In Patrick Turner's latest message replying to the thread I started on
building a high-performance AM tube tuner, he stated his skepticism
that a pure TRF circuit will, in a practical sense, meet the specs I'm
looking for (to meet the specs will require an impractical number of
RF amplifier stages, such as six or more.)

Within the design constraints he selected, I cannot disagree with his
conclusion.

However, one of the constraints he made, and that most make, is that
tuning is to be done by an infinitely variable multigang air
capacitor.

I wonder if the same conclusion would be reached if we approach this
from a single channel perspective? That is, what if we fix the
frequency we want to receive (e.g., 830khz, or 1420khz), and then for
each stage optimize the parallel RLC circuit (or use some other tuned
circuit with the right resonance and bandwidth response -- crystals?)

With the much greater design freedom this should allow, could we now
possibly meet the specs with only two or three RF amps, for any
frequency in the 500 to 1800khz range? For a simplified diagram of a
two RF stage TRF receiver, see page 7 of:


http://www.technology.niagarac.on.ca...531unit6rx.ppt

Now, if this single channel approach indeed makes TRF more attractive,
then the next step is obvious -- consider building the TRF AM tuner
with selectable channels. Obviously, the downsides to this a the
need for a switch, the complexity and cost of having lots of channels
if we want to cover the whole BCB range (e.g., 120+, every 10khz in
North America, and 9khz in Europe and elsewhere), and the issue of
oscillation due to interference between the RF stages (shielding and
proper layout design will likely be very critical.) I envision the
tuned circuits for each channel being put on a mini-board which will
plug into a slot, but then there may be other ways to handle the large
number of tuned circuits necessary to cover the whole BCB (360+ of
them -- this does sound quite expensive.)

Thoughts?

*****

Of course, the other alternative is to modernize a proven design of a
super-het tube radio from yesteryear (the final audio amplifier will
be stripped out, replaced with a line-out.) Which radio circuits
would you nominate (with improvements), which provide excellent audio
fidelity (probably with variable bandwidth control), very good
sensitivity and selectivity, and other desirable specs? In addition,
the circuit design should be "forgiving", so those who build it from
kits (with pre-made PCB boards and proper guidance on layout and
shielding) will likely be successful -- the parts, too, should be
purchasable as NS from somewhere (NO SCROUNGING for old used parts as
I've noted before). We probably need to make special arrangements for
producing the various coils, since not everyone is going to want to
wind their own coils.


Jon Noring