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About JFET Polarity

Ever since the very beginning of Boozhound Laboratories, 10+ years ago, I have been using the same software, which I recently discovered numbers the pins backwards on the default TO-92 part.  So the transistors in the thousands of kits I have sold and that people are happily listening to for years are technically “backwards”.  But it doesn't matter!

These JFETs are symmetrical devices.  The Drain and the Source are interchangeable.  They are just a chunk of doped silicon with a lead on each end and one in the middle.

I am switching to the Linear Systems part from the Toshiba part, and the datasheet for the LS part appears to show a different pinout.  In revisiting the Toshiba datasheet, it is not at all clear which pin is which.  So I have always just trusted the design software, and since the circuits work and sound great I have never had any reason to take another look.

I am currently wrestling with what to do about this.  I guess my advice is to install the JFETs backwards from how they appear on the circuit board.  It won't make any difference, but it will be more technically correct.

Actually there is a slight difference in some devices - there might be a very small difference in input capacitance depending on how the device is constructed.  This will be inaudible in my circuits as they are not particularly sensitive to capacitance.

A few people have questioned the orientation of the parts and I have referred to my schematics (which are wrong due to the error in the JFET part in the software I use) and to the Toshiba datasheet (which is confusing) and come to the conclusion that it is correct since it works and it has always been this way.  My apologies that my confirmation bias has kept me from noticing this.

I will include a discussion of the correct part orientation in all future kits that ship with the incorrectly labeled board, and all new circuit boards will have the “correct” orientation.

If you have already built a kit, there is no need to change anything.  Polarity on these JFETs does not matter at all, and the sound of the circuit is not affected.

Thanks,

Jason

(June 2017)

 

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