Medium Wave Receiver Using Only An LM386 Audio Amplifier
The circuit uses the front end of the LM386 as a Colpitts oscillator with enhanced audio gain accomplished by bypassing the internal gain determining resistors.
Performance is very good. Because of the high gain it is best to build it on a copper clad board which acts as a good ground plane although I have seen successful variations on a plastic protoboard.
The value of the choke in the circuit is not critical and I have successfully used values from 1 mH to 10 mH. The 10 mH choke which I used had a resistance of 30 Ohms which gave just the right audio output amplitude. If a lower value choke is used which will likely have a lower DC resistance it would be best to add a small resistor (22-100 Ohms) in series with the choke to limit the audio gain to a reasonable level.
Reducing the number of turns on the ferrite rod inductor makes it possible to receive amateur SSB on 160 meters (1800-2000 kHz). Raising the frequency higher to receive 80 meters (3.5-4 MHz) will require a circuit modification.
I have used it with standard 32 Ohm earbud headphones both in series and parallel. Audio volume when using earbuds is earsplitting and it will likely drive a 16 or 32 Ohm speaker to a moderate volume.
Using a 3 inch flat ferrite bar with a standard MW winding and both gangs of a standard MW Polyvaricon tuning capacitor, the receiver tuned from 540 KHz to 1700 KHz.
Using 15 turns closewound on the flat ferrite bar and only the main gang of the PVC variable capacitor (not the LO section) the receiver was able to tune from 2.9 MHz to 8.0 MHz by changing the value of the 220 pF Colpitts feedback capacitor to 33 pF. This value of capacitor provided the proper Colpitts feedback ratio required for oscillation up to 8 MHz in series with the intrinsic input capacitance of the LM386. Audio volume is adequate when receiving strong commercial SW stations. Audio output volume from SSB reception on the 80 and 40 meter amateur bands may be improved by using stereo earbuds in parallel which creates an effective load impedance of 16 Ohms.
The ability to oscillate up to 8 MHz with a 9V supply is likely dependent on the manufacturer of the LM386. I have had the best success with chips made by National and Samsung.
The links below are videos made by a radio amateur of an earlier iteration of the circuit.
