Low-noise phantom microphone supply uses capacitor “trick”

May 11, 2020 //By Thomas Mosteller, Christopher Jarboe
Ultra-Low-Noise Phantom Microphone Supply Uses Tiny DC-DC Converter Plus Capacitor “Trick”
A mini dc-dc boost converter and filter scheme using a clever arrangement of capacitor and high-beta transistor provides the 48-V phantom power source needed for a condenser microphone.

Professional condenser microphones require a 48-V supply to charge the internal capacitive transducer and power the internal buffer for the high-impedance transducer output. You can use a simple boost converter, a filter circuit to reduce electromagnetic interference (EMI), and a little trickery to build a compact, ultra-low-noise, phantom microphone supply (48 V) that operates from a 5-, 12-, or 24-V input. This voltage is often provided by what’s called a “phantom” supply arrangement, which uses the existing microphone connections to deliver power, so no additional supply leads are needed.

(Note that the condenser microphone should not be confused with the electret microphone, which sometimes employs a phantom-power source but of a different kind for its internal preamplifier. If you’re not familiar with the principle and application of phantom power for condenser microphones, see the References at the end for various perspectives.)

This phantom supply is low current, typically only a few milliamps. However, it must be very low noise because the microphone’s output levels are quite low and the microphone buffer doesn’t have very good power-supply ripple rejection. In addition, the phantom microphone supply must not inject EMI into adjacent low-level circuits, which is always a challenge in tightly packed products.

This very high-performance supply is built using the LT8362 boost converter, which features a 60-V, 2-A switch and operation up to 2 MHz, and comes in a package as small as 3 × 3 mm. The circuit is based on the standard LT8362 DC2628A demo board (Fig. 1) .

 1. This demonstration circuit, DC2628, is the basis for the phantom power supply.
1. This demonstration circuit, DC2628, is the basis for the phantom power supply.

 

The input EMI filter on the demo board, aided by the switching inductor, does a good job of taking care of high-frequency noise that appears in series with the input. The situation isn’t as good on the output. The output EMI filter effectively suppresses noise in the megahertz region, but it has little effect on noise


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