Loops (or "Let Me Hum a Few Bars…")
By Ken Simmons
You're excited! you just bought your first Home Theater setup: Pro-logic receiver, 5 speakers, subwoofer, Hi-Fi VCR, and 35" TV. You spend the next hour carefully hooking everything up, double-checking the connections with all the hookup diagrams in your User Manuals, and neatly dressing the wires and cables.
You're finally done. You stand back and admire your handiwork, praising yourself for finally having a decent system for watching your favorite videos and TV programs. Then, holding your breath, you apply power to all the components.......
...a loud "HUUUMMMMMMMM" emanating from the speakers is the reward for your efforts.
Congratulations - you're the newest victim of "ground loop". Fret not, there are ways to reduce, if not totally eliminate, this major annoyance of audio and video lovers worldwide.
Simply put, "ground loop" is a condition where all your electrical "grounds" aren't at the same electrical potential, namely zero volts AC and DC. The next sections will give you detailed information about "what is 'ground'?" in relation to your home electrical system.
In the United States, household electrical power is derived from a 4 kilovolt to 230 volt AC step-down transformer. "But Ken," you say, "all my appliances are rated for 115 VAC!" Don't panic, gentle reader, the explanation is simple. I refer you to Figure 1. As you see, the transformer has a 230V center-tapped secondary. It's this center-tap that, according to US Electrical Code conventions, becomes the "neutral return", or "electrical ground" for our appliances.
"But wait!" you say, "how do we get 115 VAC from this transformer?" An excellent question. Again referring to Figure 1, let's look at the various voltage potentials existing at the three secondary wires of the transformer as defined in Table 1.
|A||B||230 VAC (nominal)|
These are the three main measurements to make. You can undoubtedly figure out the other "permutations" of connections, based on which wire you use as your "ground", or return, connection.
"OK Ken, I see how you get 115 VAC for my normal appliances. However, some of my appliances run off 230 VAC. If I connect the 'ground' wire to the transformer center tap, it won't work. Where does the 'ground' wire for these appliances hook up to?" Excellent question, gentle reader.
The "ground" connection for these 230 VAC-operated appliances (clothes dryers, stoves, water heaters, etc.) is where the "ground loop" problem arises.
Per US Electrical Codes, every electrical supply (residential and industrial/commercial) requires an earth ground connection. For home electrical installations, this connection usually consists of a simple steel rod, 4 feet long or so, driven into the ground somewhere near the house. Other methods of earth grounding consist of using a buried cold water supply pipe. Whatever the method, somewhere in your home's electrical system is a connection to earth ground.
This creates an immediate electrical potential problem. If you were to take an AC voltmeter and connect one lead to the earth ground and the other lead to the transformer center tap (the "neutral return"), you will not read zero volts AC. To get around this "problem", US Electrical Codes require that center tap to be directly connected to the earth ground at your home's breaker/fuse panel. Refer to Figure 2 for the connections.
The "GND" is the earth ground. If you look at the side of your breaker/fuse panel, you'll see a thick piece of bare copper wire bolted to the panel "going somewhere". This is the earth ground connection connecting your breaker panel to earth ground.
For 230 VAC appliances, the "supply" wires connect to points "A" and "B" of the transformer and the ground wire connects to the earth ground/CT connection in the panel. This is how 230 VAC-operated appliances get their full 230 VAC. Confused yet? well, the next section will hopefully clear things up.
With the basics of AC power entering your home dealt with, let's deal with how your electrical outlets are wired. If you examine the wires leading to your outlet, you'll notice three wires: Black, White, and bare copper. These correspond to "hot" (115 VAC supply), Neutral (the "electrical ground" return), and Ground. Referring to Figure 3, you'll see how your outlets are wired.
Referring back to Table 1 and Figure 2, you'll see that you now have 115 VAC between Black and White wires and between Black and bare wires.
"That's redundant!" you say. You're right. However, per US Electrical Codes, the bare copper wire present in the "Romex" cable that runs from your breaker panel to the outlet must be connected at both ends. At the outlet, it's connected to the "green" screw or the junction box if it's made of metal and to the Center tap connection in the breaker/fuse panel. It's called a "safety" ground. For more information on home wiring, visit your local library or contact a local certified electrician.
With all this information, you can see that your regular 115V electrical outlet wiring contains two "ground" connections: the neutral return (white wire) and the "safety ground" (bare copper) in addition to the black 115 VAC power line. On the surface, you'd conclude there's no "ground loop" evident in your home's electrical wiring (I refer you to the definition of "ground loop" at the beginning of this article). Unfortunately, things aren't that simple in the "real world".
