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Author Topic: U47a (Gotham audio)  (Read 10837 times)

Donn Halliburton

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U47a (Gotham audio)
« on: December 03, 2008, 03:09:14 pm »

Hi everyone!

I understand that U47's having an "a" on the serial number had something to do with Gotham audio's distribution of these mics.  My question is, are they any different than any other U47 produced at the time?  Is their sound compromised in any way. Is their asking price reduced in any way due to the "A" designation?

Thanks!!


Donn
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Oliver Archut

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Re: U47a (Gotham audio)
« Reply #1 on: December 03, 2008, 07:33:35 pm »

The Gotham A indicates the GN8 x-former with 6dB less output, good for low impedance mic pres.

Best regards,

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Oliver Archut
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Donn Halliburton

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Re: U47a (Gotham audio)
« Reply #2 on: December 03, 2008, 11:10:26 pm »

Oliver Archut wrote on Wed, 03 December 2008 18:33

The Gotham A indicates the GN8 x-former with 6dB less output, good for low impedance mic pres.



Does that affect the sound in any way, positive or negative?


Donn
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Klaus Heyne

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Re: U47a (Gotham audio)
« Reply #3 on: December 04, 2008, 12:58:07 am »

Not good, in my opinion, as "normal" mic pres need to be cranked up quite a bit to compensate for the significantly reduced output of the transformer's secondary. So, the mic will have more noise, and the different transformer changes the sound to a slightly more glassy timbre.

I would be hesitant to buy that mic, unless it is so flawless in condition that its lack of output does not weigh in.

By the way: the inscription on the infamous transformer is: " BV8b"  The 'b' is the culprit here! You see it, and it means 12/50 Ohms secondary.

Also please note: ALL U47 and U48 with the 'a' suffix have the BV8b transformers. 'a's  were made from ca. late 1958 to 1960, according to my historical data.
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Klaus Heyne
German Masterworks
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Re: U47a (Gotham audio)
« Reply #4 on: December 04, 2008, 01:45:23 am »

Wouldn't the sound and level improve if the "a" type mic were connected to a proper low-impedance preamp?
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Klaus Heyne

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Re: U47a (Gotham audio)
« Reply #5 on: December 04, 2008, 02:04:40 am »

Let's revisit why Neumann did this low-imp scheme on the U47a (hint: it was NOT for High Fi purposes!)

Stephen Temmer, newly appointed exclusive U.S. importer for Neumann at the time, insisted that mics would sell better if their outputs better matched mixing board inputs of the era; inputs which were designed for ribben mic output levels (the dominant recording mics of the 1940s and 1950s), not a 40 dB hotter condenser mic!  So Neumann had to find a way to humor Temmer and bring the hefty U47 output down. They did that in two equally questionable ways (no braking without heat!):

1. Installed an h-pad resistor network in front of the XLR of U.S. delivery Gotham power supplies  (down a good 6-8 dB)  then

2. added the BV8b transformer to the mic (another 6dB down); pair that with the (medium low output) stock M7 capsule, and you had an s/n disaster which virtually obliterated the dynamic impact and robust character of the mic.

I have not tried an 'a' in a low imp input, but cannot imagine that, aside of some gain recovery, the timbre of a genuine BV8a could be recouped Let me know if you find otherwise.
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Klaus Heyne
German Masterworks
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compasspnt

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Re: U47a (Gotham audio)
« Reply #6 on: December 04, 2008, 08:54:11 am »

Surely one could replace the transformer with the proper one, and remove the pad from the PSU?

Originality be forsaken...for quality.
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Jim Williams

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Re: U47a (Gotham audio)
« Reply #7 on: December 04, 2008, 10:59:50 am »

I would believe that this version might be a winner for those wishing a more hi fi output from the mic. Transformer gain is not without trade-offs. With today's very low noise mic pre's with EIN specs up to -135 db, noise would not be a problem. With a vaiable input impedance, one could also find a sweet spot for this version.
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Jim Williams
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Donn Halliburton

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Re: U47a (Gotham audio)
« Reply #8 on: December 04, 2008, 11:57:05 am »

Klaus Heyne wrote on Thu, 04 December 2008 01:04

to pair that with the (medium low output) stock M7 capsule, and you had an s/n disaster which virtually obliterated the dynamic impact and robust character of the mic.
.


Does this mean that in addition to the BV8b transormer and the pad circuitry, that they also equipped these with M7's while other non a's of the era were K47's?

Donn
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Klaus Heyne

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Re: U47a (Gotham audio)
« Reply #9 on: December 04, 2008, 03:28:36 pm »

No. I just did not express myself clearly enough. M7 are lower in output, compared to K47, by about 3dB.

ALL mics in a given U47/48 era used the same capsule.
I.e. there are early U47a with M7 and late U47a with K47, depending when during that transitionary capsule period the mics were made.
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Klaus Heyne
German Masterworks
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strangeandbouncy

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Re: U47a (Gotham audio)
« Reply #10 on: December 08, 2008, 04:00:27 pm »

Why should the lower output tranny sound so much worse? Is it of a different lam material? The tranny is surely being hit just as hard, so the core is still reacting the same way to the input. Is it a question of loading perhaps?

Given, the noise increase is unwelcome, I can only add that some of my pre-amps actually sound better at higher gains, such as my various Helios modules. I have some 10 and 20dB pads and a constant impedance attenuator so that I can exploit this. Helioses especially really sing with a pad in, and that cannot be a function of the input transformer, which is still getting the same level. FWIW, In my musical area, noise isn't really a problem!

Kindest regards,
ANdyP
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Klaus Heyne

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Re: U47a (Gotham audio)
« Reply #11 on: December 08, 2008, 04:36:13 pm »

I am not an expert on transformers but can tell you that whenever you alter winding patterns, wire gauges or both, an audio transformer's sound will change, even if the core remains the same.

The lower output of the mic is a problem all onto itself, but probably cannot be separated from the design changes of the transformer.
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Klaus Heyne
German Masterworks
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Oliver Archut

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Re: U47a (Gotham audio)
« Reply #12 on: December 08, 2008, 04:44:46 pm »

To add to Klaus, the secondary acts as a magnetic shield, and though the magnetic field generated is equal, the size of the copper shielding is 40% thicker, due to the fact that the heavier gauge does not line up as neatly as its thinner counterpart.
(Also, for some GN8b x-formers the alloy is different.)

it is possible to rewind the BV8b to the BV8 specs that everybody wants to have, but if you have a working x-former, leave it and enjoy as is.

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Oliver Archut
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strangeandbouncy

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Re: U47a (Gotham audio)
« Reply #13 on: December 12, 2008, 09:43:12 am »

Hi Guys,


    Thank you Oliver and Klaus.


  Kindest regards,


    ANdyP
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Helicopter

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Re: U47a (Gotham audio)
« Reply #14 on: January 08, 2009, 02:30:27 pm »

Hi,
now I'm confused: the terms BV8b and GN8b and GN8 are used very confusingly in different threads I searched here. Could Oliver or Klaus clarify? I mean, in another thread Oliver mentions to a "later GN8" as not the best transformer" in other posts this is supposed to be the GN8b or the BV8b. So, is the GN8 the same as the lower impedance BV8b or GN8b or a a different name for a genuine BV8? Or is there a typo somewhere?
Thanks,

K.A. Gisen
Belgium.
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