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 on: September 27, 2017, 01:47:12 pm 
Started by BradL8068 - Last post by klaus
Neumann's KM54 capsule history is complicated.

1. I don't believe the schematic showing a KM54c with 34pf capsule (reply #5) is correct. Neumann's nickel capsules were always 45pf, probably because this metal membrane is so incredibly thin (under 1) that arcing would ensue too easily were it placed any closer than the 45pf indicates.

The schematic showing a 34pf capsule on a KM54 was issued in September 1966, more than a year after K54 with 45pf were replaced by the KM64 with 34pf Mylar capsule. There is also no schematic I have in my collection showing a Neumann pencil mic Mylar capsule with anything other than a 34pf capacitance- that capsule was manufactured unchanged from 1965 to the late 1980s.
I guess this was a schematic specifically issued for servicing the transition KM54 with K64 heads retrofitted with an adaptor.

2. You and Schneider are at least partially right. As far as I have seen, only very few KM54 were issued ex factory with the adaptor to accommodate the then new K64 Mylar heads, but plenty of KM54 were retrofitted with adaptor and K64 head. Many of these went to the German Broadcast system in the mid 1960s.

3. 1965 was a transition year. KM64 with the Mylar capsule were already shipped in spring of that year (the capsule was developed in 1964). U64 followed in 1966, and by February 1967 the first KM84 were shipped. Yet, KM54c were not immediately discontinued, but shipped as late as July 1966 to major German Broadcasters. I own one of the KM54c shipped to WDR in Cologne, and it sports the Mylar K64 head with adaptor.

4. 1965 was not the end for the K54 nickel capsule. The KM88, made from 1969 to 1986, used two of them (left-overs from the SM2/23 period).

5. Addressing the question of the KM54's frequency response:
I have never seen an "early" KM54 mic or schematic with a stronger response curve in the higher frequencies. I would love to see at least a schematic pre-dating my earliest one (1957) that would explain such rise (Uwe?). A possible explanation for an "early" KM54 with stronger high end may be the lack of  feedback network Uwe mentioned. High-cut and output networks were introduced gradually in the mid to late 1950s in all Neumann condenser mics with AC701 tubes.

 on: September 27, 2017, 08:46:47 am 
Started by BradL8068 - Last post by David Satz
Klaus, are you saying that some KM 54c were manufactured (i.e. as direct products of the factory) using a capsule with Mylar membrane, rather than being retrofitted for repair? Martin Schneider has said on the Neumann Pinboard, "No KM54/254 was originally produced with anything but nickel membranes. There is/was a conversion available from Neumann which uses current production K84/184 mylar gold capsules in conjuction with an electronic subassembly under the capsule head." Later he added for clarification, "When KM5x capsule production ran out, the corresponding KM6x capsules with a mechanical adaptor were used to repair defective KM5x's."

--Separately he made another comment which is relevant to this thread: "(O)nly the earliest KM54s had the mentioned 5 dB rise, the later KM54a/b/c had a basically flat frequency response (modified circuitry), with -3dB at 15kHz."

--best regards

 on: September 27, 2017, 02:20:33 am 
Started by BradL8068 - Last post by klaus
(...) the schematic for KM54a shows the KK54 capsule's capacitance as 45pf, yet the KM54c schematic shows the KK54 capsule impedance as 37pf.
That's an easy explanation.
The schematic date for the KM54 "c" is 1965, at which point Neumann had moved on from the nickel K54 to Mylar (the same capsule would later become famous as the heart of the KM84 and KM86 with nominal 34pf, not 37pf capacitance.

To further complicate the transition from KM54 to KM64 and beyond: the earliest KM54 with an embossed "c" behind the serial number already had the "c" cathode biasing but still used the original nickel capsules from all prior KM54 generations. Later KM54c with Mylar skins used a decoupling ring to facilitate the grounded backplate connection of all KK64, KK74, and KK84 capsules.

In this regard, the Neumann schematic Uwe posted is incorrectly deleting the components and circuit of the decoupler, as there are no 34pf Mylar heads connected that way.

Boy, what a long-winded story.

 on: September 26, 2017, 01:08:20 pm 
Started by BradL8068 - Last post by AusTex64
Uwe, thanks for your reply.

