Steven Swift wrote:

Hi Fi AM--

This was "almost" popular as a design project in the 60s. Even RCA in
their RC-19 Tube Manual have a circuit labelled "TRF AM Tuner-- for High-
Fidelity Local Broadcast Reception." (Circuit 19-8, p. 357)

John Byrns web site has the circuit diagram for the RCA design you
mention (or a related one if there's more than one of them):

http://users.rcn.com/jbyrns/pics/RC-17-8.jpg


Fidelity on an AM signal requires that most common circuits used in
radios be eliminated: [snip of good list]

All these "don't do" can be found in Terman, the Radiotron Designer's
Handbook and others.

[snip]

The RC-19 circuit uses a 6BA6 as an RF amp, followed by a 12AU7 used as a
detector and audio amplifier.

As the diagram at John Byrns site shows.


What's intriguing is how simple this design is -- it has one RF stage,
which indicates that a one RF stage TRF for local, high power stations
makes sense when audio fidelity is the overriding criterion.

Now, I wonder how much improvement in the audio quality is possible
if the channel TRF approach is used (which optimizes the bandpass for
each broadcast frequency)? Or does it not make sense by the law of
diminishing returns? Being able to use a higher order plug-in bandpass
filter (such as a constant delay/linear phase one), optimized for each
frequency, is intriguing.

Jon Noring