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#444344 - 05/15/01 05:54 AM Restoring U.S.-delivered U87s to European Output
Klaus Offline
Senior Member

Registered: 09/30/00
Posts: 116
Loc: ,OR,UNITED STATES
For those who contacted me because they wish to run their U.S.-bought U87 at full European output, (anywhere from 4 to 12 dB hotter than U.S. imports) I have enclosed a summary of the infamous "Gotham Pads", and also enclosed step-by-step conversion procedures to get rid of this utterly counterproductive strangulation of microphone output done by Neumann's U.S. importer, Gotham, for more than two decades.
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How to remove the audio pads in the U87i and/or change the output transformer from 50 Ohms to 200 Ohms.

Most U87i microphones sold in the Americas until 1990, and some other Neumann FET microphones like all KM 8x-series and the Fet 47 all were delivered with a reduced output, compared to the same models for European delivery. The output reduction was accomplished by adding a resistive pad in line with the output, and/or by strapping the output transformer’s two secondary coils in parallel, instead of in series. You may find both of these output attenuations in your mic or just one.

Microphones equipped with the Gotham pads are easily identified.
Look for a colored dot next to the serial number, which is embossed next to the XLR connector: if you find a red dot (through ca. 1978) or a light blue dot (from ca. 1978 to 1989) mics left the factory with lower output. If you see just a blank brass indentation without color fill, the mic has full European output.
The lowered output settings which were mandated for U.S.import by the then importer Gotham, New York, rob these FET mics of some of their aural beauty, and force higher gain settings from the mic preamp, resulting in a lower signal-to-noise ratio.

How to remove the U87 output pad resistors
-Remove the housing tube by unscrewing the mic’s bottom bell
-Unscrew two tiny slot screws which fasten the battery lid to the two long frame bars
-Tilt the battery lid upwards
-Look on the component side of the circuit board (opposite side of the battery lid) for two 47 Ohm resistors (color markings yellow, purple, black and gold), which are mounted on each side of the battery switch, close to the round metal piece which houses the XLR connector. Markings on the circuit board further identify these resistors: “R19” and “R20”.
-Now, rotate the mic 180 degrees and look at the solder side of the circuit board: you will find a solder spot connecting to each leg of the 47 Ohm resistors. In order to bypass each resistor, all you need to do is to bridge the two solder spots, either with a blob of melted solder, or with a piece of wire soldered to the two points.
Now you have bridged the two resistors and they are no longer active.

This procedure will increase the mic’s output between 4 and 6dB.

Note: mics which were already converted have bare bridging wires instead of the resistors, or the resistors are already bridged with solder blobs at the factory.


Changing the output transformer’s impedance from 50 to 200 Ohms

The U87’s transformer is in the center of the circuit board: you will notice two copper coils with white plastic frames. With the battery lid up, look at the back side of the transformer: below its rectangular metal core you will find four mounting posts. A colored wire is soldered to each of these posts. Orienting the mic with the head up and the connector at the bottom, the wires are, from left to right: blue, green, yellow, white.
The transformer is set up for 50 Ohms, when you see two wire jumpers: the blue and the green wires are bridged, so are the yellow and white wires.
-For 200 Ohms, remove both jumpers, and reinstall one of them to bridge the two middle posts only: green and yellow.
This procedure will increase the mic’s output about 6dB.

Good Luck!
Klaus Heyne



This message has been edited by Klaus on 05-15-2001 at 03:10 AM

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#444345 - 05/15/01 04:12 PM Re: Restoring U.S.-delivered U87s to European Output
Punchmo Offline
Senior Member

Registered: 09/23/00
Posts: 282
Loc: PA,
Thank you Klaus

Steve

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#444346 - 05/15/01 05:46 PM Re: Restoring U.S.-delivered U87s to European Output
George Cumbee Offline
Senior Member

Registered: 02/05/01
Posts: 133
Loc: Franklin(Nashville), TN/Paduca...
Klaus:

I spoke to you many years ago about this subject. I checked one of my 3 87's after reading this. All are modified as per your directions except that they also have the resistor, just below the transformer taps lifted, one side that is. I can't read the R number. It measures 755 ohms. It is on the foil side just above the back of the meter. Should that be out of the circuit?

Thanks for all you help on this forum. It is much appreciated by all...

George Cumbee
Classic Recording, Franklin, TN
http://www.classicrecording.com
_________________________
George Cumbee
Audio Creations Inc., Paducah, KY
Classic Recording, Franklin, TN
http://www.classicrecording.com

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#444347 - 05/16/01 03:58 AM Re: Restoring U.S.-delivered U87s to European Output
audiofreek Offline
Gold Member

Registered: 05/09/01
Posts: 782
Loc: Prince George,CANADA
Thanks Klaus,
My U87 was German issue(no colored dot),but still had both sets of output
pads,I bridged one&disconnected the other from one side.Transformer was
200 Ohms.

You the man!!!

Sincerly,
Rick

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#444348 - 05/21/01 04:18 AM Re: Restoring U.S.-delivered U87s to European Output
audiofreek Offline
Gold Member

Registered: 05/09/01
Posts: 782
Loc: Prince George,CANADA
Thanks to Klaus for taking time from what must be a busy schedule to
respond to new members like myself. I may have done the worst,and
sold the best mic I have ever owned,had it not been for your help.
I would also like to thank George M.and EQ mag. for having this forum.
YOU GOTTA LOVE THE INFO AGE!!!!
Best wishes,
Rick

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