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The 48-Volt Phantom Menace Returns

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[Feature] In a paper presented at the 110th AES Convention, Hebert and Thomas described the “phantom menace,” wherein phantom power faults can damage audio input circuitry. Their approach focused on the analysis of a “common-mode fault” occurring at the microphone preamplifier inputs, which was described as a “fault at both inputs, simultaneously.” However, this is not the only fault condition that can occur. New fault mechanisms are considered in this paper, and we show that commonly used protection schemes for popular integrated microphone preamplifiers do not always protect the preamplifiers as expected.

JAES Volume 58 Issue 3 pp. 197-213; March 2010
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David Josephson
David Josephson

Comment posted April 6, 2010 @ 15:43:17 UTC (Comment permalink)

We're glad to see more discussion of this problem, and Ros Bortoni's paper gives good solutions that more console and mic preamp makers should observe. However the problem occurs not only when one side of the mic cable is shorted due to a fault, or when a line level output is connected with a mic input. It is also experienced when a microphone is plugged in to phantom-powered input, or disconnected from the input, while the phantom power is on.

The additional problem is that XLR connectors are not precisely symmetrical. Either pin 2 or pin 3 of the microphone cable connector will contact its mate on the preamp first, with the other contact being made a fraction of a second later. If the microphone draws very little current, this only results in a loud thump in the output because the transient current, hence the voltage spike, is limited. But if the microphone draws a lot of current as soon as it's connected (many currently available phantom-powered microphones from a variety of manufacturers exhibit this behavior), this can result in a differential spike of nearly the full phantom voltage across the differential input, with almost as much current as the shorted-contact simulation mentioned in the article. Even with low-current microphones, it can be a problem too when long cables are used; the surge current caused by charging the cable capacitance can be substantial for an instant. Some mic preamps are well protected from this condition, and the methods in the paper provide reliable means to test this. But many high performance mic preamps are not sufficiently well protected. The practice should always be to turn off phantom power and let the capacitors discharge for a few seconds before unplugging or plugging in a microphone cable.

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