Signal Digger Regen

The Signal Digger regenerative radio receiver circuit is based on  differential pair regenerative receivers designed by B. Kainka and Alan Yates respectively. 

The collector of Q1 is connected to a tap about 1/5 of the way up from ground on L1.

The emitter of Q3 is connected to tab about 1/4 of the way up, to actually the top of L1.

Tapping about 1/4 the way up is good for SSB and CW, up higher is good for AM.

Q3 and Q4 form a current mirror. The current at the collector of Q4 is the reflected version of the current flowing through R3. 

Q3 is shorted at RF by C2.  Q3 is wired as a 'diode connected transistor' with a relatively low impedance (13k) at about 2 uA forward current flow.

Q4 detects the signal from the resonant circuit L1 C1 due to transistor non-linear voltage signal behavior. The input impedance of Q4 is very high due to its low collector current. The low collector current also results in RF being filtered out by semiconductor capacitance at the collector of Q4 and additionally C4 if you choose to use it.

If the voltage across R4 is more than a volt or two increase R3 from 2meg to 4.7meg.

You can replace R4 with headphones if you like. 

Of course you must couple an antenna signal into the resonant circuit L1 C1. The better sort of antenna to use is a loop of electrical wire about 1 or 2 square meters in area, and it's the area that is important.

The 2 ends of the electrical wire are electrically connected together.  Then you loop 1/2 or 1 turn of the larger loop around the L1's coil former.