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[klaus]

I agree.

[Marcel9206]

I totally agree, I will definitely check the mic before changing anything. But I just wanted to know what my options are when I'm not fully satisfied with the sound of the mic. Since I'm in Europe, I was wondering if there anyone here capable of tuning capsules etc... Changing the tube myself is of course no problem.

[Derek Samuel Reese]

I haven’t had any luck finding the Belden cable for my U67, however I managed to buy original male and female Tuchel connectors.
Does anyone know where I can find just the Belden cable alone ? I can then wire it up :-)
This is the hardest thing to find online.

[klaus]

Derek,
As that cable was only supplied for U.S.- based sales through Gotham Audio in New York (the U.S. importer for Neumann at the time), it will be hard but not impossible to find.

U67 sales within Germany and in other export countries were supplied with the EMT/Dörfler broadcast cables familiar from M269, M249 and so on.

[SDVIG]

Hello.
I see a 100 picofarad capacitor in all the photos.
Why is the capacitor on my U67 Reissue - 160 picofarads?
Thank you.

[klaus]

You are referring to C17, a low pass filter. It is the final high frequency adjustment, once a mic has been assembled and tested.
As you can see in the schematic detail, C17 can be anywhere from 80pf to 160pf, depending on overall high frequency content of the mic.

Depending on a U67's high-frequency response at this testing stage, the appropriate value is chosen to get within the "Sollkurve", the specified frequency response for the mic.

[SDVIG]

Klaus. Thank you.
I think the capacitor is selected in the place with the capsule installed?
And I understand that the larger the capacitance, the more low frequencies?
Thanks again.

[klaus]

As far as I know Neumann does not make the effort to fully assemble each mic, burn in the tube for a few hours, to THEN put it in an anechoic chamber to adjust the overall high frequency response.

It is far less time consuming to adjust just the amp's high frequency response via C17 on the bench, to compensate for differences in component tolerances (leaving the actual capsule off), but I could be wrong.

And I understand that the larger the capacitance, the more low frequencies?

The larger the capacitance the less high frequencies pass through.

[SDVIG]

Klaus. Thank you.

[David Satz]

The capsules have one set of frequency response tolerances, while the amplifiers are assembled separately and have their own tolerances. For both of these main parts of the U 67, at the extremes of the frequency spectrum those tolerances are wider than they are in the middle.

There are two "test points" on one of the amplifier's circuit boards. With an initial C17 in place, the contacts where the capsule head would normally connect to the amplifier are shunted by a special "test head"--a shielded arrangement of two fixed ~50 pF capacitors mimicking the capsule, but insensitive to sound. The amplifier can then be powered up and connected to a 1 kOhm load.

A 1 kHz signal (less than ca. 140 mV but not too much less) is then injected via the test points. The signal voltage at the output is noted--it should be within about 1 dB of the input signal level, since the amplifier is nominally unity-gain at 1 kHz. Then the frequency of the test tone is changed to 16 kHz for comparison. The signal level at the output should be ca. 7 dB lower now--and if not, C17 can be replaced with a capacitor of different value accordingly. Once the amplifier is up to spec, C17 is soldered into place.

There's a similar test for low-frequency gain, which at 40 Hz is spec'ed to be -5 dB relative to the 1 kHz gain. And the same setup--with the test head in place of the capsule head, and calibrated signals (or a short circuit) applied at the test points--also allows testing the amplifier for noise, distortion, and maximum level.

--best regards

[afterlifestudios]

This is GOLD!  Thank you, David.

[David Satz]

Well, I should probably explain that what I described corresponds to the methodology from when the U 67 and the original U 87 were current models. It's covered in a number of documents that have been in Neumann's "Infopool" for years (the test heads in one document and the expected measurement results in several different ones). I have no specific information on what's done at Sennheiser, but would assume that in principle it is the same.

[Juan brieva]

Hello. My first post.

I want to thank Klaus Heyne for his U67 analysis. Not only for the killer work, but for a surprise that has helped me a lot:


I was freaked out about the importance of the Belden cable when i first read it.
Recently I bought a 1968 Neumann KM86 with a very thin, long (10 meters) brown cable. It was unbranded. Didn't paid much attention to that cable, but didn't have any other cable at hand.

When listening to the KM86, we were freaked out how good it sounded.
Our KM84, in comparison, sounded bassy and bright compared to the KM86 which sounded perfectly tight in the bass and sweet in the highs.


I was selling two of 3 KM84, because I really thought I would never use them, after listening to the KM86.
Somehow, I was freaked at such difference in sound between two mics having (about) the same capsule. I was using a short length of Mogami quad and Sommer carboflex cables with the KM84, assuming these were "good" cables, and- shorter the better.


I remembered Klaus Heyne's article, took a look at the KM86 cable which seemed to be a Belden 8402 or similar older cable.


I now have made long (10m) Belden 8402 cable runs (with XLR connectors), and now the KM84 is so much closer in sound to the KM86.
 I decided to not selling the KM84 after all!!!


I really think a 5% improvement, when you talk about great microphones, is a whole world.
Thanks Klaus Heyne so much.

[König von Smichoff]

Hi Klaus, what do you think about the first U67 reissue from 1992?

[klaus]

The 1992 Reissue (just a few hundred pieces, starting with serial number 10000) is every bit as good and authentic sounding as the originals from the 1960s. The same high resale values as the first edition prove the quality as well.