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Old 2nd July 2012, 01:30 AM   #2001
jeff517 is offline jeff517  United States
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Quote:
Originally Posted by lloop9 View Post
Hiya fellows, I'm going to do some jfet matching for some 2SJ103, 2SK170, etc. I would like to know if the below schematic is appropriate for my purpose?

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would these setups work?
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Old 2nd July 2012, 01:41 AM   #2002
Salas is offline Salas  Greece
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Don't know, never used anything device special. A 9V battery a small resistor, DMM, and Ohm's law proved enough for just IDSS.
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Old 2nd July 2012, 01:43 AM   #2003
jeff517 is offline jeff517  United States
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I'm new and not very knowlegable. could you explain?
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Old 2nd July 2012, 02:07 AM   #2004
Salas is offline Salas  Greece
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Yes. That little circuit looks like its for matching Vgs(off) spec. Its the cut off voltage of a Jfet. So lets forget that. What we need match here is IDSS. I.e. top current when fully on with Vgs=0. That's the way we use it in this buffer, that's what we look for. So get yourself a 9V battery, a 10 Ohm resistor and some small crock leads. From left to right as you face the flat face of Toshiba 2SK170 you have pins DGS. Catch the two last ones to the right with a crock clip together and connect the other crock end to battery (-).
Now catch the left alone pin D with another crock clip and connect to the 10R on the other end. Connect the free 10R lead to battery (+) in a couple of minutes after you had touched the Jfet to let it lose your fingers heat. Now full IDSS current runs through. Use your DMM on DC voltage mode and autorange. Note down the reading across the resistor after it settles in a bit. Unhook that Jfet and stick it over that reading on paper. Next one ditto. After you got 'em all written down calculate currents from I=V/R. V=your note in V or mV, R=your 10 Ohm resistor or anything in hand up to 50R, I=IDSS. Make pairs of the closest ones. Use two good pairs of a nearby mA region to assemble your DCB1's audio section. Send me cherries.
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Old 8th July 2012, 05:25 PM   #2005
jeff517 is offline jeff517  United States
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Default Thanks!

Your method of matching Jfet's surprised me at its simplicity. I used it to match LED's also. You've helped me immensely and I am in your debt.
It all fits with something I try to live by." A day where you don'y learn something new is a wasted day. "
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Old 11th July 2012, 09:22 PM   #2006
atupi is offline atupi  Romania
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From what i've read for the buffer part Takman resistors are a good choice but wich one carbon or metal ? I'm preparing an order to pcx and i want to know your opinions.
Thanks!
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Old 11th July 2012, 09:31 PM   #2007
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I liked the AMRG resistors from parts connexion - an improvement over Takman, I think, and not outrageously expensive. They don't fit too well though.
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Old 12th July 2012, 03:31 AM   #2008
BigE is offline BigE  Canada
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I've just ordered an 80 ma toroid from Richard sumner in Toronto, potted, to start the ball rolling on this build.

Was considering this pot, but it's only 10K and carbon to boot.

KKB1031S28 Precision Electronic Components Ltd | KKB1031S28-ND | DigiKey

Any opinions? Would a goldpoint 20k be that much better?

I'm pretty much a newbie doing these things, having only built a couple items, but my question here is -- the BOM calls for a pot from 10K to 20K. The original B1 was 25K.

I'm a bit confused as to what value to use, and any help would be very appreciated. My readings of the threads are inconclusive, if 10K, 20K or even 25K is the "right" value.

Thanks in advance!
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Old 12th July 2012, 07:09 AM   #2009
Salas is offline Salas  Greece
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The lower value between 10K-25K your source equipment can afford loading wise the slightly better its going to sound.
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Old 12th July 2012, 01:15 PM   #2010
BigE is offline BigE  Canada
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Excellent, thank you!

The output impedance of the phono section that will be playing into this is 55 ohms. 10K makes for a input/output ratio of 181.8. That should be fine.

That opens up a LOT of potentiometer options, like TKD.
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