Hum noise: for tech people only...

[Tino]

Hello,

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 images/icons/frown.gif )... I would be so grateful images/icons/grin.gif !

Thanks!

Tino

[Gulliver]

At first glance it sounds like a ground-loop problem. If that\'s the case than your facing one of the most annoying electrical problems known to man-kind! images/icons/smile.gif

j/king.. but only half joking..

What you have to do is to carefully trace where your equipment grounds are going. Is the mixer connected to the transformer and the computer straight to the wall socket? If so, the mixer is completely isolated due to the presence of the transformer.

You might have to introduce an audio transformer between your computer and the mixer but be hesitant about this. These audio adaptors tend to distort frequencies, even the best ones. I would try to solve the problem any other way if I were you.

[Tino]

Hi,

Thanks for the reply!

So I guess it is really *just* a ground loop (this page (\"http://www.ebtechaudio.com/findloop.pdf\") 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)

Thanks a lot!

Tino

[Gulliver]

Technically, yes.. True balanced I/O with proper balanced cables should do the trick.

Good luck.

[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 images/icons/shocked.gif or go their equipment burnt images/icons/mad.gif ?) 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

[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