by Jim Tanenbaum, CAS
For many years, my sound cart has been a “cable-free zone.” Besides the talent’s radio mikes, both my boom operators are wireless, as are any plant mikes. My cart runs on batteries (105 amp-hours worth), including the built-in worklights. I have two UHF video transmitters that I connect to the video assist system so I don’t need coax cables from it to my monitors, and I send the audio out by Comtek. Director, script, producers, etc., all get Comteks, and my boom ops have their Comteks on a separate frequency. (I have a third channel available if each of them needs to hear only their own mike.) As a result, buzzes from H.I.D. (High Intensity Discharge) lights, such as H.M.I. or Xenon, 60-Hz hum from power cables, RF (Radio Frequency) pickup from radio/TV stations, audio/timecode crosstalk, and static from moving bad cables are all a thing of the past.
Or are they? Unfortunately, my cart is not, in fact, “cable free,” because all my equipment is interconnected with … (wait for it) … CABLES. And some of you still use cabled mikes, connect to video assist with cables, run some or all of your gear on AC, send/receive timecode (TC) by cable, etc. Here’s what I’ve learned in 45 years about dealing with these problems: like radio mikes, cables also work partially by magic. What appear to be similar problems often do not respond to the same solution, and equipment that is trouble-free one day may not be the next, even though everything is still in the same place.
The cables interconnecting various pieces of equipment on your cart are the easiest to deal with because they are under your complete control, do not change position (usually), and do not have to be connected and disconnected as much. Cables from your cart to somewhere else are subject to the “slings and arrows of outrageous fortune” in the outside world.
Most of this article is written at a fairly basic level, not requiring a great deal of electronics knowledge. There are a few advanced discussions here and there that can be safely skipped, or consulted with a more “techie” mixer. If the text becomes incomprehensible—just keep reading and it will soon clear up. (The other end of the cable is that I have overly simplified a few things, so I ask the technically-literate to please bear with me.) Local 695’s website has an excellent online “Ground Loop” seminar by Bill Whitlock that is incredibly detailed and technical. It is almost exclusively dedicated to AC power noise problems in fixed installations, but very useful nevertheless and well worth several hours to watch. I have made sure to cover his relevant main points in this article as well.
A final note: cables have a much higher retail markup than recorders or radio mikes. Rental cables, in spite of Herculean efforts by the rental companies, are not 100% reliable. Therefore, if you know which end of a soldering iron to grab; make your own. If not, get your local techno-nerd teenager to teach you or take the Local 695 cable construction class.
Some Basic Information to Start With
1. Mike-level signals are in the 5 to 50 mV (millivolt = 1/1000 volt) range. Line-level signals are in the 0.5 to 5 V (volt) range. AC power is in the 100 to 200 V range. If you are involved with H.I.D. lighting units, the cable connecting the lamp to the ballast (called a “head feeder”) carries thousands of volts (KV). The higher the voltage, the easier it is for the current to “leak” into some other circuit, like your mike cables.
2. Civilian AC power frequencies are in the 50 to 60 Hz range. (Hz or hertz, formerly called “cycles-per-second,” or “CPS,” a much more descriptive term.) Audio frequencies are conventionally said to be in the 20 Hz to 20 KHz (KiloHertz = 1,000 Hz) range, although few people today, especially anyone over 16, can hear anywhere near the top or bottom of that. RF (Radio Frequency, but not limited to “radio” signals) starts around 500 KHz with the AM radio band, and goes upward from there. MHz = 1,000 KHz and GHz = 1,000 MHz. The RF spectrum is further subdivided into HF (High Freq) = 30-100 MHz, VHF (Very High Freq) = 100-300 MHz, and UHF (Ultra High Freq) = 300-1,000 MHz. There is also ULF, VLF, LF, and MF, but they probably won’t concern you. Frequencies in the GHz range are often called “microwave” or “MW.”
3. All electrical conductors (except for superconductors, which you won’t be dealing with) have some amount of resistance, measured in ohms (Ω). When an electrical current flows through a resistor it loses some of its voltage, although the amount of current (measured in amperes) remains the same. The amount of this “voltage drop” is given by Ohm’s law: E = IR. (E is voltage; I is current; R is resistor; and IR means I times R.) For example, a table lamp with a 100-watt light bulb draws about 0.9 A (ampere or amp) at 110 V. If the 2-wire power cord has a resistance of 1 Ω in each wire, there will be a 0.9 V drop in each wire (0.9 A times 1 Ω), for a total of 1.8 V, so the light bulb will get only 108.2 V across the terminals of its socket. IMPORTANT: This drop is distributed such that the “hot” terminal of the socket will be at 109.1 V with respect to “ground” (the meaning of which we will discuss later), and the other (“neutral”) terminal will be 0.9 V above ground. This “IR drop” phenomenon is the cause of most of our woes with “ground loop” problems, as we shall eventually see.
