Comment

Comment by David on May 13, 2012 at 7:18pm

Hey everybody, I've been pondering M/S theory and thought I'ld throw this thought out and see what people think: Drastic changes in mixing the M/S channels definitely change the effective pattern of the mic, but would you agree that minor balances change the projection of the mics onto the stereo field of the mix more-so than the actual pickup width? I've only used M/S a handful of times but this idea seems to fit my experiences. 

Comment by Tony Tartaglia on February 13, 2012 at 4:15am

Aall great comments, Don has the best one. Your problem sounds like a bad cable making a shoeted connection on your front nine in a wall unit or breakout box. As a professional audio engineer I have come to the point where I will NOT purchase any premade cables at all. I prefer to build all of my XLR and 1/4" balanced cables with Canare quad star and real Neutrix connectors. This way I can eliminate 99% of all noise issues. My speaker cables are also overkill by using the next size than the book calls for. Once the cables meet my standards the ony thing in my systems that can cause problems is the outboard gear itself. (easy to diagnose) As Don said any premade cables from a big store are (for the most part) very poorly made, conductors touching, too much insulation stripped off,etc. They are meant for beginners, not for professionals.

 

Comment by Don Lanier on February 12, 2012 at 1:24pm

Nice explanation David, One more small thing, dont assume ANY cable thats bought from a Guitar center etc is right, you can have a connector thats got a marginal connection explaining why when you ramp things up its starts getting worse, your going to have to LOOK at each one, and be sure you dont have a Pices of Older Outboard gear thats using 1/4 connections in the middle of the rack thats using XLR connections....Variables all have to be varified....

Comment by Donovan Drake on February 11, 2012 at 4:12pm

You should double check the type of connectors used in the floor boxes. I had a problem like this once when I built a monitor amp rack. If you use the all metal switchcraft TS connectors then be aware that the (-) is wired to the chassis. This caused a lot of problems when I had a whole rack of amps with their (-) leg wired together. While I didn't wire any of the xlr inputs ground to the chassis, I imagine that if I had it would have cause a very similar situation to this one.

Just check the wiring and if the XLR ground is wired to the chassis undo it. Also if it's wired with the all metal switchcraft jacks then use the plastic neutrik trs jacks or NL4s.

Comment by David on February 11, 2012 at 3:50pm

Ooops - one addendum, XLR inputs and monitor outputs can interfere if run in parallel in the same conduit or near a cheap Di or 1/4". But that is a noise floor issue and not a serious electrical problem.

Comment by David on February 11, 2012 at 3:48pm

FOH and stage should always share the same ground but that doesn't explain his problems. If they were on different grounds he would probably get a humming as current flowed through the shields of the XLRs between the console and the amps. The monitor outputs and XLR inputs of a system should NEVER be able to interfere with each other. My best guess is that someone didn't terminate the connections at a panel properly. The connections on both XLRs and monitor drives should be completely physically isolated from everything all the way from the output of the console/amp to the input of the microphone/microphone. With amp outputs there is no such thing as "ground," the output voltage is the difference between the - and the + terminals. For example you can't wire + to the speakers and just ground the black wires - bad stuff could happen. With XLRs, the "ground" pin is really a drain for the shield, or a ground reference for phantom power. The shield/pin 1 should never touch the chassis of any panels or the shell of any connectors between the console and whatever it's plugged in to. It should be wired exactly like pin 2 and 3 all the way from the console to the microphone/Di/whatever.

An electrician can verify that the grounding of the stage & FOH is done correctly, but in this case it really seems like bad wiring on the XLRs & monitor outputs. The two most important things in the power design of a sound system is having a common isolated ground for all things audio related on stage and in FOH, and having all audio related plugs fed from their own subpanel(s). Having there own subpanel is usually enough to keep those stupid wall dimmers or cheap motion sensors from causing noise or pops. Good grounding can be tricky and requires planning and careful/firm communication with the electric contractor, it requires special types of subpanels that have an isolated ground bus. The outlet plugs are grounded separately from the conduits and j-boxes to prevent any accidental grounding to metal studs or stuff like that. As the system gets larger you'll need things like surge suppressors and isolation transformers, but those are usually secondary concerns in smaller installs.

Don is right, be methodical and check everything. Start at the console and trace every wire from end to end to ensure proper termination. If the problem is affecting multiple inputs then it's probably a systematic error and you'll find a batch of bad terminations.

Comment by Don Lanier on February 11, 2012 at 1:34pm

You could be experiencing a GROUND DIFFERENTIAL issue especially if you have Metal conduit going to floor boxes with Mixer/Eqs, FOH being on one Ground, and the amps, Stage and Backline being on another, or different LEGS/Phase of the House power. I would consult a Pro Electrician who understands these issues, and I would reinvestigate ALL the Interconnects, Speaker, Line, AC and be sure there all correctly wired in Phase, Polarity etc...ya it will take time but starting at A and working to Z is far easier then jumping into the middle and nevr knowing whats been checked, use some sort of marker to verify youve checked that cabling, dont assume ANY cable is wired correctly.

Comment by Peter Richter on February 11, 2012 at 1:22pm

Daniel, when you say "somehow feeding power back" - what sound are you actually hearing?

Comment by Daniel "RED" Fields on February 10, 2012 at 11:41pm

I also posted this in the install section but thought this area would be appropriate too, as in theory this does not normally have problems and there usually are some simple solutions to quickly fix the problem. 

 

O.K. so I ran into a problem I have never seen before today, and all the suppliers and many other install techs I know are just as baffled as I am with this. The problem the church is having is that the monitor amps are some how feeding power back down the mic lines in the floor pockets. What is really puzzling is that it only happens if any of the gain knobs, from those channels only, are above 50 percent. It then overloads the channels from all the floor pockets and lights them up better than many Christmas trees. It is an allen and heath GL2400. The problem is also fixed by simply turning off or interupting signal to the amps. All channels can be pushed to 100 with out any problems. I have gone through all the power and grounding knowledge I have and the advice I can come up with. I have taken apart almost all of the system and also rewired the installed monitor cables with heavily insulated cables, same as portable system cabling. Nothing has effected the issue. At first we thought it was the board or patch bay but by moving the channels from the floor boxes around the problem follows the change. It also stops effecting any one channel from the boxes if it is disconnected.  

 

OH and PS there are 32 Stage channels on a 40 channel console and this is only effecting the front 9 in the floor pockets. There is also monitor cabling running to the the wall plates and a large back pocket. But these are not having any problems and all of the same monitor zones are in these three plates also.

 

Any help/thoughts/ideas will be awesome

Comment by JD Myers on February 28, 2011 at 10:47pm

Hmmm, somehow lost the end of my comment. Now then, what did I just type?!

 

...

Comps seem to be the hardest things for people to learn to use, as they can have a very subtle to a very harsh affect on the sound. Some things to listen for are distortion (compressing too much) and 'pumping' (affected by the attack/release knobs). Something to help figure these things out would be to set some extreme attack/release settings, then switching back and forth with the AUTO switch to hear the difference.

 

Sometimes there is a 'knee' knob or switch, from soft to hard, which is basically a function of the threshold/ratio - soft will apply the compression a little more as you approach the threshold you have set, while hard will be more dramatic and instantaneous but might not actually start to compress untill you are right at the threshold...

 

Hope this helps!

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