N CHannel Depletion mode MOSFET

[gyraf]

Sorry, no spies around  ;D

 

[PRR]

>> Jfets were widely used in the fetrons, solid-state tube replacements
> jFETs but could not find any rated above 40V

Early JFETs were available tuned for large gate breakdown. Rheinfelder advocated 300V JFET designs in 1966.

Gate breakdown conflicts with transconductance. Most users wanted Gm, not high voltage. I have not see a hi-V JFET in decades.

 

[ruffrecords]

>> Jfets were widely used in the fetrons, solid-state tube replacements
> jFETs but could not find any rated above 40V

Early JFETs were available tuned for large gate breakdown. Rheinfelder advocated 300V JFET designs in 1966.

I did not realise that.

Gate breakdown conflicts with transconductance. Most users wanted Gm, not high voltage. I have not see a hi-V JFET in decades.

gm seems to be an order of magnitude greater than in small signal tubes so trading some for higher voltage operation should not be a problem.

I spent my entire career in electronics product development without ever using a FET as an amplifier. Used loads of BJts but not one FET except as a voltage dependent resistor in a compressor It is like going back to school again.

Cheers

ian

 

[gyraf]

Maybe 2SK373  @100V? The Hifiers seem to like it...

http://www.elenota.pl/datasheet-thumbs/big/2SK373_datasheet_74264_0.png

Aliexpress being full of it (in more than one sense)

Jakob E.

 

[JohnRoberts]

I did not realise that.gm seems to be an order of magnitude greater than in small signal tubes so trading some for higher voltage operation should not be a problem.

I spent my entire career in electronics product development without ever using a FET as an amplifier. Used loads of BJts but not one FET except as a voltage dependent resistor in a compressor It is like going back to school again.

Cheers

ian

JFETS have merit in LTP (long tailed pairs) precisely because of their lower transconductance than bipolar devices, that changes the compensation trapeze act (maybe somebody needs to make a JFET version of a DOA)..... oops did I say that out loud?.  Perhaps explaining the huge popularity of bifet op amps.

Modern JFETS are quite good but somewhat different animals than mosfets...  Back in the day mosfets would fry if you looked at them cross eyed, modern ones are more robust.

JR

 

[ruairioflaherty]

I beta tested the Force, the designer is a friend.  I can confirm that the circuit works well and has lower distortion than the all tube version of the same pre in the Core.  Both have their place, I will reach for tube gear because of its distortion signature.

 

[AusTex64]

I beta tested the Force, the designer is a friend.  I can confirm that the circuit works well and has lower distortion than the all tube version of the same pre in the Core.  Both have their place, I will reach for tube gear because of its distortion signature.

In preamps with a voltage gain stage and a current drive stage, I've found the voltage drive section to have much more effect on the sound than the current drive. I'm quite curious about that MOSFET White follower Manley is using, though.

Interesting comment about tube gear and the distortion signature. Horses for courses as the English say. I'm quite fond of The Mastering Lab mic preamp (all tube), which has THD specs as good or better than my discrete 1073's and Hardy M1. And I dare say my favorite sound.

 

[ruairioflaherty]

I've never used a Mastering Lab Pre but I did build their line amp and it measures extraordinarily well and sounded god in my tests.

 

[PRR]

> gm {of JFETs} seems to be an order of magnitude greater than in small signal tubes

And an OoM lower than BJTs. And typically some lower than thin-glass MOSFETs.

So not counting the weirdos in Audio, JFETs survive "only" as RF mixers, where the square-law curve may be cleaner than diodes or BJTs. (MOSFETs are used but I have not kept up with this.) Typically an RF-IF chain all works at 12V or less, so high-V FETs are not in demand.

Further, radios often have tight power budgets. High mixer current improves RF performance but not so the average customer would care. The fat JFETs cost more, demand is low, they been going out of production. (And ALL through-hole JFETs are gone scarce in this SMD world.)

> I spent my entire career

I used JFETs twice in custom work. The Yamaha Electric Piano preamp was a chain of JFETs and I copied that onto a guitar amp. The player liked it. I dunno if the FETs had any part in it. Later I had the inevitable idea to rig two JFETs and a transformer with adjustable bias as a "tube amp emulation". The victim said there were few good sounds in it. (I may have used a simple source follower somewhere but I do not recall it.) I've never done the JFET limiter thing; always thought photo resistors were better.