John Byrns wrote:
In article , Patrick Turner
wrote:
I might add that in real circuits, the value of R2 is quite critical for
lowest
thd, and it should be estabished experimentally for lowest thd;
too high a value will have terrible cut off distortion on the positive
peaks of the audio,
"Positive peaks of the audio"? Don't you mean negative modulation peaks
of the audio?
But this circuit still has the diode of the detector powered via a vigh
impedance
circuit of the secondary of the IFT,
and its still not a best possible outcome.
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.
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.
Try using a CF buffer to power a detector with a germanium diode, you'll hear the
difference!
Measurements will confirm the improvement.
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.
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, 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.
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.
Patrick Turner.
Regards,
John Byrns
Surf my web pages at, http://users.rcn.com/jbyrns/