FirstWatt J2

[generg]

For me the sound with only one 2SJ109 BL was too soft. Two 2SJ109 BL or four SJ74BL made the sound more energetic.

 

[EUVL]

So you are saying the F8 also has not enough energy ??

🙂


Patrick

 

[Zen Mod]

so, you're deliberately being silent about difference that F8 is not having source resistor down

🙂

 

[EUVL]

That you can also do in a J2, not ?
Plus that you can also change the push-pull ratio.


Patrick

 

[tonimxp]

Hi Alex...sorry for my mistake....i also have ECW20N20 .
My idea is only for solve the problem of reperibility of SJEP and to use something better than IRFP.

 

[generg]

In a quick and and dirty Spice the difference with and without the 0.47/0.47 in the source of the lower SS is about 8dB.

Can this count for the two pairs of J2 in comparison to the F8?

 

[EUVL]

2 pairs with same LTP bias will not bring you 6dB.
More like 3dB.


Patrick

 

[Zen Mod]

Hi Alex...sorry for my mistake....i also have ECW20N20 .
My idea is only for solve the problem of reperibility of SJEP and to use something better than IRFP.

feel free to try it

even with lower xconductance, that can be very nice amp

 

[andynor]

For me the sound with only one 2SJ109 BL was too soft. Two 2SJ109 BL or four SJ74BL made the sound more energetic.

Genergetic

Maybe I should try parallelling in my sloppy joe B1R2

 

[generg]

The J-Fet frontend of the J2 seems to have a OLG of about 22dB, the FE of F8 about 12dB, so 10dB less.

The omitted source resistor gives +6db more OLG for the OS stage.

So the we can calculate that the difference is about 4dB.

 

[EUVL]

What Yfs do you assume for Semisouth ?

Patrick

 

[generg]

Hi Patrick, I worked only with spice .... so I cannot answer your question.
Sorry

:--))

 

[EUVL]

We discussed OLG, and not distortion. Correct ?

I measured by curve tracing at 55°C Yfs of SJEP120R100 to be ~6S at 1.3A.
That is 0.17R intrinsic source resistance.
Add that to the actual (hardware) source resistor give you the totoal effective source resistance (SJEP + Rsource).
So you can now determine how much to gain by changing the value of Rsource.

It does not say anything about bias stability over temperature.
For that of course you need to contact experiments on heatsinks.

Spice is only as accurate as the model you use.
I always measure to cross check with simulations.


Cheers,
Patrick

 

[generg]

Oh yes I just remembered, we ( friend DSDjoy) measured in an original J2, at 1,4A. for the lower SS around 6.5S and for the higher SS a little bit more.

Forgot it it was 2012....

:--))

 

[schultzsch]

I searched in my parts bin today and found a matched quad at 10.6ma of j74bl. I also found 2 quads of matched k117gr. One is at 4.3ma and the other at 5ma.
Should I go with these or it’s better to order the ones with the required idss?

 

[generg]

I see a problem.... the four J74 are BL as in the original, but the k117, may not be good.
Either two 2SK170 or 2SK370 in parallel with an Idss adding to 10mA are requested.
Of course 5mA and 5mA would be the best solution, to share the current equally.

 

[generg]

Some people in the web say the SK117 is a possible replacement for 2SK170GR.
Maybe someone can say more.....
:--))

 

[Zen Mod]

if someone asks me, only LTP needs JFets

CCS for JFets ....... use whatever you have

nail, coffin, kettle, resistor, any JFets, ring of two

in short, quartet of 10mA6 is perfect for Poor Man J2 knockout

2 quartets for real deal knockout

 

[generg]

Thank you Aleksandar!

 

[EUVL]

2SJ74 is P-JFET, 2SK117 is N-JFET.
2SK117 is the TO92 version of 2SK209.
Already published in 2018.
https://www.diyaudio.com/community/threads/njfets-for-source-follower-applications.329131/


Patrick