4. Whenever two wires run in proximity, energy can transfer between them by two mechanisms. This is true whether they are internal circuitry in equipment or inner conductors in a cable. Shielding may reduce or increase the effect, and the shield can even serve as a conductor itself. Current flowing through a wire produces a magnetic field around it, and if the flow varies, the varying magnetic field can inductively couple to the other wire(s) and induce a current flow. In addition, if a wire has a voltage on it, even if there is no current flow, a voltage can be capacitively coupled to the other wire(s). Sometimes the energy source is the pair of wires of a circuit, with the signal current flowing up one wire and back down the other, or a static condition with the voltage on one wire positive and the other negative. In these cases, the corresponding magnetic or electric fields around each wire are opposite polarity and cancel out, theoretically. In reality, the physical arrangement of the two wires is never exactly identical so the cancellation is never complete. There will still be some residual field left to interact with the remaining conductor(s).
5. Cables have a characteristic impedance, also measured in ohms but using the symbol “Z” to represent impedance, instead of “R” (used for resistance). Impedance is a more complex form of resistance, having capacitive and inductive components in addition to resistive. The main thing you need to know is that for analog audio cable, its impedance is relatively unimportant. For digital audio and timecode, it may be necessary to consider impedance, especially for long cable runs (see baluns later in the article). For video and RF antenna cables, impedance definitely needs to be taken into account.
6. Input and output circuits also have a characteristic impedance, and some of the same considerations mentioned above apply. Most professional dynamic mikes are Lo-Z, about 150 Ω, but some ribbon mikes are 50 Ω. Semi-Pro Hi-Z mikes are about 1,000 Ω (1 KΩ). Professional line-level circuits are 600 Ω. High-impedance circuits are many thousands of ohms (47 KΩ is common). In general, you can connect the output of a low-impedance device to a high-impedance input without distorting the signal, though the Hi-Z input circuit may not have enough gain. (And certain types of 600 Ω line-level outputs may not deliver the full level of low frequencies to a Hi-Z load. You can usually fix this by connecting a 600 Ω resistor across the input terminals.)
7. To summarize the above, you can connect a 50-Ω ribbon mike with a 110-Ω mike cable to the 5 KΩ “bridging” input of an audio amplifier with no problems. If you are sending AES/EBU digital audio to a device with a 75-Ω BNC input, but use a 110-Ω mike cable and a simple mechanical XLR to BNC adapter at the end of the run, you may or may not have a “jitter” problem, depending on various things including the length of the cable. But if you use a 50-foot length of 75-Ω video coaxial cable to connect your 50-Ω wireless mike receiver to a 50-Ω sharkfin antenna, you will definitely notice a loss of range compared to the proper 50-Ω coax.
8. “Crosstalk” is the transfer of a desired signal to another circuit where it is not wanted. Factors that increase crosstalk are: higher voltage, higher frequency, closer proximity, less effective shielding, and ground loops. Note that digital signals (audio and timecode) are a type of “square wave” that have high-frequency components (over 20 KHz) and can more easily crosstalk into other circuits compared to analog audio signals. Crosstalk can occur between external cables, between components of multi-cable snakes, or between the wiring inside equipment. (e.g., if you’re recording TC on one audio channel of a video camera, attenuate the TC signal to at least 30 dB below full scale with an external in-line pad to prevent TC crosstalk inside the camera to the audio channel.) Even though some of these signals are above the audible range, they can still cause audible problems, as discussed in the next section for RFI (Radio Frequency Interference, but used for any type of noise signals in the MHz range.)
9. “Noise” refers to any unwanted addition to a signal (whether it is immediately audible or not). Beside audio and TC signal crosstalk, interference from AC power is another major offender.
