Neumann U87ai with extra SMD PCB

Today I got a Neumann U87ai that looks a bit unusual.
At the place where the FET used to be, there is now a small add-on PCB with some SMD components.
Anyone familiar with this? Is there a circuit diagram available of this version?
(There is a sticker on the transformer with the text: "Circuit diagram 04")
Very curious!

not curious in fact, it's the most recent evolution of the U87 Ai
stock , they sound sterile and i've modded a pair of similar U87 Ai recently.
on the SMD board , there's a JFET on the first stage , and a coupling capacitor before it (after the red wire),  i've changed it for a Styroflex 1nF.
i've changed the source capacitor (the big one) as well for a silmic 2. (22uF 6.3V)
There's some bipolar transistors on the second stage : nothing to improve here
usual modifications on the other classical "through hole" board are recommended also
At the end the sound was on par with the previous versions...

Cheers
Fred

granger.frederic said:

not curious in fact, it's the most recent evolution of the U87 Ai
stock , they sound sterile and i've modded a pair of similar U87 Ai recently.
on the SMD board , there's a JFET on the first stage , and a coupling capacitor before it (after the red wire),  i've changed it for a Styroflex 1nF.
i've changed the source capacitor (the big one) as well for a silmic 2. (22uF 6.3V)
There's some bipolar transistors on the second stage : nothing to improve here
usual modifications on the other classical "through hole" board are recommended also
At the end the sound was on par with the previous versions...

Cheers
Fred

Click to expand...

Got to love progress and improvement.  ???

micaddict said:

Got to love progress and improvement.  ???

Click to expand...

"Never change a winning horse!"

Ruud, i don't supposed you've bothered tracing out the circuit on that small board, have you? Or maybe tried to read the markings on those sot23's (if visible)...

No, I didn't. Some SMD components have no value printed on them.
The bottom-right component is obviouly a FET.
But I have no idea what the 4-pin SMD component could be.
I have scanned the internet, but no 'Circuit diagram 04' schematic...

Let's see...

Wire from capsule - ceramic coupling cap (470p-1n) - 1G(?) resistor to ground - 470n feedback coupling cap (to JFET drain) - JFET gate.

4753 = 470k resistor
51B =  3.32k
01E = 1M
81A = 681 ohm

Is the component next to the 51B resistor a LED? Seems sort of translucent, definitely no ceramic cap...

If you shine the light at an angle, you juuuuuust might be able to read some markings off the SOT23's. I can kinda-sorta see something on the JFET and the transistor next to it, but very unclear. This board's conformally coated, isn't it? Judging by the sparkle on some of the components...

Yes, there is no available schematic online .

I would request schematic for  'Circuit diagram 04' "for self repair" here:
https://service.sennheiserusa.com/
With description that it isn't available online

I would guess two jfet cascode with a two transistor or transistor and diode(s) CC in the source leg.  I don't see a source RC in the one picture(rs marked on the main PCB in the one picture)
A CC would allow  jfets to bias without any trimming
EDIT
The 22uf would bypass the CC so you will have gain with a CC in the source leg looks like the + goes to the cascode source.
Does the 4 terminal device measure as two Si diodes?
That could be used as one diode to cancel the Vbe drop and another as a fixed about .7 VDC as ref voltage for the CC
About 33VDC 87 circuit? (28.5VDC measured on board)
I think it can be easily traced even not knowing the caps if you can look at the back or take a guesses based on cascode and CC circuit and check for connections with a meter

Is the part that looks like a cap under resistor 51B (top left) a diode of some kind like a led(IR)?
Is the part that looks like a cap to the right of the 4 lead device a diode(led?)?
Anyone know the what the numbers and letters are on the two 3 lead devices at the bottom right.

I have a  schematic drawn with a few guesses I am not happy with
I guessed wrong about the 4 pin device in the above post.

After some internet search without response, i'm back to the U87Ai small board. It is series 97000. Circuit diagram 4. Trying to help, i'll add some infos. The mic still is with low gain.

Does anyone know if the U87ai -circuit 04 with smd daughterboard, has an equivalent noise level lower than the normal U87ai? Was it just an attempt to use jFETs in cascode configuration, with modern, more accessible, better jFETs?