I've taken the plunge and ordered two boards for a 3-way speaker project. I've read the entire thread, and strayed into a couple of side quests like balanced operation and JFET buffers.
I've also ordered a bunch of LSK170's because clearly I like the deep end. Mostly the conversation around FETs in here is about biasing, so have that well understood. Where I'm still missing context is around matching. It's been mentioned in passing a few times, but there
seems to be contradictions. I've also learned that when smart people seem to spout contradictions, there's something interesting to learn. It was never contradictory, but lacking context.
There looks to be some fuss around matching among builders here, then N.P. drops this nugget in post
#932:
Matching is nice, but not crucial. It is convenient to consider 2SK170 / LSK170 in the
follower positions and continue to use J113 as constant current sources.
Everyone is scrambling to "match" their JFETs, when Mr. Pass is casually hand-waiving it away and blasting Idss values into the stratosphere, well beyond the J Fringe. And to my layman, tiny brain it seems logical that pairing a J113 (Q2, CCS) with an LSK170 (Q1, Follower) would be quite
impossible to match, they are quite different parts. For instance, J113 has an Idss of 2-? (I've seen 40mA+ in other's measurements?), whereas the LSK170 is 6-12mA.
Quote below is not about matching per se, but builders seem to want to match for the purposes of staying within Idss "limits". From
this article elsewhere on the Internet:
It should be apparent that expecting to operate a JFET with an IDSS of 1mA with a drain current of more than 1mA won't work - ideally the quiescent drain current will be somewhere between 50% and 85% of the rated (or measured) IDSS for minimum distortion.
Huh? No wonder there's confusion around this topic. There's a devil lurking in the details, I suspect. On to the questions:
1. In what context is matching FETs important for this particular circuit? Vp? Minimum Idss? Between Q1 and Q2? Horizontally across channels? Vertically along one signal/filter path?
2. I see Beyond the J Fringe has LS parts in a push-pull circuit, and Toshiba parts in another. Can drain current overdrive work safely in the context of this crossover?
3. If the answer to the above question is no, what are the considerations/pros/cons for dropping drain currents from the stock 8-10ma to my lowest common denominator ~6.5mA?
Wow long post. Thank you Nelson Pass for this fantastic design, and thanks to all the folks in here for sharing your knowledge and builds!