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Author Topic: U87Ai Sub-Board: How Bad Is It?  (Read 1724 times)

klaus

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #15 on: July 24, 2017, 01:21:39 pm »

(...) Compared to the original single 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.
It was my understanding that Neumann was unable to source a high-quality discrete 3819 FET, therefore designed the "work-around" daughter board. Once they found a quality 2N3819 (Fairchild), they dropped the daughter board again.

But assuming your contention is correct that the mics with the daughter board have better specs, I am sure you already anticipated the next question: why would Neumann have dropped the daughter board?
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Klaus Heyne
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Kai

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #16 on: July 24, 2017, 02:43:33 pm »

... why would Neumann have dropped the daughter board?
So the current production U87 microphones are back to single 3819 FET again?
If yes, is it possible to identify those from the outside?
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uwe ret

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #17 on: July 24, 2017, 04:11:57 pm »

To the best of my knowledge the U87A is NOT available with the 2N3819. If anyone cares to compare the datasheets for the 2N3819 and the SST309 (used in the FET replacement daughter board) you will find improved noise specifications and tighter tolerances for the SST309. Then add consideration for the added optimization circuit and ask yourself why take a backward step to the previous, albeit simpler circuit?
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klaus

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #18 on: July 24, 2017, 04:58:15 pm »

The daughter board was introduced with circuit revision 04 in 2002 (ca. 8x.xxx ser. number) and discontinued with circuit revision 06, ca. 2014/15 with serial numbers in the low 120000.

To the best of my knowledge the U87A is NOT available with the 2N3819.
Starting with rev. 06 Neumann uses a discrete Fairchild 2N3819 FET in all U87Ai. That FET is also used in the high impedance stage of the recent U47fet reissue.
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Klaus Heyne
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uwe ret

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #19 on: July 24, 2017, 07:11:27 pm »

Since 2002 (rev. 05) the FET daughter board has been used to replace the specially selected 2N3819 or equivalent. The board is still used, as evidenced in the latest schematic for rev.07:
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klaus

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #20 on: July 25, 2017, 01:12:22 am »

I am sorry to contradict, but that drawing is from January 2004, with "Circ. Diag. 04" marked on the right side. To my knowledge there is no rev. 7 out yet.

Current U87Ai (circuit #06) do no longer use the daughter board, as I have confirmed by inspecting new U87Ai over the last two years. Please check with Martin Schneider to confirm what date the transition occurred.

In addition, I will post a picture of a current mic amp layout, next time I purchase a new U87Ai.
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Klaus Heyne
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uwe ret

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #21 on: July 27, 2017, 04:20:31 pm »

The latest schematic I have available is still for circuit diagram 04 (with the daughter board) revision 07 from January 14, 2004. To the best of my knowledge, this still does reflect the current production. Please see the ID at the lower right corner of the schematic diagram.
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klaus

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #22 on: July 28, 2017, 01:32:41 am »

See arrows, and get confirmation from Berlin. I'll post a photo as soon as the next 06 arrives here. Worked on about a dozen so far.
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Klaus Heyne
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uwe ret

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #23 on: August 29, 2017, 04:25:24 pm »

To end the apparent confusion over this subject, I did contact Neumann's engineering department in Berlin, and received the following clarification about the various versions of U 87A circuits:

Circuit Diagrams:
02:         Standard-U87A Neumann (FET: 2SK10x selected)
03:         DC/DC-converter board without those components only required for U89 (from ~1995)
04:         with additional PCB 4 in place of discontinued FET (from 2002 to 08/2013)
05:         2 values of components on DC/DC-converter board changed (2011)
06:         return to single FET in place of daughter board PCB 4; same as Circ.Diag.03, plus changes from 05. (since 08/2013)

My apologies to Klaus for the confusion originating from the various definitions between Circuit Diagram # and release #.

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klaus

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #24 on: August 30, 2017, 01:41:06 am »

...and, as promised, here is a photo of the current edition U87Ai (circuit #06) which shows the re-introduced discrete FET on the lower right, below the silver attachment screw.
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Klaus Heyne
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Jim Williams

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #25 on: August 30, 2017, 12:03:20 pm »

To the best of my knowledge the U87A is NOT available with the 2N3819. If anyone cares to compare the datasheets for the 2N3819 and the SST309 (used in the FET replacement daughter board) you will find improved noise specifications and tighter tolerances for the SST309. Then add consideration for the added optimization circuit and ask yourself why take a backward step to the previous, albeit simpler circuit?

Besides the fet and transistors, there are also low quality surface mount passives used. The mono ceramic coupling caps are especially troublesome, the sonics are awful compared to a quality polystyrene cap. A better jfet will not benefit from using a ceramic signal capacitor.

"Two steps forward and one step back"  ~ Johnny Winter
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klaus

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Re: U87Ai Sub-Board: How Bad Is It?
« Reply #26 on: August 30, 2017, 02:49:36 pm »

Since discontinuing Tantalums (mid 1980s), Neumann uses polarized electrolytic coupling capacitors, not ceramics.

So if you want to keep the mic stock, you can keep the coupling cap but you may want to throw out the daughter board of versions #03 though #05 and replace with discrete FET, 47kΩ drain resistor, source cap and resistor (which needs to be biased) and add a 1 gig Ω shut at the FET gate.

All this is fairly easy to do (at least for a mic tech).
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Klaus Heyne
German Masterworks®
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