Save Complementary J-FET's - F4 Input Alternative


2004-03-12 8:42 am
Herewith I declare Nelson Pass guilty for drying up the Toshiba J-FET market by proposing power amps which use those!

As we have to save those complementary Toshiba J-FET's (they seem to dry out earlier than crude oil fields....) I thought of a different input for the F4.
OK, it needs 4 MOS-FET's, so...nothing for resource savers. :D

Match M1/M3 and M2/M4 for Vgs, take into account that the input resistance is only around 2.5k (who has such girlie preamps which cannot drive this load??) and there you are (hopefully).

Ciao, Tino

PS: to increase input Z you could always replace R1 and R2 with current sources, or raise the front-end supply voltage and use higher value resistors.

PS2: no, the different Vgs multiplier than in the original F4 (TL431) has nothing to do with my proposal...


  • f4 with mos-fet input.gif
    f4 with mos-fet input.gif
    39.8 KB · Views: 735


2004-03-12 8:42 am
gl said:
How does it sound Tino?

I have not tried it as a power amp (need to buy 2x18V "power" trannies before, all I have are 2x40V, too much for F4 :hot: )

But a short test with headphones (Bayer DT990, which are rather analytical, almost sharp treble) seemed to produce more soft treble and dry bass, so with these, it is a good match. I used only low volume and increased the current to 10mA, to avoid crossover.

I did not filter the supply well. PS was around 20V, not regulated, and i think that i should better thermally couple the pairs, as offset was not constant.
(i used a coupling cap, big 220µF, yes, I know...).

But for the next 4 weeks or so, i'll not be able to do much investigations, i'm out for work most of the time.

But may be it inspires someone here do give it a shot?

Steen, thank you for the compliment.