AC power noise consists of hum, a low-frequency (usually the AC power frequency) tone, composed of a single, pure sine wave signal, or buzz, which adds harmonics to the basic hum. Because hum is a pure tone (60 Hz, or 50 Hz in some other countries), it can be more or less easily filtered out in post. A buzz, with its harmonics, can sometimes still be removed with a sequence of filters at 60, 120, 180, 240, 300 Hz… (or 50, 100, 150, 200 Hz…), but if there are nonlinear elements in the source, there will be non-harmonic components that cannot be readily eliminated. WARNING: Many military vehicles and installations use 400 Hz AC power or even higher frequencies. Working in this environment is extremely challenging because any AC pickup is almost impossible to filter out.
When AC power circuits have a bad connection point (loose or corroded; whether visibly/audibly arcing or not), it can create “static,” which is heard as a sputtering or ripping sound.
Static (actually a form of radio signal) is created whenever electrical current flows through a mechanically imperfect (e.g., a rubbing or lightly touching) joint, as compared to a solidly clamped or soldered one. Note that static can also be created if one of your own cables has an intermittent connection at a plug, or a broken conductor or shield wire(s).
Finally, “inaudible” RF audio and video signals can produce audible noises, especially from high-powered commercial radio and television stations, even at a considerable distance. (Or a nearby video assist or remote control transmitter.) While their frequency is far above the audio range (MHz vs. KHz), if they infiltrate audio equipment, nonlinear components in the audio circuits can “detect” these signals and produce audible interference based on their AM modulation. RF signals can also interact with wireless mikes by heterodyning, creating sum and difference frequencies that fall in the audible range. And if their level is great enough, RF signals can overload lower frequency circuits, causing distortion or complete muting.
A Balancing Act—It’s Easier to Balance on Two Wires Than One
Audio circuits can be either balanced or unbalanced. Balanced circuits have their signal carried by two conductors, neither of which is connected to “ground” (to be defined later, but we’re getting closer). The two wires can be surrounded by a metallic shield, or not. Unbalanced circuits have only one conductor, surrounded by a metallic shield that is used to complete the circuit as well as to keep out interference. Almost always, balanced circuits are less susceptible to noise than unbalanced.
There are two forms of noise: common-mode (C-M) and transverse-mode (T-M). (Transverse-mode is sometimes called differential-mode, or normal-mode, from the geometric term “normal,” which means “perpendicular to,” but I will use “transverse” in this tutorial as it is less confusing.) Transverse-mode noise involves the interfering signal appearing between the single conductor of an unbalanced circuit and ground (or some other point), or between the two conductors of a balanced circuit. Common-mode noise involves the interfering signal having identical voltages (referenced to ground or some other point) on both the single conductor and the shield of an unbalanced circuit, or on both conductors of a balanced circuit. Like phantom mike powering voltage, C-M noise will not affect a balanced signal, but unless blocked, it can travel along with the signal until it reaches a susceptible circuit component and causes trouble there.
Noise can get into audio circuits by various methods: directly, by means of an electrical connection (or leakage through insufficient insulation); or indirectly, by means of an electric field (capacitive coupling) or a magnetic field (inductive coupling), or both. Any metallic substance can shield against electric fields, but only certain magnetically-conductive materials can block magnetic fields. A radio wave consists of crossed electric and magnetic fields, and it is sufficient to block just the electric field to shield against it. (Or just the magnetic field, but that’s much harder to do.) An isolated electric field is most often encountered as “static electricity,” such as when you pull off a sweater on a cold dry day and your hair stands on end. Magnetic interference was a common problem in the days of tape recording, when the recorder’s heads would pick up hums from nearby AC motors or transformers. Today, the problem usually occurs when dynamic microphones, or certain condenser microphones that use audio transformers, get too close to an overhead fluorescent light fixture with a magnetic ballast.
On the Ground at Last
The term “ground” has many meanings (the U.K. uses “earth” for some of them):
1. The physical substance on the surface of our planet. Most AC electrical power systems have their neutral conductor “grounded” at the service entrance, usually by bonding to the underground metal water-main piping and/or an eightfoot metal stake driven into the earth next to the structure.
2. The metal case and/or chassis of a piece of equipment. Often called chassis ground.
3. The zero-potential circuitry of a piece of equipment. Often called circuit ground. This may or may not be connected to the unit’s case/chassis.
4. The metallic shielding of cables or other components.
5. A separate wire included in a cable or conduit for grounding purposes. This is done in some AC power cables as well as some audio cables. In the United States, power ground wires are colored green; other countries may use other colors. The ground wire may also be bare (un-insulated) or replaced by the metal conduit through which the wires are run.
