Re: Iso Transformers

Dale Farmer wrote:

There is a lot of misunderstanding out there about power in our sorts of situations with inducing noise into audio and video circuits.

The real root cause of much of this is actually a design fault in the equipment. Known usually as the Pin 1 problem. ( Read AES-48 for a much more technical and accurate explanation.) Basically when you have equipment that is plugged into different power centers, say a mixer board out at FOH and the amps sitting under the stage, you have the potential of having the grounds at different voltages compared to each other.
That power in the different grounds will want to flow in one direction or the other. this flow in the signal ground conductors is known as a ground loop. This current will inductively add noise to the signal in unbalanced lines, usually video or low end audio lines. It also flows through the circuit boards of the gear at each end if the gear has the pin one problem, inducing *lots* of noise in the affected gear.

You can put signal isolating transformers in the signal lines (Humbuckers and so on) which block these ground loops. You can break the safety ground on the power cable, which is a very bad idea, sometimes in a darwinian fashion. You can stop using gear with the pin one problem. You can add additional ground conductors that route that unwanted current away from the sensitive signal electronics.


So, you have this ground loop problem. Since going out and getting gear that complies with AES-48 and doesn't have a pin one problem is not really an option for many situations, I'll talk about mitigation strategies.

The goal is to stop that ground loop current flow inducing noise into your audio or video system while still leaving you with a safely operating system.
The easiest way in balanced lines (such as the ubiquitous XLR-3 microphone line) is to break that ground conductor in one and only one spot. Many microphone splitter boxes come with switches labeled Ground Lift for exactly this purpose. You have to leave the ground conductor still connected at one spot, as this is also the signal shield, and needs to have a connection to earth ground to block interference.
You can't use a ground lift on an unbalanced line, as the ground is not only the shield, but is also part of the signal path. Cutting that would stop the signal you want as well. So you can put in an isolation transformer. Handwaving the details, it passes the signal but blocks the ground loop by sending the signal through a 1:1 transformer. (There are thick books out there on transformers in engineering libraries.) Some video gear have isolating transformers built into their input lines because this is such a common problem.
You can take the signal and break that ground loop by making the link between the systems an optical fiber or something like the whirlwind E-Snake which digitizes the stuff and runs it down a cat-5 cable, which provides the needed isolation.
You can go at the problem from the other end, reducing the differentials between the different grounds. this is done by two methods of grounding system. the Star and the mesh ground system. The star system you build everything electrical such that it comes from one single source. The One True Ground. You see this taken to extremes in some recording studios where they have an entirely seperate technical ground system, and the gear is carefully installed such that there is ony the one connection between the technical ground and the electrical safety ground.
The mesh ground system goes the other way, where you run additional grounding wires from everything to everything. This gives that ground imbalance many additional paths to travel, reducing the current in the sensitive gear to a low enough level that it is unobjectionable.

So far I've just talked about ground loops, since that is the vast majority of what we end up dealing with. Now I'll get into power quality. Basically, dimmers end up throwing all kinds of crap back into the power feeds in the building. Again, handwaving the technical details, as at the level of meatball theater, they really don't matter. If you are designing dimmers/audio/video/whatever, then they do matter, but you probably already know lots more about them than my rusty memory can regurgitate here.
Here again, poorly designed gear will show the problems earlier. The hundred kilobuck mixing console will not even notice the interference that will drown the guitar center deal of the week no-name mixer.
Your friends in this circumstance are distance and attenuation. Putting the lighting dimmers on one power feed in the facility and the audio video gear on a different feed will help a lot. Mostly because the hash and trash from the dimmers will attenuate some as they travel back to the building's main panel, and the rest of the building's electrical distribution system provides more avenues to absorb that hash, leaving less to come back up the feed to the audio and video power distro. ANd no, putting dimmers on one phase and AV on the other phases really doesn't do that much to help, and sometimes will even hurt.
A power isolation transformer just upstream of the dimmers or the AV power distro will help a lot. If you have room in your truck for an additional several hundred pounds, go for it. They are standard gear in mobile recording trucks. Having good grounds improves things, electrical safety grounds. Undersizing your ground and neutral conductors, which is allowable under come circumstances in the various electrical codes (consult a local authority for local details) makes things worse, as now that hash and trash has a higher resistance path back to the transformer or generator where they ultimately go to die.
IGBT dimmers, I have read, are much less abusive of the power supply. I haven't really investigated them yet.

In short, lots of good grounding practice, and if you need to break a ground conductor, break it on a signal line, don't break off the electrical safety ground. The life you save might be your own, or even more importantly, mine. Local practices differ, just because they are different, does not mean they are wrong, unless you are FW.

--Dale
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