23 Jan
2015
23 Jan
'15
12:47 p.m.
I don't know whether some US televisions do this too but I have a Japanese radio
that has two screw terminals for a 300 Ohm balanced feeder and alternatively
allows a 75 Ohm unbalanced feeder to be connected between one of those terminals
and a ground terminal which is also provided.
One of my old FM tuners (a Leak Trough Line 3 [1]) has that arrangement. It's quite
simple
electrically, since the impedance ratio goes as the square of the turns ratio, you need
twice as
many turns to match 300 ohms as to match 75 ohms. And since the 300 ohm input is
balanced,
it should really be centre-tapped to earth. These conditions are both met by having a
centre-tapped
input coil (matching to the right impedance on the secondary side of the RF input
transformer) with
the tap grounds. I am sure the Trough Line 3 schematic is on the web (every other old Leak
schematic
I've wanted is) if you want to see the circuit.
[1] So called because the oscillator 'coil' is a concentric line soldered to the
tuning capacitor. It is
therefore very stable.
-tony