Phantom Powered Microphone Preamplifier – Low Noise – Professional Quality

The project presented here is a professional quality Phantom powered Microphone pre-amplifier. The project is built using the INA163 chip from TI. The Chip is a very low-noise, low-distortion, monolithic instrumentation amplifier. R2 and R3 provide a current path for a conventional 48V phantom power source for a remotely located microphone. A slide switch SW1 allows phantom power to be disabled. C6 and C8 block the phantom power voltage from the INA163 input circuitry. Additional input protection against ESD and hot-plugging, four 1N4148 diodes D1, D2, D3, D4 connected from the input to supply lines. R7 and R8 provide a path for the input bias current of the INA163. Gain is set with a variable resistor, PR1, in series with R4. R4 determines the maximum gain. The total resistance, R4 + PR1, determines the lowest gain. A special reverse-log taper potentiometer for PR1 can be used to create a linear change (in dB) with rotation. LED D5 is used as a power indicator. L1, L2, C7, C10, and C11 components protect the inputs from EMI and RFI noise.

Optional DC Output Control Loop

Input offset current (typically 100nA) creates a DC differential input voltage that will produce an output offset voltage. This is generally the dominant source of the output offset voltage. With a maximum gain of 1000 (60dB), the output offset voltage can be several volts. This may be entirely acceptable if the output is AC-coupled into the subsequent stage. An optional circuit is provided to tackle this problem. An inexpensive FET-input op-amp U2A in a feedback loop drives the DC output voltage to 0V. Op-Amp is not in the audio signal path and does not affect signal quality.

Install the Following Components for DC Output Control Loop

• U2A = OPA2134 SOIC8
• C9 = Ceramic Capacitor 0.1uf/50V SMD Size 0805
• R6 1Mega Ohm 5% SMD Resistor Size 0805
• Omit Resistor R5

Connections and Other Details

• CN1: Pin 1 = +48V DC Phantom Power Input, Pin 2= GND
• CN2: Pin 1-2 = VCC +15V DC, Pin 2-3 = GND, Pin 4-5 VEE -15V DC
• CN3: Pin 1 = BB Microphone – VIN, Pin 2 = GND, Pin 3 = AB Microphone + VIN
• CN4: Pin 1 = Audio Output, Pin 2 = GND
• D5: Power LED
• SW1: Phantom Power Enable/Disable Switch

Features

• Power Supply Dual 15V DC (+/-15V DC)
• On Board Power LED
• Slide switch for Phantom Power Enable/Disable
• 3 Pin Screw Terminal for Microphone Connections
• 2 Pin Screw Terminal for Audio Output
• 2 Pin Screw Terminal for Phantom Power Input 48V DC
• 6 Pin Male Header for Power Input
• Gain Up to 1000/V (60dB)
• Potentiometer PR1 Gain Adjust
• PCB Dimensions 60.17 x 39.85mm
• 4 x 3mm Mounting Holes

Schematic

Parts List

Connections

Gerber View

Photos

Video

Datasheet

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