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OpenRefMic is an open hardware design for a reference microphone that fits in the popular 1/2” reference microphone form factor, works with consumer microphone interfaces, and meets or exceeds the performance of professional reference microphones at a vastly reduced total system cost.

Critical specs, performance
- Frequency Response: 10Hz-25kHz (±1dB after EQ)
- Noise Floor: 18dBA
- Acoustic Overload Point: 118dBSPL
- Dimensions: 12.7x99mm
- Parts cost: $40 (not including 3D printed parts)
- Interface: mini-XLR, 48V phantom power
- Total system cost: <$250 (with Focusrite Scarlett 2i2)

The core of the OpenRefMic design is a preamplifier that biases an electret microphone from 48V phantom power and buffers the microphone signal to send it to a standard microphone interface. The circuit was designed for the PUI AOM-5024L-HD-F-R low noise microphone capsule, and has been built and tested with that part, but should work with most other electret mics. The schematic and PCB layout were done with KiCAD and are available in the Preamplifier section of the project, along with the BOM for all electrical and mechanical parts.

OpenRefMic construction is simple. The case is 3D printed with any standard material, and the mic capsule is held in place with a press-fit grille printed in TPU. Threads modeled into the case mate with the threads of a mini-XLR connector on the back of the mic body, and everything is held together with 4 small screws. CAD in Fusion360 and STEP formats are in the Construction section, along with STLs for the printed parts and a step-by-step construction guide.

The performance of the complete OpenRefMic prototype is excellent (after applying equalization). Raw measurement data, compensation curves, and recommendations for applying calibrations to measurements and recordings are in the Calibration section.”

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