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.
Is there any new mic you can buy or a kit to build that sounds as good as a vintage U87i?
- Thread starter living sounds
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RuudNL
Well-known member
rock soderstrom
Tour de France
Really sad news. Ruhe in Frieden Andreas.
ubxf
Well-known member
Sad news indeed, I talked with him on the phone a few years back. Such a nice and generous man RIP Andreas
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
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
Well-known member
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?
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
Well-known member
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...
Vishay bought Siliconix at some point, but i'm not sure they ever bothered changing the logo on components.
https://www.vishay.com/en/company/brands/siliconix/
https://www.eetimes.com/vishay-completes-acquisition-of-siliconix-common-stock/
https://kaimte.com/upload/brand/20210112/e21ff7e2c5757354af16ace31b20f613.jpg
https://www.vishay.com/en/company/brands/siliconix/
https://www.eetimes.com/vishay-completes-acquisition-of-siliconix-common-stock/
https://kaimte.com/upload/brand/20210112/e21ff7e2c5757354af16ace31b20f613.jpg
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
Well-known member
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)?
living sounds
Well-known member
Interesting, thanks. At least in the case of this mod, the drain resistor (R109) was not changed.
living sounds
Well-known member
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.
living sounds
Well-known member
Thans for that link and thank lord we have waybackmachine.
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living sounds
Well-known member
What would be a good trimpot value to try in a U87 type circuit? 10k? 50k?
RuudNL
Well-known member
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.
(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.
