Electronics
How to build a microphone preamp with an op-amp
A microphone gives only about 20 mV, so a preamp boosts it before anything else can use it. One op-amp does the job from a single 9 V battery: bias its input at 4.5 V so the signal can swing both ways, couple the mic in through a capacitor, and set a gain of 101 with two resistors, about 40 dB.
Bias it at half the supply
On one 9 V supply the output can't go below 0 V, and a typical op-amp can't reach the rails either: say 1.5 V to 7.5 V. Rest the output halfway, at 4.5 V, and it has 3 V of room each way.
Two 100 kΩ resistors, one to +9 V and one to 0 V, hold the + input at 4.5 V. To the audio the battery counts as ground, so the signal sees them in parallel: 50 kΩ.
Couple the mic in
A capacitor passes the voice but blocks DC, so the mic can't upset the bias. With Cin = 1 µF into 50 kΩ:
That's far below the lowest voice, so every note gets through.
Set the gain
The op-amp is a non-inverting amplifier with Rf = 100 kΩ and Rg = 1 kΩ:
20 mV × 101 ≈ 2 V peak, so the output swings from 2.5 to 6.5 V, inside its limits.
Live experiment
Testing, testing
An op-amp on one 9 V battery, biased at 4.5 V, with a gain of 101 for the voice and 1 for DC. Hum, talk, hiss and shout into the mic and watch the output. Then leave out the gain capacitor, or the bias, and see where the output gets stuck.
“Project: Microphone Preamp” is the project that ends Amplifier Peaks, Electronics World 6. Free with an account. Lessons unlock in order, so you’ll do the 41 before it first.
Keep the DC gain at 1
There's a catch: a gain of 101 would multiply the 4.5 V bias too, and 454 V won't fit on a 9 V supply. The output would stick at its top limit.
The fix is Cg = 10 µF in series with Rg. At DC it's an open gap, so the op-amp is a follower and its output rests at 4.5 V. For a voice it's nearly a wire, and the gain is 101 again. Its corner, 1 ÷ (2π × 1 kΩ × 10 µF), is about 16 Hz.
Check the bandwidth
An op-amp's gain-bandwidth product is roughly fixed. With 1 MHz and a gain of 101, the gain holds up to about 1,000,000 ÷ 101 ≈ 9.9 kHz: enough for speech, and a sign that a faster op-amp, or two stages of ×10, would suit music better.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Project: Microphone Preamp” is the project that ends Amplifier Peaks, Electronics World 6. Free with an account. Lessons unlock in order, so you’ll do the 41 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: gets lower. Gain and bandwidth trade off, like a bike's gears. Their product, the gain-bandwidth product, stays about the same, so more gain means a lower corner.
- Why not “stays the same”? It's the gain-bandwidth product that stays the same. Raise the gain and the corner has to fall.
- Why not “gets higher”? Gain and bandwidth trade off. Their product stays about fixed, so more gain pulls the corner down, not up.
Show the answer
Answer: Runs open loop: enormous gain, and the signal is anything but ×2. Feedback is what sets the gain. Without it the op-amp amplifies the difference between its inputs by everything it has.
- Why not “Does nothing”? The op-amp still amplifies, hugely: the slightest difference between its inputs slams the output to a limit.
- Why not “Gives ×1”? ×1 needs the output fed straight back to the − input, as in a follower. With no feedback path, nothing sets the gain.
- Why not “Gives ×2”? The ×2 comes from the feedback resistors. Take one away and only the op-amp's enormous open-loop gain is left.
Show the answer
Answer: 26 dB. ×20 = ×10 × ×2. Multiplying ratios means adding decibels, so 20 + 6 = 26 dB. Doubling the 20 dB for the ×2 would give 40 dB, which is ×100. A ×2 only adds 6 dB.
- Why not “20 dB”? 20 dB is a voltage gain of 10. A gain of 20 is 10 × 2, and the ×2 adds 6 dB more.
- Why not “40 dB”? 40 dB is a voltage gain of 100. A ×2 adds only 6 dB, not another 20.
“Project: Microphone Preamp” is the project that ends Amplifier Peaks, Electronics World 6. Free with an account. Lessons unlock in order, so you’ll do the 41 before it first.
Questions people ask
How much gain does a microphone preamp need?
It depends on the mic and what follows. A dynamic or electret mic gives a few millivolts to tens of millivolts, and line level is around a volt, so 30 to 60 dB is common. A gain of 101, about 40 dB, turns 20 mV into 2 V.
Why bias an op-amp at half the supply?
On one supply the output can't go below 0 V, but audio swings both ways. Resting the output in the middle, 4.5 V on 9 V, leaves equal room above and below, so the waveform isn't clipped on one side.
Does an electret microphone need power?
Yes. An electret capsule has a tiny transistor inside, so it needs a resistor, a few kilohms, from the supply to its output, and a coupling capacitor to pass the audio on while blocking that DC.
Related topics
- Non-inverting amplifier explainedAn op-amp, two resistors and gain G = 1 + R_f ÷ R_g: bigger, in step, and clipped at the rails.
- Op-amp gain-bandwidth product explainedGain times bandwidth stays fixed: ask an op-amp for more gain and its corner frequency falls.
- High-pass filter explainedA capacitor in series that blocks DC and the bass, passes the treble, and couples audio between stages.
- Decibels explaineddB as a ratio on a log scale: 10 log for power, 20 log for voltage, and why gains just add.