Introduction: Electret Mic Preamp

By Bantam Tools
About: Bantam Tools desktop CNC machines provide professional reliability and precision at an affordable price. (Bantam Tools was formerly Other Machine Co.)

In this project guide, we show you how to use your Desktop PCB Milling Machine to make an electret mic preamp. This circuit, based on an LM358 operational amplifier, is essential for a number of audio-reactive projects. Let’s dive in!

Step 1: Gather Your Tools, Materials, and Files

TOOLS

MATERIALS

FILES

Step 2: Set Up Your Job

Turn on your Desktop PCB Milling Machine, open the Bantam Tools Desktop Milling Machine Software, and home the mill. Next, install the alignment bracket onto the spoilboard. In the Bantam Tools software, under Fixturing, select Locate and follow the instructions on the screen. Using the alignment bracket will ensure that your board is perfectly squared.
Note: If you haven’t installed the alignment bracket before, follow the steps in this support guide.

Now it’s time to set up your job. We’re going to run through these steps quickly. If you need more guidance on how to load your tool or enter information into the software, refer to the Light-Up PCB Badge project.

  1. Double-check to make sure it says Bracket under Fixturing.

  2. Select the 1/32” Flat End Mill, load it into the mill with the bit fan attached, and locate the tool.

  3. In the Material Setup tab measure and enter dimensions in the X, Y, and Z values under Material for the FR-1.

  4. Then apply high-strength, double-sided tape on the backside of the PCB, and place it onto the spoilboard so it aligns with the corner of the alignment bracket.

  5. Be sure to account for the tape’s thickness (Z height) in Material Offset Z. The thickness of the high-strength, double-sided Nitto tape we sell in our store is typically 0.006 in.

Step 3: Import Your File

Under Plans, click Open Files and select the Electret-Mic-Preamp.brd file. Then, select the 1/32" Flat End Mill and the 1/64” Fat End Mill. Your mill time will vary depending on the speeds and feeds recipes you use. For this project, we used the following recipes.

For the 1/32" flat end mill:

  • Feed Rate: 59 in/min
  • Plunge Rate: 15 in
  • Spindle Speed: 25,000 RPM
  • Stepover: 49%
  • Pass Depth: 0.010 in

For the 1/64” flat end mill:

  • Feed Rate: 50 in/min
  • Plunge Rate: 15.00 in
  • Spindle Speed: 25,000 RPM
  • Stepover: 50%
  • Pass Depth: 0.007 in

If you’d like to adjust your speeds and feeds to match ours, click File > Tool Library > Add Tool. Name your new tools and then input the speeds and feeds recipes. Learn more about customizing your Tool Library in our dedicated guide.

Step 4: Start Milling

When you’re ready, click Start Milling.

Remember, this job will require a tool change. When prompted, install your 1/64” flat end mill with the bit fan attached, locate the tool, and select Start Milling to finish your circuit board.

Step 5: Solder the Components

Time to solder! Grab your components and your soldering iron. It’s easiest to solder the components in the following order:

  1. 100K, 10K, and 1K resistors
  2. 0.1uF ceramic capacitors
  3. Electret mic
  4. 2x4 dip socket
  5. LED
  6. 10 uF electrolytic capacitors
  7. 1x2 screw terminal
  8. Potentiometer

Step 6: Design Notes

While this is a dual operational amplifier, you’re only utilizing one of the amplifiers on Pins 1, 2, and 3 in this circuit (see schematic below). Pin 4 is the ground and pin 8 is VCC. Pin 8 is powered by a 5V source through two capacitors (C2 and C3) for filtering out high and low-level noise.

The electret mic is powered through a 10K resistor (R1) where the signal goes through a coupling capacitor (C1) and the 1K resistor (R2) before entering Pin 2. A 200K variable resistor (VR1) serves to create feedback, and the relationship between R2 and VR1 establishes the potential gain of the signal being. This is represented by Gain = VR1/R2. There is a potential gain of up to 200x of the original signal (200K/1K = 200) in this circuit, allowing for a wide range of adjustment. Two 100K resistors (R4 and R5) are connected to GND and VC prior to connecting to Pin 3 for setting the potential voltage (2.5V). The circuit also has an LED to show when the unit is powered on.

Step 7: Start Recording!

You’ve done it! You’ve built your own electret mic preamp using your Desktop PCB Milling Machine. Record a drum solo. Build a voice-activated light. Sky’s the limit!