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SEN I/V stage for BIII


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the real requirement is the higher Idss of V-grade JFETs

Not clearly so.
We need higher transconductance, just because it straightly defines input impedance. And latter preferably to be as lower as possible.

not the lower noise of the 2SK369

It doesn't have huge amplification as common opamps, so noise factor are less matter than higher transconductance.
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Maybe I'm missing something, but the Pass(ish) I/V stages seem a bit silly. The input impedance of a JFET is always going to be higher than that of a BJT past a few mA, because its transconductance is lower. The matched JFETs are necessary for input offset, but a JFET running at IDSS makes a lousy current source. Just about all of them use at least one output coupling cap, if not two.

If you haven't come across it already, highly recommend checking out the Hawksford paper on I/V stages.

I would try running a common base BJT very hot, without feedback.

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The SEN I/V is on it's way.

I got the unit in this post:  http://www.diyaudio.com/forums/digital-source/195483-zen-cen-sen-evolution-minimalistic-iv-converter-62.html#post2952599

The idea is to get it working and compare to the existing I/V stages by twisted pear.  If it's better, I'd like to make a board that the BIII will stack on top of.  One for stereo and one for dual mono.  I have found a source for matched quads of 2SK369V so the next step is to buy one set and test those.  The only problem I see is getting heatsinks like the one in the picture.

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  • 2 weeks later...

I got the boards in the mail.  I'm sure I can clean up the implementation.  I also noted that 2x 100R gate stoppers between Gnd and JFET gates were added.  They're surface mount and there were pads on the original PCB.  These aren't shown in the schematic that was posted from Xen.  It appears that these were added to help eliminate distortion.  

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  • 4 weeks later...

I got 4 matched quads from a legit seller on ebay.  I'll protoboard a backplane for stacking one BIII module on top of the extra components needed for the ESS9018 to work with the V18 boards.  If I can get it sounding good I'll work on a dual mono board.

Probably won't get to it until the end of summer so this is a work in progress.  I'll see what happens

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