The 48 Volt Phantom Menace

The 48 Volt Phantom Menace (153k)
By Gary K. Hebert and Frank Thomas, THAT Corporation
AES Convention Paper 5335, 110th Convention, May, 2001

The authors encountered anecdotal evidence suggesting that field failures of existing line driver and microphone preamplifier integrated circuits (ICs) were correlated with accidental connections between line outputs and microphone inputs with phantom power applied. Analysis showed that the most probable mechanism was large currents flowing as a result of rapid discharge of the high-valued ac-coupling capacitors. Commonly used protection schemes are measured, analyzed, and shown to be lacking. More robust schemes that address these shortcomings are presented. It is concluded that the small additional cost of these more robust protection schemes is likely outweighed by the reduction in field failures and their associated repair cost.