the superregenerative detector is designed around TR1, a BF191 VHF transistor. The base has a tight fixed bias by the diode string, forcing a stable operating point over a wide tuning range, aiding the squelch to maintain good performance. The audio is low pass filtered to remove the 30kC quench frequency and amplified in TR2. IC1a is the main gain block, its low output impedance preventing any unwanted interaction between the squelch and volume controls. The audio then is amplified in the LM386 gated audio power amplifier. The signal from IC1a is also passed to the multiple feedback bandpass filter around IC1b, the second half of the LM358. The filter has zero gain at the Fc of 12kC, dropping rapidly from each side. This filters out any residual quench, and any audio modulation. The output is rectified and the recovered vo
ltage used to gate the LM386 off, muting the radio. Set correctly, the squelch will "spit" off signal, and fully open on even the weakest transmission. This design is the classic "noise" squelch method, relying on the "quieting" effect a transmission has on the severe background hiss of any high gain RF amplifying system.
Purists will note the absence of any isolation between detector and aerial. My experiments have found little to no attenuation of SR hash with a buffer stage. Measured on my old spectrum analyser, the 6 microwatts of RF hash is spread over a roughly 200kC spectrum at the quench frequency , reducing any radiated power at any one point. My Yupiteru 7200 is struggling to detect any interference at a 3 meter distance, and that is one sensitive receiver.
It is also interesting to note, the performance of this version is very similar to the three valve version of the same thing, the former using a high active component count, the valved set using just 6 active stages.
Edit.... I forgot the pin numbering for the LM386... the transistor collector connects to pin 7, when switched on, it deprives the preamp stages within the IC of operating current. The capacitor bypass on pin 8 slows the voltage change to remove switching clicks, making the squelch "softer" with the attack and decay.