Induced hum won't usually be cured by using baluns or transformers, it will be cured by shielding or moving the susceptible conductor(s) away from the source, or moving the source away from the conductors. Electrical isolation is used when a ground loop exists and it's impractical to cure without re-wiring the electric service & cable infrastructure or the solution violates the NEC. Lifting the power cord ground pin of a device that has its power supply grounded to the chassis is one of the latter.

Are your racks grounded and if they're connected to different panels, is a bonding conductor in place between the panels?

Sometimes, loud humming is a sign that the grounding of the electric service is bad and a significant AC voltage is being conducted by audio, video or control wiring. It could also be caused by using so many power filters- you would be better off using one large unit for each rack and then, using a long un-filtered power strip (Middle Atlantic makes them and they're mounted vertically in the rack).

How many power circuits is fed to each rack? If more than one (I can't see using one for each rack- determine the load at each rack and plan accordingly. It's possible that the load is causing enough voltage drop that a difference in potential is causing the problem.

It's also possible that the feed to the cable box isn't grounded or their line is passing DC to the cable box. Try disconnecting the cable from the box and if the hum stops, use an isolator on the cable feed. Or, you could have the cable company fix the problem they caused.

Absolutely, 100%.

We just experienced this at a property. Cable tech on site insisted it was our problem and something was wrong with our gear or with the electrical in the house. He swore up and down all of his items were correctly grounded. I requested that he show me everything and while walking with him I asked to open the pedestal so he could show me it was grounded...low and behold it was not. Of course he didn't have the ability to ground it or to provide an isolator so I ordered one and the hum is gone.

Have you tried turning down the amps, increasing the gain on the input/output of the matrix and slowly turning the amps up?

Just fixed a system carefully going over every source doing this.

For the less experienced: "Current" is movement of electrons. This movement is accompanied by an electric field and a magnetic field. We usually think of the magnetic or electric fields as forcing the electrons to move, but a stray electron blasting through the area (even in a vacuum) will be associated with an electric and a magnetic field. When we talk about "Induced hum" in our electronics, we usually are implying magnetic field coupling where an external magnetic field generates current in the wire that adds to the desired signal.

In an electric generator we are causing relative movement between a magnet and a wire. This movement forces electrons to move in the wire -- current. If there is no movement, there is no current. (and no voltage)

In a power transformer we are using the alternating magnetic field created by electrons moving (current) in the primary winding, to induce a magnetic field into the secondary winding (forcing electrons to move in the secondary -- current), and if we are clever about things, the secondary voltage will be more convenient in some context.

If your signal wire passes through a region with a relatively strong magnetic field (possibly created by a motor, power transformer, or current carrying power wiring) this magnetic field will create a low grade transformer that will add hum to your delicate little signal.


Again, for the inexperienced: In order to understand "Ground Loop" (by the way, a "loop" is simply a piece of wire or a circuit trace that goes from here to there) let's define a perfect four terminal device, an amplifier, where the output is simply a multiple of the input. Let's keep things simple and label the input and output terminals as "i+", "i-", "o+", and "o-". Our theoretical amplifier is buried deep inside one of the boxes that we mount in our rack. Actually, there might be multiple "amplifiers" mounted inside our box. (Phono preamp, power amplifier, RF amplifier, tone control, etc.)

We usually call the zero point in our circuit "ground". Unfortunately, we are very sloppy with this term and "ground" could be at the "i-" terminal of our theoretical amplifier, a point on a circuit board, the shield on a coax cable, a connection to the chassis, the third wire in a power cord, a rod driven into the soil, or any other reference point we might use to measure voltage.

As you can imagine, the sloppy use of the term "ground" will lead to trouble.

Remember, whenever current is flowing in a conductor, there will be a voltage drop due to resistance. When we are modeling circuits we often draw a little generator symbol in this conductor to remind us that there is a voltage difference from end to end when we use the same reference point while measuring the voltage on both ends of the wire and this "generator" is the result of some physical process. (Actually, there is a voltage drop in the leads of the voltmeter too)

By the way, it can be very helpful if you draw a little generator symbol in every little scrap of wire, shield, circuit trace, transistor lead, etc. in your project. A physical process will be driving each one of these little "generators". Of course, many of these voltages will be insignificant, but many will be quite significant.

