As recordists, our goal is to make the best sounding recordings possible. That hinges largely on the talent and skills of the participants and the quality of the equipment. But much more mundane yet hugely important issues come into it: Ground loops, bad shielding, improper cabling, haphazard levels or poor quality power can drag even the best performance down. Know how to address these issues, and how to set up the studio properly.
TCRM 6 discussed audio levels and cabling and connectors. Now, we’ll focus on the complexities of ground and ground loop issues as well as power, tie lines, patch bays, and digital audio connections.
Ground is a very large electrically conductive body, often the earth itself, which is used as a zero reference of electric potential. Wires, or other conductive mediums, which are connected to ground share earth’s zero reference and so are often also referred to as ground. Additionally, the term ground is often used as a verb to express the action of connecting something to a ground reference. (Ex: “Before you turn on the power, be sure to ground the amps properly.”)
There are two main types of grounded signals in the studio, safety (power) and audio. The difference between these two is simply the type of signal they are meant to deal with. Safety grounds are intended to handle the more potent power signals. They usually ground directly through the third pin of AC-3 power cables. The term safety is applied because it is these connections that keep large amounts of electrical current from finding an undesirable path to earth ground… such as through you.
Audio grounds are designed to deal with the much weaker electrical currents of audio line and microphone signals. These are generally not strong enough to cause permanent bodily harm, although they can sting a bit. The main danger here is caused when there is no good safety ground available, so electrical current uses an audio ground instead. As discussed in TCRM 6, proper audio grounding is key in reducing the amount of hum and noise that are induced into audio cables. Audio cable is not intended to cope with power-level signals (with the possible exception of heavy gauge speaker cable).
A painful lesson
I first discovered the true nature of the safety ground while rehearsing with my cover band back in high school. It was nearly impossible for anyone to hear my vocals over the guitars, drums and bass down in our boomy basement rehearsal space. To solve this we plugged my microphone into a second input of the bass player’s old 350-watt 8x10 rig as a sort of poor man’s PA system. Unfortunately, this setup caused an incredible amount of hum. The keyboardist announced, “I know how to fix that!” and proceeded to insert one of those notorious 3-to-2 prong power adapters. The hum magically disappeared.
After strumming the introduction to the next song on my electric guitar, I stepped up to the mic to sing. That’s when I saw the bright arc of electricity as it sprang from the mic to my lips. The next thing I knew, I was lying on the floor writhing in pain. Since the current from the amp had no safety ground, it discovered the next best way to travel to ground… through me! Trust me when I tell you: that’s a feeling you can do without.
The lesson: never circumvent an AC-power safety ground connection (as many do by removing the 3rd prong on a power chord or using a 3-to-2 prong adapter). This can create an unsafe condition in which someone can get seriously hurt or even killed. Though this approach can be a quick way to remove unwanted hum (by breaking a ground loop) the possible consequences are not worth it.
Being grounded… and not loopy.
Well, if you’re not supposed to use the all-too-common method of removing hum by lifting the safety ground, just how should that pesky hum be avoided? In answering this question, the first thing to consider is the cause. The primary culprit here is known as a ground loop.
A ground loop can occur when audio equipment is connected to ground through more than one path. For instance, when an effects unit is connected to a mixer by way of the aux sends and returns, the two devices are connected by the audio grounds. When each is plugged into separate power outlets, they both share a ground reference, but the electrical potential of each may be slightly different. This difference can cause current to run from the effects unit into the mixer, from the mixer to the outlet, from the mixers outlet to the effects units outlet, and from the outlet back into the effects unit, where it starts the whole process over again. This ground-based feedback loop is heard in the audio as a hum or buzz.
There are two basic methods for avoiding ground hum:
- Equalizing the electrical potential to ground.
- Having a single ground reference per device.
Equality—One of the primary ways to make the electrical potential to ground equal is to connect the power cables of every device to a single outlet on a single breaker circuit. In most project-type studios this should probably be a 20 Amp circuit with 12-gauge wiring in the walls. Keeping cable runs as short as possible, yet equal in length, will also help maintain equal potential.
Singular Reference—Another approach, but certainly not a mutually exclusive one, is to ensure that no loops are possible. Each device should ground through a single path, with no loops in the chain. Since it is unwise to disconnect safety grounds, this method requires creating a scheme where audio devices do not share ground through shields. One method of doing this is to disconnect the shield from cabling connectors on the input (load) end. The shield should remain connected on the output side. In the above example of a mixer and effects unit, first try just disconnecting the ground wire from the cable connector at the aux input of the mixer. This may be enough to remove the hum: if not, you could also try removing ground from the line input of the effects unit.
It should also be noted that hum and/or buzz can also be caused by issues other than ground. Be aware that fluorescent lights, dimmers, power lines, power supplies, electric motors, and phones, as well as nearby amplifiers, speakers, electrical wiring/cabling or video monitors may also be to blame. Try to minimize and isolate these other types of factors to reduce problems and help determine if a ground loop is really to blame.
