Ground Loops (or "Let Me Hum a Few Bars...")
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.
What Is a Ground-Loop?
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.
Electrical Power and Grounding
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.
Table 1
| From | To | Voltage |
|---|---|---|
| A | B | 230 VAC (nominal) |
| A | CT | 115 VAC |
| CT | B | 115 VAC |
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.
Home Wiring Basics
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".
Ground Loops
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.
Ground Loop Reduction/Elimination
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.
Another method that will break the ground, while still passing the signals, is using capacitors as couplers. 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)
While ground loop is the primary cause of unwanted "injected" power-line noise, there are other noise sources that need to be mentioned. Light Dimmers. Your innocent triac-based lamp dimmer is a major source of unwanted power-line noise. With dimmers, the noise is injected directly on the Neutral return (white wire) as well as radiating said noise. There is only one practical way to eliminate this noise source: Removal of all dimmers from your home's wiring. Miswired Outlets. The possibility exists that you have one or more "miswired" AC outlets where the Hot (black) and Neutral (White) wires are reversed on the terminals. This condition can be immediately ascertained by using an "outlet checker" (Radio Shack #22-101 or equivalent). Any outlets determined to be "miswired" should be immediately corrected as it only takes one miswired outlet to "screw-up" every outlet. "Lost" Saftey Grounds. This condition can also be determined with the "outlet checker". If your A/V equipment uses 3-prong (grounded) plugs, this condition can cause no end of trouble as well as expose you to potential shock hazzard. These outlets should be corrected ASAP for your safety and for the safety of your 3-prong A/V equipment as mentioned earlier!
While the article mainly focuses on the technical aspects of ground loops in audio and video equipment, it highlights the importance of understanding electrical grounding and potential solutions to reduce or eliminate these annoyances. It's crucial to ensure safety and consult professionals when dealing with electrical wiring and connections. Having a solid understanding of ground loops and their causes allows for a more enjoyable and optimized experience with your home theater setup.