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R/E/P => Klaus Heyne's Mic Lab => Topic started by: J. Mike Perkins on October 06, 2021, 08:52:39 PM

Title: Neumann KM74 - Forgotten Classic?
Post by: J. Mike Perkins on October 06, 2021, 08:52:39 PM
I own a pair of Neumann KM 74 mics and they have been my go to "desert island" small diaphragm mics for the last 30 years. 

You can use them on almost any sound source and get good results. I think they are a forgotten classic design. They are the same mic as the Neumann KTM (Neumann's first solid state mic from 1965) with a slightly smaller barrel. Although similar to the KM84, they sound slightly different, but you would use them on anything you would use a KM84 on. 

Does anyone else use these mics? 
I know You Tube videos are not the best way to hear a mic, but at least this is my You Tube video and I think it gives you a clue. Although I would not normally use the KM74 as a vocal mic, I am using one here (and the other one on acoustic guitar). 
Mine are powered by a 2 channel 12 volt T Power supply from Drefahl Audio. 

The signal chain is 2 Telefunken V376 mic pres, a tiny amount of compression from a Manley ELOP and then into the video camera (no reverb, no eq, vocal in 1 channel and guitar in the other). I'm not a professional, but your posting guidelines encourage people to report their actual experiences using specific microphones they own.
Title: Re: Neumann KM 74 - forgotten classic mic
Post by: klaus on October 06, 2021, 11:02:37 PM
"we have all been here before... we have all been here before" (quick: what song?)

17 years before, to be precise:
Title: Re: Neumann KM 74 - forgotten classic mic
Post by: David Satz on October 07, 2021, 11:54:58 AM
In the United States, Gotham Audio would provide Neumann fet 70-series microphones (KM 73/74/75/76 and U 77) on special order, but recommended them only for use with Nagra recorders--not for studio use, since the powering was "incompatible". This meant more specifically: incompatible with dynamic microphones, especially ribbon microphones, since there was a difference in potential between the two modulation leads--and in this case "incompatible" was a euphemism, since expensive damage could be caused if such microphones were plugged into "live" inputs with this type of powering enabled.

The fet 70 series and the fet 80 series were introduced at the same time, but the fet 70 series was discontinued in 1976 while some parts of the fet 80 series continue to this day.

--While re-reading the old thread that Klaus referred to, I noticed something in one of Oliver Archut's messages: He offered to send out a copy (in German) of an IRT study that had compared parallel vs 48-Volt phantom powering. Did anyone here take him up on that offer? I'd love to read that study, but Oliver is sadly no longer with us, and the IRT closed up shop at the end of last year.

--best regards
Title: Re: Neumann KM 74 - forgotten classic mic
Post by: klaus on October 07, 2021, 12:57:26 PM
Inquiry sent to TAB's Dennis Maximova.
Title: Re: Neumann KM74 - Forgotten Classic?
Post by: Kai on October 08, 2021, 06:56:52 PM
Fact is, German Broadcast “ARD” (which IRT was linked to) ended up using P48, and so called “Tonaderspeisung” (“T-power“) became a niche product.
Unfortunately in Germany and France a multitude of phantom power variations were popular:
P12, 12 V phantom power, almost 100% of all church installations,
P12 polarity reversed (-12V) on “Sogie”-connectors in France, or small Tuchel.
I even have a Schoeps CMT340p with XLR (that I converted to normal P48).

Here‘s a Neumann document describing how they happened to invent P48 Microphone Phantom Power Supply in 1966.
I couldn’t locate the original source, so it’s on manualzz.com:
Title: Re: Neumann KM74 - Forgotten Classic?
Post by: uwe ret on October 14, 2021, 05:37:16 PM
This is a first for me! But I feel compelled to correct David Satz:
Neither did the KTM microphone use Germanium transistors, nor did it operate on the RF-principle. That was almost the exclusive domain of the Sennheiser condenser microphones of the MKH series.

Like these early contemporaries, they also were T-powered. The KTM used a simple FET amplifier (2N3068 or2N3819) and silicon BJT impedance converter/line driver (BC109B), and a DC-DC converter for the capsule bias and FET supply with another silicon BJT  BSY72.
Title: Re: Neumann KM74 - Forgotten Classic?
Post by: David Satz on October 15, 2021, 06:47:06 AM
Thanks, Uwe. Actually I realized my mental lapse and corrected my posting a week ago. But it's good to see the schematic as well, which has never been available from Neumann's Web site to my knowledge.

Kai, the Neumann history document on the KM 84 can be found on https://en-de.neumann.com/file-finder?product=KM%2084&category=historical -- the last document shown. There used to be a similar page about the KTM, but it doesn't appear to be on the site any more. You might also want to visit https://en-de.neumann.com/file-finder?product=KM%2074&category=historical .

The phantom circuit principle was already used in telephone systems long before it was used in studio technology; there's a nice YouTube video of a U.S. Army training film about it from the World War II era. And phantom-powered condenser microphones had been introduced three years earlier by Schoeps--the CMT 20 series, which incidentally used RF audio circuitry. Neumann's actual innovation in the KM 84 was the low-current 48-Volt arrangement for phantom powering, compatible (via an inexpensive adapter) with existing supplies for microphones based on the AC 701 tube. The latter aspect was important in gaining acceptance for the new microphones at German broadcasting organizations.
Title: Re: Neumann KM74 - Forgotten Classic?
Post by: klaus on October 16, 2021, 01:54:06 PM
FYI: The circuitry of Neumann's first transistor small-diaphragm mic "KTM, mentioned without further explanation in a couple of previous posts, is identical to that of its immediate successor, KM7x. Only the housing and head shapes changed.