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Dynamic microphones do not need phantom power. Err... this one does!

It is a known fact of sound engineering that dynamic microphones do not need phantom power. But there is one that does. Why?


One of the first things any aspiring sound engineer learns is that dynamic microphones do not need power; capacitor (condenser) microphones do.

The reason why a dynamic microphone does not need power is because it generates its own electricity. The capacitor microphone uses a different principle of operation that requires electricity to work.

So that seems like an advantage for the dynamic microphone. But along come microphone manufacturers Blue with The Ball, a microphone with a name, not a number, and yes it does require phantom power.

In this case, phantom power is used to power an internal amplifier, similar to that found in capacitor microphones (although capacitor microphones would still require power to work, even if the amplifier could run on thin air).

But normally, a dynamic microphone is considered to produce a strong enough signal for any practical length of cable. Why should an amplifier be necessary?

The answer is in the output impedance of the microphone. Output impedance is a measure of the output's ability to supply current. Lower output impedance is better.

For preference, any output should have an output impedance that is as low as possible so it can drive anything easily. But even if the output impedance isn't all that low, the one thing it should be is consistent across the frequency range.

Guess what? The average dynamic microphone has an output impedance vs. frequency curve that is about as flat as the Himalayas. The output impedance changes widely with frequency.

Now, if the microphone was connected to a high impedance input that doesn't require much current, then all would be well. There would be enough current at all frequencies.

However, this degrades noise performance. So there is a less-than-desirable scenario where a microphone with a varying output impedance is connected to an input that demands quite a lot of current.

The result is an uneven frequency response. So even if the microphone measures fine when connected to test equipment, in practice the frequency response will be poor. What's more, it will be different according to what preamplifier the microphone is connected to.

Having an internal amplifier solves this at a stroke. The output impedance can be as constant as you like, and absolutely comparable with capacitor microphones.

Capacitor microphones have always had the upper hand over dynamics. But for Blue's The Ball microphone, that is one advantage that is simply swept away.

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By David Mellor Sunday September 4, 2005

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