3.4 Simple Radio Receiver with LM386 IC
Listening the programme over the headphones has its advantages, but the true radio receiver is certainly the one that with the loudspeaker. In this project the simplest such receiver is being described, consisting of input circuit, diode detector and audio amplifier with LM386 IC, that allows the loudspeaker reproduction to be performed.
The author has firstly met with the LM386 circuit over twenty years ago, and has ever since been using it very successfully in various devices. The fact that this IC has survived at the market for such a long time is a considerable proof of its quality, however, its most significant advantage remaining extremely low price. The readers can find more information on this circuit in book 4 of PE. Purchasing this IC, one must have in mind that it is being manufactured in several versions, marked as LM386, LM386N-1, LM386N-3 and LM386N-4, that differ themselves by the supply voltage values and the output power. In case the supply voltage being no greater than 12 V, any of these can be used in this receiver.
Electronic diagram of the direct radio receiver that has a LM386 IC in its LF stage is given on Pic.3.18. The resistor in the detection circuit is the log type 470 kOhms potentiometer. The LF voltage is being led over its moving end and the coupling capacitor C2 onto the inverting input (leg No.2) of the LM386. Other, non inverting input (leg No.3) is connected to the ground. The output is on the leg No.5. On this leg the load (i.e. 8 Ohm impedance loudspeaker) is being placed, connected over the C6 capacitor to the ground. With smaller battery power supply voltages a 4 Ohm impedance loudspeaker can also be used. If there's nothing in between the legs 1 and 8, the voltage amplification of the IC is Au=20, in which case the capacitor C7 can be omitted. If, however, a 10 ìF electrolytic capacitor is connected between legs 1 and 8 (+ end on leg 1) as shown on Pic.3.18, the amplification is Au=200. Adding a resistor in line with the abovementioned capacitor, any amount of amplification between 20 and 200 can be achieved. This resistor is shown in dashed line on Pic.3.18 and is being marked with "*" sign.
The PCB and components layout for the receiver shown on Pic.3.18 are given on Pic.3.19. Miniature loudspeakers from the pocket-size radio receivers should be avoided, since their efficiency and sound quality are poor, especially in the low frequencies area. On Pic.3.19 a 1W loudspeaker is being shown, whose membrane is 8 cm in diameter. That isn't such a bad solution, but even better would be using the loudspeaker with greater power and membrane diameter (During the testing the 3 W , 8 Ohm loudspeaker has also been used, and the sound quality was much better than with the one that is shown on the picture). As you can see, the cables connecting the loudspeaker with the PCB are firmly twisted around each other. This is a must, especially for the cables being longer than a dozen centimetres. The same has to be done with cables that connect PCB with the battery and the main switch Z.
Significantly louder reception, with bandwith being narrower in the area of the lower bound frequency, can be accomplished with the coil that has smaller inductivity and the coupling capacitor CA with greater capacitance. For example, if you are interested in radio stations that emits the signal in the MW area from 750 kHz to 1700 kHz, you could use the coil that has been previously described, by using only its' part between the legs 1 and 2, as shown on Pic.3.20. For the antenna that is about 6 m long use the CA=33 pF, and the reception is going to get better. Try increasing the CA capacitance (47 pF, 100 pF, 200 pF), the reception is going to get louder and louder, but the reception bandwidth will be getting smaller and smaller, some stations will not be heard any more, others will start to "mix" etc. it is up to you to find an optimum that suits you best. If using very short antenna, you should omit CA, and connect the antenna directly to the oscillatory circuit (at pt. 1).
