R/E/P > Klaus Heyne's Mic Lab Stickies

Mic/Pre impedance "matching"- What Are The Rules?

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Inlinesix:

--- Quote from: polypals on December 25, 2014, 08:03:28 PM ---Hello George,

My sincere compliments to jump into the deep.

May I suggest for you to get acquainted with electronics before you go on designing circuitry.
This is not meant to discourage you but to open a whole new world of possibilities to you.

The dc resistance of a transformer is not of major importance for the application in audio.

--- End quote ---

I know I have to study more electronic circuits. Whenever I learn something new I think I'm just an ignorant.

I always learned everything myself, I never had a teacher.

You said dc resistance is not so influences the audio, but it is not what it seems when it comes to OT for guitar amplifiers.

In such cases I noticed much difference. So I asked the question.

I understand that in low-level signals that does not make a significant difference.


tanks

klaus:
David Satz posted on the Neumann forum a really clear and informative summary of microphone impedance strapping found through the 1980s on U87, KM84 etc.

There are actually three possible settings for the output impedance of the KM 84 and most other Neumann "fet 80" microphones: 50, 150 or 200 Ohms. In Europe, 200 Ohms was the de facto norm, with 50 Ohms as an alternative if you needed to drive long microphone cables; the lower source impedance helps avoid high-frequency losses and other problems that can occur due to the distributed capacitance of the cable. The 50 Ohm setting also has 6 dB lower output levels for any given SPL, which can help prevent overload in the input stage of a marginal preamp, mixer or recorder.

Most fet 80 microphones sold via Gotham Audio Corporation (the long-time U.S. distributor) were set to the third option, 150 Ohms, although many have been rewired in the years since then. The two secondary windings of the output transformer were paralleled as in the 50 Ohm setting, and a pair of ca. 50-Ohm series resistors, normally bypassed with solder bridges, were "un-bypassed" by opening (unsoldering) the bridges.

That setting has the lowest output levels of the three--more like those of a dynamic microphone. When the KM 84 was introduced in the 1960s, some U.S. broadcast stations still had old mixing desks with microphone inputs designed for dynamic microphones. Mr. Temmer, the head of Gotham Audio, was concerned about overloading those inputs, and also about some high-frequency response variation that occurs in certain preamps when the output impedance of the microphone isn't what the preamp's input circuit was designed for. He didn't want Neumann microphones to be blamed for any possible adverse interactions with other equipment. As a result, if you were a U.S. customer, you had to special order the microphones if you wanted them set for 50 or 200 Ohms--and then (as I experienced myself) someone from Gotham would call to quiz you about what kind of equipment you intended to use the microphones with.

But nowadays, if you're using a good modern preamp or recorder, it should be perfectly safe to use the 50 or 200 Ohm settings. Either one is preferable to 150 Ohms for avoiding interference in the microphone cable; if resistive pads are needed to prevent overload, they should always be placed immediately before the part of the circuit that is threatened with overload, rather than at the output of the microphone. 

Hermetech Mastering:
Just got a European/Tuchel pair of KM84s from 1977, and am presuming they have the 200 ohm outputs. Question is, regarding the mic pre, (a Chandler TG2 which has switchable input impedance), which setting will give me the flattest/best sound, 1200 ohm (standard) or 300 ohm? Have just been using 1200 so far and liking the results, but am about to ship the mics off to Andreas Grosser in Germany for a service. The 300 ohm setting is about 5dB louder on the same source, but am worried it won't sound as good/have higher noise/be less flat etc.

Theoretically, which setting would be best for these mics?

Thanks in advance,

Gregg

klaus:
The old rule of 1:5 still applies in most if not all cases: with  200Ω output strapping, the mic pre's input should be at least 1000 ohms.

As to output strapping on the KM84: I still like the sound of secondaries of the output transformers best in series (200Ω) rather than parallel (50Ω), with in-line pads removed.

Hermetech Mastering:
Thanks Klaus!

How easy it to remove the inline pads on a KM84?

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