Hum noise: for tech people only...

[Tino]

***This topic is not directly related to VSL or mixing, but since the knowledge from the people here is huge, maybe someone can help me with this!***

I use a Mackie mixer (bought in the USA), and brought it back home (Switzerland). It gets its power from a robust transformer (240 VAC -> 120 VAC, working fine), and is connected to the sound card of a computer working on 240 VAC.

I now have this annoying hum noise, which is perhaps not a surprise for some of you. I tried to solve this problem, but found no solution so far. The computer alone (mixer unplugged and not connected to the mixer) works fine -no hum noise in my sequencer-, and the same with the mixer alone -no hum coming from the speakers- (computer unplugged and no connection to the computer). But as soon as both are under tension and connected: huuummmm.
Nobody in either the music or electric field over here can help me (but everybody says "oh yeah, it's typical!").

I analysed the frequencies: peaks at (Hz): 65, 151, 236, 387.

FRQ (Hz): Stereo Energy (dB):

22 -65.9
65 -52.2
108 -71.5
151 -69.4
194 -80.0
236 -74.2
281 -80.0
323 -80.0
387 -77.4
472 -80.0
... -80.0

There are no perfect multiple of a frequency (harmonics), but it seems to be around 60 Hz.

This is a technical/hardware question, but if someone has a solution (which doesn't involve buying a new mixer [:'(] )... I would be so grateful [:D] !

Thanks!

Tino

[KingIdiot]

Sounds like a Ground loop.

For quick solutions, you either need to lift the ground off of one of the power supplies, or "lift" the ground from the cables that are connecting the mixer and the computer.

[Mattias Henningson]

What King said (it's a very common problem with computers btw), but don't get into lifting ground from the power supplies...they are normally there for a reason which would be to protect you from an electrical shock... A DI box with ground lift between the computer and the board will eliminate the hum. A passive one will do just fine and won't cost you a fortune.

/Mattias

[Tino]

Hi,

Thanks for the replies!

So I guess it is really *just* a ground loop (this [URL=http://www.ebtechaudio.com/findloop.pdf]page[/URL] is actually quite useful to understand how it happens.

I am about to change my sound card for a new one with TRS balanced I/O. Since the Mackie also has TRS I/O: will balanced lines between the mixer and the card solve the problem? (the problem lies in this connection)

If not, Mattias, what is a passive DI box? (sorry for the newbie question). What brand/cost?

Thanks a lot!

Tino

[Tino]

Ok,

By lifting the groung from the mixer power supply, the hum/groung loop went away. Now, is this a dangerous thing to do? (did people died [[:|]] or go their equipment burnt [:@] ?) I know that the ground is there for safety issue, and I don't want to take any risk damaging the equipment nor myself (specially myself). This would be the easiest way to go though. After plugging everything to the same power outlet, the hum was still here, and that is a little weird.

Will a DI box alter the quality of the signal? Another solution would be to lift the ground from the balanced cables, making them unbalanced (I will try this when I got my new sound card with TRS I/O).

Many thanks for your help, I am getting closer to a hum-free world!

Tino

[KMuzzey]

I had the same problem, and if you can't fix it, there's a cheap little gadget out there called the Hum Eliminator, and you should get a couple of them. They eliminate any ground loops in your system, and it's safer & better than doin gthe ground lifting thing. I think Sweetwater.com has them for about $60 apiece, or for $200 you can buy a unti that takes up 1/2 a rackspace, and has 8 ins and outs. Great investment!

[KingIdiot]

MAttias is correct, lifting the ground from the powersupply is the second choice in the options I mentioned.

lifting it from the audio cables is usually the first but more difficult to do. There are websites that will tell you how to do this. Direct boxes are also an option, but get to be a PITA when you have lots of connections from the mixer to the computer (or other units).


Ground lifting form the power supply isn't "extremely" dangerious IMO, but you should be careful, when things are disconnected, as there wont be a ground ANYWHERE in the chain.

[Simon Fox]

In the end I'd suggest going balanced I/O if you can. You get better signal to noise ratio, and lifting the ground will NOT unbalance it! That's the whole point: you have ground, + signal and - signal. The ground only needs to be connected at one end, so no more hum loop. The extra boxes you could buy to solve the loop problem won't improve your sound quality; going balanced actually will.

Good luck!
Simon

[Dietz]

@Tino said:

Ok,

By lifting the groung from the mixer power supply, the hum/groung loop went away. Now, is this a dangerous thing to do? (did people died [[:|]] or go their equipment burnt) [...]
Tino

Yes, definetly. Don't, DON'T (did I say: DON'T!?) lift the ground (which is actually "earth") of your powersupplies. If there is a gound-loop between single items of your equipment, get rid of it by seperating them galvanically AUDIOWISE. This means: Using DI-("Direct Input" or "Direct Injection")-boxes with a ground-lift (or - if you feel brave enough for operations like this - cutting the ground of your lines on the proper ending). There are literally dozens of DIs available on the market (a safe bet is BSS, for example).

Another reason could be faulty part inside one of your things or a broken cable, but actually this is not very likely.

HTH,

/Dietz - Vienna Symphonic Library

[Tino]

I ordered the Hum Eliminator from Ebtech (passive DI box). I'll let you guys know how it performs when it arrives.

Cheers!

Tino