Henry Kolesnik wrote:

How many stages you need depends on the selectivity you need because
of your geographic location and antenna. Two stations close in
frequency will interfere with each other unless you have enough
selectivity. A weak local station that is strong enough to be heard
may get splatter from a distance station close in frequency that has
50KW or more and a pattern that concentrates on your area! High Q,
double tuning, addtional stages all add to selectivity. The superhet
solved this problem.

Hmmm, the "channel TRF" approach may help with one stage selectivity
since it appears we can now use a perfectly optimized higher order
tuned filter on the channel mini-board, while in a traditionally tuned
circuit, implementing that same bandpass circuit to apply across the
whole BCB will be much more difficult, and I would guess be near
impossible (too many circuit components which need to be varied
simultaneously as one varies the reception center frequency.)

With the channel TRF approach, the tube-o-phile can mix and match
bandpass filter types from station to station depending upon the
circumstances. For example, they could use the default, wider-band,
gentler, bandpass filter plug-in board (one which has better linear
phase) for a local station which doesn't have adjacent interference,
and for a more difficult station (with adjacent interference) they can
use a bandpass filter plug-in board with a shape factor closer to
unity (which probably has more ripple and worse linear phase). (Even
for the default "wider-band" filter, because we can now use a
frequency optimized higher order filter, we should be able to achieve
reasonably good selectivity, at least sufficient for local station
reception, even with one RF amp stage.)

There appears to be a lot more freedom given to the circuit designer
when the necessity of tuning a fixed set of tuning components over a
frequency range is removed, such as using higher order bandpass
filters. (Of course, this is one reason for IF, but even superhets
have at least one tuned RF amp before the mixer, so the same issue
applies to superhets, but is not as critical.)

I now wonder that with a single TRF RF amp stage, and with a higher
order bandpass filter optimized for a particular frequency, if we can
now dispense with the RF transformer? Or does an RF transformer confer
other benefits that it should remain? I thought its main benefit was
for improved bandpass shaping, but then I may be wrong here (likely
with high probablity -- RF transformers do help with isolation of
stages for DC, so I've read, but don't know how that would benefit
real tuner circuit design.)

Jon Noring