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.
- 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.
2N5457 JFET BC182 NPN
Neutrik NTE4 transformer datasheet
Bill Of Materials
|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|
Simulations performed using Simetrix
Gain at 1kHz = 4.04mV / 500uV = 18.1dB
Measured performance on bench 9.9dB
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