on their famous CMC5 circuit.
http://recordinghacks.com/images/mic_extras/schoeps/cmc5-schematic.gifThey did add protection diodes across the output emitter followers though....
Looks like they considered damage to the output transistors from 'hot plugging' more likely than damage to the FET from 'arcing' ?.....