https://github.com/dunkelstern/electret_preamp

Electret Microphone Preamp based on Texas Instruments Single-Supply, Electret Microphone Pre-Amplifier Reference Design by John Caldwell
https://github.com/dunkelstern/electret_preamp

electret jupyter-notebook kicad microphone pcb-layout preamp schematic

Last synced: about 1 year ago
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Electret Microphone Preamp based on Texas Instruments Single-Supply, Electret Microphone Pre-Amplifier Reference Design by John Caldwell

• Host: GitHub
• URL: https://github.com/dunkelstern/electret_preamp
• Owner: dunkelstern
• Created: 2020-05-31T00:15:29.000Z (almost 6 years ago)
• Default Branch: master
• Last Pushed: 2020-06-02T10:20:34.000Z (almost 6 years ago)
• Last Synced: 2025-03-27T04:41:34.633Z (about 1 year ago)
• Topics: electret, jupyter-notebook, kicad, microphone, pcb-layout, preamp, schematic
• Language: Jupyter Notebook
• Size: 6.29 MB
• Stars: 5
• Watchers: 2
• Forks: 3
• Open Issues: 0
• Metadata Files:
  • Readme: Readme.md

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README

          
# Electret Microphone Preamp

This design is purely based on [Texas Instruments Single-Supply, Electret Microphone Pre-Amplifier Reference Design by John Caldwell](http://www.ti.com/lit/ug/tidu765/tidu765.pdf?ts=1590780945605)

The Jupyter notebook is basically a interactive Version of that reference design PDF. I claim no copyright on this.

## What you will find here

### Jupyter Notebook

The included Jupyter Notebook is an interactive version of the reference design PDF from Texas instruments. You can

play with all values and let the program re-calculate all relevant values to be used in your own design.

There are some areas that will be most important:

- Operating Voltage of the Preamp

![Operating Voltage](img/intro_vcc.png)

- Parameters of the Microphone capsule in use

![Microphone parameters](img/intro_mic.png)

- Acceptable high-frequency attenuation

![Acceptable Attenuation at high frequencies](img/intro_attenuation.png)

- Acceptable low-frequency cutoff in output highpass filter

![Output capacity](img/intro_highpass.png)

- The List of passive component values

![Passives](img/intro_passives.png)

- The Op-Amp parameters to look out for

![Op-Amp Parameters](img/intro_opamp.png)

### LT Spice simulation

I have simulated the circuit in LTSpice with some different Op-Amps I had in stock.

I deviated from the `270k` resistor in the Op-Amp feedback loop and opted for a `180k` value. This

means the amplification of the Op-Amp will not reach `0dB` (max. seems to be around `-1.8dB`) but the circuit seems

to run more stable at that amplification.

Here are some bode-plots of the frequency response for some tested Op-Amps:

#### OP07

![Bode plot for OP07](img/bode_plot_op07.png)

The OP07 is the worst possible of the bunch. The bad response is a product of the sub-optimal slew rate of ![0.3V per uS](https://render.githubusercontent.com/render/math?math=0.3%5Cfrac%7BV%7D%7B%5Cmu%20S%7D).

Additionally this Op-Amp will add noise to the signal as it's input voltage noise is around ![10](https://render.githubusercontent.com/render/math?math=10\frac{nV}{\sqrt{Hz}}). Ideally

it should be below ![6.6](https://render.githubusercontent.com/render/math?math=6.6\frac{nV}{\sqrt{Hz}}) as we can see in the calulations.

#### TL072

![Bode plot for TL072](img/bode_plot_tl072.png)

The TL072 is a bit better than the OP07. The slew rate of ![16V/uS](https://render.githubusercontent.com/render/math?math=16%5Cfrac%7BV%7D%7B%5Cmu%20S%7D) is at least enough.

This Op-Amp will add a lot of noise to the signal as it's input voltage noise is around ![15](https://render.githubusercontent.com/render/math?math=15\frac{nV}{\sqrt{Hz}}). Ideally

it should be below ![6.6](https://render.githubusercontent.com/render/math?math=6.6\frac{nV}{\sqrt{Hz}}) as we can see in the calulations.

#### NE5532

![Bode plot for NE5532](img/bode_plot_ne5532.png)

The NE5532 is really good at this. All parameters are within the parameter range we calculated.

#### LM318

![Bode plot for LM318](img/bode_plot_lm318.png)

You can see why the LM318 is used for these applications a lot, as it is a dedicated Audio-Op-Amp the response from

this thing is the best of the bunch.

**Attention**: The `LM318` needed `5.3V` operating voltage to actually do anything in the simulation, I have not

verified if this is an artifact of the simulation or an actual problem!

### KiCAD design

The KiCAD design is based on surface mount components to save as much space as possible. You could use tantalum

capacitors instead of the electrolyte I used here, but they are expensive and I don't have them in stock.

![KiCAD schematic](img/schematic.png)

The board design is optimized for space and to be single sided as it is easier to prototype this way.

I will probably add a better two sided design in the future which will be optimized for noise performance and

will probably get even smaller (but I have to order the PCBs then)

![KiCAD board layout](img/board_layout.png)

This is what the board looks like rendered, I will add a photo of the finished product when I have built one.

![KiCAD board render](img/board_render.png)