Introduction to the Lm386 Amplifier
The LM386 is a low voltage audio power amplifier designed for use in low power consumer applications. It requires minimal external components, making it ideal for simple and compact audio circuits. Some key features of the LM386 amplifier include:
- Voltage gains from 20 to 200
- Very low quiescent current drain (4 mA)
- Low distortion
- Wide supply voltage range (4V-12V)
- Minimum external parts required
- Available in 8-pin mini-DIP and SO packages
The LM386’s combination of low distortion, low power consumption, and ease of use make it a popular choice for small audio projects and devices.
Basic LM386 Amplifier Circuit
Below is the schematic for a basic LM386 amplifier circuit:
+Vs
|
+-+
| |
+-------+ | |
| | | |
| +--+ | | |
+--+--+ | | | | +--------+
| | | | +-+ | |
| 10uF| | | | |
-+ +--+ 10K+---+ IN |
| | | | |
+-----+ | +--+--+--+
+---+ | |
+--+ |
| |
+-----+--> Out
| |
+--+--+
|
=== Gnd
-
This basic configuration provides a voltage gain of 20 (26 dB). The key components are:
- LM386 amplifier IC
- 10uF input coupling capacitor
- 10K ohm volume control potentiometer
- 10uF bypass capacitor between pins 1 and 8
- 250uF output coupling capacitor
- 8 ohm speaker
The input signal is AC-coupled via a 10uF capacitor to block any DC component. The 10K pot acts as a volume control. The 10uF capacitor between pins 1 and 8 helps improve stability and reduce noise. Finally, the amplifier output is coupled to an 8 ohm speaker through a 250uF capacitor.
Adjusting the Gain
The LM386 provides different voltage gain options by connecting certain pins:
| Gain | dB | Pins |
|---|---|---|
| 20 | 26 | None |
| 50 | 34 | 1 and 8 |
| 200 | 46 | 1 and 8 |
To increase the gain to 50, connect a 10uF capacitor between pins 1 and 8. For a gain of 200, leave pins 1 and 8 open. Higher gains allow lower input signal levels but increase noise and risk of feedback.
Power Supply Considerations
The LM386 can operate on a supply voltage range from 4V to 12V. This allows it to be powered from batteries or a DC wall adapter. Some guidelines for the power supply:
- Voltage should be well-regulated
- Supply should be adequately bypassed close to the LM386
- Watch the power dissipation at higher voltages and lower speaker impedances
- At 9V, max output power is 325 mW into an 8 ohm speaker (before clipping)
To minimize noise and hum, use a regulated supply that is well filtered. Bypass the supply with a 100uF capacitor close to pins 6 and 4 of the LM386.
Input Signal Conditioning
For best results, the input signal to the LM386 should be:
- In the range of 2 mV to 50 mV
- AC coupled to block DC
- Low impedance (less than 50K typ.)
- Properly scaled to avoid clipping
Use an input coupling capacitor of at least 1uF, preferably non-polarized. Keep input wiring short and away from power supply and output traces to minimize noise pickup.
If the input signal is too high, distortion will occur as the amplifier output clips. Reduce the input level or input impedance in this case. On the other hand, extremely low input levels (< 2 mV) will have a poor signal-to-noise ratio. Increase the gain or input signal level if possible.
Minimizing Noise and EMI
Proper circuit layout and grounding is critical to achieving low noise operation with the LM386. Some tips to minimize noise, hum, and interference:
- Keep input wiring short and away from supply/output traces
- Make a single-point ground connection, usually at the output coupling capacitor
- Physically separate input and output sections of the PCB
- Use shielded wiring for lowest noise
- Include bypass capacitors on supply pins
- Keep component leads short
- Use capacitors with low ESR for power supply filtering
Carefully evaluate the circuit layout to identify potential noise sources and coupling paths. Breadboarding the circuit first can help identify noise issues before committing to a PCB design.
Bass Boost Circuit
To add bass boost, you can create a low-pass filter network on the input pin 3:
+Vs
|
+-+
0.033| | 15uF
uF| | |
+-------+ | | |
| | | | +--+
+--|--+ +--+ | | | |
| | | | +-+ +-+
| | | | 15K | |
-+ | +-+ +--+---+--> Pin 3
| | | |
+--+ | +-+
+--+ |
| | 10K
+-----+
|
===
-
This bass boost filter has a low frequency gain of approximately 6 dB and a crossover frequency around 100 Hz. Adjust the component values to tailor the bass response as needed.
