Re: ZN414 and TA7642 radios
I made a radio for our bathroom that only receives 198 kHz Radio Four. It's a ZN414 and LM386, completely standard circuit, although with an adjustable 1.5V to set the gain of the ZN414 (actually it's an MK484 equivalent) for LW. No idea how much longer 198 kHz will be in use (at east a few more years I believe), but it gets a great deal of use.
General points using ZN414 or equivalents: make sure the shaft/moving side of the variable capacitor is connected to the earthy side of the tuned circuit and not to the IC input. This helps reduce hand-capacity effects.
The capacitor on the output of the IC has various uses in conjunction with the AGC resistor (See ZN414/MK484 data sheet) and can also effect treble cut. I've found 100nF is about right.
It's a good idea to have some sort of voltage adjustment (and many circuits show this) for the ZN414 to adjust its gain for your location because they can overload if you're near a strong station. Likewise you can turn up the gain if there's not much around you. More than about 1.65 volts on MW leads to instability - depending on component layout.
Ian
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