Small stereo amplifier to a power a set of desktop speakers for a PC.

Requiring a small stereo amplifier to a power a set of desktop speakers for a PC. The amplifier is built around two LM380 integrated circuits that will deliver about 2 Watts per channel of clean audio.

The LM380 is a classic audio power amplifier integrated circuit that has remained popular for over 40 years. Although originally intended for applications such as intercoms, portable radios, and communications equipment, it also makes an excellent low-power stereo amplifier for a desktop computer.

By using two LM380 integrated circuits, one for the left channel and one for the right channel, it is possible to build a simple, reliable stereo amplifier capable of delivering around 2 watts per channel into 8-ohm speakers from a 12 to 18-volt power supply.

The circuit requires very few external components, making it an ideal project for a quality desktop amplifier.

The LM380 is a monolithic audio power amplifier with several useful internal features including:

• Fixed voltage gain of 34 dB (×50)

• Internal thermal shutdown

• Short-circuit protection

• Low component count

• Good power supply rejection

• Stable operation without external compensation

Circuit description.

The 14-pin LM380N was chosen as opposed to the 8 pin variant because this package is designed to help remove heat from the amplifier. 

Unlike many integrated circuits, the LM380 has eight ground pins (pins 3–7 and 10–12). These pins are all connected together inside the chip and are attached to the metal frame inside the package. This allows heat to flow from the silicon chip into the printed circuit board (PCB).

By connecting these ground pins to large copper areas on the PCB, the board acts like a heat sink, helping to spread and release heat into the surrounding air. This often removes enough heat that a separate metal heat sink is not needed for low- to medium-power applications.

Fig 1  The stereo amplifier schematic

The 22 µF input coupling capacitors (C1 and C7) block any DC voltage while allowing the AC audio signal to pass through to the amplifier. Without them, any DC voltage from the audio source could affect the LM380's input bias and reduce its performance.

Together with the 10 kΩ input resistance created by R1 and R3, the 22 µF capacitors forms a high-pass filter with an approximate cut-off frequency of 0.7 Hz. Since this frequency is well below the human hearing range, all audible bass frequencies pass through the amplifier without noticeable attenuation.

The 10 µF capacitors (C2 and C8) are connected to the bypass pins of the LM380s. They help reduce electrical noise and make the amplifier run more smoothly.

The 0.1 µF capacitors (C3 and C10) are connected across the power supply to filter out high-frequency electrical noise. The 470 µF capacitor (C4) acts like a small energy storage tank, supplying extra current as reqired.

The amplified signal then passes through the 470 µF output capacitors (C5 and C11). These capacitors block DC voltage from reaching the speakers while allowing the AC audio signal to pass through.

Finally, the 27 Ω resistors (R2 and R4) and the 0.1 µF capacitors (C6 and C12) form a Zobel networks. This helps keep the amplifier stable and prevents unwanted high-frequency oscillations that could reduce sound quality.

The purpose of the Zobel network is to keep the amplifier stable, especially at high frequencies. The speaker is not a simple resistor - it also has inductance, which means its electrical resistance changes as the frequency changes. This changing load can sometimes cause the amplifier to become unstable and produce unwanted high-frequency oscillations.

The Zobel network provides the amplifier with a more predictable load at high frequencies. This helps absorb unwanted high-frequency signals, preventing oscillation and improving the overall performance of the amplifier.

Without the Zobel network, the amplifier may still work, but it is more likely to become unstable when connected to certain speakers or long speaker cables.

Construction

The circuit is assembled on single sided Vero-board PCB  

Fig 2  The stereo amplifier components layout for Vero-board construction.

Photo 1  The stereo amplifier Vero-board completed construction.

The PC headphone or line output connects directly to the amplifier inputs using a standard 3.5 mm stereo socket or rear RCA connectors.

Photo 2  The stereo amplifier front showing simple left and right channel gain control, on/off switch, on indicator LED and stereo input jack.

Photo 3  The stereo amplifier rear showing speaker out connection, left and right channel RCA connectors and 12VDC supply plug.

  
Testing Audio Quality

Simple test method used an audio signal generator, an oscilloscope and 8 Ω dummy load (or speaker).

Photo 4  The stereo amplifier out put display on the oscilloscope showing the ideal perfect sine wave.

 

Frequency Response Test
 
The sine wave peak to peak voltage values were recorded from the oscilloscope measurements per the below table and charted as shown in Fig 2.

 Fig 2  The output level remain very constant from 40Hz through to 150kHz easily satisfying the human audio range.
 

The pleasant surprise of the LM380 is its sound quality. Although introduced in 1972, it produces surprisingly clean audio when operated within its design limits.  

The amplifier has a frequency response extending well beyond the audible range. Typical performance is: 20 Hz to over 100 kHz

The practical response is essentially limited by the loudspeakers rather than the amplifier itself.

The LM380 remained only lukewarm after running the amplifier continuously for many hours at a good volume, therefore requiring no further heatsinking.

Conclusion

With a pair of second-hand Sony bookshelf speakers this amplifier sounds fantastic and is way better than the typical basic PC speaker set that I was use to. The LM380 integrated circuit amplifier is perfectly suitable for this task, however if a little more power is required a LM384 chip can be dropped into this circuit with no other changes needed for double the power (About 5 watts).

References  

Texas Instruments LM380 audio power amplifier integrated circuit specifications:

LM380 data sheet, product information and support | TI.com

Texas Instruments LM384 audio power amplifier integrated circuit specifications:

LM384 data sheet, product information and support | TI.com