Remember the definition of ground loop: "electrical grounds aren't at the same electrical potential, namely zero volts AC and DC." This now brings us to what generates these loops. Refer to Figure 4.
As you see, each outlet has its' own separate Neutral and Ground wires to the panel. As Figure 2 detailed, all these bare ground wires are eventually connected to the Neutral wires of the same outlets. Because these are wires, they're affected by Ohm's Law, E=IR. Since any wire has resistance, when you pass a current through it, you'll develop an electrical potential across that wire.
"Ground loop" arises when you have current flowing through one ground wire developing a voltage across that wire and you connect another ground from a different source to that same ground wire. Referring to Figure 4, assume there's a small current flowing through the wire from outlet 1 to the panel. There will be a small voltage potential across that wire (typically less than 10 VAC) if you measure it using ground 3 as the return path for your meter and measuring at outlet 1's ground terminal.
"So? I'll just connect all my audio gear to one outlet and avoid this problem!" you say. Not a problem - in fact, that's what you should do as long as you don't exceed the current rating of the breaker supplying that outlet (15 Amps per US Electrical Code).
NOTE: The "safety ground" connection is the most problematic one, especially if you're blessed with metal junction boxes rather than plastic. In most (if not all) cases, the bare copper wire is simply surface-clipped to the junction box rather than securely screwed to the box or outlet's "Green" screw (i.e. a "friction" fit). This "friction fit" method uses the outlet's mounting "ears", physically and electrically connected to the outlet's "ground pin", to make the electrical connection to the metal box. The problem? there's a relatively "high-resistance" connection between the outlet's "ground" pin and the breaker panel. Unfortunately, "outlet checkers" only check for the presence, not the "quality" of this electrical connection. Unless your equipment actually uses the ground pin (i.e. 3-prong plug), it's nothing you need to directly worry about. If your equipment does use that third wire ground, you'll have to take measures to improve the "ground" connection to the "ground" pin of the outlet.
Now that we understand what "ground loop" is, here's where it comes into play. Per US Electrical Codes, all audio and video gear must have a DC "soft ground" connection to the AC Neutral Return. This "soft ground" consists of a large value resistor connecting the device's metal chassis (if it has one) and DC power ground the AC neutral return. In many (if not all) cases, the audio signal grounds are also connected to the device's internal DC power ground. Figure 5 illustrates these connections.
This resistor, which is internal to the device (TV, VCR, Receiver, etc.) can usually be seen near the device's power transformer if you open the device. The resistor's high value, somewhere between 1 megohm and 2.2 megohm, is dictated by Underwriter's Laboratory (UL) US Electrical Code safety standards. Again referring to Ohm's Law, if there's any current flow whatsoever through that resistor, there will be a voltage developed across that resistor. All it takes is less than100 Millivolts of AC "ground voltage" across that resistor to create that nasty "HUUUMMMMM" you hear in your system.
"So, I'll just clip that resistor and be done with it!" you say. Fine and dandy. However, doing so will void any warranty on the device and create a possible electrical shock hazard if you should be touching the device's metal case and another "ground". No, leave that resistor in place because that is not the prime source of ground loop we all experience at one time or another.
The prime source of "ground loop" noise comes from none other than your omnipresent and innocent cable TV hookup! Figure 6 clearly illustrates the problem.
As you see, the Coax shield is grounded at the cable provider's main distribution outlet, but not at your house! that cable "ground" connection is usually miles away from your house. Since earth grounds can be at different potentials, even at different houses on the same street block, it's a given that there will be an AC potential across the coax shield if you measure it using your own house's ground as a reference. Refer to Figure 7 for an illustration of this problem.
As you see, it's the different physical ground connections between your TV/VCR and the resultant differing electrical potentials that create the annoying "HUUUMMMMM" many are plagued with.
Now that you understand what ground loops are, let's now deal with how to eliminate them. Since the main culprit of ground loop noise is the cable TV coax feed, let's deal with it first.
Since we know ground loop is caused by differing ground connection potentials, let's deal first with "breaking" that ground connection at the "source" without losing the cable TV signal feed and required signal return back to the cable company. By "source", I refer to the "wall plate" where the cable comes into the house.
Let's start with Figure 8. This is called a "double balun" connection using a pair of 75-300 ohm baluns generally available at drug stores, "small" department stores (i.e. Target, K-mart) "large" grocery stores, and your local Radio Shack.
