have a serious audio problem

[fiadmimedia]

we are trying to record our services to dvd and also broadcast the audio feed to ustream but from the foh mixing board the audio signal is very clean no humming etc. we then go into another small 12 channel mixer for all our recording and broadcast stream. That's when the problem starts there is this constant buss and humming in the signal in the mix. the only other equipment between the foh mixer and the broadcast mixer is a compressor. any ideas that would the problem be or if i am doing the right thing. the issue with using another mixer is that the audio is too low when we play back the dvd or listen to the broadcast stream.

[tedanderson]

It could be a ground loop problem if all of the devices are plugged into separate outlets. One way to test this is to use a 2-prong to 3-prong adapter for the electrical outlet for your 12-channel console. Do not use this as a permanent solution (or even temporary one for that matter). This is just a method to narrow down the source of the problem. If this does not remove the symptom then connect an active DI between both consoles for only one channel to determine if it is a ground loop in the audio signal.

[daver2]

If you disconnect the small 12 channel mixer from the foh mixer is the audio stream now clean? If so, plug things back in until the humming comes back. Disconnect the humming unit for the time being and continue plugging the remaining items back in one at a time. You should be left with the equipment that is causing the problem - or the last link in a loop. It should point you in the right direction.

The most likely cause (as Ted has stated) is a ground loop or using different power circuits or phases.

[fiadmimedia]

the foh of system is running on 4 different breakers. 1 breaker for only 3 amps, another for 2 amps plus FX, then 1 for all compression and eqs, and the last one for the mixing console. all is running on a power conditioner except the mixing console. after doing the process of elimination foh is clean but the output from the foh mixer to the 12 channel mixer that when the humming starts

[daver2]

You state that your kit is running off four different breakers. Do you know if you have 'three phase' at your place and are all of the four breakers running off the same breaker further upstream? The main idea is that one phase should feed a large breaker, which then should then feed smaller breakers as the power supply feeds separate pieces of equipment. We had a situation in our Church where we had two power phases (yellow and blue) that were feeding our audio equipment (not by design I hasten to add - the electrician got it wrong). This introduced hum and noise quite nicely!

The simple check is to work your way back to the next higher-level distribution board and turn off the breaker feeding your audio kit. There should only be one breaker! Obviously, your audio kit should all be turned OFF when you perform this test. It would also be wise to ensure (as far as you can) that there are no other consumers on this same circuit - otherwise their computers will all die with power failure when you flip the breaker off! When the higher level breaker is OFF - none of your audio kit should have any source of power. If there is some equipment that does have power - it means that it is fed from somewhere else and this could be the source of your hummmmm.

Good hunting.

[fiadmimedia]

the main breaker is a 100 amp breaker that supplies the whole church building. the thing is that i am trying to remember if this started after the a/c unit was put up on the roof. i can't remember at all

[daver2]

The plot thickens...

If it's mains wiring induced (e.g. from electrically noisy motors etc.) then fitting ferrite cores to the mains cables of the audio equipment may help - however, this will mask the problem - not cure it at source. Some audio equipment is more sensitive than others to electrical noise. One of our keyboard players has a 'cheap' mini mixer desk (he got it free from a friend) which is incredibly sensitive to noise. Now we have tracked down what was causing us problems we have banned it! A word of warning from my experience. We had some electricians in that didn't tighten up the main incomming neutral bar connection on a distribution board they fitted. We were trying to pull 60 amps from a cable that was not screwed in. That produced some noise (mainly arcing)!

See if you can find out what your A/C installers did and when - the two issues may be connected as you summise!