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Author Topic: KMS84, wire connections  (Read 425 times)

RuudNL

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KMS84, wire connections
« on: November 28, 2019, 03:52:57 am »

Recently I was given a Neumann KMS84 microphone that had been dropped on the floor.
The microphone did not produce any sound.
When I opened the microphone, I noticed that the wires to the XLR insert were not directly connected to the PCB, but through 0 ohm SMD resistors. One side of the SMD resistors were soldered to the PCB and the wires were soldered to the other end of the SMD resistor.
Because of the shock some SMD resistors had broken and interrupted the connection to the output.
What could be the reason for using these (fragile) SMD resistors in series with the output wires?
I soldered the wires directly to the PCB and everything is now working as it should.
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klaus

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Re: KMS84, wire connections
« Reply #1 on: November 28, 2019, 12:03:01 pm »

It's a pretty rare and obscure model. I will take a look at the schematic and try to identify the section of the circuit that corresponds to your description.
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Klaus Heyne
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uwe ret

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Re: KMS84, wire connections
« Reply #2 on: November 28, 2019, 06:47:51 pm »

The broken components are most likely SMD RF-chokes from an aftermarket modification. They may have been added in an attempt to ameliorate RF-interference.
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RuudNL

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Re: KMS84, wire connections
« Reply #3 on: November 29, 2019, 02:34:56 am »

It is indeed possible that they are chokes. The one that wasn't broken measured a very low resistance (that is why I assumed they were 0 ohm resistors). But... it is a strange construction and makes the microphone very fragile.
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klaus

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Re: KMS84, wire connections
« Reply #4 on: November 29, 2019, 12:15:05 pm »

Close-up photos, please!
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Klaus Heyne
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RuudNL

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Re: KMS84, wire connections
« Reply #5 on: November 30, 2019, 04:07:33 am »

I already gave the microphone back to the owner. But the components look like (black) SMD resistors.
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uwe ret

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Re: KMS84, wire connections
« Reply #6 on: December 01, 2019, 02:32:42 pm »

Might have been an alternate after-market modification of a passive pad to reduce the microphone's output level?
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RuudNL

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Re: KMS84, wire connections
« Reply #7 on: December 03, 2019, 09:49:22 am »

I don't think so. The output of a KMS84 is already pretty low, so I don't see a need for a 'pad'.
The way the small SMD 'thingies' were mounted looked pretty professional.
(Also I think that an amateur would have used 'normal' through hole resistors.)
And the SMD components that were not broken measured 0 ohms.
I have seen (and used) SMD inductors, but the components inside this KMS84 looked to me like resistors.
It is a mystery!
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klaus

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Re: KMS84, wire connections
« Reply #8 on: December 04, 2019, 02:32:30 am »

It's a bit tedious to continue to discuss a mic which is no longer owned by the OP, who also no longer has access to inspect the parts in question.

But, for the record: the predecessor to the KMS84 was the KMS85 (despite the numbers going backwards). Both mics use op amps, in difference to most other fet-80 21mm pencil mics of the era. The KMS85, whose schematic I have, employs no RF chokes.
 
Furthermore, none of Neumann's mics of the era had SMD components, including choke coils; but, where coils were used, they were traditional discrete point-to-point soldered parts.

So, my guess is as Uwe's: someone probably tried to get rid of excess RF, which was not uncommon in Neumann's op amp mics of the era, especially when mic cable terminations are not meticulously following Neumann's ground/shield standard.
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Klaus Heyne
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uwe ret

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Re: KMS84, wire connections
« Reply #9 on: December 05, 2019, 01:35:09 pm »

Correction, neither of these models (KMS 84 or KMS 85) uses integrated Op-amps, rather they used good old fashioned reliable discrete components! Deviating from the usual Neumann microphone model designation where the last digit signifies the directional pattern, the KMS 85 does NOT use the hypercardiod KM 85 capsule, but was built around the standard cardioid KM 84.
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klaus

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Re: KMS84, wire connections
« Reply #10 on: December 05, 2019, 02:55:22 pm »

Uwe, I did not mention "integrated" op amps (i.e. where the op amp components are embedded in a chip, as in current red-badge Neumann mics).

Both models are powered by op amps - in this case, a close variant of the U47fet discrete op amp, which provides superior headroom, for applications that demand it.

Thanks for the schematics! Now I finally have one of the KMS84.
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Klaus Heyne
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