The only problem is, while the signal is indeed coupled from cable co. to TV, there's no sheild isolation! unfortunately, these are the most prevalent type of cable baluns around because they're the cheapest to produce. No, what we're looking for is illustrated in figure 9 – total shield isolation using one or two totally isolated baluns.
Parts list for balun-based isolator
2 ea 75-300 ohm isolated balun
When you go shopping for these baluns, take a multimeter with you. Using the highest resistance scale on your meter, measure continuity between the threaded shell and either of the "tail" wires. If the reading is less than open circuit (i.e. "dead short" as read on the meter), that balun is unsuitable. As shown in Figure 9, the idea is to get at least one balun with "total isolation" between the 75 ohm and 300 ohm ends. If you can find two that are isolated, so much the better! The only problem with this method is this: 75-300 ohm baluns with total isolation are rare and expensive. Be prepared to shop around a long time as well as pay a "premium" price for them.
Another method that will break the ground, while still passing the signals, is using capacitors as couplers. Figure 10 illustrates this method.
Parts list for capacitor-based isolator
2 ea "Chassis mount" F-61A connectors (Radio Shack #278-212 or equivalent)
2 ea .01Uf ceramic disc capacitors, 500WVDC rating (Radio Shack #272-131 or equivalent)
This method, while providing the required signal coupling, also provides an "AC return" back to the cable company. Furthermore, this methos is very inexpensive in terms of parts cost (59 cents for both caps and 99 cents for both connectors at Radio Shack).
NOTE: Both capacitors must be the same value to maintain a "balance" in the signal and return. Also, make sure they have a high AC voltage rating (200V or higher) for safety reasons. If you assemble it in a case, do not use a metal case or the shield will not be isolated!
Once you assemble the parts, test it by carefully connecting it in between your cable's "wall plate" and "main" TV/VCR feed (i.e. before any splitters, etc.) to verify its' operation. Make sure the capacitor wires and jack terminals are isolated from each other, or the isolation won't be there (measure with your meter if necessary).
NOTE: if you experience too much signal attenuation through the unit, substitute larger value capacitors for both units until the attenuation is minimal (I experienced no appreciable signal loss with .01uF caps). Try to avoid using polarized electrolytics as they may not "fare well" with the high-frequency RF present on the cable, plus high-voltage units are physically very large.
Once you're sure the assembly works, "seal it" with heat shrink to give it some rigidity (again, verify isolation after "sealing").
IMPORTANT: keep the physical length of the final assembly as short as possible to minimize induced power-line noise infiltration. Example: I assembled mine as an "in-line" unit, covering it with heat-shrink for a measure of rigidity and it came out to 2" in length. If you plan on installing it in a "hostile" environment (i.e. outdoors or under the house), encapsulate the completed unit with epoxy or silicone caulking material to provide a measure of weather-tightness. If it's impractical to add epoxy or caulk, wrapping with black vinyl electrical tape after installation (extended past both connectors roughly 1" or so) will provide some environmental protection, further isolate any exposed shield connections and add to the overall rigidity.
While ground loop is the primary cause of unwanted "injected" power-line noise, there are other noise sources that need to be mentioned:
To reduce/eliminate these noise problems, try one (or more) of these ideas:
The subject of Ground Loops, and other sources of noise in A/V gear, as well as their reduction/elimination is not an "exact science". In fact, luck plays a large part in tracking down and eliminating unwanted power-line (and other) noise. Consider a course in "Voodoo" as a baseline in understanding this field of A/V headaches!
I know all my "overseas" readers may feel cheated because I didn't deal with their power systems. Don't fear - all the "basics" apply to you as well. Just substitute the necessary power distribution concepts (i.e. 220/230 VAC, 50 Hz wiring) in place of the American AC power concepts and you'll essentially be working along the same lines as I outlined for my North American readers.
There's one extremely important point I want to stress in all this is:
Do nothing to compromise electrical safety - your life, as well as the "life" of your gear, is way more valuable than that one foolish "disconnection" or "connection" that eliminates the hum but puts full power-line voltage potential on the chassis of your gear. If the best you can do is reduce hum to a barely audible level (i.e. you have to "crank" the amp to hear it), then congratulate yourself for that.
I hope this little piece helps your understanding of noise, the sources of noise, and how to reduce/eliminate it in Audio and Video equipment. I enjoyed writing it and I hope it's valuable to all who read it.
Any questions can be directed to me via E-mail and I'll do my best to answer them.
© Ken Simmons 1997.
This file has been reproduced from SMR Home Theatre
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