Now, let's imagine a printed circuit board where our delicate little signal enters on one side and the shield is connected to the board's "ground" trace at this point, while our ideal amplifier is located on the other side of this board. Next imagine a sloppy bit of design work where a fan is also connected to this "ground" trace and the fan is located on the same side of the board as our ideal amplifier and amplifier and fan share the same power supply. (We now have a little noise "generator", associated with the fan, introducing a "signal" into our "ground") Meanwhile, our signal travels straight to the ideal amplifier's "i+" terminal. We have created a classic grounding issue because the current for the fan is added to the signal presented to the "i-" terminal of our ideal amplifier and our amplifier output is a multiple of the voltage drop in the "ground" caused by the fan current plus a multiple of our signal. If the design is really sloppy, the ground signal might swamp the audio signal.

There have been experiments where the same circuit is built by different teams and the resulting equipment performed differently due to one team's better layout and management of "grounds".

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The above is a quick overview.

The is enough meat in the "ground" drama to keep specialists employed for a career -- just straightening out subtle and gross oversights by others.

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A consideration for our shielded wire: The shield is used to protect the coaxial central wire from stray electric and magnetic shields. Actually, the shield is not very effective at protecting against magnetic fields. (this is why we don't want to run our signal cable beside high current power wiring) The idea is that the shield represents an easier path for the fields than the insulator around the central wire and the fields will tend to remain in the shield and stay away from our signal.

Now consider the idea that the shield is connected to the chassis at both ends of the cable and one can measure a voltage difference between the chassis. Now, we have current flowing in the shield caused by the different chassis potentials. You might come across this sort of situation labeled as "circulating current" because it is present with or without our audio signal. This is really bad because there will be current flowing in the shield due to this voltage difference. Since there is a magnetic field associated with this shield current, a signal is induced into the central wire that the shield should be protecting. (never mind that there could also be messy "ground" management in either box that contributes to our troubles) Because this chassis to chassis voltage difference is usually associated with the power line, we experience "hum".

This suggests a quickie technique that many of us have used: Break the shield connection at one end of the cable and make a chassis to chassis connection outside of the shield. This will eliminate chassis to chassis current in the shield. I'm not suggesting that this is universally the best technique, but sometimes it will result in a quickie miracle.

BTW, this is a technique used by some premium audio cables.

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Whew! I'm out of time at the moment, maybe someone will follow on and explain why "balanced line" is so helpful when dealing with and preventing noise issues. And, why that missing cable system "ground" can be so bothersome.

Link the two racks with fiber and go home to your family.

No copper no hum.

Lifting the ground of a three-prong plug is DANGEROUS. Only do so as a temporary troubleshooting measure to see if the problem is ground-related. If you determine that it's ground-related, then you have to find a solution other than lifting the ground. Some products are built with switches that allow you to remove the connection between the chassis and the power ground, and it is safe to use those switches to disconnect the chassis from the power ground. All other products with three wire power cords are designed to be used with the ground pin connected. More importantly, they are UL approved only if the ground is connected, and if you remove that ground yourself, you are liable for any and all damages that even MIGHT happen as a result. Fire comes to mind as something you could be blamed for.

Now, on to tonight's next topic:


buzz, taking that path is fraught with danger!

I'd be careful using that approach here. Someone or other is going to feel insulted, or will argue that what you're talking about here isn't germane (and I don't mean Jackson). For instance, the entire subject of induced hum was brought up in a discussion of ground loops hums, and the two things are barely related at all. Now just wait for the argument against that statement.

Plus, you'll end up writing long posts, because once you start getting technical, you'll probably feel like you need to write defensively in advance, including all the technical terms and provisos that will keep someone from arguing with you. Oh, and people will argue that your posts are too long.

Now I'll read that post of yours. Thanks for it

buzz,
Good post, man!

A concept that has helped me in the amplification thinking is the concept that the input to the amplifier is not what's fed in on the -+ lead. Instead, it's the difference of voltages between the i+ and the i-. That, all by itself, explains noise being injected via the ground, without any fancy inducing stuff.

The technique used by some audio cables is slightly different -- there's a hot wire and a neutral wire and a shield. Using your designations, i+ of the cable goes to its other end, o+, i- of the cable goes to its other end, o-. The shield, however, is connected to i-.

The purpose of this is to route electrostatic noise to ground only at one end. Since the source end of any well-designed pair of component will have a lower impedance, the overall system will have less noise if the shield is grounded at its source end.

This is how those puzzling first Monster Cable cables with the arrow were wired. The arrow was to make you plug the source end into the source. I worked briefly for Ryder Sound (Academy Award winning sound company) in the late 60s; their double-phone plug to phone plug rack interconnection cables were wired this way, indicating that this practice goes back at least to the 1940s.