6. A specific terminal on a device to which “ground wires” are connected. In theory, a ground has no voltage on it, and can accept unlimited amounts of unwanted signals and dispose of them completely. Unfortunately, things in the real world are not so easy. In AC power outlets, there is a third opening (in the United States, it is round to distinguish it from the power slots) for the safety ground circuit. Any leakage current from connected devices flows through it to the bonding point at the service entrance, where it returns to the neutral wire. But this current flow creates an IR drop of hundreds of millivolts or higher, and this voltage will be different at each outlet. Where the electrical conduit serves this purpose, it often has high resistances at the mechanical joints, which may increase further with age, so the IR drop can be several volts. In addition, normal AC current flowing through the power and neutral wires can inductively couple to the ground wire and raise its voltage even more.
Ground Loops (Not the kind an airplane does when it crashes on takeoff)
Since any current flowing through a conductor (whether AC power, a desired audio signal or unwanted noise) will experience an IR voltage drop, if the audio and noise share a common circuit path at any point, their voltage drops may be added together, with unpleasant results for the audio. Let’s take a simple example. A small nightclub has a singer performing in front of a stand mike. The Hi-Z mike is connected to the house sound system with an unbalanced mike cable. The PA amplifier is connected to an AC outlet with a three-wire power cord, which grounds the amp’s chassis to the electrical service entrance ground. The shield of the mike cable is also connected to the amp’s chassis. The singer sings and all is well. Then she grabs the mike with one hand to remove it from the plastic clip, while she steadies the metal mike stand with her other hand. A loud hum blasts out into the audience. What happened?
WARNING: Using 3-to-2 pin adapters without connecting their ground lug/wire to “lift the ground” of all the 3-wire cord units may sometimes actually stop the hum/buzz, or reduce it to usable levels. DO NOT DO THIS! NO SHOW IS WORTH YOUR LIFE OR THAT OF A FELLOW CREWPERSON! Floating safety grounds are a disaster waiting to happen—even if only a “mild” shock results, it can cause someone to fall or jerk back into a serious or fatal accident.
The mike was in a plastic clip that had insulated it from the metal stand. When the singer touched both it and the stand, her body now provided an electrical connection from the mike to the stand, which was resting on the concrete floor (concrete is not a very good insulator). While it is true that the AC-powered amplifier’s case and chassis were grounded, the ground connection was implemented through a long length of wire running down conduits in the building, and the AC leakage current flowing through it produced an IR drop of almost one volt, raising the amp’s chassis above ground by that much. This “hot chassis” voltage in turn caused current to flow through the alternate path to ground created by the mike cable shield, the metal mike housing, the singer’s body, the mike stand, the concrete floor, and the damp soil beneath it. Remember that the mike’s unbalanced cable has a center conductor surrounded by a metallic shield. The few millivolts of audio from the mike travel down both of them to the amp. But now a far larger voltage is driving 10 or 100 times more 60 Hz current down the cable shield, and its IR drop adds to the mike’s audio signal—indeed it completely overpowers it.
In fact, the singer was lucky that a loud noise was all that resulted from her actions. If the amplifier wasn’t properly grounded (e.g., the plug’s third prong broken off, a mis-wired outlet, or a 3-to-2 pin adapter used without attaching the grounding wire), she could have received a severe electrical shock instead of just a nervous one from the sudden loud noise.
If you plug several pieces of your equipment (with 3-wire power cords) into different AC outlets, the outlet’s “grounds” will be at different voltages, and therefore, so will the equipment chassis. Now, when you interconnected the gear with audio cables, AC current will flow through the cables’ shields to equalize the difference. 100 mA (milliamp) flowing through a 1 Ω shield will give an IR drop of 0.1 V. This is about a third of consumer or “semi-pro” line level, and only about -25 dB below professional. If it gets into the audio circuits… Using equipment with 2-wire cords won’t necessarily prevent problems because they still have electrical leakage, and it will flow down the audio cables to any 3-wire units. Using all 2-wire gear can still give trouble because their leakage voltages will be different and will produce equalizing currents. Furthermore, 2-wire equipment is required to have its leakage current limited by higher impedance, to protect you from electrocution hazards. (Unless there’s a design defect that got by the UL inspectors. You do trust them absolutely, don’t you?) But because of the higher impedances involved, touching a particular unit can increase or decrease the hum or buzz, which terribly complicates troubleshooting.
