LM386 Amplifier - TA7642 AM Radio

LM386: The No Win Scenario

Just like James T. Kirk, I do not like the no win scenario, and I do not believe in it. Hence, I cheated. I used an LM386 amplifier IC because it solves many problems. Granted, that from an educational point of view, you are not going to learn anything from using this chip! And granted, that this chip is too simplistic, however, I figured that the needs of the many whippersnappers must outweigh the needs of the few or the one. Many poor little whippersnappers out there would like to build an affordable first radio, and this project will introduce them to using a chip at least!

In order to keep the cost low, I even considered the TDA7052 because this chip does not require a capacitor on the output pin, and no other components, however it is more expensive than the LM386. I was able to find sellers on eBay offering ten LM386 chips for around two pounds, which is good value, and using the LM386 chip solves many of the following problems:

1. It eliminates the need to buy biasing resistors. I was using four per each stage in the transistor amplifier circuit, and with this chip, I can cut the component count to less than 50 %.
2. There is no requirement for decoupling capacitors, which is great because they are expensive as well!
3. With all the biasing resistors, there is always the chance that someone might make a mistake in the build. There is always the possibility of some resistors being slightly off tolerance influencing the circuit.
4. The gain from a two-transistor amplifier is very small and simply not worth the cost compared to an IC.

With the LM386, I can get a warp gain factor of 200! Moreover, a power output of 250 mW, whilst using only five components including the chip itself! A gain of 200 is obtainable by placing a 10 µF capacitor between pins 1 and 8. In addition, this chip has a special secret weapon that you cannot resist, known as frequency shaping. This capability allows a cheap speaker to produce a nice bass sound, and to enable this, all you have to do is to place a series RC between pins 1 and 5. For a 6 dB bass boost, R=10 kΩ, and C=0.033 µF. Hey, I will take three chips... Warp factor 200! Punch it! Hence, if I were to be publishing a new Ladybird book for an AM radio in the year 2022, when who knows we may even be suffering from a pandemic, and the world will be coming to an end, then it would have two chips and a few discrete components. That is progress for you!

LM386 Circuit

Here is the circuit that I am using for the LM386 amplifier chip. Cg is a 10 µF electrolytic capacitor, which enables the gain to be 200. R2 and C4 form a Zobel network, and are most definitely required to provide stability to the output. I removed them to see what would happen, and I was getting all sorts of feedback noise. These two components come into play when the inductive reactance of the loudspeaker coil becomes excessive.

The audio output capacitor C5 is 220 µF (electrolytic type). I am using a recycled one that is normally used in high-end equipment. Silmic and Elna usually make some of the best high-end capacitors. C3 is the inter-stage decoupling capacitor, which is 1 µF in value. You could use anything up to 4.7 µF here, as long as it is an electrolytic type.

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