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Electrical Engineering Dictionary
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Mono Amplifier
This is an example of a mono amplifier circuit, single channel audio. The circuit is shown with minimum components and can be used with op amps using a single voltage supply. Be sure to watch the Gain Bandwidth Product of the selected amplifier to insure it will operate over the entire audio range. That includes both the desired gain of the amplifier and the load that it is required to drive.
Op Amp Mono Amplifier
Single Channel Amplifier
This circuit uses an LM386 as the example Op Amp. Although the input is applied to the plus input, the LM386 does not require the standard [Rf / Ri] resistors to provide feedback and gain. The gain pins [pins 1 and 8] set the gain at 20 [when left open]. Of course the 10k potentiometer sets the voltage level reaching the Op Amp.
The schematic does not show pin numbers for the Op Amps because the circuit works for either a single or dual package Op Amp and of course a large number of different Op Amps. However to the right, some different pin numbering is shown.
The LM386 has two gain control pins [not used in the diagram]. With pins 1 and 8 open an internal 1.35 k resistor sets the gain at 20 (26 dB). If a capacitor is put from pin 1 to 8, bypassing the 1.35 k resistor, the gain will go up to 200 (46 dB). If a resistor is placed in series with the capacitor, the gain can be set to any value from 20 to 200. Gain control can also be done by capacitively coupling a resistor (or FET) from pin 1 to ground.
The application notes indicate that when using the LM386 with higher gains (bypassing the 1.35 k resistor between pins 1 and 8) it is necessary to bypass the unused input, preventing degradation of gain and
possible instabilities. Pin 7 of the package should have a 0.1 uF capacitor or a short to ground depending on the dc source resistance on the driven input to bypass the LM386.
Regardless of the Op Amp used or the number of devices in a package, always bypass the power pin Vcc [Vs, V+] to ground using a .01uF capacitor [at a minimum]. Bypass capacitors are not shown in many of these schematic diagrams to make them less complicated to read.
Speaker Drive
A capacitor should be connected between the output of the Op Amp and the speaker when a single supply is used. The capacitor prevents any DC off-set developed by the Op Amp from reaching the speaker. A typical capacitor value may be 250uF.
Small 0.1uF capacitors should also be connected between the volume control and the input to the Op Amp when using a single supply. The 0.1uF capacitor blocks DC voltages from the input of the Op Amp.
LM386 Output Power
Normally operational amplifiers don't deliver a lot of power, and the LM386 is no exception. The LM386 can produce around 700mW of power into a 8 ohm load. Common speaker impedances range between 8 and 16 ohms, head-phones might have a bit higher impedance. The point being that the circuit isn't meant to drive a large speaker or produce much power, maybe a 2 watt speaker at best [only because the speaker would be over-rated for protection].
Audio Circuit Enhancements
The amplifier shown above is the most basic circuit possible using the fewest number of components. The amplifier is a single channel Opamp with a simple volume control. A more complex midrange control could be added in place of the single potentiometer. The midrange control adds better frequency response than the single potentiometer, although at the cost of a dual-ganged stereo volume control [100k is available]. With the addition of a switch, or switch-potentiometer combination a loudness control might also be added to add more flexibility.
For best results the potentiometer should be an audio taper and not a linear taper, although depending on the final volume or usage it may not matter. A standard potentiometer changes resistance in a linear fashion as the taper or control is moved across the resistive element. An audio taper, on the other hand changes value in a logarithmic manner. As the wiper is moved, the resistance increases or decreases logarithmically, which follows how the ear hears or responds to sound.
Related topic; Audio Mixer Design using an OpAmp.
The LM386 is also used in other circuit designs on the site, use the search bar to locate additional schematics or design tips.
