In article , Patrick Turner
wrote:
John Byrns wrote:
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.
I will NEVER build anything because " we only ever did it that way before ..."
or any other equally gutless and stupid reason.
But your reason is equally gutless and stupid, you are enamored with your
circuit and refuse to look at how a diode envelope detector actually
interfaces with the secondary of an IFT in the traditional approach, and
whether it has any merit.
Like others, yourself included, I am just prejudiced against some ideas,
and a cathode follower between the IFT and detector is just something that
I have little intention of trying.
Try something different, and try abandoning your prejudice,
just for an hour it takes to make something; maybe you hear something good.
I may try the Selsted Smith detector given in the back of the RDH4. I'm
not sure what hearing something good means, sometimes what sounds good is
actually bad, or as some people call it euphonic.
Measurements will confirm the improvement.
Or they may only confirm that the cathode follower helps with your
detector design for some as yet unexplained reason, but doesn't help in
the general case, see my comments further along. Have you measured
identical detectors with and without the cathode follower?
I don't need to make the comparisons with measurements.
Its so plain obvious with CRO experiments and dual trace
tracing of the envelope shape against the detected signal.
Now your prejudices are coming out, you say "I don't need to make the
comparisons with measurements."
I think I finally
understand what you are doing with the bias on your detector, and how it
works. I believe I have misunderstood what your biased detector was all
about and you haven't explained it. You have made statements like "This
method means that detection of weak signal lower than the forward voltage
of the Ge diode of 0.27v peak approx are not subject to the non linear
turn on of the diode, ie, there is no clipping by the diode." This lead
me to believe that you were using the bias to somehow "linearize" the
diode, which I didn't understand.
In a tubed diode detector used so often, the value of ripple voltage
varies a lot
between
the bottom of a detected sine wave to the top of the crests of the wave,
and its a distortion mechanism, worst when modulation % is high, which it is,
on most transmitted AM signals these days; since efficient use of a carrier is
wanted.
In my biased detector, the variation in ripple voltage along all parts of the
audio wave
form is very nearly the same amplitude.
The figure from the RDH4 which John
Stewart posted finally made me realize that you were doing exactly the
same thing with your bias as the RDH4 figure, except that you used a fixed
bias and left out the tracking feature.
My CF plus following Ge diode is totally different to anything in RDH4.
Only to the extent that you have added a cathode follower to the
compensated diode circuit in the RDH4, but the cathode follower serves no
useful purpose, and of course you have removed the tracking feature from
the RDH4 circuit.
The distortion reduction you
claim makes sense in that context, because your receiver as described by
the schematic you posted has an extremely poor AC/DC load ratio and I am
sure the distortion is extreme without the bias. Your bias scheme
presumably partially compensates for the poor AC/DC load ratio, rather
than somehow improving the "non linear turn on of the diode" as I had
erroneously assumed from what you have said.
The diode detector schematic I did post does have a poor AC/DC load ratio, but
still works fine
to make a few volts without any wave clipping or added distortion
because of the
ratio.
That is the bias at work compensating for the poor AC/DC load ratio, just
as explained in the RDH4, it has nothing to do with the cathode follower.
Even better results with capacity for a much higher undistorted output
voltage is
possible
with the same first CF and Ge diode and CRC filter, but then directly
coupled to a
second CF,
which is the other half of a twin triode, and behold, there is zero AC
loading on
the detector.
Yes, I have said repeatedly that is where you should be using a cathode
follower.
Now the only remaining question is, does the apparently pointless cathode
follower driving the diode also compensate in some way for another
unnoticed design flaw?
I give up.
If you cannot see the benefits of intelligent buffering, and you won't try an
idea before roundly condemning it, then it becomes pointless for me to
provide any more justification than I already have.
I can easily see the benefits of intelligent buffering, but a cathode
follower between the IFT secondary and the diode is not "intelligent
buffering".
Regards,
John Byrns
Surf my web pages at,
http://users.rcn.com/jbyrns/