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Peter's Phantom Powered Condenser Microphone Pre-Amplifier

This is being hosted on behalf of Peter Miles who designed this circuit for his church's sound system and used it for the basis of a presentation on SPICE simulation at work. The pre-amp is in regular use and has proved very reliable.

Words and circuit design by Peter Miles. Any errors are likely to be mine!

An audio pre-amplifier designed to interface an electret condenser microphone to XLR balanced microphone input at mixing desk which provides phantom power.

     

Specification

  • Bandwidth: suitable for good quality speech 100Hz – 10kHz ±3dB
  • Gain: in range 12dB – 20dB
  • Low noise
  • Output impedance << 3kohms (to drive into 3kohms input impedance at mixing desk).
  • Transformer coupled balance output on to pins 2 & 3 of XLR audio circuit.
  • Powered from 48V phantom power coupled on to pins 2 & 3 of XLR audio circuit via 6k8 resistors
  • Provides DC bias in range 1.5V – 5V at input for the electret microphone module.
  • Construction: shielded inside metal box. Box to be grounded to pin 1 of XLR audio circuit.

Some useful information on powering electret microphones

     

Schematic

schematic schematic

2n5457 jfet
bc182 bipolar
2N5457 JFET
BC182 NPN

neutrik nte4 datasheet for transformer

List of key items and sources

Item
 Supplier Part Number Description
Veroboard Farnell
 1201473 CIF AJB16 RBP STRIP PROTOBOARD 100X160
Copper clad plain board
 Farnell
 149054 KELAN PCB, PLAIN S/S 100X160MM
Box for system Farnell 1171664 BOSS ENCLOSURES DIECAST BLACK Length / Height, external:62mm; Width, external:112mm; Depth, external:31mm
JFET Farnell 682366 ON SEMICONDUCTOR 2N5457 TRANSISTOR, JFET N TO-92
Zener Farnell 9844627 FAIRCHILD SEMICONDUCTOR BZX55-C5V6 DIODE, ZENER 500MW 5.6V
NPN Bipolar Farnell 9558462 ON SEMICONDUCTOR BC182G TRANSISTOR, NPN TO-92
XLR Cable Farnell 4259348 LEAD, XLR MIC P-S 1.5M Connector A:3 Way XLR Plug; connector B:3 Way XLR Socket; Length, lead:1.5m;
Transformer CPC LS01463
 NTE4 NEUTRIK AUDIO TRANSFORMER-YELLOW

    

Simulation Results

Simulations performed using Simetrix

gain

Simulation

  • Gain at 1kHz = 4.04mV / 500uV = 18.1dB

Measured performance on bench

  • Vin = 12.3mV pk-pk
  • Vout = 39mV pk-pk
  • Gain = 39/12.5 = 3.12 = 9.9dB
frequency response Frequency Response

     

Finished Assembly Photos

case 1   case 2

  • Copper clad board on back of vero board to provide ground plane under the entire circuit
  • Circuit located inside diecast box by the copper clad board being cut larger than the veroboard and providing tabs that slide into the groves in the box inner walls.