PC headset to XLR phantom voltage adapter

This adapter makes it possible to connect a standard PC headset or microphone to a professional audio interface with XLR connectors and 48V phantom power. In my case the noise floor of the microphone was improved by about 30dB over the standard microphone inputs of my laptop and PC.

Microphones intended for use in conjunction with a laptop or standard pc sound card usually come with a 3.5mm TRS style jack and take 2.5V – 5V of phantom voltage. To use them on the professional XLR style microphone connections not only the pulg type has to be adapted, but also the phantom voltage has to be reduced from the professional 48V level down to 3.3V. This circuit works well from 12V to 65V phantom power.

Here is the schematic:

Microphone phantom adapter: 48V to 3V conversion. Schematic
Schematic of the XLR to TRS, 48V phantom voltage to 3.3V adapter, used to connect PC headsets to professional audio interfaces

A few notes:

  • The Frequency response starts at 2.5Hz. (-3dB point if the interface provides only a 2kOhm input impedance. Even lower for higher input impedances.)
  • On my headset the noise floor is about 45dB below the normal speech level.
  • Everything fits into a standard XLR connector.
  • I first tried an LDO regulator but they are way too noisy without extensive filtering. Extensive filtering is too big to neatly fit inside an XLR connector. An emitter follower with a Z-Diode for reference is better by several orders of magnitude.
  • I went with a 5.1V Zener diode because this is the first diode in the series which voltage is very constant over a large range of currents. The large range is needed in order to provide a good stability over a large range of input voltages. If possible use this specific type.
  • The two LL4148 together witht he base-emitter voltage reduce the output voltage to about 3.3V.
  • The 10kOhm resistor in parallel to the 100µF output capacitor provides a base load, so that the LL4148s and the BE-juntion are biased into their steep region, i.e. the voltage drop over them is stable for any output load.
  • The ZPD75 between the signal coming from the mic and GND is for protecting the microphone when the 48V phantom voltage changes very abruptly. You might want to use lower values (e.g. 5.1V). The 100kOhm is for DC level biasing of the Zener in case the mic has a decoupling cap in the signal line.
Microphone phantom adapter in action
The finished adapter in action.
Microphone phantom adapter: 2x 3.3µF + 2x10k
2x 3.3µF + the two 10k resistors fordecoupling the 48V phantom power
Microphone phantom adapter: Zener diode
5.1V Zener disode + 22nF connected to GNd (Pin 1)
Microphone phantom adapter: Transistor and base-collector resistor
Transistor + base-collector resistor (2x 100kOhm in parallel)
Microphone phantom adapter: 100uF output capacitor + 10k load
100µF output filter capacitor with 10kOhm base load resistor in between its legs
Microphone phantom adapter: 2x 47uF phantom block
47µF 63V decoupling caps
Microphone phantom adapter: 2.2k output resistor and 2xLL4148
2.2kOhm resistor as required for the PC headset / microphone. The two LL4148 are connected using enameled wire to keep mechanical stress off of them
Microphone phantom adapter: Signal connection
Connecting the signal on the back side
Microphone phantom adapter: Finalized
Add some isolation tape and glue the wire to the capacitors (reduces mechanical stress)
Microphone phantom adapter: The finished product
The completed adapter

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