2sj109/2sk389 too low Vgs range ?

[gebi]

Hello

I'm planing to build a simple line stage based on the n-jfet 2sk389.
I'm confused. In the data-sheet the Vgs range is from -0.6-0 V.
The same for the 2sj109 with 0-0.6V V_gs range.

Is this not a too low voltage range for audio ?
E.g with a bias of around 5mA I just can use a Vgs range of +-0.3 V.

As far as I know - in the Aleph J a 2sj109 is used in the input stage.
How can that work ?
For full output swing you need around U_in_peak=2.2V
( calc: U_out_rms=sqrt(P R), U_peak=U_rms * sqrt(2),
U_in=U_out/Gain, with R=8Ohms, P_out=30Watt, Gain=10 => U_in_peak=2.2V)
at the input ?!
Similar for line levels with are typically around 1V_rms.

Sorry maybe I missed here something.

thx
gebi

[jackinnj]

Erno Borbely has his JFET articles from The Audio Amateur 5/99 and 6/99 archived on his website -- these are really helpful in understanding what you seek to accomplish

http://www.borbelyaudio.com/adobe/ae599bor.pdf

http://www.borbelyaudio.com/adobe/ae699bor.pdf

[gebi]

Thanks for the link. Beautiful articles.

Stupid me !
The variation of Vgs is not equal to the variation of the input voltage.
It depends on the way how the transistor is used - common source or drain...
E.g If the circuit has a gain of
G1=Vout/Vin and the transistor a gain of G2=Vout/Vgs
then it follows that Vgs=Vin G1/G2.
In case of a follower with G1 around 1 is the Vgs variation
very small.

sorry

gebi