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-   -   Matching Jfets (http://www.diyaudio.com/forums/solid-state/23546-matching-jfets.html)

NickC 2nd December 2003 01:45 PM

Matching Jfets
 
I need a circuit that can help me match Jfets to a specific current range of Idss 4-6ma and another circuit than can match them for gate source voltage. Can anyone point out a circuit that can do so. Thanks

Netlist 2nd December 2003 01:53 PM

http://www.passdiy.com has a brilliant tool for that job.

/Hugo :)

john curl 2nd December 2003 10:52 PM

I hope that Nelson has shown how to do this, BUT it is fairly easy with just a multimeter. You just short the gate to the source and add a 9V battery from the drain to the source. This gives you Idss. But how to measure? Put a 10 ohm resistor between the drain and the battery and measure across it. 0.1V=10ma and so forth. Matching Vbe is much more difficult.

peufeu 4th December 2003 02:23 PM

DMM
 
I use a multimeter set on 200 mA caliber and short grid to source...

I then take a sheet of paper, mark a scale from (for instance) 15 to 40 mA, and drop the transistors on the scale after measuring them. That's an easy way to group them without headaches...

johnferrier 4th December 2003 08:44 PM

Re: DMM
 
Quote:

Originally posted by peufeu
I then take a sheet of paper, mark a scale from (for instance) 15 to 40 mA, and drop the transistors...
Can you recommend good JFETs in the 15-40mA range? How many would it take to find 4 that match at around 20mA? Is 10% considered an adequate match? And how much better would 2% matches be???

What I have in mind is matching 4 JFETs for the outputs to a headphone amplifier (for 300 ohm Sennheisers). I'm thinking about 2sk170, but don't know if I would need to buy 20, 50 or a 100 (to find enough that "match").

Sorry for a barrage of questions (I don't know if someone is reading that knows such things "like the back of their hand"). TIA.


JF

peufeu 5th December 2003 06:41 AM

JFETs
 
I have only used JFETs to cascode bipolars so can't comment directly on their sound qualities.

I have used J309 in my DAC and that works well (and is cheap, too).

I think J309 would fit your bill, maybe. Look at the IDss values...

johnferrier 5th December 2003 10:58 PM

Re: JFETs
 
Quote:

Originally posted by peufeu
I have only used JFETs to cascode bipolars so can't comment directly on their sound qualities.

I have used J309 in my DAC and that works well (and is cheap, too).

I think J309 would fit your bill, maybe. Look at the IDss values...


: )


JF

Jocko Homo 6th December 2003 01:38 AM

2SK170s........
 
If you really need that much current, assuming that you have the BL group, you can put 4 in parallel to get that amount of current. Maybe with 3, but 4 to be certain.

Jocko

fdegrove 6th December 2003 01:44 AM

Hi,

Quote:

you can put 4 in parallel to get that amount of current. Maybe with 3, but 4 to be certain.
Current sharing resistors not included? Or are they?

Cheers,;)

johnferrier 6th December 2003 02:28 AM

What comes closest to what I have in mind is Borbely's "Super Buffer" which is figure 8 at the following link:

http://www.borbelyaudio.com/borb502.pdf

I'm thinking of parallelling 4, 5, 6...or whatever of the jsk170/jsk74 to get 100mA. It seems that violet parts would help me get there at a lower part count (and are just a little more cost). Maybe you're suggesting 5 ohm series resistors to allow for differences between JFETs. Figure 9 shows a MOSFET version (with higher current output), but Borbely suggests that an all JFET amplifier would be the best.

My question is how closely matched (10, 5, 2%) do the k170/j74 pairs need to be?


JF


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