3.13 AM Receiver with ZN415E & LM386 IC’sThis FRT AM receiver combines the features of two magnificent IC’s, ZN416E (HF amplifier, detector and the pre-amplifier) and LM386 (audio power amplifier). Add a power supply based on 7809 voltage stabilizer IC, use a loudspeaker with membrane diameter being about 15 cm, put all this in one of the boxes described in the Appendix, and you will have a new and improved silicone - based model of an old - fashioned radio receiver, built somewhat less than 100 years ago.
The electrical diagram of this device is given on Pic.3.38. The voltage reduction (and stabilization) from 9V to 1.5V, as needed by ZN415, is being done over the R2 resistor and D1 and D2 diodes, although any circuit given on Pic.3.34 can be used instead. Universal - type diodes 1N4148 are used here, but any other low-power silicon diodes will do. With 250 Ohms trimmer TP the fine - tuning of the voltage on the pin 6 is being done (the receiver is set to some station in the upper part of the reception bandwidth (app. 1500 kHz), and the slider of the TP is then used to obtain the optimum reception). After that, the receiver is switched off, the trimmer is disconnected, its resistance measured and an equivalent resistor soldered in the circuitry. On the PCB, this resistor is marked as R3.
The PCB, component layout and connections with other parts of the device ar shown on Pic.3.39. If the receiver is made for the household use, you should certainly put it into some bigger box where bigger loudspeaker (~15 cm) can be installed. The ferrite antenna should be mounted on the rear or side panel, as far from the loudspeaker as possible. All the conductors, especially those that connect the capacitor to the board, should be as short as possible. The wires that connect the loudspeaker and the adapter to the PCB have to be firmly twisted, as shown on picture for the loudspeaker. The potentiometer is to be connected with the shielded microphone cable, if it is more then 10 cm away from the board.
A connector for 64-Ohm headphones can also be mounted on the box. It is then connected to the PCB via a microphone cable, connecting its “live” end directly on pin No.5, and the shield wire to the closest ground. Headphones can also be used instead of loudspeaker, in which case such connector must be chosen, that will switch off the loudspeaker when headphones are plugged in.
* Some more modern IC, other than LM386, can also be used in this device. The author recommends TDA7052A. Its main features are:
1. Extremely stable operation
2. Implemented output short-circuit protection
3. No cooler necessary
4. Small power consumption, IO=4 mA
5. No “clicking” during switching ON/OFF
6. Voltage amplification is 39 dB
7. Output power Piz=1.2 W (3x bigger than LM386)
8. The 220 mF capacitor isn’t needed if the power rectifier that has an electrolytic capacitor on its output is used, and the cables connecting it with the device are short. In that case the amplifier is practically made without the external components, if we exclude the (inevitable) 100 nF capacitor, which is in every way great advantage.