Ground Loop / hum

[radardoug]

In a "properly"  designed small studio (all balanced outs and all differential inputs) the shield on the cables is not needed at all.

You are not correct. The sheilding is needed to reject rf and similar signals. Please try to give accurate information on the web.

 

[radardoug]

Well, I removed all the channels from the console and kept only the master channel to be able to isolate the problem.
Now when I listen to the headphone I have no noise, exept when I plug my interface to the aux return and solo the aux return.

This could be due to two different effects.
1/ Having removed most of the modules, the current load on the power supply is much less. So the supply may be delivering much less ripple.
2/ Grounding and bussing effects between modules. When you plug all the modules in, you create multiple earth loops depending on the console design.
In chasing this, you need to have some serious test gear including a good millivoltmeter. Do you know what your signal to noise is now? If its 70 odd dB, then that might be the best this design can do. Without this knowledge you are really wasting your time.
You need as a minimum a good scope, a good oscillator, a good N&D meter.

 

[Andy Peters]

Hi,
I have a problem with my mixer (Mitec EX31 - 48tracks) It generates too much hum. I think it comes from a ground loop.
I unplugged everything, muted every channels and just plugged my headphones and the hum is very low but still audible, it starts growing when I plug grounded gear like my audio interfaces (echo audiofire 12) my computer and monitors.
The hum is a also present (even more audible) in the monitors out when I plug my headphones in the aux output.

The advice everyone else has given you is excellent.

One more thing, coming out of left field: Is the mixer plugged into a surge-suppressor power strip? If so, get rid of the power strip and see if the problem goes away.  The components which do the surge suppression (MOVs) tend to die in an interesting way: they get leaky and ground noise infects the mains.

 

[JohnRoberts]

The advice everyone else has given you is excellent.

One more thing, coming out of left field: Is the mixer plugged into a surge-suppressor power strip? If so, get rid of the power strip and see if the problem goes away.  The components which do the surge suppression (MOVs) tend to die in an interesting way: they get leaky and ground noise infects the mains.

+1

I just repaired a power strip with "protection" (cough) built in. The "protection" was leaking 2mA AC of mains current into the ground.

This alone "should" not contaminate signals with a solid low impedance ground and good input differentials, but it is a bunch of extra noise current that has to be disposed off and/or ignored.

JR

 

[moamps]

You are not correct. The sheilding is needed to reject rf and similar signals. Please try to give accurate information on the web.

I did that twice in experiments about 5-6 years ago. We made wiring of two small studios (wires counts below 100) and got S/N better for about 4-6dB if cables was unshielded. The all connections was balanced-differential types, and each piece of equipment in the system was powering using proper power cable. For two microphone cables shielded Mogami cables was used. The spectrum analysis of the residual noise in shielded system showed problem with ground loops currents (mostly noise from SMPS and 100Hz residuals). For the conclusion, we found that more noise we have received in the system from ground loops than from the outside world. There is a lot of change in RFI spectrum in last 30 years, from high power AM transmitters working on LW, SW and MW bands to nowdays small power spread spectrum sources as mobile phones, wifi etc. worked on  few GHz  which obviously don't interfere in audio systems so much. 
Of course, your mileage may vary. Anyway, you can ask mods to delete my posts if you are not satisfied with my accuracy. 

 

[fjgaston]

Hi guys,
Thank you all for the advices 
Good news !!! I repaired the damaged power cord, while doing this I noticed the wires of the PSU fan went through the entire PSU carrying 230V AC, as I know the fan has been changed I thought I may twist those 2 wires (as I have done for heater wires in a tube amp) While the PSU was opened and the soldering iron still hot, I did it.
I conducted some tests, with the master channel and 4 input channel  (each module is 4 channels) I used it with only the Audiofire 12 and the mac plugged there is only a very very low noise.
Then I remember the noise used to be very noticeable in the stage monitors in the other room, plugged in the aux send so I tried it again, there is only some noise when using unbalanced jack.
I can't test now with everything plugged, but I'll do it tomorrow during the day and let you know. (fingers crossed)

The advice everyone else has given you is excellent.

One more thing, coming out of left field: Is the mixer plugged into a surge-suppressor power strip? If so, get rid of the power strip and see if the problem goes away.  The components which do the surge suppression (MOVs) tend to die in an interesting way: they get leaky and ground noise infects the mains.

I do have a surge suppressor power strip, I'll also try to remove it, thanks.

 

[gridcurrent]

Steve Lampen of Belden has done extensive research on the subject of UTP for audio.

http://www.aes-media.org/sections/pnw/pnwrecaps/2013/jun_cat5/

from above:

"From an American perspective, the European obsession with shielding cables is overblown, and a response to a problem that has not been demonstrated to exist at a significant enough level to need a solution. Unshielded cables work just fine in almost every application, except for specific environments with very high levels of specific RF interference. The vast, vast, vast majority of users will not benefit from using shielded cables, nor will they suffer ill effects from not doing so."

 

[radardoug]

That article is refering to data transmission in Cat5 and the like, not audio. Data by its construction is not worried by signals at - 70, audio is. In a perfect world, with differential transmitters and receivers balanced to -100, then sheilding might not be required, but show me that perfect world!

 

[gridcurrent]

That article is refering to data transmission in Cat5 and the like, not audio. Data by its construction is not worried by signals at - 70, audio is. In a perfect world, with differential transmitters and receivers balanced to -100, then sheilding might not be required, but show me that perfect world!

are you  sure ?  lets not give inaccurate information on the web.
http://www.belden.com/docs/upload/Non-Data_Applications_for_Category_5_Cable.pdf

 

[radardoug]

Thats a different article!

 

[JohnRoberts]

Of course, your mileage may vary. Anyway, you can ask mods to delete my posts if you are not satisfied with my accuracy.

While the mods have above average knowledge about electronics we do not serve as technical editors, so all advice posted is caveat lector (reader beware).  I wouldn't edit/remove a post unless it was clearly dangerous to human safety, all else remains for the community to ponder. 

I will comment when I see something obviously wrong but life is too short to vet every piece of advice. That is the community's job by outcry, and we have a wealth of experience around he.

Some anecdotal experiences can misunderstand the cause and effect, or just be wrong...  Part of learning how to function on the internet is learning to evaluate multiple inputs then decide for yourself.

Good luck...

JR

PS: I am already on record suggesting that we ground both ends of all shields, but that can aggravate other related problems.