USB Soundcard / Noise Filter

[pill]

Hi,

Not sure if this is right section for this question, admin can you move if i'm wrong please!

I have the problem of getting a horrible ground loop hum (or at least I think that's what it is) whenever I have a Laptop plugged in to my sound system.

At my last gig it was particularly nasty when I had the microphone turned up, the hum was so bad that it sounded like there was a mashed up song playing in the background.

The guy at Kosmic told me I had no choice but to buy a $300 USB DI (direct injection) box. I'd never seen one of these except for with high level signals when i did some stage production audio engineering at high school.

http://www.onlinemusicstore.com.au/index.php?page=showProduct&id=ARXUSBDI-VC

Anyone know whether one of these will solve the problem? I'm not keen to drop that kind of cash for what's basically an external sound card.

Note: I already have a ground loop isolator in the system between the mixer and EQ (which then goes to the amp) but this does not solve the problem.

[SourceRaver]

OK Pill...

Start your journey here:

http://www.ausdjforums.com/modules.php? ... pic&t=1469

I bought the ARX USB/DI a few months back for work but didnt like the quality. I swear i could hear the digital processing (ie a slight 'flutter' in the resulting audio). The local distributer came out and said he couldnt hear anything. I have since picked up a replacement unit and its sitting on my desk. I'll test it today and let you know how it goes.

Otherwise its a great unit if you want audio straight from your computer into a mixing desk (computer - USB - ARX/DI - balanced XLR cable - mixing desk) . Are you sure this is what you want it for because most Dj systems are a bit more complex?

Could you describe how your system is set up (ie what devices are you using and how are they connected)?

[SourceRaver]

I've found in most cases that an isolation transformer between the computer and desk is all you need. It can be as simple as:

Headphone jack - 3.5mm Jack to RCA cable - Isoloation transformer (ie from Jaycar) - RCA to 6mm jack adaptor - line inputs on mixing desk

Another more professional option would be to replace the isolation transformer mentioned above with two (one for left and one for right) DI boxes. DI boxes have an inbuilt IT (ie 'ground lift') and Behringer sell them for arround $70 each:

http://www.behringer.com/EN/Products/DI100.aspx

[pill]

Sure,

The Audio lines on my setup go:

2 Laptops: 3.5mm Jack to RCAs - One splitting right/left from Virtual DJ to channel 1 and 3 in mixer (Numark M4) and one going straight to line 2 on mixer.

Then flows Rca Mixer -> Noise filter -> Unbalanced 30cm jack-jack to eq -> unbalanced 30cm Jack-Jack eq to amp (and speakon to speakers).

As mentioned in the other thread, the noise disappears if i unplug the laptops' power supplies.

Also interesting to mention is if i plug my laptop in with a car laptop power supply and in to my car's speaker system, i get the same noise.

[SourceRaver]

Now if all else fails...

Remove the power supply from your computer and check to see if the hum goes away. If it does then:

a) I've found that some laptop power supplies are prone to ground loop hums while others (even within the same brand?) arent affected.

Some folks say that buying another 'after market' power supply has cured the problem.

c) Removing the earth pin on the power supply (or buying an international adaptor) cures the problem. I would only do this as a last resort particularly if your using the equipment for public performances.

Let us know how you go!

[pill]

hum goes away when removing power supplies, i'll investigate the cutting off a ground idea next, just got to find an old power board to chop up

[SourceRaver]

OK...so i'm thinking two pooders into the desk (one running VDJ and the other more or less acting as a media player)?

Based on that assumption...

As a quick experiment, try removing your isolation transformer from between the mixer and EQ and insert it between one of the lappies and the mixer. If this works you:

1) May only need a few more ITs and/or

2) You may not need the IT between the mixer and EQ

I dont think more than one IT will a problem audio quality wise and its a lot safer than removing an earth pin.

[pill]

yep that's what my set up is; i have a few spare ITs from Jaycar in my car somewhere that aren't doing anything, I'll give them a shot when I re-do the stereo in a few weeks (Hail Storm Payout = new head unit =D )

[SourceRaver]

Hey Pill...

Is this why you have the second pooder:

http://www.ausdjforums.com/modules.php? ... highlight=

[pill]

Ok, long time but here's the update:

without the power cable plugged in, sound is fine.

With the power cable plugged in and earthed, there is both a hum and some kind of noise that comes through which I think is when he hard drive is being accessed.

With the power cable plugged in and not earthed (through a powerboard with the ground cut), the hard drive noise is still there, but there is no hum.

On this logic, a ground loop isolator won't fix the problem of the fuzzy noise if there's no ground loop hey?

I wish this damn noise would go away!

[SourceRaver]

Yeah the computer noise is quite common but its usually at a low level in the background. You may need to experiment with the volume/gain on laptop and/or mixer to get the best result (ie lowest possible noise from the PC).

[FabDJ]

I had this problem too and cut the earth pin off a double adaptor for pc power. no more hum.

Caused a rucus on a US forum with half opposed to the idea and others agreeing it was low risk as we are dealing with 12-15 volts here.

Its called negative loop. I have the double adaptor clearly marked to show for PC use ONLY.