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u87ai HPF confusion
Old 3rd June 2015
  #1
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loveinoctober's Avatar
u87ai HPF confusion

Hi,

I'm confused about the the HPF on the u87ai. On this website it says that the HPF is selectable at 75 and 150 Hz, but on the microphone there is only a on/off switch for the HPF? What am I missing?

Moreover, when studying the frequency plot for the cardioid pattern, it looks like the -3dB corner frequency is around 400 Hz, much higher than the 75 or 150 Hz listed in the specs. What's going on here?

I find the HPF unusable for vocal recording because the -3dB cut off frequency is too high. Is there a mod to lower the cut off frequency to around 75 or 80 Hz? It should be a matter of just changing 1 resistor and changing the tuning of the -6 dB/octave filter. Let me know if you have any tutorials/schematics for this mod.

Thank you in advance.

-E
Old 3rd June 2015
  #2
I haven't read anything about the HPF... but it was clear at the very first time, that it was useless for vocals because the cutoff freq is too high!!

:(
Old 3rd June 2015
  #3
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loveinoctober's Avatar
Quote:
Originally Posted by Prabha View Post
I haven't read anything about the HPF... but it was clear at the very first time, that it was useless for vocals because the cutoff freq is too high!!

:(
Agreed, but it seems to me that there should be a simple mod for correcting the correcting the cut off frequency and restoring balance to the audio universe.
Old 4th June 2015
  #4
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Yep, HPF on the 87 is way to high
Never engage it, it really makes the mic sound thin
Old 5th June 2015
  #5
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Klaus's Avatar
 

Quote:
Originally Posted by loveinoctober View Post
I'm confused about the the HPF on the U87Ai. On this website it says that the HPF is selectable at 75 and 150 Hz, but on the microphone there is only a on/off switch for the HPF? What am I missing?
The information on the website is not correct. Next time, just go to the Neumann's own website and view their mic specs there.
Quote:
Is there a mod to lower the cut-off frequency to around 75 or 80 Hz? It should be a matter of just changing 1 resistor and changing the tuning of the -6 dB/octave filter.
Yes, simple enough: increase R102 (150kΩ) until you are happy. I cannot calculate an exact value for arriving at the exact roll-off you desire because, 1. it's a shelving eq you engage with the switch, and 2. the overall bass response of K870 capsules varies quite a bit.
I would start with doubling the current value to 300kΩ, then increase further, if the mic is still too bass-shy for your ears when the roll-off switch is engaged.

Good luck!

Last edited by Klaus; 5th June 2015 at 02:57 AM..
Old 5th June 2015
  #6
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loveinoctober's Avatar
Quote:
Originally Posted by Klaus View Post
The information on the website is not correct. Next time, just go to the Neumann's own website and view their mic specs there.

Yes, simple enough: increase R102 (150kΩ) until you are happy. I cannot calculate an exact value for arriving at the exact roll-off you desire because, 1. it's a shelving eq you engage with the switch, and 2. the overall bass response of K870 capsules varies quite a bit.
I would start with doubling the current value to 300kΩ, then increase further, if the mic is still too bass-shy for your ears when the roll-off switch is engaged.

Good luck!
Awesome! Thank you.

Does R102 form the HPF with capacitor C106 (0.47 uF)?

Cheers.
Old 5th June 2015
  #7
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Klaus's Avatar
 

Feedback path R106/C105 determines most, but not all, of the upper and lower eq limits of the mic. When R102 is engaged via roll-off switch, it parallels R106, which increases feedback and decreases low end.
Old 6th June 2015
  #8
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Cathedral Guitar's Avatar
The 87 HPF seems to be one of the main ingredients of "the NPR sound."
<A top audio engineer explains NPR’s signature sound | Current.org>
Old 7th June 2015
  #9
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Spede's Avatar
 

Quote:
Originally Posted by Retinal View Post
Yep, HPF on the 87 is way to high
Never engage it, it really makes the mic sound thin
This might be the source of confusion which was going on few years ago (at least in Finland) where people considered the TLM 103 somewhat resembles the U87.
Old 7th June 2015
  #10
Gear Nut
 

The HPF is totally fine if you are in anything other than omni and very close to the mic.
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