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Author Topic: U/I versus E/I form factor in microphone transformers  (Read 12859 times)

Jim Williams

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Re: U/I versus E/I form factor in microphone transformers
« Reply #15 on: June 01, 2013, 12:56:46 pm »

Bill Whitlock of Jensen is the authority on this subject. Maybe he will read this and contribute.

Whenever coils are stacked there is a magnetic interaction, that can be lessened by spacing, wind geometry, etc. Whenever a coil is placed next to another coil, there is magnetic coupling. Rotating one coil 90 degrees helps minimize that. That is why speaker crossover coils are positioned at 90 degree angles, to avoid that interaction.

AKG is another mic manufacturer that aways used UI transformers with split, seperate coils. That didn't help them much, most of them don't sound very good, including those used in the 460 and 414 models.
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Oliver Archut

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Re: U/I versus E/I form factor in microphone transformers
« Reply #16 on: June 03, 2013, 09:32:03 pm »

Hello Guys,
The shape of the BV8/12 type transformer was specially developed by the NWDR and came in UI and LL shape (same overall size). It was first used in mic pre inputs (for 1:40 application), but was later implemented in mics too, first in the Hiller mics, then later in the M49 (1950 started the development), before Neumann started to use it in 1951 in the U47 too.

The lamination was only made by two companies, Herasus in Hanau as well as Krupp Stahl (in my hometown of Essen). Neither of those companies are still making that style of lamination, due to the fact that the production ended in the late 70s. 

Standard transformer lamination is made with little to no cut-off waste, the "I" are normally the cut outs from the two facing "E" or the "U" to be as efficient as possible, but efficiency is not always good for sound or better sound specs, so the NWDR worked closely with Krupp to develop a side ratio that yields the lowest amount of distortion and magnetic loss, but the side effect was a 40% waste. The heat-treating process was also a big secret of how that type of lamination was made.

The UI and LL transformer is not like a guitar pick-up 'Humbucker' in any form or way, but it does cancel noise differently than the pick-up does. This forum is not the best way to go into such technical details, but keeping it simple: the transformer has two legs, that are magnetically quasi out of phase. If both coils are wound the same way and then hooked up anti-paralleled, the noise cancels; this might sound similar how how the pick up cancels noise, but because no static magnet is involved, there is a bunch of additional issues that are too complex to roll out here. Aided by the incredible magnetic property, it is also quite easy to manipulate the frequency response (compared to EI types) just by arranging the windings in a different way, as well as stacking the lamination in sets of a 3: 5 ratio.
Some M49 as well as all U47 have a build in-low cut filter, a LC cut off that works with the coupling cap as primary inductance.

There is virtually no printed matter available on this subject, other than the NWDR "Hausmitteilungen" (internal communications, K.H.) and the Herasus spec sheets, both of them are in German and nearly impossible to find these days.

Neumann did several revisions of both transformers, but all of them work very similar. Trying to work on the transformer should be avoided, Neumann (they made all U47/M49 transformers in-house) used double-built silk wire that is then varnished, for lead-in wire. It crumbles away after 50 years of use in a mic, so if your transformer is still working properly, do not touch or play with it....

Best regards,

Oliver
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Oliver Archut
www.tab-funkenwerk.com

We are so advanced, that we can develop technology that can determine how much damage the earth has taken from the development of that technology.
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