IMPORTANT: If you crimp a ring or hook lug to your wires, be sure that the crimper is properly sized for the gauge of wire, AND the particular type of lug. Lugs for the same gauge wire can have barrels of different wall thickness, and using a thin-wall lug in a crimper designed for thick-wall lugs will result in too little crimping pressure and an unsatisfactory joint. Make a test crimp and try to pull the wire out of the lug. The wire should break instead of pulling out. As an added protection, some mixers will solder the lug after crimping the wire in it.
Ground loops can also occur with the low-voltage DC current that powers your equipment, or even audio signals themselves. If you have an audio signal from one device passing through an unbalanced cable to another device powered from the same DC source, and the power cables to these pieces of equipment do not have exactly the same current times resistance, the IR voltage drop in the power cables will be different and the ground voltage at the equipment end will be different. If the equipment’s cases are connected to one side of the power (as most are, usually the negative), they will be at different potentials, and DC current will flow through the shields of the audio cables to equalize them. Unbalanced audio circuits can be severely disturbed by this situation, and even balanced audio circuits may be affected.
This DC current flow can upset circuits regardless of whether they have active or transformer inputs. A direct-coupled active input can have a DC offset introduced that is sufficient to swamp it, or at least, add to the audio to the point of overload.
Capacitor-coupled inputs can have the same problems if there is sufficient leakage current through the capacitor, or transient charge/discharge currents if the DC voltage level changes. DC current flowing through a transformer input can fully or partially saturate the magnetic core, producing much the same kind of distortion. Another problem occurs when HF noise such as from a switching- type (inverter) power supply travels out of the device over the DC power wiring, and then into another device over its power cable because both units are connected to the same battery. Using a power distribution panel that has filters at each output socket will help to eliminate this noise source. To be most effective, the filters should incorporate a parallel capacitor (to short out most of the higher frequencies before they can pass though to the next stage), followed by a series inductor (to block the remainder of the higher frequencies from passing through). The lower the high frequencies you want to block, the larger (physically and electrically) these two elements must be. You may have seen ferrite traps, split ferrite cores in a plastic housing designed to snap around the outside of a cable, but they are suitable for stopping only radio frequency interference.
While not a ground loop problem per se, if one piece of equipment has inadequate power supply filter capacitors, it may have its current drain modulated by the audio (or some other function) and cause the output voltage of the common battery to fluctuate. This variation may in turn affect other devices connected to the same battery (or power supply if you’re running on AC). You can solve this problem with “decoupling caps,” large electrolytic capacitors (1,000-5,000 MFD @ 20 VDC) installed across the power circuit, as close to the offending device as possible. WARNING: These capacitors are polarized and must be connected correctly. They may explode, or at least vent hot gasses if hooked up backward.
Be sure to use adequately-large wires to carry DC power to the various gear on your cart. A 1-volt drop is not significant in a 110-volt circuit, but definitely is in a 12-volt one. Also, stranded wire can have several strands broken in the process of stripping off the insulation, further increasing the voltage drop. Be careful to avoid this when making connections. As an example, 10-gauge wire is rated at 30 A for a 100 feet of household electrical wiring, but for 12 VDC it should be limited to no more than 10 A for runs of 10 feet. (Easy to remember: 10-10-10.) 10-gauge copper wire has a resistance of about 1 milliohm (mΩ) per foot, so 10 A and 10 feet gives a voltage drop of one tenth of a volt (0.001 x 10 x 10 = 0.1), or slightly less than 1% of 12 V. But remember that there will be this drop in both power wires, for a total loss of 0.2 V. (Another example: a smaller 20-ga wire is about 10 mΩ/ft, so it is good only for 1 A @ 10 ft, or 2 A @ 5 ft, or 4 A @ 2.5 ft, etc.)
The most common connector used for low-voltage power distribution is the four-pin XLR, with Pin 4 positive and Pin 1 negative. Each pin is rated for 5 amps. This is sufficient to operate most devices, but may not be for recharging larger 12-V batteries. On my battery charger cable and the mating receptacle on the battery pack, I jumper Pin 1 to Pin 2, and Pin 3 to Pin 4, thus doubling the current-carrying capacity. WARNING: Some power systems use Pin 2 and/or Pin 3 for other voltages or charger inputs, so be careful not to use this high-current jumper format with them.
Editor’s note: Subsequent installments will deal with transformer balancing, optimal cable wiring practices, sound cart construction to minimize grounding problems and other practices to assure safe and noise-free operation.
Text and pictures ©2012 by James Tanenbaum. All Rights Reserved.