Cool write-up. Thanks, Winston!
I love JFETs in small signal devices. The little bit of additional noise in low source Z applications (compared to BJTs) never bothered me. I’ve found the 1108 to be amongst my favorites. (What’s not to love though? JFET input glued to a DC coupled, class-A 2N3053 output section. Color my world.)
Love the idea of an all-JFET mic/line stage. I’ll definitely be protoboarding this at some point.
As I wrote in a post today on the McCurdy AT-284 (another FET mic pre that came out in the late 60s) it’d be interesting to find out which came first: the 1108 or the AT-284.
dogears said:
2SK170s / LSK170s these days also seem to be unobtanium. Simulating the circuit with various other JFET combos shows lower gain - any other subs suggested? What’s the set of parameters to match the two for the cascode?
LSK170s are still being manufactured by Linear Systems. Check Trendsetter Electronics.
The usual subs for 170s are 2N5457 and PN4393. Mouser had both last time I checked.
But with the "usual subs", you'll have to be selective about which ones to use in the specific circuit you need it in. I have found that 2SK184 actually comes closer to 2SK170 than do the others above in most preamp circuits. You can get 184s at Small Bear Electronics.
One problem is that the 170 is a high admittance part. Finding subs that fit from an Idss and Vgs perspective may not be enough in this application - that's probably why you saw lower gain from the ones you used. That's why I find the 184 a better sub even though you rarely see it suggested as one - it has high admittance like the 170.
A couple of JFET comparisons I did attached. Steeper slopes = higher admittance. Chart shows Id to Vgs.