To say it simply, the reason that balanced cables are quieter than other cables is that the wires are arranged with opposing voltages on the two wires, and are twisted such that neither wire is closer, on average, to any surrounding fields that might induce a voltage. As a result, when voltages are induced in the wires, the same voltage is induced into each wire; at the other end, the input "sees" the different voltages on the two wires, which are our signal, and rejects the same voltages on each wire, which are noise. Identical voltages induced into both wires are called common mode voltages, since the voltage is the same on each. The ability of an input to ignore the same voltage on both wires is called CMRR, meaning common mode rejection ratio.

And yes, that's saying it simply.

Run a long extension cable through the house and connect the 2nd rack to the same outlet as the first rack. If the hum goes away it's a power phase issue. So moving the breaker to the same phase should fix the issue.

In the future, I suggest not having two racks. If it's an absolute must, you should be using fiber to connect the two.

Right- sometimes, the cart is placed ahead of the horse.

I was referring to induced current being a problem, often caused by cables in close proximity and parallel to other wiring that may be conducting much higher current and/or voltage. I did a job in the late-'90s where the audio system would make a loud snapping sound if the level from the mixer reached a certain level. It was a 70V system with previous wiring woven through other cables without any care about how that could affect things.

WRT "Whew! I'm out of time at the moment, maybe someone will follow on and explain why "balanced line" is so helpful when dealing with and preventing noise issues. And, why that missing cable system "ground" can be so bothersome. ",

In addition to what Ernie posted about CMRR, and that one aspect is the main reason true balanced lowZ circuits are less noisy, the audio circuitry and chassis aren't at the same potential in a true complimentary balanced system- the audio is usually floated ~50 Ohms from the chassis and the shield is only supposed to do what the name implies- shield the audio or video from stray noise. Devices using unbalanced signal don't even need more than the center conductor to use the signal if they have polarized power plugs and the chassis is connected to the neutral or if the chassis are making good contact. Look at the shorting jumpers on most of the integrated amps from the '70s-'90s- it was a piece of wire, bent so it could be inserted in the Pre Out and Power Amp In jacks. No shield whatsoever and no noise was introduced, either.

Also, EMI shielding can be used, but it's a bit less certain as to whether it will work when it's protecting cabling. Speaker magnets, tape heads and things like that can be surrounded by something like Mu-metal but that's not a good solution for cabling because it's stiff and the edges are very sharp although it would be a good thing if they have come up with a way to make this in a foil but I'm not aware of it. Another way to protect cables is to use copper tape with stainless braid (looks a bit like screen material) and grounding it at both ends, assuming the ground points are equipotential. If they aren't, it would be grounded at the end with the sources and preamp.

EMI was more of a problem with car audio in the late-'70s and early-mid '80s. I think it was the Chevy Celebrity that had a service bulletin for re-routing the main heater wire because it passed behind the head unit and would induce noise through the tape head. The Cavalier had its own bulletin for the hood bolts- the OEM part was coated with a material that didn't stop the hood from being bonded to the chassis and when body shops would replace the hood or its bolts, they often used the less expensive oxide-coated bolts, which provided electrical resistance and therefore, allowed noise to exit the engine compartment.

The NEC requires bonding the racks but most people don't. IIRC, it's Article 25 or 725.

Too late for that unfortunately. The racks are about 300 ft apart in a fully finished home.

I will be back there his week and I will try out the suggestions in this post and get back to you.

Had a similar problem years ago. Went all the way from CA to IN to CEDIA to take a class on troubleshooting audio buzz/hum in systems. Low and behold, it was taught by the owner of Jensen transformers. I learned great insight, not only as to the cause, but how to troubleshoot. I went back home and ordered some parts from Jensen (Ironically, about 40 miles from my home in CA). Between an audio isolator on an LFE sub input wire and another isolator on the coaxial input to the cablebox, it fixed the problem.

No common ground, slight difference in ground potential, and reference point looks like voltage, or an audio signal to the amplifier and it give you buzz/hum.

Dtruxal,

Using an Ohmmeter, is there any conductivity from any contact on the mini-phone plug on the Muxlab devices that you are using and any pin on their CAT-5 side? (This is not so hard to do, just make up a CAT-5 plug, strip and mash all of the wires together)

After talking to my Prewire guy we did run fiber lines between the racks. That sounds like the best solution.

I have never ran fiber between racks, what is are some good converters to use? The only ones I could find ran 8 zones for $6,000. Is there a cheaper option?