Best way to implement a low cut in a c12ish microphone?

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

hop.sing

Well-known member
Joined
Jun 21, 2005
Messages
160
Hi

I built that Mic with a Haun ck12 a Haufe t14/1 and an e81cc years ago. It is a fantastic sounding microphone, but every once in a while it bothers me that it captures rumble as well as musical sound.
The frequency response goes easily down to dc :)
So what about a low cut that sits around 30HZ and interferes as little as possible with my music signals?
Wouldn`t  a u67 implementation be possible, even without the tertiary winding?
Just the feedback from the plate to the capsule, lowpassed at the right frequency?
Or are there other elegant ways to reach my goal?
Thanks for help
Tobias
 
Your question talks about performing a low-cut (or high-pass) function, but then you mention a feedback capacitor and a low-pass filter, which is the opposite.

With a C12, the easiest way to make a high-pass is by lowering the grid resistor down, however this also tends to increase noise.  You can also shrink the size of the transformer coupling capacitor.
 
Easiest way is to use a smaller grid resistor, as Matador mentioned. If it's C12'ish and you're using 250M, try 100M instead. 250/251's used as small as 8M (AC701) and 30MΩ (6072A).

In self-bias circuits, you can also use a smaller bypass capacitor. Change 47 to 22uf, or 22 to 10uf etc.
 
Matador said:
Your question talks about performing a low-cut (or high-pass) function, but then you mention a feedback capacitor and a low-pass filter, which is the opposite.
If you look at the u67 you will see it has negative feedback with a lowpass filter which leads to bass cancellation... elegantly implemented...
But yes, I will try the other ideas, too.
 
Floating this thread. Did you end up implementing a low cut in your microphone?
I'm about building a 251 clone and thinking out a good way to implement a low cut my self.

Guess the easiest way is to make a small board in with two resistors and a switch right? :)
 
I don't think you would want a regular switch and board in those hi-z areas with such high value resistors. But someone might sugest painless way to do it. Maybe relay?

Regular switch and board should be fine if you want to switch capacitors. 
 
To get the least sonic artifacts I'd use a smaller value coupling capacitor between tube and transformer. I would not introduce capacitor feedback...

Best, M
 
kingkorg said:
I don't think you would want a regular switch and board in those hi-z areas with such high value resistors. But someone might sugest painless way to do it. Maybe relay?

Regular switch and board should be fine if you want to switch capacitors.

That a valid point with the high-z, you don't always know if you need the low-cut prior to recording and it will take some time to let the microphone "cooldown" before you can open it.

Yeah a relay might be the best method, put the switch on the PSU instead or use the switches from the alctron boards. Even perhaps implement a pad  as well the same way.

Guess that's the way that Microphoneparts have done it. Haven study there 251 design though.

Marik said:
To get the least sonic artifacts I'd use a smaller value bypass capacitor between tube and transformer. I would not introduce capacitor feedback...

Best, M

Could you describe more in detail, with reference to the schematics, how you would go about doing so? :)
 
Zebra_PD said:
Could you describe more in detail, with reference to the schematics, how you would go about doing so? :)

Sorry, late at night and typing too fast. Not bypass, but coupling capacitor. Please post your schematics for correct reference.

Best, M
 
Well, messing with the coupling capacitor value may or may not be the greatest idea...

http://sound.whsites.net/articles/audio-xfmrs.htm#s3

Marik said:
To get the least sonic artifacts I'd use a smaller value coupling capacitor between tube and transformer. I would not introduce capacitor feedback...

Best, M
 
I wouldn't bother with feedback if it need to be still c12ish or 251ish microphone. Feedback will change sensitivity.
For M250/M251 type of circuit the easiest and most effective way is just cathode capacitor value. If like original 20uF (more common these days 22uF) then you could try 10uF, if still not satisfying then 3.3uF or 4.7uF or 6.8uF and here you can go easily with film capacitor than electrolytic (also there pretty small 10uF film caps these days).
It will doesn't hurt to reduce output capacitor to 1uF anyway.
 

Latest posts

Back
Top