[Obligatory Telamon rec.radio.shortwave on-topic statement: The
following reply touches upon aspects of TRF design as they apply to
MW DXing, particularly sensitivity and selectivity of tube-based TRF
designs. MW DXing is on-topic to r.r.s. discussion. I appreciate
Telamon's efforts to keep r.r.s discussion strictly on-topic per the
written and published r.r.s. Charter.]


craigm wrote:

Don't forget you will have to switch those modules [all 120+] in and
out of circuit. The switch and all the associated wiring will
significantly add to the space.

Granted, it will. Some of the wiring hassle is removed if we use a
ready-made PCB motherboard (see, for example, the ST-35 clone board at
http://www.diytube.com/ ). But, unless one can think of some clever
electronic switching arrangement, there will be wires from the
switch to the board. One would make sure the wires from the switch
would solder on to pins along one side of the motherboard (and
appropropriately shielded), but for 120+ channels could be 480+ wire
connections (assuming we can get by with four wires per channel to
feed the bandpass filter(s), which is unclear at the moment) --
definitely formidable, but not outside the realm of solvability by
clever design and utilization of modern components.

With 15-20 channels (for local listening purposes), however, the
situation is much more reasonable all around.

As I've noted recently (a shift in the requirements), the channel
TRF concept makes the most sense for an audiophile-acceptable tube
tuner to listen to strong, local stations. Even the so-called "RCA
"high-fidelity AM tube tuner" is a TRF design, not an IF. Thus, having
it optimally tuned for 15-20 stations (channels) will be acceptable
for this purpose, and in fact may be a good selling point among these
people.

If one wants all 120+ stations soldered in, that means the user is
interested in tuning the whole BCB, which automatically means DXing.
Now we are in a different ballpark, and the user expectations are
different.

[Here, I would consider John Byrns advice and design a doubly tuned
TRF design, such as his proposed "modernized" Western Electric 10-A.
One interesting twist, to get more optimum bandshaping across the BCB,
divide the wide-ranging BCB into five or more tunable bands; thus, for
example, band 1 would tune from 500-650 khz, band 2 from 650-850 khz,
band 3 from 850-1100 khz, band 4 from 1100-1400 khz, and band 5 from
1400-1800khz (or whatever makes sense.) Each band would have its own
singly or doubly tuned bandpass circuitry, optimized to that band (but
not optimized to any particular frequency). In fact, with a narrow
enough sub-band, one should be able to get pretty good bandshaping
with single tuning, I presume, which is a welcome simplification.]


The wiring for the switch will also probably affect the tuning of
the modules forcing them to be tuning 'in place'.

Yes, that is definitely a consideration which I've noted before, the
affect of the interwire/interconnect RLC on the tuning circuit. With
continuous tuning, this is not an issue.

With the 15-20 channel system for local tuning, it may not be a
problem either if we add to the circuit some calibration indicator
(like the tuning light of old.) For example, the user makes or buys a
mini-board for 830 khz (a local station they want to listen to.) They
plug it in. They then turn on the tuner (letting it warm up fully).
Once sufficiently warmed up, they then calibrate the mini-board by
turning a trimmer on the mini-board to fine tune the station, until
either the station sounds as if it is "in tune", or the calibration
indicator light shows it to be tuned to the correct center frequency.

(Also as I've noted before, it will no doubt be important for there to
be a fine tuning control on the tuner itself, to fine tune +/- 1 khz
(or thereabouts) to account for warmup and for long-term drift of the
component values inbetween calibrations.

But many audiophiles and especially kit-building tube-o-philes love to
tweak their stuff -- they'll enjoy this, and they will also be
enamored in having the most optimum bandpass tuning circuitry for that
frequency -- it's a performance/sound issue. Some may even wish to
swap plugin boards, to try different bandpass types, order and
bandwidth (for some stations they may have to because of adjacent
interference.)

In a sense, an IF design is boring when looked at from this angle.
laugh/.


If the long end sticks up, you have the inout and output of the modules on
the same end. This could result in unwanted coupling.

Granted. One of those problems which needs to be sorted out in the
design of the whole "tuning box" architecture. It is a problem, but so
far does not appear to be a show stopper. It's one of those items that
still falls under the category "to be solved by appropriate board and
wiring design".


Conceptually it may appear clean, but the proposed switiching of
many modules adds a new complexity. All the wiring associated with a
swicth will cause more problems that the design solves.

For 120+ channels, yes, it looks like a plumbing nightmare unless
someone can come up with a clever idea (and that is certainly
possible).

But with 15-20 channels, it is entirely workable. Old TVs worked with
12 channel switches (channels 2-13.)

And I'm not familiar with what could be done with modern electronic
switches.


If you are looking for broad band with reasonable attenuation of
other stations, a superhet is much better as there is only one
signal you are optimizing for.

Well, the same applies to the channel TRF concept. When we switch in a
particular bandpass tuning circuit, it is calibrated for a single
center frequency, with the optimum tuning circuit for that frequency.

Note again that Patrick himself said that IF is not needed when one is
building a single frequency receiver -- and from his comments he is a
very strong advocate of superhet design for a tunable receiver. That's
all the channel TRF is: a single frequency receiver, duplicated n
number of times (where n is the number of channels one wants to tune,
which are switched in and out.)

The downsides of a channel TRF are obvious: plumbing (wiring)
complexity for an all-channel BCB tuner, and not being able to
continuously tune all frequencies within the BCB.


I would think that a kit should be simple, the proposed solution is not.

At 120+ channels, the channel TRF is intimidating (the switch box and
individual tuning circuits), but at 15-20, with the plugin
architecture I am thinking of, it does not look that complicated,
especially if a lot of the architecture and components we see used in
PCs can be utilized.


And that know-how does not all need to be explained here. There are
plenty of resources on the internet, look for them and study them.

I've definitely done that!


IMO this thread and the related theads are more on-topic for rrs
than much of the stuff posted by one of the people considering this
to be off topic.

I've put in the obligatory Telamon preamble stating this message is
on-topic to r.r.s., so all should be fine with the Usenet gods (tm).

Thanks for your informative feedback.

Jon Noring