Peter's Phantom Powered Condenser Microphone Pre-Amplifier

Another imported page from my old website which I'm winding down.

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!


Summary

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

2N5457 JFET           BC182 NPN

Neutrik NTE4 transformer datasheet

Bill Of Materials

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

Gain at 1kHz = 4.04mV / 500uV = 18.1dB
Measured performance on bench 9.9dB

Frequency Response

   

Finished Assembly Photos

• 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