Integrated Circuit Audio Amplifiers
Our first IC amp is listed in Figure 1 and uses a LM386 IC. The LM386 comes in 3 flavors now; LM386-1, LM386-2, LM386-3 with output power levels of 300, 500 and 700 milli-watts respectively. The type sold by Radio Shack is the LM386-1 and is the one we used in this circuit. Perhaps the most unique feature is that it is available at any Radio Shack and can operate at voltages as low as 5 volts. Just like regular op amps, audio amp IC's have an inverting and non-inverting input. Input signals are normally fed to the non-inverting input while the inverting input is normally tied to ground. Because of the high gain of IC audio amps, it is highly recommended to isolate them from the power supply to prevent oscillations. In this circuit, R1 and C1 accomplish this task very well. Resistor R3 controls the gain and Capacitor C3 couples the output to the speaker. Output capacitor coupling is mandatory in just about all IC audio amp designs.
The LM386 IC is unique in that the gain can be modified by changing Resistor R2 and Capacitor C2. This configuration will give us a gain of 20. By removing R2 and connecting C2 across pins 1 and 8, we can increase the gain to 200. It is important to understand that increasing the gain does not increase the output power. The increased gain is only used when a very low input signal is to be amplified.
Our next IC is the LM380 and it also comes in two flavors; LM380-8 and LM380 with output powers of 700 milli-watts and 2 watts respectively. Figure 2 depicts the LM380-8 and Figure 3 depicts the LM380. The LM380-8 comes in an 8 pin package and its basic circuit is virtually identical to the LM380 except for the different pin out. The LM380 comes in a 14 pin package and pins 3,4,5,10,11 and 13 are connected to ground to act as a heat sink. Experience has shown the LM380 should be soldered directly to the circuit board (no IC socket) if it is going to be operated at its full rated 2 watt output. This IC can become quite warm and it's important to get rid of excess heat through the pins. The primary advantages of the LM380 series IC's are higher output power, very low distortion and low external parts count.
No matter how much volume, an audio amp provides, there are still those who require even more. The circuit in Figure 4 uses a LM383 IC amp and will provide up to 7 watts of output power for those who want to really experience their audio. The LM383 comes in a TO220 type package with 5 pins as indicated in Figure 4. My experiences with this IC revealed that it must be heat sinked at all times due to it's high standby current. If you plan to use this IC keep all components as close as possible to the IC and be certain that your power source can supply up to 1.3 amps of current. The main advantage to this IC is its 7 watt output which is why it has found its way into many inexpensive car radios. This IC offers low distortion and is a real bargain compared to discrete transistors.
The LM386 IC has been designed to work with low voltages...between 4 to 15 volts. The current taken from the power source or the batteries when standing by, is also quite small, something around 3 mA's. If you add to these qualities the possibility of changing the amplification power, you realize that the LM386 chip is an attractive circuit to work with.
In the schematic to the right, you see that there is a capacitor between pins #1 and #8. This capacitor can be varied from 0.5uF to 10uF. With higher values employed, greater amplification can be obtain...though the risk of oscillation also increases.
To avoid oscillation of the circuit, it is a must to connect a 100uF electrolytic capacitor between pin #6 and ground. This capacitor is shunted in parallel with another one of 0.1uF. It can be either ceramic or mylar.
You can attach to the output of this circuit, speakers from 8 ohms to 16 ohms. I experimented with small speakers and even one of 7 inches in diameter showed excellent results!
The picture to the left is all of the components used in the LM386 audio amplifier project.
The LM386 IC (Integrated Circuit) is manufactured in an 8 pin dual in-line package and it is good practice to mount it onto an 8 pin socket. This IC can deliver up to 0.5 (audio power) watts without any kind of heatsink. This is when a 16 ohm loudspeaker is attached to the output, together with a 9 volt power supply.
The LM386 IC has an ample field of experimentation...not only as an audio amplifier, but also as a Wienbridge oscillator and a square wave oscillator.
The picture to the right shows the 'copper tracks side' of the LM386 audio amplifier printed circuit board (PCB).