Patrick Turner wrote:

John Byrns wrote:



In article , Patrick Turner
wrote:



John Byrns wrote:



In article , Patrick Turner
wrote:



Better of course would be to have a CF tube to accept the IF envelope,
and the low impedance output from the CF can then power a crystal


diode, or


a tube diode
in a variety of ways I have previously explained in post on the matter.


You still haven't explained how this added cathode follower, to drive the
detector, helps matters? Many experts even make the claim that a finite
source resistance can be beneficial in reducing distortion, especially
high frequency distortion.


IN my case there *is* a finite source resistance which is the 100k
R across each IFT winding.


Yes, but the diode is driven from the low source impedance of the cathode
follower, not something on the order of 100k. Why not eliminate the
cathode follower and choose the diode detector load so that it looks like
100k to the IFT at 455 kHz?



Because a CF does it better.
With 100k source impedance, one still gets distortion to the audio signal
where the diodes conduct to charge the 100 pF C1 of thre CRC detector filter.
I prefer to go the extra country mile at every part of the audio chain,
and it all adds up to a good total outcome, rather than one which is medicocre
along the way, which then adds up to a poor total outcome.



Pat, I just ran a simulation of the use of two cathode followers (I used
12AU7's) and
a 6AL5 diode ddetector. I got very good results, using a 1KHz
audio sine wave, modulated onto a 95% modulated 455KHz carrier, got 55dB
down
of the 2nd and 3rd harmonic. I also threw in a little bit of positive
bias to the detector
diode to partly get it up above the "Knee". About 300mV worth of bias.
Of course
this requires that the AM signal be at least 2Vp-p and that the
modulation index be
about 95% (leaving 5% carrier at the valleys). As I'm pushing a bit
into the "other
side" of the AM signal. IOW, I moved the zero crossing voltage a bit to
make "fake
exaulted carrier".

See a screen capture of my simulation schematic and results in ABPR