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I have the M-Audio Mobile Pre soundcard which I will be using to send signal from my PC to a mix console. What I don't know, though, is whether I need to put a DI BOX in between?

The sound card does not have an XLR output, it has only the 6.3 connectors, but I'm still not sure whether this automatically means that the signal needs to be processed through DI BOX first.

Alternatively maybe I could use the headphones output on the soundcard, which is amplified and therefore shouldn't require DI BOX?

Any ideas?

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migrated from Feb 14 '14 at 15:19

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The MobilePre's line outputs are balanced, so it should be perfectly ok to plug it straightly into a console, either with two stereo TRS cables into symmetrical line inputs or with stereoTRS-to-XLR cables into microphone inputs. It should even work with mono cables, though you then lose the symmetricity, so you might pick up noise.
If you're unlucky, you might even pick up noise with a balanced connection: PC mains adapters often introduce very nasty static at around 2 kHz; if you experience something like that a DI-box should fix it but I'd first try it without.

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Oh, so if I understand this correctly, a stereo TRS cable may be used in a mono output/input to transmit a balanced mono signal? On condition that the outputs/inputs are balanced, that is. – Richard Rodriguez Dec 17 '11 at 1:29
@RiMMER Exactly. – leftaroundabout Dec 17 '11 at 19:35
@RiMMER see below, actually your MobilePre doesn't have balanced outputs at all, but that's not a bad thing! – JayP Dec 22 '11 at 16:19
@JayP: according to the manual here, the "2nd generation" model has "2 back panel 1/4-inch TRS Balanced line outputs with dedicated output volume control". I figured this feature would already have been present in the 1st generation, but apparently I was wrong. So it depends on what model it is. – leftaroundabout Dec 22 '11 at 19:59
up vote 3 down vote accepted

The MobilePre actually has line-level unbalanced outputs (according to their website here).

Many people shy away from unbalanced connections, as if they are some sort of terrible mistake that should be avoided at all costs. In fact, they have their place, just as balanced cables do too. In fact, in this case, over short cable runs at least, they could even be considered desirable!

As you may know, the process of "balancing" involves duplicating and inverting a signal, in order to filter out noise at the receiving end. This process in itself and actually induce some level of noise, which can be undesirable. Bob Katz, in his book on audio mastering, mentions that many mastering engineers will actually modify their equipment to remove unnecessary amplification and balancing sections, all in the name of reducing noise.

This is precisely why audio interface designers tend to provide unbalanced line level outputs. Higher end interfaces will provide digital outputs, which don't suffer from noise in the same way as their analogue counterparts.

Unless you run your speaker output cables in parallel with a mains cable (this actually induce noise even in a balanced cable), you will most likely not get any problems at all.

Introducing a DI box into the equation will increase clutter, increase noise, increase complication, and worst of all, probably not make any difference whatsoever.

Most mixers have dedicated line level inputs for stereo sound sources, just like your M-Audio MobilePre, so I suggest you use these.

The issues of "static" or "hum" in hardware connected to computers is more likely to be caused by poor earthing. One way to fix this is to not rely on the USB connection to provide power - plugging your MobilePre into it's own dedicated power supply can reduce this.

Edit I just noticed that the MobilePre doesn't have connections for an external power supply, which is a shame! For other interfaces that do have an external connection, this can be useful. End edit

Another thing to consider is that your mixing console might actually be unnecessary here - without knowing your setup, it's hard to say, but have you considered simply connecting the outputs of your MobilePre directly to your monitors? This has obvious benefits in terms of simplicity, but removing extraneous equipment from your signal chain can also reduce noise.

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What you say about balanced vs unbalanced correct for an ideal studio situation, however the disadvantages of balanced connections are always very subtle. On the other hand, one bad unbalanced connection can produce a very problematic amount of noise, so I'd only ever go for unbalanced voluntarily when I'm 100% sure everything else is prefectly under control – which is hard to achieve in the studio and virtually impossible live. +1 for emphasising simplicity, because complex setups with many cross-connections are the number 1 source of noise. – leftaroundabout Dec 22 '11 at 20:09
@leftaroundabout; you're right, my points do represent an "ideal" where bad cables don't exist and everyone has a perfectly clean power supply! – JayP Dec 23 '11 at 9:19

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