You can use two detector diodes in a solid state AM radio to get better weak station reception. Using two diodes keeps the AVC filter cap from back biasing the detector diode. I kept the old silicon diode (D3) (600mV diode drop) that came with the radio for the AVC circuit (R29 and a new 0.01μF cap to ground), and used a second diode that has a much lower diode drop, around 200mV for the audio signal detector function. I split off the audio signal path (R30, C38) off the now AVC diode and put it on the new diode. A sharper diode knee will make for better reception of weak stations, but you'll likely need to add bias to overcome diode drop voltage. This is done with some forward bias current to improve weak signal detection, as this bias will help overcome the diode drop loss of signal present at the diode. A diode with a higher drop but sharper knee would be the better choice. Too many variables to calculate the resistor value to use (what current on the diode knee will make it detect best, what's your supply voltage, what's the voltage of the diode drop voltage, what's the resistive load on the diode circuit such as the volume control, and so on), so trial and error would be quickest.

Also try a small signal silicon transistor with its collector and base tied together. Think doing it like this makes the collector turn on once the base is biased on, making a sharper knee. And collector saturation doesn't seem to be an issue. The emitter on such an NPN transistor would be the cathode. Here I used a 2SC2785 salvaged from a VCR or such. Try various transistors. I found that 680K seems to hit the sweet spot in this radio, but it's not a critical adjustment on a trimpot (which you can use to quickly find the value you'd want in a fixed resistor). I had thought that using a transistor this way was the mark of a sloppy company (did they run out of diodes?), but I got the best weak station detection performance with this. Oh, check strong stations too, which worked well too.

Ultimately you are better off making an infinite impedance detector using a FET & RFC.