IntermediateAudio
LM358 Line-Level Audio Preamp
Build a low-voltage non-inverting preamp for a small line-level signal. It boosts voltage, not speaker power.
Parts from the database
- LM358P × 1Dual op amp; one section provides gain
Other supplies
- breadboardCommon build supply
- 9 V supplyCommon build supply
- 10 kΩ resistors ×3Common build supply
- 47 kΩ resistorCommon build supply
- 100 kΩ resistorCommon build supply
- 1 µF input capacitorCommon build supply
- 10 µF output capacitorCommon build supply
- 100 nF bypassCommon build supply
- powered line inputCommon build supply
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Connections
- DIP-8 pin 8 +9 V and pin 4 ground; bypass. Make a 4.5 V reference from two 10 kΩ resistors between rails and bypass midpoint with 10 µF.
- Amp A: pin 3 non-inverting input, pin 2 inverting, pin 1 output. Bias pin 3 to 4.5 V through 100 kΩ and AC-couple input through 1 µF, positive toward pin 3.
- Connect 10 kΩ from pin 2 to 4.5 V and 47 kΩ from pin 1 to pin 2; nominal gain is about 5.7.
- AC-couple pin 1 through 10 µF to a powered line input, positive at pin 1. Never connect a passive speaker. Tie unused amp B as a grounded-input follower.
Build steps
- Measure the 4.5 V reference first.
- Build the feedback and verify output rests near mid-supply with no input.
- Use a quiet source and powered speaker/recorder; raise level gradually.
- Reduce source level if the waveform clips.
Software and firmware setup
This is a hardware-only circuit; no firmware is required.
Test your build
An amplified AC signal is centered at 4.5 V before the output capacitor; this is line-level voltage gain, not speaker power.
Troubleshooting
- Output at a rail: check reference and pins 2/3.
- Hum: shorten input wiring and share source ground.
- Distortion: lower input level; output cannot reach the positive rail.
Safety and limitations
Use isolated 9 V and line-level sources only; do not connect mains-referenced signals or a raw speaker.
Manufacturer and platform documentation
Community Notes for This Project
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