[AMRadio] infinite impedance detector vs diode

[Jim Tonne]

Brett:

I ran your circuit through Spice and it is quite nicely
behaved.

As you submitted it, there is a huge amount of carrier
leaking through.  (I used a carrier of 500 kc for this
test.)  But that is easy to get rid of, and once it is
down to a low percentage it can be considered
harmless.

I have never seen that topology before.  It is rather
like a "tube" version of the Belar AMM- series of
modulation monitors.  A diode detector direct-coupled
to an amplifier.  After that you can break out the signal
and do as you see fit.  Although the Belar uses an
L-C lowpass filter prior to that amplifier.  

I used a 12AU7 tube, one half as a diode-connected
demodulator and one half as a triode connected as a
cathode follower per your writeup.  The distortion was
in the vicinity of a percent at all frequencies.  As you
lowered the input level it got a bit worse.  But I would
expect that demodulator to sound very nice indeed.  It
works acceptably well down to just a couple of volts
of input RF.   

- Jim Tonne   WB6BLD   TonneSoftware.com