Phantom Power and Magnetized Transformer Cores

A discussion on the Ampex listserve led to some further thought vis a vis the issue of hot patching (connecting or disconnecting a microphone without first turning off the phantom powering). Although the theory says that you can do this, practice says that you need a few more details before deciding what the practice shall be in your own facility.

This issue at hand is the magnetization of the transformer core caused by unbalanced current pulses through one of its windings. Once this occurs, it is quite difficult (although not impossible) to undo. Once the core has been magnetized, the transformer's low distortion characteristics go by the wayside.

First, let's look at the different ways that this can happen:

  1. Hot patching a phantom powered microphone. If you can guarantee that both pin 2 and pin 3 make at *exactly* the same time, then this is not a problem. Problem is, you can't. This isn't as bad as #2 or #4. If the phantom feed system uses a resistor bridge across pins 2 and 3 of the XLR, then this cause is pretty much a non-issue.
  2. Using a shorted mike cord with phantom power turned on. Varying degrees of problem result depending on the nature of the short and what happens at the instant of plug insertion. Transformerless microphone inputs are more of a hazard in this scenario because the coupling capacitors are (by necessity) large, and in a short circuit condition, they act as 50v batteries capable of considerable current (for a short time).
  3. A dufus with an ohmmeter
  4. Plug in a mike having a grounded center tapped output with phantom power turned on. This is the same mechanism as #1.
So, if your board has input transformers, and/or if the mike has an output transformer, then it's good medicine to turn that phantom power off and wait a bit before plugging things in. If you know for sure that both the board and mike use resistor bridges to apply and get powering to/from the line, then it doesn't matter if you turn the phantom powering off first or not. If this isn't the case, it would be worth the piece of mind to modify the phantom power switching to ensure that there was a discharge path for the input stuff, and/or monitor it with a meter to see how long it takes things to die.

I guess there IS a GOOD reason to have phantom power switches on every input!

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Last modified 04/14/2011. 18:27:16