i just built the filter/buffer with parts i got at hand and i get better drive and the sound is smoother at first sight.
thanks for sharing joachim.
by the way, i m using k170 and 2n5459.
got a few questions:
in the lastest schematic, does the resistance before the inductor includes the resistance of the coil?
can someone explain why it is important to match the the cascode jfet? is not the bf862 match that is important and that dictate the current?
how bf862 Vds is important in this topology? i see that the bf862 operates on a tiny small load line so i m wondering if the value of Vds has to be whithin a certain value. i guess the higher the better?
or maybe all this is already discussed in some Pass paper that i d missed?
ok i think i failed to see the importance of the driving fet s Vds. At such a low Vds, current varies a lot with Vgs, so we want the same Vds for the current source and the upper part. Hence, having high pinch-off cascode device minimizes this effect and helps zero the Dc offset.
And 2sk369 for example is more tolerant to this because Vds vs current are less dependent at low Vds compared to bf862, i.e hwat you said about early voltage
does this makes sense or am i in need of some reading?
edit: thus the 2sk369 has less distorsion than bf862 (for instance) because its Vds varies less for the given load and "works less"?
The more horizontal the drain current to drain source voltgae is the higher the ouput impedance of the Fet is. You could also say that it is a good constant current source.
The less the drain current varies with variations of Vds , the less distortion. The cascoding helps anyway because it forces a constant voltage ( more or less ) over the input Fets. My buffer has really minuscule distortion, BF862 or not.
Another thing with this floating cascode buffer is that the voltgae over the input Fet is controlled by the gate of the cascode Fet by sensing the voltage at the source of the input Fet so you have a driven cascode that reflexes changes in voltgae over the input source Fet into the Vgs dynamically.
It looks simpel, but ....
Our team member who is taking care of the buffer has a serious family emergency which we cannot forecast how long it might take.
He has our fully moral support, and the team reassured him that hobby is at this moment of no importance.
If Joachim does not want to wait, we can send you a couple of PCBs, which will be with you in a week or two.
But you would have to match your own FETs, and acquire the inductor from Mouser.
Thank you for being patient with us.
But we have maintained, and will also in the future, that this is only hobby.
There are things in life that deserves much higher priority.