Improving Stability
In some cases, the LM386 can become unstable and oscillate, especially at higher gains. To improve stability:
- Keep supply leads short and well bypassed
- Isolate input and output sections of the circuit
- Minimize output coupling capacitance
- Use a Zobel network on the amplifier output
A Zobel network consists of a series resistor and capacitor connected from the output to ground. Typical values are 10 ohms and 0.1uF. The Zobel network isolates the amplifier from the effects of speaker impedance variations and long speaker wires that can cause instability.
PCB Layout Tips
When laying out a PCB for the LM386 amplifier, follow these general guidelines:
- Make a single-point ground connection
- Physically separate input and output sections
- Keep traces short and direct, especially for input and output
- Use ground plane to provide shielding between stages
- Provide adequate heatsinking for the LM386 if needed
- Locate bypass capacitors close to supply pins
- Use large, low ESR capacitors for supply filtering
- Keep speaker wires away from input section
Pay close attention to potential ground loops and noise coupling paths. Use a star grounding scheme to avoid creating unintentional signal paths.
Bridged Mode Operation
For more output power, two LM386s can be operated in bridged mode:
+Vs
|
+-----+-----+
| |
+-+ +-+
| | | |
+-------+ +---+ | | | |
| | | | | | | |
+--|--+ +---+ +-+-+ | |
| | | | +-+
| | | +--+--+--+ |
-+ 10uF| Input | | | | |
| | >-----+--------+ | +---+--> Out
| | | +--+ | |
| | | +--+--+--+ +-+
| 10K| | | | |
+--/\/+ +---+ +-+-+ | |
| | | | | | | |
+-------+ +---+ | | | |
+-+ +-+
| |
+-----------+
|
===
-
In this configuration, each LM386 amplifies the input signal but with opposite polarity. This doubles the effective output voltage swing, quadrupling the max output power.
A few things to keep in mind for bridged mode:
- Input signal still sees 10K impedance
- No output coupling capacitors are used
- Load is connected between the two amplifier outputs
- Minimum load impedance is 8 ohms
- Max supply voltage is still 12V
- Max output power is 500 mW at 9V supply
Bridged mode does increase component count and circuit complexity compared to a single-ended design. However, it provides much more output power for driving lower impedance speakers.
Troubleshooting Common Issues
Distortion
If you hear distortion, some things to check:
- Input signal level too high, reduce input level
- Volume control turned up too high
- Supply voltage too low to provide enough headroom
- Speaker impedance too low for the output power
- Loose, damaged, or incorrect component values
- Defective LM386
Low Output Volume
For low or no output volume:
- Check input signal and wiring
- Test with known good input source
- Check that supply voltage is applied and correct
- Use a multimeter to check DC voltages on each pin
- Inspect solder joints for bad connections
- Test speaker and wiring
- Try a different LM386
Noise or Hum
Potential causes of noise/hum:
- Poor circuit layout allowing noise pickup
- Noisy input source
- Lack of shielding on input wiring
- Ground loop between input and output
- Insufficient supply filtering
- Ripple or noise on DC supply voltage
- Failing component, especially capacitors
- Oscillations due to poor layout or improper routing
Careful PCB layout that separates input and output sections and keeps signal wires short is essential for low noise. Use shielded wires for inputs when necessary. Verify all components are in good working condition.
LM386 Project Ideas
The LM386 is a versatile amplifier IC that is useful for countless small-scale audio projects, such as:
- Portable guitar amp
- Desktop/computer speakers
- Headphone amplifier
- Intercoms
- Hearing aid
- Megaphone
- Alarm/beeper
- Musical instruments
- Radios and receivers
- White noise generators
- Audio mixers
- Ultrasonic sensors
- Electronic stethoscope
- Piezo transducer amplifier
Many of these can be built on a small PCB or even a breadboard. Wherever a compact, low power audio amplifier is needed, chances are the LM386 will do the job.
LM386 Amplifier FAQ
Q: What is the maximum output power of the LM386?
A: The LM386 can deliver 325 mW of output power into an 8 ohm load with a 9V supply. Output power can be increased to 500 mW by using two LM386s in bridged mode.
Q: How much voltage gain does the LM386 have?
A: The LM386 can be configured for three different voltage gain settings: 20 (26 dB), 50 (34 dB), and 200 (46 dB). Gain is set by connecting pins 1 and 8, or leaving them open.
Q: What is the input impedance of the LM386?
A: The LM386 has a relatively high input impedance, typically around 50k ohms or more. For best results, the source impedance should be less than 50k.
Q: How much quiescent current does the LM386 draw?
A: The LM386 has a very low quiescent current of just 4 mA. This makes it well-suited for battery powered applications where current consumption must be minimized.
Q: Can the LM386 be used with a single supply?
A: Yes, the LM386 is designed to operate on a single ended power supply ranging from 4V to 12V. For lower supply voltages, output power and headroom will be reduced accordingly.
0 Comments