LM358 microphone amplifier
After the unconvincing performance of the LM386 mic amp, another design is put to the test. This time, based around a LM358 operational amplifier.
Op-Amps
Operational amplifiers are high-gain voltage amplifiers. There’s a huge amount of information available on how to use them and the guide I followed is TI’s“Op Amps for Everyone”. The schematic is an “Inverting AC amplifier” (A.3.18, p. 424). From the equations there, the voltages on the two outputs are:- V_OUT_UC = – V_IN * R5 / R4 + VCC / 2 = – 100 * V_IN + VCC / 2
- V_OUT = -100 * V_IN, as the C3 capacitor is blocking the DC component
(V_IN is the voltage at pin 2)
Op amps do not provide a lot of output power. In-ear headphones can be driven from V_OUT, but it’s unlikely that a LM358 will power even a small speaker. For a microcontroller, connected to V_OUT_UC, output power does not matter, only voltage does.
LM358 sound sensor performance
As in the LM386 mic amp setup, V_OUT_UC is connected to Arduino’s A0 pin and the Min-Max sketch is uploaded. By using different R5 resistor values (10K, 47K, 100K), gains of 10x, 47x and 100x are achieved:
| Gain | (Silence) | Loud knock | ||
| Readings | Amplitude | Readings | Amplitude | |
| 10x | 510 ~ 512 | 2 | 307 ~ 735 | 428 |
| 47x | 505 ~ 517 | 12 | 17 ~ 754 | 737 |
| 100x | 500 ~ 525 | 25 | 7 ~ 755 | 748 |
Pretty good! The noise level in silence increases linearly with gain. Moving from 10x to 50x gain gives a good increase in the maximum output swing. Pushing the gain to 100x provides only a minor improvement. Overall, the 50x setup gives the best noise vs. output swing results.
The LM358 does not have a true rail-to-rail (0V to VCC) output. The datasheet specifies an “output voltage swing 0V to (VCC – 1.5V)”. This means that when VCC = 5V, the largest possible analog reading is in the 715 – 750 range. In order to get an wider output range, a better op-amp should be used.
Current draw was 1.52mA, in the 100x setup
LM358 sound sensor on a breadboard (click for high-res)
Parts list
| Part | Value | Description |
| C1 | 10uF | Microphone coupling capacitor |
| C2 | 100nF | Power supply decoupling |
| C3 | 220uF | Output coupling |
| MIC | Electret microphone | |
| R1 | 1 ~ 10K | Microphone load resistor |
| R2, R3 | 10K | Voltage divider: 1/2 VCC |
| R4 | 1K | Gain = – R5 / R4 |
| R5 | 100K | |
| VSS | 3 ~ 30V | Supply voltage |
Downloads
- EAGLE schematic [.sch]
- Min-Max sketch code [.pde]
Related posts
Links
- LM358 Datasheet [.pdf], STMicro
- “Powering microphones” by Tomi Engdahl
- “Op Amps for Everyone”, Texas Instruments
- A similar LM358 mic amp design
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Otro Diseño
http://circuitdiagram.net/
Here the simple mic pre-amp circuit design based on single IC LM358. The circuit is very simple, inexpensive and easy to built. This mic pre-amplifier circuit operated with 9V DC voltage, you may use 9V battery for this circuit.
Component Parts List:
R1, R3, R4 = 10K
R2 = 1K
R5 = 100K-1M Potensiometer
C1 = 0.1uF
C2 = 4.7uF/16V
IC1 = LM358 dual op-amp single supply
Mic = Electret Microphone
Notes:
Simple Mic Pre-Amp based LM358
Component Parts List:
R1, R3, R4 = 10K
R2 = 1K
R5 = 100K-1M Potensiometer
C1 = 0.1uF
C2 = 4.7uF/16V
IC1 = LM358 dual op-amp single supply
Mic = Electret Microphone
Notes:
- Use R5 to adjust the gain of op-amp LM358.
- The LM358 has dual op-amp module, you may build stereo audio pre-amp using single LM358. These devices consist of two independent, high-gain frequency-compensated operational amplifiers designed to operate from a single supply over a wide range of voltages.
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Otro Circuito con TI en pdf
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