01
Input and volume
RV1 is a 10 kΩ potentiometer between VIN and ground. Its wiper feeds pin 3; pin 2 is grounded. Start with the volume down and keep the signal at pin 3 inside the ±0.4 V input rating.
Audio Amplifiers
LM386 Audio Amplifier with Bass Boost
TI Figure 9-9 redrawn as an editable LM386 bass-boost amplifier: a 10 kΩ and 33 nF feedback branch, volume control and an 8 Ω speaker powered from a 9 V battery.
- Operating Supply Voltage
- 9 V battery as drawn; 5 V to 12 V design range (TI Table 9-5)
- Components
- 9
- Formulas
- 3
- Gotchas
- 4
Circuit overview
- Operating Supply Voltage
- 9 V battery as drawn; 5 V to 12 V design range (TI Table 9-5)
- Components
- 9
- Formulas
- 3
- Gotchas
- 4
Click any component in the schematic above to inspect its electrical ratings, function, and connections.
Circuit overview
This circuit follows Figure 9-9 of the Texas Instruments LM386 datasheet SNAS545D. A series 10 kΩ resistor and 33 nF capacitor connects pin 1 to the amplifier output at pin 5, before the output coupling capacitor. It changes the feedback with frequency, giving bass more prominence relative to the midrange and treble. Pin 8 stays open: this is not the separate gain-200 circuit. The 9 V battery and 8 Ω speaker are explicit choices within Table 9-5. The optional, unvalued BYPASS capacitor shown dashed by TI is omitted. TI recommends local supply decoupling but does not specify its value here. Application performance must be checked in the final build.
Key Technical Specifications
| Operating Supply Voltage | 9 V battery as drawn; 5 V to 12 V design range (TI Table 9-5) |
|---|---|
| Voltage Gain | Frequency-shaped by R2/C3; pin 8 open; no gain-200 bypass (TI Figure 9-9) |
| Load | 8 Ω speaker as drawn; design range 4 Ω to 32 Ω (TI Table 9-5) |
| Output Power | 325 mW typical, 250 mW minimum at VS = 6 V, RL = 8 Ω, THD = 10% (LM386N-1); the datasheet does not specify LM386N-1 output power at 9 V |
| Quiescent Current | 4 mA typical, 8 mA maximum at VS = 6 V, VIN = 0 (TI §6.5) |
| Input Voltage at Pin 3 | −0.4 V to +0.4 V (absolute maximum and recommended range) |
| Low-Frequency Corner | ≈ 80 Hz, calculated from C1 = 250 µF and an 8 Ω load; not measured |
| Bass Feedback Branch | R2 = 10 kΩ, C3 = 0.033 µF (33 nF), in series between pins 1 and 5 |
Theory of Operation & Working Principle
02
Frequency-shaped feedback
R2 (10 kΩ) and C3 (0.033 µF) form a series path from pin 1 to pin 5. TI describes this path as paralleling the internal 15 kΩ feedback resistor. At low frequencies C3 impedes the added path; at higher frequencies it conducts more feedback, reducing gain relative to the bass region. This is a relative tone contour, not extra output-power capacity.
03
Pin 8 and stability
Pin 8 is open. TI specifies 10 kΩ as the lowest feedback resistance for good stable operation in this configuration. The separate capacitor-between-pins-1-and-8 modification for gain 200 is not fitted. The component RC benchmark is about 482 Hz; this is not the complete amplifier response or a measured bass cutoff.
04
Output coupling
Pin 5 biases to approximately half the supply, about 4.5 V at 9 V. C1 (250 µF), positive towards pin 5, blocks that DC from LS1. With an ideal 8 Ω load its calculated high-pass corner is about 80 Hz, so very low bass is still attenuated.
05
Output RC network
C2 (0.05 µF) and R1 (10 Ω) form the separate series output-to-ground network drawn by TI. Keep it distinct from the pin-1 feedback branch; R2 returns to pin 5 before C1, not to the speaker terminal after C1.
06
Supply and bypass
The drawing uses a 9 V battery; Table 9-5 gives 5 V to 12 V and loads from 4 Ω to 32 Ω. Pin 7 is left open, omitting the optional dashed BYPASS capacitor. Add local pin-6-to-ground decoupling and follow TI layout guidance; the reference does not assign its capacitance.
