"patrick-turner" wrote in message
...
Recently we heard about Mr Pogossov's "boosted cathode follower" using a bjt
between the cathode and 0V rail where the bjt has base current from an anode
R between anode and B+. Current production of bjt is regulated by Vg-k of
the follower tube, and all sort of positive claims are made about lower
Rout, lower THD etc, but not one measurement of Vac signals possible or
distortion have been offered to prove the claims are valid. In the fullness
of time, the voltages and THD measurements may follow, we might hope.
Meanwhile, instead of using a bjt as an active current source in the cathode
circuit, we can always use the bjt as a passive current source, as seen in
my preamp circuits at Fig4 Schematic at page
http://www.turneraudio.com.au/Line-preamp-2003.htm
Alex:
Of course we can use a current source, but it is a more or less standard
solution. You can also use just a 10K...20K resistor from the cathode of
12AU7 to GND and bias the grid to +40...80V.
It is easy to drive a hi-Z load, but what if someone wants to drive a 600ohm
load? Or a line coupling transformer which is nonlinear at low F? Then your
follower on the lousy 12AU7 will have gain of 0.5 instead of 0.95 plus
several % of THD.
Boosted cathode folower with Rout about 5 ohm or lower will just laugh
maintaining almost the same 0.95 gain and low THD.
If you just rewire your venerable MJE340 from a CCS into a booster common
emitter configuration, you will vastly improve performance and NOT increase
parts count. (You will be able to get rid of -125V rail as well.) Something
for nothing? Sounds too good to be true, you might think.
And the last thing -- no one forces you to use this or that circuit. The
website is only for "educational" purpose. Perhaps it might give some reader
some hints.