dave.harper wrote:

K7ITM wrote:


There is such a thing as TOO HIGH a loaded Q. Let's say you start off
with a coil with unloaded Q of 500, and couple lightly to it with your
circuit (antenna and detector), so the loaded Q is 250. That means the
bandwidth at 3dB points, if you tune a station at 1MHz, is 4000Hz. If
you've tuned to the center of the station, your demodulated bandwidth
will be only 2kHz. Since the rolloff is gradual with a single-tuned
circuit, voice should be OK, but you'll be missing out on a lot of the
highs. (Mind you, it's not easy at all to get an unloaded Q of 500 at
1MHz!)


Ok, I'm back with a couple more questions, since y'all are so
informative... I know it's not a true xtal set if I add an amp, but
wouldn't the best way to minimize the drop between the unloaded and
loaded Q be to add an amp? With a transistor/op amp, it seems that you
could tailor the load imposed on the tank circuit so that it's minimal,
while it seems a headphone load would be pretty significant (depending
on the headphone)...?

In my set I simply use an audio matching xfmr. I'm using sound-powered
phones which are in the 200 ohm impedance range, and around 50 ohms DC.
A little xfmr like the Calrad 45-700 is a good choice. Some of the
guys use a switchable matchbox using a tapped xfmr like the Bogen. You
can see some of these at http://www.crystalradio.net/

As for an amp...mine plays great into my computer's sound card! I tried
a little one chip amp but it only had about 20db of gain which isn't
really enough to do a lot of good.

-Bill