Design Equations & Calculations
Feedback RC Benchmark
fRC = 12π R2 C3
Calculated frequency at which the magnitude of C3 reactance equals R2: about 482 Hz for 10 kΩ and 33 nF. This is a component benchmark, not the full bass-response cutoff.
| Symbol | Parameter | Typical value |
|---|---|---|
| R2 | Feedback resistor | 10kΩ |
| C3 | Feedback capacitor | 33nF |
| fRC | RC benchmark (calculated) | 482Hz |
Output Coupling Corner Frequency
fc = 12π RL C1
Frequency below which C1 and the speaker attenuate the signal (−3 dB), treating the speaker as a pure 8 Ω resistance.
| Symbol | Parameter | Typical value |
|---|---|---|
| RL | Speaker impedance | 8Ω |
| C1 | Output coupling capacitor | 250µF |
| fc | Corner frequency (calculated) | 80Hz |
Output Bias Voltage
VOUT(DC) ≈ VS2
The output biases itself to about half the supply voltage; C1 blocks this DC level from the speaker.
| Symbol | Parameter | Typical value |
|---|---|---|
| VS | Supply voltage | 9V |
| VOUT(DC) | Output bias voltage | 4.5V |
Bill of Materials (BOM)
Every part needed to build this circuit. Select a reference designator to inspect it on the schematic.
| RefDes | Component | Value / rating | Package | Role in circuit | Reference |
|---|---|---|---|---|---|
| Low-Voltage Audio Power Amplifier TI LM386N-1/NOPB. LM386N-3 and LM386N-4 share the pinout but have different supply and output-power ratings. | LM386N-1 | PDIP-8 | Audio power amplifier with frequency-shaped feedback | Low-Voltage Audio Power Amplifier pinout & datasheet | |
| Rotary Potentiometer | 10kΩ | Panel / PCB mount | Volume control and input attenuator | — | |
| Aluminium Electrolytic Capacitor Datasheet value. 220 µF is the nearest common value and moves the calculated corner to about 90 Hz. Voltage rating above the supply; + towards pin 5. | 250µF | Radial | Output coupling to the speaker | — | |
| Ceramic or Film Capacitor Datasheet value; 47 nF is the nearest common value. | 0.05µF | Radial / 0805 | Output RC network | — | |
| Resistor 1/4 W | 10Ω | Axial / 0805 | Output RC network | — | |
| Loudspeaker 4 Ω to 32 Ω per TI Table 9-5. | 8Ω | Wired | Load | — | |
| 9 V Battery Any supply from 5 V to 12 V (TI Table 9-5). | 9V | PP3 / 6LR61 | Supply | — | |
| Feedback Resistor TI Figure 9-9 value. With pin 8 open, TI specifies 10 kΩ as the minimum for good stable operation. | 10kΩ | Axial / 0805 | Series bass-feedback branch | — | |
| Feedback Capacitor 33 nF, the 0.033 µF of TI Figure 9-9. Non-polarized. | 0.033µF | Radial / 0805 | Frequency shaping in series with R2 | — |
Practical Applications & Target Domains
AM-FM radio amplifiers
Intercoms
TV sound systems
Battery-powered speaker output for portable audio projects
Critical Design Traps & Engineering Gotchas
Traps found in real builds, with the fix for each.
Overdriving the Input
The absolute maximum input voltage is ±0.4 V. A headphone or line-level signal can be larger; fed straight to pin 3 it can exceed that rating, and the output clips well before the input reaches it.
Mitigation
Keep RV1 between the source and pin 3 as drawn and start with the volume turned down; keep the signal at pin 3 within ±0.4 V.
Missing Supply Decoupling
Figure 9-9 draws no capacitor on the supply, but TI recommends one close to pin 6. Without it, long battery leads and supply ripple reach the output as hum or instability.
Mitigation
Add a capacitor from pin 6 to GND right next to the IC (the datasheet does not give a value) and keep the speaker leads short (TI §10, §11.1).
C1 Fitted Backwards
Pin 5 sits at about half the supply, so C1 has a steady DC voltage across it. An electrolytic fitted the wrong way round is reverse-biased.
Mitigation
Fit C1 with its positive terminal towards pin 5 and the negative terminal towards the speaker, and use a voltage rating above the supply voltage.
Feedback Resistance and Pin 8
A smaller R2 with pin 8 open can reduce the closed-loop gain below the range for which the LM386 is compensated. TI gives 10 kΩ as the minimum for good stable operation in this arrangement.
Mitigation
Keep R2 at 10 kΩ or above with pin 8 open, as in Figure 9-9. Connect the RC branch to pin 5 before C1. Do not combine it with a gain-200 modification without redesign and validation.
Primary sources
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