Gus

HPF in microphones
« on: December 29, 2020, 08:06:07 AM »
I don't understand about why people remove the HPF in a microphone like a U67.

I would think you would want a HPF in the range of 20Hz to 40Hz if you don't have a treated room.

I would think it is better to remove the low frequency rumble etc. before sending it to a preamp. Less junk to cause IM distortion.

I have added a 12dB HPF at about 30Hz in microphones and on the list of things to build is one with a 4th order HPF at about 20Hz.




abbey road d enfer

Re: HPF in microphones
« Reply #1 on: December 29, 2020, 11:58:55 AM »
I don't understand about why people remove the HPF in a microphone like a U67.

I would think you would want a HPF in the range of 20Hz to 40Hz if you don't have a treated room.

I would think it is better to remove the low frequency rumble etc. before sending it to a preamp. Less junk to cause IM distortion.

I have added a 12dB HPF at about 30Hz in microphones and on the list of things to build is one with a 4th order HPF at about 20Hz.
Certainly, filtering out subsonics from the very start makes a lot of sense. However, the way it is so often done has serious drawbacks.
It is a very simple scheme that puts a resistor in parallels with the capsule for increasing the -3dB LF cut-off frequency, but it has the big disadvantage of increasing noise.
With a very low cut-off frequency, the "capsule noise" (or QTC noise) spectrum is dominant at infrasonics, so inaudible.
By raising the cut-off frequency, the spectrum is shifted in a way that it becomes audible and questionable.
The proper way consists in operating the capsule and first stage with a very low frequency cut-off and following it with a separate filter. that's the way it's done in the AKG414 or Neumann U89, for example.
Others do it by introducing selective feedback.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: HPF in microphones
« Reply #2 on: December 29, 2020, 01:31:24 PM »
The capsule went straight to the grid (through a dc blocking capacitor) in some of the AC701 powered mics and there was a few pf feedback from the anode to stabilize the response... for all that matters, 5 pf is a couple of twists of hook up wire...and the Messeingang (calibration input) allowed one to sweep response through the internal preamp....

RuudNL

Re: HPF in microphones
« Reply #3 on: December 29, 2020, 02:51:02 PM »
The capsule termination with a (relative) low value resistor was what Rode did in the NT2.
It did cut the low frequencies, but it boosted the self noise!
This is probably the worst way to implement a high-pass filter!
There is a solution for every problem!

http://www.vansteenisaudio.nl

Gus

Re: HPF in microphones
« Reply #4 on: December 29, 2020, 10:06:37 PM »
I was not posting about 1st order HPFs like reducing the input resistor value
I did have "like a U67" in the first post
I also have used the HPF between two stages like abbey road d enfer posted

abbey road d enfer

Re: HPF in microphones
« Reply #5 on: December 30, 2020, 08:30:04 AM »
I was not posting about 1st order HPFs like reducing the input resistor value
I did have "like a U67" in the first post
I don't know that people did "remove" the HPF in the 67. How do they do it? Do they simply disconnect S2?
For proximity use, it's adeqaute to HPF, but for distant micing, many SE's want to have the full LF response.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Gus

Re: HPF in microphones
« Reply #6 on: December 30, 2020, 12:22:13 PM »
If you do a search for a string like "u67 disconnecting s2 in an u67 microphone"
you will find posts about disconnecting S2




Re: HPF in microphones
« Reply #7 on: December 30, 2020, 04:53:23 PM »
One of the well known microphone "gurus" out there usually suggests removing the HPF (S2) in U67's hence, that may be one of the reasons folks do that.

I haven't recorded anything with a U67 that needed such low freq. pick up...  Is there really such a difference when recording vocals etc. (or anything other than a pipe organ) that neccesitates removing the filter?

I always thought the U67 HPF was quite an elegant way to do things, but maybe I'm an idiot   :o
D. J. H.

abbey road d enfer

Re: HPF in microphones
« Reply #8 on: December 30, 2020, 05:20:27 PM »
One of the well known microphone "gurus" out there usually suggests removing the HPF (S2) in U67's hence, that may be one of the reasons folks do that.
I guess it's the one that can hear phase shift at 16 Hz...

Quote
  Is there really such a difference when recording vocals etc. (or anything other than a pipe organ) that neccesitates removing the filter?
I would bet there is. The HPF in the U67 was designed to fix the issue in the U47 with pops that would make the mic to break up.

Quote
I always thought the U67 HPF was quite an elegant way to do things, but maybe I'm an idiot   :o
I would think you're the same kind of idiot as Neumann engineers... ;)
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: HPF in microphones
« Reply #9 on: December 31, 2020, 09:01:23 AM »

 I would bet there is. The HPF in the U67 was designed to fix the issue in the U47 with pops that would make the mic to break up.

The S2 position filter?  Are you saying there would be a benefit to removing this if recording normal source material other than deep, deep bass?

I would think you're the same kind of idiot as Neumann engineers... ;)

Haha, well I've never had the honorific title "guru" which makes all the difference ;) 
D. J. H.


Re: HPF in microphones
« Reply #10 on: December 31, 2020, 09:08:58 AM »
P.S.  Today I learned that implementing the (user switchable) 6dB octave, low cut filter at the capsule is a noisy way to do it, thanks chaps! 
D. J. H.

abbey road d enfer

Re: HPF in microphones
« Reply #11 on: January 01, 2021, 03:18:35 AM »
The S2 position filter?  Are you saying there would be a benefit to removing this if recording normal source material other than deep, deep bass?
Well, there may be a language barrier here. What I mean is this HPF has been implemented by neumann at the request of their distributors following complaints of uses that didn't like the way the U47 behaved when faced with plosives, but for distant micing would not be necessary, in fact a tad thin when deep lows are expected. That's the main reason, I guess, for the possibility to disengage the HPF, by opening S2.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: HPF in microphones
« Reply #12 on: January 07, 2021, 12:20:26 PM »
Well, there may be a language barrier here.

Haha, yeah I get what you're saying now, my bad :)
D. J. H.

Gus

Re: HPF in microphones
« Reply #13 on: January 07, 2021, 07:04:32 PM »
Thanks for the posts.

I don't think of a u67 as a distance microphone, I understand maybe removing the jumper then however, why not use a different microphone?

It is interesting to look at the meter using a microphone with a nice HPF and one without.
I have done this with a microphone using most of the same circuit and the same capsule and grill.
A 12dB octave HPF also does not seem to change a voice being recorded if it is set at 30Hz to 40Hz.
Higher frequencies do change the recorded low end.

Again unless the room is treated I don't understand removing a nice HPF design for vocals, podcasts etc.

Re: HPF in microphones
« Reply #14 on: January 07, 2021, 08:24:02 PM »
That sounds right to me Gus.
Perhaps much the same as the U67 capsule being used in the SM69 tube without HF de-emphasis, also probably due to being used as a distance mic.

I've no experience with the 67 as anything other than a mid to close microphone myself.  I do see pics of them being used at distance for orchestral type pick-up in older recording session pics though.   
D. J. H.


 

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