Last night I posted a couple messages regarding the proposed "channel
TRF" AM tube tuner, focusing on the plug-in mini-board idea as one way
among several possibilities to implement it.

The idea underlying the channel TRF concept is to build bandpass
tuning circuitry specific to, and optimized for, each frequency in the
BCB, instead of fixing that circuitry to some "average" value and
trying to vary it using a traditional variable air capacitor (or
variable inductor) for continuous tuning. A switch would be used to
select the bandpass circuitry for the particular frequency channel the
listener wants to hear.

This would allow, in principle if not in practice, the ability to
very precisely optimize the bandpass circuitry (to maintain a quite
constant bandwidth and shape) for every broadcast frequency in the BCB
(from 500 khz to 1800 khz.)

The "mini-board" variation of the concept would place the bandpass
circuitry for each channel (frequency) onto a small plug-in PCB board.
Depending upon the type and order of bandpass filter used, the number
of components on the mini-board may be quite small, maybe a couple
capacitors, a resistor or two, an inductor, etc., having the optimal
values, and with one or more trimmers for fine adjustment of the
center frequency.

Clearly there are several implementations of the general concept, one
of which is a well-known hybrid that allows continuous tuning in the
more traditional and familiar way. The ones I think of at the moment
a

1) Traditional continuous tuning: Divide the wide BCB into several
sub-bands, such as 5 or even more, each sub-band having optimized
bandpass circuitry for the sub-band, and then use the traditional
variable capacitor or inductor to tune within the narrow sub-band.
Although each channel will no longer have the most optimal bandpass
configuration, it will be closer to optimal.

2) Single Board, True Channel: It may be possible, instead of having
120+ totally independent channel circuits each placed on a separate
mini-board, to put them all onto one larger board, but still keep
all circuits otherwise separate on the board. A lot of components,
and probably a lot of trimmers.

3) Single Board, Shared Components: As a combination of items (1) and
(2), channels which are adjacent to each other (in their own
"sub-band") could probably share a lot of common bandpass
components, thereby reducing the number needed on the board. Only
the large number of trimmers for individual channel calibration
will remain.

The original idea of mini-boards is most advantageous when the user
of the TRF tube tuner only plans to listen to 10-20 stations (such
as local, higher-power stations). They only install the channel
mini-boards they want to listen to.


*****

I do have a couple questions of both John and Patrick (and anyone else
caring to chime in) related to this.

1) In the single frequency TRF tube receiver (a TRF designed strictly
to listen to a single frequency), is there a need for double tuned
circuits? Or will singly tuned circuits be sufficient for
excellent performance (audio quality, sensitivity and
selectivity)? If not, how do double tuned circuits benefit the
overall performance of the single frequency TRF receiver?

2) Let's assume that we decide to design a Mark I TRF AM tube tuner
kit designed solely for more local, higher power stations (thus the
sensitivity is less critical than a tuner to also be used for
casual DXing.) How will this further simplify the optimal single
frequency TRF receiver design? Will only one RF amp stage be
necessary, or will we still need two? The focus now will be on very
high-quality audio reproduction of local stations, which I believe
tubeophiles will be most interested in.


Thanks.

Jon Noring