Hi Fi AM--

This was "almost" popular as a design project in the 60s. Even RCA in
their RC-19 Tube Manual have a circuit labelled "TRF AM Tuner-- for High-
Fidelity Local Broadcast Reception." (Circuit 19-8, p. 357)

Fidelity on an AM signal requires that most common circuits used in
radios be eliminated:

1: No AVC. This distorts the low frequency frequency response
2: No cathode bias bypass.
3: No diode detectors, unless the signal feeding them is greater than 10Vrms.
4: No AC coupling if diode detector is used (the "AC-loading" distortion
described in Terman, et al).
5: Speaker resonance 30Hz. Assumes the line out goes to a real "Hi Fi"
system.

All these "don't do" can be found in Terman, the Radiotron Designer's
Handbook and others.

If you are willing to live with about 5-10% THD, then you can use more
common circuits. However, there are dozens of "Hi Fi" AM circuits published
by the hobby magazines, tube vendors and kit makers. Have a look at them.

The RC-19 circuit uses a 6BA6 as an RF amp, followed by a 12AU7 used as a
detector and audio amplifier.

Good luck.

Steve.

--
Steven D. Swift, , http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA