Members W. M. Hellinger Posted September 7, 2010 Members Share Posted September 7, 2010 This weekend I had a situation where phantom power from a Peavey 16FX mixer taking input from the DI out of a Roland KC150 keyboard backline amp was causing a pronounced humm in the PA system. My first attempt at a solution was to meter the AC power. All was fine there. My second attempt at a solution was to use a pin 1 lifted XLR cable in-line between the keyboard amp and mix board... which resulted in "no change... humm the same". My third and finally 100% successful solution was to turn off the (global) phantom on the board... the humm from the keyboard amp DI connection disappeared entirely... but the use of our condenser mics also disappeared, so I hadn't fixed the problem... just masked the symptom. I have experienced this sporatically over the years (as I suspect many of us here have) and I don't know the cause & cure (I suspect many of us here are equally perplexed). Can anyone here offer any insight as to what the real problem was and what a real solution(s) might be? Link to comment Share on other sites More sharing options...
Members Coaster Posted September 7, 2010 Members Share Posted September 7, 2010 individual phantom? Link to comment Share on other sites More sharing options...
Members W. M. Hellinger Posted September 7, 2010 Author Members Share Posted September 7, 2010 individual phantom? Yea... I know... and that is an option... cept "they" don't want to use my big, old, heavy reliable, individual ch phantom powered analog board. BTW: The new lightweight high-tech board, with the global phantom power, surfaced some other major PITA issues this weekend: 1) for some reason "ch's 12-16 were completely dysfunctional" 2) for some reason a total clusterfk of "full onboard FX" was the only thing that would come out of the aux sends. and we didn't have the manual with us... attempting to fix the problems didn't seem to be intuitive... and we were probably 100 miles from the closest internet connection. So... we played 10hrs. or so this weekend without approx. 25% of our inputs and no monitors. Link to comment Share on other sites More sharing options...
Members agedhorse Posted September 7, 2010 Members Share Posted September 7, 2010 Could be that the DI outputs on the keyboard are tied pretty hard to circuit ground through loading resistors, and if this ground is not clean (or is tied to neutral) then hum is a definate possibility. The phantom power is modulated by the neutral ripple and the result is that the mic preamp can't reject that much noise. Could be other causes too, but this is one that I would look at. Another is that a mic line has failed pin 1 to 2 or 3 short, leaving any noise as differentialk mode rather than common mode, and why lifting the ground pin 1 on the output didn't change anything. Here's where an audio isolation transformer is your friend. Link to comment Share on other sites More sharing options...
Members agedhorse Posted September 7, 2010 Members Share Posted September 7, 2010 Which console? Link to comment Share on other sites More sharing options...
Members JohnnyGraphic Posted September 7, 2010 Members Share Posted September 7, 2010 individual phantom? Could've been...The Humming Phantom!!! j/k Link to comment Share on other sites More sharing options...
Members W. M. Hellinger Posted September 7, 2010 Author Members Share Posted September 7, 2010 Which console? The problem this weekend involved a Peavey 16FX like this: Link to comment Share on other sites More sharing options...
Members agedhorse Posted September 7, 2010 Members Share Posted September 7, 2010 I vote for a cable problem or a cable + power problem. Link to comment Share on other sites More sharing options...
Members dboomer Posted September 7, 2010 Members Share Posted September 7, 2010 Yep ... my guess is that the problem was not caused by the phantom but rather just a grounding issue between the two. Link to comment Share on other sites More sharing options...
CMS Author Craig Vecchione Posted September 7, 2010 CMS Author Share Posted September 7, 2010 Mark, next time this happens, plug the Peavey into the same outlet as the keyboard amp (yeah, I know...lonnng extension cord...) and see if that ends teh hum. I Spy a ground loop. Link to comment Share on other sites More sharing options...
Members StratGuy22 Posted September 7, 2010 Members Share Posted September 7, 2010 Would a separate DI box have helped? Link to comment Share on other sites More sharing options...
CMS Author Craig Vecchione Posted September 7, 2010 CMS Author Share Posted September 7, 2010 Would a separate DI box have helped? A 1:1 isolation xformer on that cable would be better. A DI is expecting high-impedance unbalanced input, and the keys amp probably has a low-impedance balanced output. Link to comment Share on other sites More sharing options...
Members agedhorse Posted September 7, 2010 Members Share Posted September 7, 2010 Would a separate DI box have helped? A passive transformer isolated DI would certainly help, slight noise penalty over a 1:1 isolation transformer provided the console can handle a real +4dBu input. Some can not. Link to comment Share on other sites More sharing options...
Members Unalaska Posted September 7, 2010 Members Share Posted September 7, 2010 I don't like the roland KC amps DI out. I don't trust them to be clean. There is a ground lift switch on the back of the KC500 but even then it wouldn't have solved the problem. I always use an external DI, usually the EWI stereo DI with a dual 1/4 TS cable. Often I put FOH processing on the same AC as backline. Link to comment Share on other sites More sharing options...
Members kevinnem Posted September 8, 2010 Members Share Posted September 8, 2010 input from the DI out of ... real solution(s) might be? I don't know what the "real" solution might be, but was it a transformer isolated DI? Link to comment Share on other sites More sharing options...
Members Keyrick Posted September 8, 2010 Members Share Posted September 8, 2010 I have a KC 550 and I would experience this problem on occasion. I purchased two inline Isolation transformers from Andy and that solved the problem. I no longer use them as our mixer changed and now I use a stereo input channel on the board which terminates on 1/4 inch TRS balanced connectors. As there is no phantom power on the 1/4 inch connectors, I am under the impression that the neutral ripple on the Phantom source that was mentioned earlier was the problem. I still keep them in my gig bag though as you never know when this situation could arrise. Get a couple of them as they are worth the cost in being able to quickly resolve the issue. Rick Link to comment Share on other sites More sharing options...
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