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I have a pair of phantom powered small diaphragm microphones. However, they output unbalanced signal (pins 1 and 3 are shorted) and I've had intereference / whine on my output. I am considering to purchase a DI box to make the signal to the mixer unbalanced and more resilient to external offending radio waves.

Question: Can I expect the DI box to provide the phantom power also to the microphone it is connected to (while itself being powered by it as well)?

I've searched the web and read the manuals of some products but haven't found anything so far.

EDIT

I'm using low-end t.bone EM 700 microphones and a (rather versatile) Behringer mixer FX1622. If I swicth off phantom on the board, the mics go quiet. On the mics, pins 1 and 3 definitely connected to each other, tested both with multimeter (I guess the mixer possibly detects this). If I google "phantom power wiring", one can see that Phantom power is applied to pins 2 and 3. My guess is, the mic uses just two wires. DI box has two jobs: converting line level high impedance input to mic-level low impedance input AND converting 2-wire unbalanced input (from the device) to 3-wire balanced output. Since my mics apparently use 2 wires only, I had hopes it could also pass on the phantom voltage.

  • What kind of mixer or mic preamps are you using? Have you tried moving things around to get rid of the noise? Is there anything else connected that could be causing a ground loop? As Laurence wrote, a direct box would be useless in this situation. You couldn't even plug a mic into the direct box. – Todd Wilcox Apr 1 '17 at 6:13
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You are confused. The microphones you have specified use phantom power, so they cannot possibly have an unbalanced signal (phantom power is injected in both signal lines equally with the ground line as reference so it requires a balanced line).

Were ground and negative signal line shorted, the microphone would very likely be defect (shorting out one phantom powered line would force a large DC offset to the output circuitry of the microphone): this cannot really be part of a useful design.

A DI never passes phantom power since it connects balanced to unbalanced signals and only balanced signals can carry phantom power. An active DI may have its circuitry powered by phantom power since it is generally unidirectional (unbalanced input signal to balanced output signal).

There are special converters providing plugin power on an unbalanced input derived from phantom power on the balanced output lines. Plugin power is used for driving simple electret condensor microphone circuits (typical for lap pin microphones) but is just something like 3V rather than 48V. Those converters are not called "DI" and they would not apply to the microphones you mention.

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You've got this all back to front. Phantom power comes through a balanced connection. If your mic has pins 1 & 3 shorted it isn't receiving phantom power.

A DI box is used to convert an unbalanced signal to a balanced one. A balanced signal is more immune to interference than an unbalanced one.

Tell us presisely what microphones they are, make and model number. And what you're connecting them to. Then we might be able to help.

Whine is often a computer problem. Is there a computer in this setup? Particularly a laptop working on a mains adapter rather than battery?

  • It's t.bone em 700. 48V is applied to pins 2 and 3, so I see no reason why it could not do so even if ground (pin 1) is connected to 3. Am I wrong? – Ain Mar 31 '17 at 23:33
  • Think what 'connected to ground' means! – Laurence Payne Apr 1 '17 at 11:50
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A DI box isn't the right tool for the job of converting an unbalanced mic to balanced output.

A pre-amp with phantom power and balanced output would be the better solution.

Generally a DI is providing isolation from the Phantom Power, and most are designed for connection to instruments like guitars and keyboards.

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I've had intereference / whine on my output. I am considering to purchase a DI box I'm using low-end t.bone EM 700 microphones and a (rather versatile) Behringer mixer FX1622.

No. A DI-box will not help. They are used to solve a completely different problem.

My guess is that you are using cables of too low quality. Get good quality, shielded mic cables with good quality connectors.

There is a slight possibility that the problem can be solved by isolating one (or both) microphone from from surrounding metals, simply use a bit of insulating material or tape.

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