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Late 80's Stephen Paul U87Ai
Old 1 week ago
  #1
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Late 80's Stephen Paul U87Ai

A Neumann U87Ai mic landed on my workbench, with reports of it distorting too easily. Lo and behold, it had been customized with a Stephen Paul Audio capsule from the late 80’s, and various changes to the stock circuitry. I assume all the changes were made by SPA way back when. A couple of their orange stickers were still in place. But if that was 30+ years ago then who knows whether other changes have been made since.

Since the Stephen Paul capsules are more sensitive than the stock ones, perhaps this mic is working as intended. The internal voltages seem fine, and it sounds distortion free at a medium singing level. A loud voice definitely has to back off.

Is this an appropriate forum to ask whether the circuitry changes I am seeing are typical of a Stephen Paul U87Ai?

For example, the transformer in this one appears to be a stock Neumann, a 90415. But those are supposed to have an resistance of 100 ohms on each coil of the secondary, so you can get a 200 ohms wiring them in series, or 50 ohms wiring them in parallel. On this one, however, I measure each secondary as 30 ohms, and they are wired in series for a total of 60 ohms. The primary reads 1.5k and the turns ratio appears to be about 18:1, twice what I was anticipating.

Cheers
Old 1 week ago
  #2
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Hopefully, one of the real Neuman Experts... Like Klaus/Eric Heiserman/Tim Campbell/et al, will respond.

IIRC Massenburg was a big fan, including the Neumann Stephen did for Linda Ronstadt.

I know some of his ideas/ mod work, are somewhat controversial, among (some) afficionados...But Stephen was always very kind to my questions.
As for me, I think he was a genius!
Chris
Old 1 week ago
  #3
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Klaus's Avatar
 

Contrary to common perception, SP generally did not modify the stock U87/Ai circuitry, but banked on his thin diaphragm concept as the path to improving the mic's sound.

Your measured DC resistances are correct, and indicate that the transformer is stock. (Do not confuse DC-resistance with impedance!).

Stock-U87Ai circuitry is very sensitive to overload, regardless of capsule/diaphragm used. The distortion you reported is either due to the mic's overload characteristics, a FET that has drifted out of calibration, the effect of SP's thin diaphragms, or all of it.

Last edited by Klaus; 1 week ago at 01:01 AM..
Old 1 week ago
  #4
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Stephen also commonly altered the backplates by scratching them up and sometimes reaming the holes out some more.

He modded the circuit in *some* cases, as an option; for example, there are added switches on some of his mod mics to reduce the added top end.

Stephen Paul’s shop also did normal repair work as an approved Neumann repair center (maybe the only approved center outside of Neumann corporate at the time?).

Last edited by toledo3; 1 week ago at 02:00 AM..
Old 1 week ago
  #5
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Quote:
Originally Posted by toledo3 View Post
...
Stephen Paul’s shop also did normal repair work as an approved Neumann repair center (maybe the only approved center outside of Neumann corporate at the time?).
Not. Don't forget your friendly German-Oregonian, who's been honored with Neumann's stamp of approval since the late 1980s.
Old 1 week ago
  #6
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Quote:
Originally Posted by Klaus View Post
Not. Don't forget your friendly German-Oregonian, who's been honored with Neumann's stamp of approval since the late 1980s.
I think I had recently read about his repair shop status in an old REP mag from the 80s (maybe 85-86ish), presumably before your much deserved stamp of approval.
Old 1 week ago
  #7
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Chris, coincidentally I’m in the middle of building some parametric EQ’s — that sounds right up Mr. Massenburg’s alley.

Klaus, thank you, that explains the transformer winding resistances I measured. And will hopefully be helpful to future readers, as I could not find anything other than 200 vs 50 when looking. I wish I had a datasheet for that part.

Does the 18:1 voltage drop match expectations? I measured a 1kHz test tone with an oscilloscope and saw 2 volts peak-to-peak on the primary compared to 110mV on the secondary.

C105 was removed, and R106 bumped up to around 20 meg, pretty well killing the negative feedback unless the high pass filter switch is engaged.

C107 appears to have been swapped out for an axial cap of some kind, instead of radial. I didn’t remove it to measure its value.

Those are the only changes I’ve spotted. I’ll have to have another look with the scope and see where the distortion first appears. If it’s at the JFET output, maybe I ought to replace it with newer, quieter part.
Old 1 week ago
  #8
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C105 should not be removed. It makes the mic too bright and pointy and ugly sounding in the upper mids. Likewise, an R106 value that high will increase noise. The winding ratio of the transformer is correct.

Last edited by Klaus; 6 hours ago at 04:18 AM..
Old 1 day ago
  #9
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Quote:
Originally Posted by Klaus View Post
C105 should not be removed. It makes the mic too bright and pointy and ugly sounding in the upper mids. Likewise, an R106 value that high will increase noise.
That's what I suspected; it would more or less seem to disable the HF de-emphasis. I saw somewhere that the aim might have been to pair it with gear that already creates HF losses, and avoid de-emphasizing twice. Perhaps it's time to audition some caps and get de-emphasis going again.


Quote:
Originally Posted by Klaus View Post
The winding ration of the transformer is correct.
Thanks for the confirmation. Sounds like that part is ok.

Had a chance to look at this mic again, and the capsule is just putting out a lot of signal. The transistor tries to to amplify it, but can't make the voltage swing. There's currently about 8.5V available for the downswing, and 12V for the upswing. I could re-bias it and get them evened up, but that's not going to add a lot more headroom compared to how it is now.

I could put in a newer, quieter transistor, perhaps replace some old caps & carbon comp resistors in the signal path too, to lower the noise floor. That'll improve S/N ratio but still wouldn't help with max SPL.

Could aim for less gain. Maybe that tradeoff would be ok with a quieter signal path.
Old 12 hours ago
  #10
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Quote:
Originally Posted by toledo3 View Post
Stephen also commonly altered the backplates by scratching them up and sometimes reaming the holes out some more.
The front backplate does indeed appear to have been scratched up a bit. Good call! The rear backplate looks clean and I don't see any manipulation to the holes on either, but maybe it's difficult to make out through the membrane or I'm not looking closely enough.
Attached Thumbnails
Late 80's Stephen Paul U87Ai-front_backplate.jpg  
Old 8 hours ago
  #11
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Call Saul! Oops I meant Klaus.
If Klaus has time, I'd send it to him for a Physical.

I'll hold off on sending Klaus my MXL V69,
just to give you more time!
We were planning to make it sound better than any Primo C12.
(sorry Klaus)
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