BTW,
I think one of the original posts had a schematic. Can you repost it?
Thanks,
------
Bob La Rocca
Lindenhurst, NY
"Bob" wrote in message
t...
Just curious here. I've been following the debate. I have a radio I
built
that uses a separate AVC amplifier to separate the AVC function from the
detector, and give me some control over AVC action. I used an infinite
impedance detector so as to not load the last IF secondary. So I pretty
much have the cathode follower and would only need to add a diode to
convert
to the discussed detector. would there be any noticeable improvement over
the infinite impedance detector?
thanks
------
Bob La Rocca
Lindenhurst, NY
"Patrick Turner" wrote in message
...
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.
I will NEVER build anything because " we only ever did it that way
before
..."
or any other equally gutless and stupid reason.
I can see where a cathode follower could be
beneficial if we were trying to build a radio with an IF as narrow
as
possible, in which case it would help keep the Q of the
transformer
secondary as high as possible, but we are talking about a radio
with
wide
band audio, and are probably talking about adding loading
resistors
across
the transformers anyway, so why the cathode follower, why not just
let the
load of the detector diode do the job? A cathode follower after
the
IFT
seems like a waste to me, better to use it after the detector,
with
a
negative cathode supply voltage, to buffer the detector from the
AGC
and
audio lines.
I do things to suit the desire for wide as possible AF bw, and the
R loading of the IFTs helps achieve that end.
I don't want severe selectivity and IFT gain; that only belongs
in Z grade AM radios and communications sets.
Yes of course, I was simply trying to point out one situation where a
cathode follower driving the diode might be useful.
Try using a CF buffer to power a detector with a germanium diode,
you'll
hear the
difference!
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.
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.
The same diode
driven at the same level, with the same DC bias applied, and the same
total load reflected to the IFT secondary at 455 kHz?
While many AM receivers have been designed in a cost conscious
way,
there
have also been a few where no expense was spared, and parts were
freely
used, and yet I have never seen a cathode follower used as you
propose in
a commercial design, I would think if it were beneficial someone
would
have used it commercially, anyone know of any examples?
I have NEVER seen any ancient commercially produced radio or audio
product where
the sound quality was not compromised, often severely, with many
lies
told
by the market cowboys, after the maker had reduced the parts count
to
reduce costs, to be able to compete.
There were certainly commercially produced AM tuners where the maker
didn't reduce the parts count at all in order to reduce the costs, the
sound quality may or may not have been compromised by your standards,
but
if it was, it was due to a poor use of the parts, rather than to a
lowered
parts count. There are certainly commercial AM tuner designs that use
significantly more parts than your tuner uses.
99% of surviving old radios are crap, and were crap when they were
foisted
onto a
gullible public.
I have never seen anything here which remotely was above the lowest
common
denominator,
except perhaps the Quad AM tuner I aquired.
But the one I built measures and sounds better on the BCB.
Sure some AM tuners use more parts than I have, and I recently posted a
few SS
design
schematics at ABSE and ABPR to let folks know there is more to AM
reception
than RDH4 ideas and a few tubes.
Some designs add
other relatively expensive parts to the detector circuit, one
trick
I have
seen whose effects might be worth looking into is replacing the
second
capacitor in the peak detector & RF filter network with a series
LC
network tuned to 455 kHz. I don't know how this circuit actually
works,
but I assume that the idea is to improve the tradeoff of the total
peak
detector capacitance vs. tangential clipping at high frequencies.
A series LC tuned to 455 kHz needs to be driven by a low impedance
to get a decent Q to reject the 455 kHz ripple,
This is patent nonsense, if anything just the opposite is true, if a
series LC to ground were driven by a very low impedance it would have
virtually no effect.
I suggested low impedance, not zero ohms impedance.
You would find that with a high source impedance that the attenuation
curve is
too broad, and as you reduce Rg, the attenuation of the series LC
becomes
sharper
and deeper,
but below a certain Rg the attenuation is restricted.
I have tried this sort of idea for a 9 kHz whistle filter for better DX
listening.
I found the shape of the null, or the Q of the null was very variable
with
Rg,
and a CF with a series R was best to find the right sort of null.
R can't be too small, lest the filter overload the tube.
But a broad null was needed to not only reduce the 9 kHz whistle
but also reduce the monkey chatter a bit.
Nothing was really effective 100%.
Another type of much better null filter is a bridged LC type, with two
caps and an
L,
and an R to 0V from the CT between the two caps, and this is less
dependant
on any critical value of Rg, although for a deep null, the two C have to
be
matched,
and the R value to 0V is very critical.
But in the case of detectors, there in no need for null filters or
traps,
and in any case, if you wanted good rejection of 455 kHz, with LC, you'd
use a
critically damped low pass filter using LC which would not only
reject 455 kHz very adequately, but any other RF noise.
but it simply
is far easier to achive in well known ways with R&C.
Usually, the CRC arrangement of 100pF, 47k, and 100pF is entirely
adequate for removing RF detector ripple voltage.
Just goes to show that those manufacturers that included the series LC
weren't among the ones you are speaking of that reduced the parts
count
wherever they could.
Depends how they used the parts which were not "rationalised out of the
package to
save $$"
The 47k could be replaced with a choke in the CRC filter,
but the CRC does enough to reduce the 455 kHz ripple to
such low levels that any remaining ripple at say -40 dB will not cause a
bother in
the audio amp.
And the audio amp will have almost no gain at 455 kHz.
This is
something I will have to look into further. There are other
detector
circuit subtleties like this that may, or may not, be worth while.
You need to use a soldering iron to find out about what I am
promoting
about AM detection.
There is no other way.
Actually I am fairly certain that there are several other ways, like
me,
you are simply prejudiced towards your own ways.
I prefer my own methods, and I welcome anyone to post a schematic which
works
better.
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.
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.
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.
If the load resistances of the CF are exchanged for transistor CCS
the bulk of the small thd caused by the CF transfer character is also
reduced
to perhap 0.02% at 10v of audio output, depending on the triode used.
6DJ8 would be ideal...
12AU7 is OK, 6AQ8, ECC85, 6201, 12AT7 are probably better,
since the gain reduction with 12AT7 is around say 35 times for a given
load R, so if the open loop thd was 1%, its reduced to 1/35 % by
the NFB effect of the follower.
A CCS reduces the thd at least several times more.
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.
Patrick Turner.
Regards,
John Byrns
Surf my web pages at, http://users.rcn.com/jbyrns/
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