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Author Topic: U47a (Gotham audio)  (Read 9478 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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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
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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
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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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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
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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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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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Klaus Heyne

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Re: U47a (Gotham audio)
« Reply #15 on: January 08, 2009, 04:52:58 pm »

The writing on the transformer's top, inside a U47 short body, is visible when you push the tube out of the way, and will be: "BV No. GN 8"  or "BV No. GN8b".

The latter is the one to watch out for, if you don't want the low-output model.

If you look at the wire visible as the outer layer of the two half-transformers, you can also confirm which transformer you have: the secondary windings of the BV No. GN8b are made with really thick copper-colored wiring, compared to the hair-thin wire of the original BV8.
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Klaus Heyne
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Helicopter

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Re: U47a (Gotham audio)
« Reply #16 on: January 08, 2009, 06:29:28 pm »

OK, I see. Thanks a lot Klaus.

K.
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Edvaard

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Re: U47a (Gotham audio)
« Reply #17 on: October 02, 2010, 04:00:47 pm »


Hello,

Sorry to drag this thread up again but I have an associate who has suddenly got the bug to get a U47 or U48, and is asking me for advice regarding the 'most authentic' for what era, etc.

To the issue of Gotham's U47a with the BV8b transformer:

Klaus Heyne wrote on Thu, 04 December 2008 02:04

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.


Terry asked, but no one answered directly; can the transformer be replaced with, say, an AMI BV8 and the PSU resistor network be properly removed? If so, would that now make it an 'authentic' and 'proper' U47 in all regards? One U47a I saw recently was available with the kk47 assembly; would there be any further considerations vs. the M7 capsule as spoken of in Klaus's post?

I know that Oliver said "if you have a working x-former be happy with it", but after reading all the downside of Gotham's modification and knowing how much authenticity of the sound (as relates to classic/historical recordings) people are looking for in a U47, it's hard to be happy with a working BV8b! Especially for the still high price of the Gotham-altered U47.

If doable, I know that all the above would not be 'cheap' but I would then be able to at least get an idea of the cost involved.

So then another factor in deciding whether to take that approach or rather wait until a 'proper' U47/48 (non-"a" version) comes up for sale is: in the US, how much more or less common is the U47a vs. a regular U47?

If I have rough estimates of costs to compare, and the relative rarity of one as compared to the other, I can then have some reasonably defined alternatives from which to choose.



Any knowledgeable responses to the questions above would be highly and gratefully appreciated here.  

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Klaus Heyne

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Re: Neumann U47a (w/Gotham Audio mod)
« Reply #18 on: October 02, 2010, 06:47:37 pm »

1. From serial number calculations I estimate that at most 5%-7% of all U47 had a BV8b, and ca. 20% of U48 did.

2. I use Oliver's transformers in some of my work, and found them to be very good so far. I tested his copy of a BV8 (old style, full output) and found its sound to be close to that of a real Neumann transformer. (Or to be honest: I could not tell the difference the one time I tested his version.)

So using a good aftermarket transformer may be more an issue of "authentic" and "period correct" than necessarily one of sounding correct. Such a mod will of course no longer make the U47a authentic.

But in the case of that particular aberration of an ingenius mic design, it may not be a weight on the collectible value, as long as you keep the original transformer handy, in case the next owner desires an authentic collectible item rather than a good sounding microphone.
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Klaus Heyne
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Edvaard

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Re: U47a (Gotham audio)
« Reply #19 on: October 02, 2010, 07:33:31 pm »


Thank you very much Klaus.


I was already leaning towards "wait for the real thing" and your clarifications and information on the relatively small number of "a" versions (especially) make it an easier decision.

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New Orleans Steve

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Re: U47a (Gotham audio)
« Reply #20 on: October 07, 2010, 02:19:03 am »

  I read this thread with little more than passing interest. I don't own or even realistically aspire to own a U47 at this point, But just love the discussion as this knowledge is so rarefied I feel privileged to be a party to the conversation.

 Do I have this correct? That All Gotham (and hence all U.S.) U47s have this "a" designation and the lower output?

 Would it then be safe to say, that all the major studios U47 inventory were these? Like at Capitol records? And Frank Sinatra?

 Thanks, Steve Daffner

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Klaus Heyne

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Re: U47a (Gotham audio)
« Reply #21 on: October 07, 2010, 01:34:27 pm »

No, you misunderstood. Very few  U47/48 came with this transformer.

After Stephen Temmer, the Austrian impresario, started Gotham Audio with some of his friends, he convinced Neumann that he was the man to re-ignite sales momentum in the countries for which he negotiated exclusive representation: Canada and USA.
He immediately drove hard bargains with Neumann, and one of them was, that all mics delivered to his territory would need to be much lower in output, to better match the input sensitivity of antiquated stateside mixing consoles, hence, increase sales.

Initially, Neumann complied by simply switching its strappable outputs to 50 Ohms (-5dB to -6dB). But somehow, after a couple of years started to wind a special BV8 transformer exclusively for U.S. export: the BV8b. That shaved another 6dB off the output.

That damage was done very late in the life of the U47/48  (see my percentage estimates, how many mics were affected, in an earlier post of this thread.)
By that time, Capitol, etc. had probably already a full contingent of mics without the dreaded transformer, so chances are very slim that your favourite Sinatra records made between 1960, when the transformer first showed up, and 1962, when production of the U47/48 by and large ceased, would have used such mic (besides, by that time, Sinatra had switched to AKG's ELA M251 and various other mics).
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Klaus Heyne
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