Is there any new mic you can buy or a kit to build that sounds as good as a vintage U87i?

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Rossi

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I have sad news: Andreas Grosser, who did this mod, is no longer with us. He passed away in his sleep, unexpectedly, aged 65. I didn't know Andreas personally, but this info is from someone who was in close contact with him.
 

rock soderstrom

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I have sad news: Andreas Grosser, who did this mod, is no longer with us. He passed away in his sleep, unexpectedly, aged 65. I didn't know Andreas personally, but this info is from someone who was in close contact with him.
Really sad news. Ruhe in Frieden Andreas.
 

ubxf

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Sad news indeed, I talked with him on the phone a few years back. Such a nice and generous man RIP Andreas
 

Gus

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Was the output drain to transformer coupling cap value changed?

Most of the mod that has been shown or posted is standard stuff except the gain reduction with the 3.9pf that is something different in a 87 and that is why I asked about the output cap.

Note the 4pf in a KM84 schematic
 

living sounds

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It's 3.3pf in the modded mic, not 3.9pf.

No, the output coupling cap was kept, a cheap Jamicon 1uf/63V.

I wonder why the mic seems to have more low end than before the mod, since the filtering network there wasn't change. Can the added feedback cap have this effect?
 

living sounds

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BTW, it definitely says VISHEY FET Transistor on the invoice, but the markings don't look like Vishay and there doesn't seem to have ever been a Vishay branded one. The closest thing would be Siliconix, but the logo again looks different...
 

Khron

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kingkorg

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It's 3.3pf in the modded mic, not 3.9pf.

No, the output coupling cap was kept, a cheap Jamicon 1uf/63V.

I wonder why the mic seems to have more low end than before the mod, since the filtering network there wasn't change. Can the added feedback cap have this effect?
Just changing the fet will affect the negative feedback network. The values might be the same but depending on the fet/bias filter might vary A LOT!
 

living sounds

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Just changing the fet will affect the negative feedback network. The values might be the same but depending on the fet/bias filter might vary A LOT!
Interesting, so does this only depend on FET type or would there be variation from - say one 2N3819 to the next - since all of them have to be individually biased (and I think I even read about selecting FETs for U87 circuits somewhere)?
 

kingkorg

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It can be dependent on individual fet no matter the type. 2N3819 is just a number, doesn't actually mean anything.
 

kingkorg

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If it helps, my experience has been that when contemplating different JFET's, it's important to draw the intended load line on a typical VDS/IDS plot, then consider how that same line will look on the replacement JEFT. Often, tweaking of drain AND source resistances is in order.

To make things more difficult, JFET parameters across a handful of the SAME DEVICES can span a 300% range across common parameters like gate-source off voltage, IDSS. For example, for the J305 device, VGS(off) can span from -0.5V to -3V, and IDSS can span from 1mA to 8mA! For the 2N3819, VGSoff can be anywhere between -0.2V to -8V, and IDSS can be anywhere between 2mA and 20mA! This often means that you need to buy a handful and measure the one that will work best in your intended circuit.
 

living sounds

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What would be a good value to aim for to keep the drain resistor and only adjust the source resistor in a U87AI circuit? Also, how does one measure these values? I've got a cheap multi-function tester that spits out the following values on a random 2N3819: Id=5.3mA, aVg=196mV and on a random J305A: Id=1.6mA, aVg=1.10V.
 

RuudNL

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Not higher than 10K. I haven't seen a U87 with a higher source resistor than 10 K.
(Most of the time I end up with a value between 3K and 5K.)
But since the 2N3819 (and other FETs) have a very big tolerance, it is possible that a FET can't be biased correctly in a U87 circuit.
 

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