Here is what I'm trying to figure out. I understand KK54 and Gefell M94 to be very similar, but not identical. The freq plots for M94 show a +5dB boost around 8kHz. I built 4 mics, using MV691 bodies, M94 capsules and a custom tube impedance converter. Topology is cathode biased, plate follower design with output transformer. I call them "my poor man's KM54's".

The issue is my mics seem pretty bright, probably because of the +5dB HF rise in the M94 capsule. However, I see no such rise in freq plots for KM54. Does KK54 not have the HF rise present in M94? If it does, how does KM54 eliminate it in order to achieve pretty flat freq response, at least until falloff starts around 12kHz? I'm assuming KK54 does have the rise due to the similarity to M94, and the feedback through the R1/C1 combination is what was making the correction. But perhaps my assumption is incorrect. Thus my question.

Also, the schematic for KM54a shows the KK54 capsule's capacitance as 45pf, yet the KM54c schematic shows the KK54 capsule impedance as 37pf. Yet NF loop values are the same. ?

I'm sure I'm missing a piece of the puzzle here. Thanks for your patience.

 on: September 26, 2017, 11:07:49 am 
Started by Gabor A. Erdelyi - Last post by Noah Scot Snyder
I bought some new custom cabling from Leo this summer. I was able to email him at the address indicated in his eBay profile. He wrote back and made the cables (for M250s to 6pin XLR) in a reasonable amount of time. He was nice and attentive to me. I hope you reconnect and it's some sort of misunderstanding; a Decca M49 is a special piece!

 on: September 25, 2017, 02:17:19 pm 
Started by klaus - Last post by klaus
Though even Neumann, the leader in microphone standardization, could never quite stick to one color scheme through the decades, I have found that certain (logical) color choices for conductors are prevalent.
Here is my suggestion how to terminate multi-pin microphone cables.

1. 7-pin Tuchel tube mic cables usually Gotham GAC 7 or Doerffler equivalent. Ground pin assignement may vary, depending on the amount of conductors

Pin #1 Audio (+) = white
Pin #2 Audio (-) = black or brown
Pin #3 + Pin #7 = blue (thick)
Pin #4 Heater = red (thick)
Pin #5 B+ - yellow
Pin #6 = variable polarization voltage = green
Always connect cable shields on both ends to ground pins and metal connector sleeves

Do not follow the above pin-outs assignment blindly, but verify that the cable pin-outs correspond to that of the mic in question

4-conductor cables (U47) used with Gotham GAC 7 or equivalent Neumann tube mic cable

Pin #1 Audio (+) = white
Pin #2 Audio (-)  = black or brown
Pins #3, 4, 6 ground and shield = thick blue, all interconnected with jumpers plus a jumper wire to the connector's metal sleeve
Pin 5 B+ = thick red

3. XLR 3-conductor interconnect cables usually Gotham GAC 3, Neumann IEC 3, or equivalent

Pin #1 Ground = Green
Pin #2 audio (+) = white
Pin #3 Audio (-) = brown
Always connect cable shields on both ends to ground pins and metal connector sleeves


 on: September 25, 2017, 12:08:28 am 
Started by BradL8068 - Last post by uwe ret
In the KM 54 the R1/C4 combination (250kΩ/160pF) provides negative feedback and suppresses tendencies of high frequency oscillation in the circuit caused by the complex transformer load. The phase correction becomes effective above frequencies above 4kHz. (f = 1/2π x R x C)

 on: September 24, 2017, 11:20:19 pm 
Started by BradL8068 - Last post by AusTex64
Sorry to post to such an old thread. Will someone please explain the implications of C4 and R1 in KM54? It looks like feedback to me. Is this to help tame the HF rise on the KK54 capsule? Thanks.

 on: September 24, 2017, 04:40:02 pm 
Started by klaus - Last post by David Satz
In addition, the B & K measurement microphone is a pressure transducer with an omnidirectional pickup pattern, while the KM 184 is about 50% a pressure-gradient transducer and only about 50% a pressure transducer, resuting in a cardioid pickup pattern. No one would ever mistake the one microphone for the other, I think, even if the free-field, 0-degree frequency response of both microphones was identical (which I'm rather certain it is not).

--best regards

 on: September 24, 2017, 04:34:07 pm 
Started by Gabor A. Erdelyi - Last post by David Satz
For what it may be worth, he has almost 100 active auctions for microphone cables, adapters and power supplies on eBay at the moment. His seller name there is "leo-eugen-schall". So I think he is still actively in business.

--best regards

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