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Author Topic: KM84 FET Replacement: Type and Procedure  (Read 11963 times)

radardoug

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Re: KM84 2N3819
« Reply #15 on: January 30, 2013, 03:07:46 pm »

Any thoughts from the panel on a replacement for the 2N3819 seeing they are getting hard to get?
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klaus

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Re: KM84 2N3819
« Reply #16 on: January 30, 2013, 07:45:15 pm »

They are readily available for little money from major electronics online shops under "Fairchild 2N3819".
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

radardoug

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Re: KM84 2N3819
« Reply #17 on: January 31, 2013, 02:58:03 am »

I did a quick check before I posted, none at Digikey, Farnell, Mouser.
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usattler

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Uwe Sattler
Technical Director - Neumann|USA (retired)

klaus

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Re: KM84 2N3819
« Reply #19 on: January 31, 2013, 09:38:10 am »

To my knowledge, Fairchild is the only manufacturer which restarted production on discrete FETs. Hence my recommendation to google Fairchild + 2N3819
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Klaus Heyne
German Masterworks®
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Jim Williams

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Re: KM84 2N3819
« Reply #20 on: February 01, 2013, 12:07:38 pm »

I wouldn't waste time on Fairchild jfets. As I mentioned, their specs are all over the map so selecting them is a PITA, IF you want the highest performance.

A better source is:
www.linearsystems.com

This is a company in Fremont, CA that specializes in high performance jfets for high end audio. They have their version of the 2N3819 jfet, the LS846. This part is higher performance with a low 3 nv noise spec and only 15 pa gate leakage. I suggest that a $1000+ EU microphone is deserving of a better jfet, they only cost a buck or two.

Linear Systems also makes their version of the 2SK170, another common jfet used in condenser microphones. This is the LSK170, only 1 nv noise and a great sub in an U-87, if you want to lower the noise floor.
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klaus

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Re: KM84 2N3819
« Reply #21 on: February 01, 2013, 05:28:54 pm »

Thanks for the tip. I've been happy with the batch of Fairchilds I have been installing ( a couple of duds, here and there, but overall happy). But will give these a try as well.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

squareneck

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Re: KM84 FET Replacement: Type and Procedure
« Reply #22 on: February 05, 2013, 06:35:29 am »

Hi again!
I have now soldered in a new resistor (R3). But still the output is very weak,  I tried with another 2N3819 but there was no difference so I decided to measure some voltage (according to the schematics) and compare to my other KM 84 which is fine and I found out:

Working one: R10(46V) = 45,8V. GR1(24V). =23.8V. C5(21.5V)=20.6V. = Drain(10V)=6V
Bad one: R10(46V) = 45,7V. GR1(24V). =22.5V. C5(21.5V)=18.7V. Drain(10V):=1.7V

So I thought I must have done something wrong when I was trying to find out the value of R3. What I did now was once again placing a variable resistor on R3 and then adjusting until I had 10V at the drain. This gave me 8.2kOhm instead of 4.7 kOhm. So I will order a resistor close to 8.2 kOhm and then I'll see if that solves the problem. Do you think I'm on the right track?

Best/Bo

ps. I've also replaced C3, C4, C5 and C8 since I heard they could be old and leaky. ds.
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klaus

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Re: KM84 FET Replacement: Type and Procedure
« Reply #23 on: February 05, 2013, 02:45:16 pm »

Do you think I'm on the right track?

I don't think so. After you have undertaken many attempts which included four different FETs, proper FET biasing etc. You have so far not  gotten full output of the mic. 

I would start looking somewhere else for the reason why you are down so much: Have you made sure that the pads are bypassed, and the mic's output transformer is strapped to 200Ω? Does the capsule measure roughly 32-36pf capacitance, as measured between contact needle and outer capsule shell?
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Jim Williams

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Re: KM84 FET Replacement: Type and Procedure
« Reply #24 on: February 06, 2013, 11:15:08 am »

Get out a scope and feed a 50 mv, 1k sine wave into the fet gate. Observe each stage for proper signal levels. You will find where it stops.
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squareneck

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Re: KM84 FET Replacement: Type and Procedure
« Reply #25 on: February 12, 2013, 02:30:12 am »

Finally I've got the mic to work properly again :-)
I must have done something wrong the first time I did Klaus method for finding out the value of R3, cause when I repeated the procedure this time I found out that 8.2 kOhm did it for me and that also corresponds well to what I found out by measuring voltage which gave me 8.2 kOmh. So when I now soldered in the new R3 (8.2 kOhm) the mic sounds perfect again.

Thanks a thousands for all your help I've learned a lot on this project and looking forward to my next one!
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