Henry Kolesnik wrote:

I haven't seen a technical reason whatsoever that a single channel
TRF perhaps switchable would have any verifiable advantage over a
superhet. But the thread continues to perhaps imply that there might
be something. Did I miss something? I'd sure like to know.

Patrick Turner noted the following in a thread from early this year,
when someone asked about a single frequency BCB tuner:

"Since you only want one channel, there is no need for a frequency
converter or any IFTs or IF amps, and a TRF with four tuned circuits
in the form of two critically coupled RF trannies will do nicely."

My understanding of the primary reason why superhet was designed was
to allow the most important amplification to be done at a single
frequency (the IF frequency), so the tuning circuit can be optimized
for that fixed frequency. (I'm sure Patrick and John Byrns will be
able to more accurately explain the advantages of the superhet, but
that's how I understand it in 10 words or less.) But if we already
have a single-frequency tuner, there's no need for an IF stage since
we can optimize the bandpass tuner sections for that particular
frequency.

So for a single frequency tuner, adding an IF stage only complicates
the circuitry -- it is superfluous -- and will add more distortion to
the final audio signal (albeit small, I assume, for a well-designed IF
stage.) So why use it? (It's been said a superhet may confer better
stability, whatever that means -- again a topic for Patrick or John to
address.)

Now, comparing a multichannel TRF tuner (with each channel having its
own optimized bandpass filter circuitry) to superhet tuner, then one
compares the complexity of switching individually tuned optimal RF
bandpass circuits with the complexity of adding a multigang tuning
capacitor (or inductor) and an IF stage. Also, there is the factor of
audio quality.

As I see it at the moment (subject to change as everything comes into
better focus over time), a cross-over point between choosing the
"channel TRF" and the traditional superhet for a tube-based BCB tuner
appears to lie between:

1) Listening to local stations, wanting the highest possible audio
quality, and

2) Casual to medium-serious DXing.

For (1), the listener only needs 20 or so channels, and the number of
RF amps can be kept to one or two (two for some added sensitivity to
pull in fairly weak local stations), so the "channel TRF" is more
attractive for this purpose (particularly for audio fidelity.)

For (2), for a "channel TRF", there'd probably have to be three RF
amps, with the full complement of optimized bandpass filters for all
130+ channels installed, so at this point a traditional superhet is
strongly indicated.

Nevertheless, even for DXing, the "channel TRF" is still intriguing
for those who might want to experiment, especially for the ability to
quickly swap bandpass filters (for changing the type and order, and
not only the bandwidth.)

Just my $0.02 worth.

Jon Noring