This afternoon I had a little play with a simple crystal set to see what sort of minimum discernible signal (MDS) was possible. My previous experiments with single germanium detectors had yielded an MDS of around -55 to -57dBm in the 5-30MHz range with a high impedance crystal earpiece.
Today I tried the same test with both germanium diodes and a hot carrier diode (HP2835) with and without some bias applied to the diode(s). Best results were with the HP2835 and with about 0.11V forward bias applied. Only a few uA are needed so battery drain is next to nothing. MDS was around -62dBm for a well modulated AM signal. For the tuned circuit I used a T50-6 toroid with around 20 turns with an antenna link winding adjusted for best sensitivity/selectivity and a 365pF variable capacitor .
Incidentally, forward bias for the diode could be obtained by rectifying other signals available on the antenna such as MW/LW broadcasters using a separate tuned circuit, rectifier and reservoir capacitor. I must give this a try sometime. Several hundreds of mV DC should be possible.
Then I tried switching the diode with a second signal generator at a power of between 1-10mW to see what sort of sensitivity could be obtained on a CW signal. MDS improved by around 25dB to around -86dBm i.e. I could just hear down to around 10uV. This would be a just usable level as a basic HF SSB/CW receiver.
I also tried the same tests with a pair of high sensitivity ST-3 headphones that Michael AA1TJ kindly managed to find for me. These are much lower impedance than the crystal earpiece so I experimented with antenna match and the match of the diode to the tuned circuit, expecting a better performance at the best settings. Disappointingly, the results were several dB worse than with the crystal earpiece.
Now, the results with the crystal earpiece are very close to those (without bias) that I obtained some years ago, so I don't think my hearing (at 63 years old) has changed much for the worse. So, Michael must have incredibly sensitive ears as there is no way I was able to hear down to the levels he could with his passive receivers.
At the moment I am playing with the AM crystal set with -62dBm sensitivity listening to shortwave broadcasters. Fun.