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Author Topic: U67 low-cut filter S2: to cut or not to cut?  (Read 1087 times)

RuudNL

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U67 low-cut filter S2: to cut or not to cut?
« on: March 08, 2022, 04:03:34 PM »

Several times I have read your comments about the weak reproduction of low frequencies by Neumann K87/K870 microphone capsules.
Since I own a number of microphones with the same capsule (U67, U87ai, TLM67), I have made a number of measurements to get an impression of whether there are differences in low frequency reproduction. (Capsule heads on the same microphone body!)

To my (pleasant) surprise, all K87 capsules turned out to show almost the same frequency response in the low frequency range, which gives me the impression that they are fairly identical (the microphones are from different years). It is noticeable that all microphones reduce the lowest frequencies (starting below 60 Hz.) as a result of the fixed high-pass filter.

Is it recommended to remove the S2 jumper in the U67 for 'normal' use, or are there drawbacks to doing this when using the microphone in a well-insulated studio, without 'rumble'?
(It takes little effort to solder that jumper back on!)
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klaus

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Re: Fixed U67 low-cut filter: to cut or not to cut?
« Reply #1 on: March 08, 2022, 06:06:11 PM »

Unlike you, I have observed noticeable variations of recent K67 capsules in their low-end response, even between front and rear of the same capsule.

Neumann uses a rather wide ±2dB tolerance  as acceptable for their LD mics. That wide range is almost entirely attributable to the capsule, and within the capsule, to bass response variations which are inevitable in the manufacturing process of any handmade LD capsule.

And herein lies the answer to your question: make the determination whether to open or close S2 dependent on the capsule you are using in a particular mic. In some instances you may find the overall sound too boomy with S2 open, or too choked sounding with S2 closed.

P.S.:  to make the low-end decision fast and user-friendly, I install a microswitch (see picture) instead of that awkward wire soldered in place as part of my U67 custom modifications.

P.P.S.: You mentioned that all of your K870/K67 capsules have a similar low end response. Please share the testing conditions and response @ 50Hz in reference to 1KHz.
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Klaus Heyne
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RuudNL

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Re: Fixed U67 low-cut filter: to cut or not to cut?
« Reply #2 on: March 09, 2022, 03:13:32 AM »

I will check if I still have all measurements, not sure if I saved them all.
I used REW and a loudspeaker with a frequency 'sweep' to check the frequency response.
Microphone was 25 cm (10") from the speaker.
Although this may not be 'scientific', it is a way to see if there are differences.
(Not really interested in 'absolute' values.)
If I don't have all measurements available, I will measure again.

By the way: nice solution Klaus, a miniature switch for the filter!

Edit: measurement results added.
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afterlifestudios

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Re: Fixed U67 low-cut filter: to cut or not to cut?
« Reply #3 on: March 11, 2022, 01:42:03 AM »

I snipped both of my u67's S2 jumpers and have never missed the roll off.
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RuudNL

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Re: Fixed U67 low-cut filter: to cut or not to cut?
« Reply #4 on: April 20, 2022, 04:09:35 AM »

Quote
as part of my U67 custom modifications I install a microswitch instead of that awkward soldered wire switch, to make the low-end decision fast and user-friendly (see picture).

A question totally off topic: what kind of glue do you prefer to attach the miniature switch to the side rail?
(It seems two component glue doesn't attach very well to the extremely flat surface of the switch.)
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klaus

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Re: Fixed U67 low-cut filter: to cut or not to cut?
« Reply #5 on: April 20, 2022, 12:32:50 PM »

I use Eclectic  E6000 (https://eclecticproducts.com/product/e6000-craft/) for attaching these switches and for attaching all kinds of components in microphones. It is also excellent for vibration-damping of parts that are sensitive to microphonics.

The product has several properties I like:
It is easy to apply, even in minute quantities, self-distributes to a smooth surface, stays elastic over years, has a very high surface resistance which is important when applying the adhesive in the high-impedance sections of condenser mics, and, when needed, parts attached with E6000 can be peeled of with needle nose pliers with one pull, leaving no residue.

E6000 is a styrene-based adhesive and has different properties than fully-hardening epoxies, which I only use for permanent installations that are exposed to high tensile forces, i.e. when I know that I will never want to remove the part again.
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Klaus Heyne
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RuudNL

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Re: Fixed U67 low-cut filter: to cut or not to cut?
« Reply #6 on: April 20, 2022, 01:01:07 PM »

Great! Many thanks!
I will see if I can find it here.
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ji43

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Re: U67 low-cut filter S2: to cut or not to cut?
« Reply #7 on: July 01, 2022, 07:30:50 AM »

As one of S2's functions is as a plosive-filter/pop filter, has anyone found that the U67 doesn't require a pop filter with S2 in line?

I snipped mine, and use a pop filter, though I also recall needing a pop filter before I snipped S2 to prevent unwanted pops in the audio.

I understand snipping S2 improves the midrange, but also adding a pop filter affects the midrange.

Considering installing a switch so I can experiment for myself, does anyone know a tech in the New York area with experience modifying mics? After snipping the S2 wire, I don't know how to solder the switch in, as the wire is really small, and as it's jut cut at one end, I am not sure how I would get the solder to connect...so hoping to find a good tech who can do it for me.
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RuudNL

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Re: U67 low-cut filter S2: to cut or not to cut?
« Reply #8 on: July 01, 2022, 07:54:18 AM »

It is very easy to do!
You have to remove the tube in order to (better) access the points where the S2 wire is soldered.
Unsolder the original wire at both sides and solder two (thin) wires to the points where the original wire used to be.
Solder the other ends of the wires to the switch.
Put back the tube, glue the switch to one of the rails and...done!
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ji43

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Re: U67 low-cut filter S2: to cut or not to cut?
« Reply #9 on: July 01, 2022, 08:27:40 AM »

What type of wire is recommended to replace the original S2 wire? I have the switch, and glue as recommended by Klaus; so not just need the proper wire. I'll still probably have a tech do it for me, but would be good to bring the parts needed to the tech.
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RuudNL

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Re: U67 low-cut filter S2: to cut or not to cut?
« Reply #10 on: July 01, 2022, 01:01:13 PM »

Any piece of thin insulated wire will do.
(The switch isn't in an extremely critical point in the circuit.)
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ji43

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Re: U67 low-cut filter S2: to cut or not to cut?
« Reply #11 on: July 14, 2022, 08:57:03 AM »

Seems to be some debate on whether the snipping S2, while improving the midrange, requires the use of a pop filter, which also affects the midrange apparently. So which affects the midrange more? And can the U67 be used without a pop filter, if S2 is closed.

Also, if I use a HPF while mixing at any point down the line (at 30hz or above lets say 60 or 80hz), whether on my preamp, or ITB with Fab Filter ProQ3 on the U67 signal, does that just bring the "midrange" back to where it started with S2 in line? If I'm going to filter, might I just be better off to do it at the mic by leaving S2 in line?
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RuudNL

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Re: U67 low-cut filter S2: to cut or not to cut?
« Reply #12 on: July 14, 2022, 12:47:44 PM »

Simple test: unsolder one end of the S2 wire and see if you like it.
(If you don't like it, simply solder it back... A listening test is worth a thousand words.)
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