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I use an AUX male to XLR cable to play music out of a PA at gigs. This seems to work so far.

At a recent gig, the sound person mentioned that the cable I was using should not exist, and that I should pick up a direct box instead. So now I'm here, trying to find out why I should be using a DI box.

After doing some research on direct boxes, it looks like they allow a signal to travel over long distances with little resistance [which I assume degrades the signal]. Also it appears that DI boxes reduce humming through the PA.

Am I missing out on these benefits by using an AUX to XLR cable? Are there other benefits I might be missing out on by not using a direct box?

Here's an example of the type of cable I'm currently using: https://www.amazon.com/GLS-Audio-Cable-Stereo-Male/dp/B006LRPF7Y

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    \$\begingroup\$ No, you use an DI box to make an unbalanced signal balanced and isolate the grounds. \$\endgroup\$ – winny Jul 12 '16 at 20:56
  • \$\begingroup\$ @winny Ok, so from what I'm reading a balanced signal cancels out noise that occurs during the trip from one end of the wire to the other. Ground isolation prevents current from multiple circuits from going through the same ground line, which apparently causes problems. So if an AUX to XLR cable is balanced and its ground line is isolated, there would be no reason to go through a direct box, correct? \$\endgroup\$ – Daniel Neel Jul 12 '16 at 21:08
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    \$\begingroup\$ Theoretically yes, but your grounds are never isolated. Sometimes if you star connected your entire power distribution from a single point, that 50/60 Hz hum + harmonics will be -60 dB down and never cause any issues to start with. Also, that adapter does not create a true hot and cold without proper transformer inside it. If it does have a transformer, you have got yourself a passive DI. \$\endgroup\$ – winny Jul 12 '16 at 21:30
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    \$\begingroup\$ @DanielNeel: My answer to XLR to microphone wiring may help explain why balanced lines are used in the first place. Edit your question here to seek clarification on any points raised relevant to this question or ask a new one. \$\endgroup\$ – Transistor Jul 12 '16 at 21:33
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    \$\begingroup\$ The main thing you're lacking (ignoring valid concerns about abuse of balanced inputs) is real stereo sound. The advert says Combines 3.5mm Left and Right Channels Into XLR Single. It is summing the left and right channel into a single channel which defeats the point of stereo. \$\endgroup\$ – David Jul 12 '16 at 22:04
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There is the assumption that if most gigs have clean sound without a DI box, then there should be no problem. Until the day comes when you have hum in your signal and maybe buzzing noises from stage lights.

It is worth it to have a simple passive DI box on hand just in case. These have an isolation transformer with a high-impedance input and a 200 ohm differential XLR output that makes it immune to noise, even if it is a 200 foot run to the sound tower.
This article helps with the history and adds more details.

Purpose:
The direct box takes a high impedance, unbalanced signal and converts it to a low impedance, balanced signal. This allows the signal to be sent over long cable runs with significantly less signal loss (especially in high frequencies) due to the lowering of the impedance, and greater rejection of interference due to the benefit of common mode rejection in a balanced signal.

Furthermore, the lower impedance (around 600 ohms is normal) allows an insignificant load to the input of a mixing console or preamp which is also designed to accept input from low impedance microphones.

There are battery powered versions for microphones that need phantom power and/or a gain boost as well. You may not need a DI, but if you do and you have one it can save you from a disastrous gig.

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