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Thanks Winston. 

I have a couple of original 2108’s from a old UA console out of LA.  They show a photo cell external gain control that I don’t have.  It’s just a remote variable resistor (pot) that can be subbed I guess.  I think the voltage shown for power was 28 volts from memory.  Any way I should put them with a power supply and get them running. 
 
Yep 1108's.    I need to get them going.  I have an old wood box from a balintine ph meter. I kept it to put them in.  Just get a power supply together.
 
I've racked up several pairs of those "gold brick" 1108 preamps over the years.  Pretty easy and they sound great.

What was the idea behind the 2108, if not to be a faithful recreation of the 1108?
 
Winston, awesome explanation and great read. Thank you.

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?
 
Winston O'Boogie said:
The initial idea came from Bill Putnam Jnr. I think.  It was to take the 1108 and make it more modern and useful.  Since the 1108 had a J-Fet on the input (might've been the first do so?), I just took it to extreme and used nothing *but* J-Fets for the added IP stage.

Yes, it may have been the first.  I am more familiar with the bipolar transistor circuits of the era, like RCA BA series.  Speaking of firsts, the first preamp cards I ever racked were a pair or Urei 1109!  All bipolars, used the same input transformer as an LA-3A.

Thanks for sharing this circuit, I'm always interested in learning the how and why.
 
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.

 

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that ain't no Heathkit tracer, that's a Textronix Jazzbone 360 fresh from a Swedish calibration  lab with the Pixar TT5500 23 mega hertz side car re-bake.
 
No strings, photo attached of actual item found on Dobbin St in Brooklyn, NY.  Inside is a handwritten date of Jan 7, 1975!  Replaced the 9 electrolytics and will fire it up today to see if it still works.  (Sorry for the thread hijack!)
 

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