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Author Topic: U87Ai Sub-Board: How Bad Is It?  (Read 1609 times)

FoxFoxFox

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U87Ai Sub-Board: How Bad Is It?
« on: July 18, 2017, 03:54:12 am »

Okay I see you do not like this circuit board  ;D (Sub board, also called "daughter board, as used in U87Ai with circuit # 04 and #05. K.H.) the mic sounds really full, fuller than I expected on the other hand! Anyway to get rid of that daughterboard and make it as stock as the ones without the daughterboard... ?
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klaus

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #1 on: July 18, 2017, 11:27:24 am »

In circuit revision # 4 and #5, the 'daughter board' replaced the previous discrete FET circuitry on U87Ai circuits.
It can easily be removed, and a few missing passive components and a new 2N3819 FET (properly biased) installed. Once the FET is properly biased, you would be back to the simpler, better-sounding circuitries # 2 and #6.
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Klaus Heyne
German Masterworks
www.GermanMasterworks.com

FoxFoxFox

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #2 on: July 18, 2017, 11:41:17 am »

Okay which one is mine then because I thought the daughter board was the floating tray, because mine is circuit 5, and it def has that "floating tray" on top of the regular board
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klaus

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #3 on: July 18, 2017, 12:12:00 pm »

Yours has the daughter board. I forgot to include circuit #5 as the other revision that used the daughter board (previous post now corrected).
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Klaus Heyne
German Masterworks
www.GermanMasterworks.com

FoxFoxFox

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #4 on: July 18, 2017, 07:53:05 pm »

Thanks Klaus!! I want to do the mod to get it back to #2 and #6, who do you recommend in NYC? also how much do you think this job typically cost
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klaus

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #5 on: July 18, 2017, 11:53:16 pm »

You can try Tom Onofrio in Connecticut. He used to work at Neumann/Sennheiser in Old Lyme.
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Klaus Heyne
German Masterworks
www.GermanMasterworks.com

soapfoot

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #6 on: July 19, 2017, 08:47:00 am »

Second the recommendation for Tom O.
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FoxFoxFox

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #7 on: July 19, 2017, 10:43:09 pm »

i want to, but its quite expensive
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FoxFoxFox

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #8 on: July 21, 2017, 09:31:23 am »

does 300-500$ sound a little high for this job?
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klaus

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #9 on: July 21, 2017, 11:30:03 am »

I am questioning the wide range of the estimate. Unless there are other issues your mic has*, FET conversion should be a well defined, straight job.

*For that reason I avoid giving estimates before I see a mic. But there are rare cases, where the scope of work is clearly defined, because other problems the mic may have can be safely excluded (capsule cleaning, cable manufacturing, etc.)
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Klaus Heyne
German Masterworks
www.GermanMasterworks.com

FoxFoxFox

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #10 on: July 21, 2017, 11:46:08 am »

Strange, respectfully this is a quote from Tom O. He also doesn't do it either but that was the range he gave me. The mic is rock solid, the signal to noise ratio sounds astonishingly good
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klaus

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #11 on: July 22, 2017, 02:04:30 am »

He also doesn't do it either but that was the range he gave me.

Maybe I misunderstand this sentence, but shouldn't an estimate be somehow connected to its service provider?
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Klaus Heyne
German Masterworks
www.GermanMasterworks.com

FoxFoxFox

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #12 on: July 22, 2017, 09:20:11 am »

Thats what I thought too... All i need is the mod, and I was quoted 300-500$ by Tom O. That doesn't sound right. Anymore recommendations in the area would be great! thanks
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uwe ret

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #13 on: July 23, 2017, 11:25:39 pm »

If it isn't broke, don't try to 'fix' it! Contrary to what may be expressed here and elsewhere, the single FET substitution with the daughter board is actually a real improvement. There is still effectively only one active FET (selected for minimal noise) in the signal path, the other seemingly cascaded one in conjunction with a dual PNP-pair automatically optimizes the operating point, and the single NPN forms the load resistance for the FET. Compared to the original sindle FET circuit the result is lower noise, slightly better dynamic range and/or lower distortion due to the automatically optimized and more stable operating point. Going out on a limb, I would argue to replace to replace any (defective) FET in the older U87A models with this daughter board!
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Jim Williams

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #14 on: July 24, 2017, 12:26:08 pm »

It's not the SOIC semiconductors, it's the SOIC passives. You do not want to run a $3000+ microphone into a mono ceramic coupling capacitor in place of a polystyrene film.

If one wants to investigate why those are a poor choice, read up on the Marsh-Jung papers in the AES journals.
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