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Author Topic: KM84 First Generation - Low Output  (Read 525 times)

NVS

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KM84 First Generation - Low Output
« on: October 09, 2022, 01:16:01 PM »

Does anyone have any knowledge/comments on this mic? It only says KM84 us 62, no serial, unless us 62 is the serial. It doesn't have a -10dB switch, but it has an XLR, not Tuchel connector. It has a fairy low output compared to a newer KMi 84 that I have, but when I engage the -10dB switch on the newer one, they sound very similar. I swapped the capsules on them, but the output on the old one was still low, so I assume the capsule is ok. Does anyone know is the old ones had a lower output than the newer ones?

Thanks
Narve Vik-Strandli
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Kai

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Re: Old KM84
« Reply #1 on: October 10, 2022, 05:43:39 AM »

According to the catalog Neumann Archive offers, there’s an early version without the -10 dB switch mentioned.
It’s the first one up left:
https://en-de.neumann.com/file-finder?product=KM%2084&category=historical&area=84

It has a 6 dB lower output of 0,5 mV / µbar (= 5 mV / PA) compared to the later version’s 10 mV / Pa.

µbar (Micro-Bar) is the old unit for (sound-)pressure, before today’s Pa (Pascal) was used.
10 µbar = 1 Pa.

The use of the old unit even shows the catalog refers to an earlier version.
If you don’t get noise from the mic compared to the newer one, everything is OK.
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klaus

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Re: Old KM84
« Reply #2 on: October 10, 2022, 09:36:13 AM »

Looking at the schematic documentation for the first edition KM84 (930-0, 1967), I see how the circuitry in your mic would account for lower output, compared to all later versions: feedback C2 was 4.7pf, whereas later versions (those with -10dB switch) used 3.3pf.

The transformer of your mic did not yet have the option for switching between 50Ω and 200Ω.It also lacked the pads that were introduced in the next version (with -10dB switch).

P.S.: I corrected my earlier version of this post after discovering new information in an official schematic/data sheet for the first generation KM84.
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Klaus Heyne
German Masterworks®
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NVS

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Re: KM84 First Generation - Low Output
« Reply #3 on: October 13, 2022, 03:13:39 PM »

My apologies for my late reply, life happens every day.

Thank you very much for your excellent answers, they explained a lot.

However, I was wrong on one thing. The first testing of the mics, like I described above, was done quickly, and I obviously did not test well enough. Turns out that the mic in question, has a very poor low end. It’s not the capsule, because swapping capsules didn’t help. There’s something going on in the electronics of the mic. I have heard that old tantalum capacitors can go bad when they get old, and was thinking maybe that could be the reason, but I do not know if this is true or not. If I’m not mistaking, the rest of the circuit is just poly and film caps, resistors and the transformer, which rarely fail, or am I wrong?

I have been unable to find the schematic for this mic, and there’s no value written on the Tantalum caps.

The mic has a BV10 transformer, by the way.

Thanks
Narve Vik-Strandli
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klaus

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Re: KM84 First Generation - Low Output
« Reply #4 on: October 13, 2022, 06:07:58 PM »

Lift one leg of each of the two Tantalums, and measure capacitance: the coupling cap should read around 1mfd, the source cap 4.7mfd. If lower by 20% or more, replace with a modern polarized electrolytic like Nichicon or Panasonic with a 50V minimum rating.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

NVS

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Re: KM84 First Generation - Low Output
« Reply #5 on: October 13, 2022, 06:42:35 PM »

Excellent, Klaus, I will look at it this weekend and post a reply as soon as I know more.

Thanks
Narve Vik-Strandli
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