Swap K47 for K67 voltage question

[Ken A]

I have looked around trying to find some discussion on this and have yet to find it specifically.

I'm looking to swap a stock K67 type capsule for a K47 type capsule on an NT-1. So this question would not pertain to the voltages for a tube mic.

In a recent discussion with Michael Joly he did mention to me that the polarization voltage to the K47 has to be lowered to keep the diaphragm from sucking to the backplate. He didn't really go into details and I'm not one to push for them. But figured I could ask here.

Does anyone have any experience doing one of these swaps, and does anyone have any insight into how to go about getting the voltage to the capsule where its needed. I have read that the FET mics using a K47 have 42V for polarization. From what I have read, the K67 capsule is designed to operate at 60V polarization but the U87 schematic that I looked at shows 47V. What throws me off even more is that plugged in my NT-1 and couldn't get a voltage reading of more than 1V at the capsule leads. So now I'm thoroughly confused.

Perhaps I'm in over my head.

Any assistance would be greatly appreciated.

 

[crazydoc]

I don't know anything about the capsule question, but you won't be able to measure voltage at the capsule due to the high resistance there (G ohm) - your measuring device will suck the electrons from the circuit, giving you a falsely low voltage reading. However, if you know the resistance of your meter, you could calculate the voltage using parallel resistors and Ohm's law.

 

[The Suntrap]

I want to do the same, I have a nady 1050 tube mic and a k47 style capsule that I would like to use in it and I have read loads of posts on many forums but i cant find the answer to getting the capsule voltage low enough. I changed both of the zeners in the power supply to 39v but the back of the capsule still gets sucked to the backplate does anyone have the soulution?

 

[dgatwood]

Interesting. The folks at

The FAR FET 47

are limiting their design to not do figure 8 because they can't get the polarization high enough for a K47 (desiring something closer to 60V). if you're having trouble at 39V, it makes me wonder if they didn't under-tension your capsule or something....

 

[The Suntrap]

Silly me I had a 75v zener and a 39v zener but I installed them pooly an they didn't work properly thats why the capsule was getting sucked to the capsule
It's working fine now using Lucio's resistor choices from his nady 1050 schematic!

I have also put a k47 capsule in my Rode NT2 wich is working fine there seems to be a peak in the frequency responce in the 4-5 k regeon could this be that the polarisation voltage is too high and affecting the responce of the capsule?

Does anbody know how to reduce the voltage to the capsule on a 1990's Rode NT2?
any help would be great.