Another place ground loops are created is through the connection of the metal chassis of various audio devices. To shield the inner circuitry of an audio device from external interference (as well as for some measure of safety) the outer chassis is connected to power ground. When two devices touch, a loop is created between them and their ground connectors. Again, it is unwise to remove the safety ground connectors; instead, it should be ensured that every chassis should be electrically isolated from one another. The various audio devices should not be in physical contact with one another or conduct indirectly through another means (such as a metal rack or audio cabling).
Homemade wooden racks are a good way to ensure electrical isolation, and are inexpensive to build. When conductive metal racks are used, vinyl or rubber washers can be used to keep the equipment from contacting the rack. Since the screws used to hold the gear in place can also conduct, great care must be taken to be sure they do not touch the chassis directly, only through the pressure exerted on the washers. Special non-conductive screws are also available.
Another method for separating audio grounds is through the use of the transformer isolation and ground lifts on commercially produced DI (direct inject) boxes. Not only do these devices isolate input from output by way of an electrical transformer, but also the included ground lift switch allows disconnection of the audio ground without having to perform surgery on expensive studio cabling. If using a DI this way, be sure it does not to attenuate the signal too much unless that is also desired. Attenuating a line level signal just to boost it back up again later can add unnecessary noise, and many passive DIs attenuate signals to mic level, a drop of 20 dB or more. Many active (powered) DIs can make up this gain loss, but check the wiring of the model you plan to use to be sure it’ll still separate your grounds properly.
Also be aware of how the DI is powered; some draw power from the mic preamps phantom power supply, which uses the ground pin to supply 48 volts. In this case, you cannot disconnect the ground from either end of the DI line output cable and still power the DI with phantom. (see TCRM #’s 2, 5 , and/or 8 for more on phantom). If phantom cannot be used, either a battery or a wall transformer (wall wart) may be viable alternatives.
Note: high quality isolation transformers are also available in dedicated units without all of the bells and whistles of DI boxes. Both solutions can work well for balanced and unbalanced connections.
The True Test
No studio should be without a high-quality, reliable cable tester. These can be used to confirm which pins are being used as positive, negative, and ground. They can report if there are any shorts (when two or more of the cables conductors are somehow in contact with one another). The good ones will also test for continuity (to be sure there is no interruption in the signal path) and even for intermittency (momentary interruptions). Intermittent shorts or continuity problems can wreak havoc on studio work and can be hard to diagnose without a good cable tester.
Tying it all together
Tie lines are permanently installed multi-channel audio connections between two or more rooms in a studio. For instance, the isolation booth contains a wall-mounted panel with numerous inputs that are sent to the control room (through cable in the walls). This offers convenience, organization, and greater sonic isolation than running separate cables or a snake each time the room is used. They can be used to send audio in either direction.
Spiffy home-made types are rather easy and inexpensive to make, but do take some time and plenty of soldering. The wooden plaques found at hobby and crafts stores make an excellent backing for audio connectors and discourage ground loops. (See pic)
Relying on the patch
In a studio control room it is often necessary to change the routing of signals or add and subtract audio devices as the work dictates. To facilitate making these numerous connections, all inputs and outputs can be wired to a convenient panel of audio jacks. This panel is called a patch bay. Here, small lengths of cable, called patch cables, can be used to make connections between studio equipment. Use of a properly configured patchbay can save a lot of time and energy connecting, disconnecting, and reconnecting gear.
Patch bays are available in a wide variety of configurations using various connection types. The most popular professional balanced kinds use either 1/4-inch TRS or smaller TT (tiny telephone) jacks and cables. Unbalanced units use either 1/4-inch TS or RCA connections. For those concerned about audio fidelity, these should be avoided as they will unbalance any balanced signals and add more noise and hum to the audio.
A variety of schemes are also available to connect the equipment to the back of the patchbay access jacks including female connectors, soldering posts, screw terminals and punch points.
Female connectors—Some patchbays simply include female connectors of the same variety as used to make patches in the front. This way of working is by far the most convenient to configure, but has a reputation for being less reliable and a bit more noisy than others.
Soldered posts—Raw binding posts are available on some patchbays. Since the wire leads are soldered directly to these, they are a bit more permanent (offering greater reliability) and keep fidelity high. Soldering each patch point can be both tedious and hard on the fingers (with all those annoying soldering iron burns).
Punched—The most recent method for making rear-panel connections is by special punch points. These require a special tool to make connections which some find tricky to use. When used properly, this method does offer great audio fidelity as well as ease of setup (compared to soldering). However, these connections are certainly not as hardy as soldered ones and should be kept in a rack where the wires in the back will not be pulled or bumped. Cable ties must be used to ensure they are secure and that gravity is not pulling too strongly on them.
Signal flow configurations
Often, patchbay jacks are set up into two-row pairs. The top row is used for audio outputs and the bottom for inputs. These rows can then be configured in the following ways:
Normalled—Here, when no patch cables are inserted, the output signal from the top jack is automatically sent to the bottom input jack. When a patch cord is inserted into either jack, this connection is broken and signal no longer passes from top to bottom in this fashion. A normalled configuration is convenient for patch points where the output needs to run directly to the associated input a significant amount of the time. It’s where the signal “normally” goes. Some common examples are:
- Out from the studio tie lines into the mic/line inputs.
- Out of the channel insert into a compressor
- Out of the multitrack recorder into the console tape returns.
- Out of a reverb unit into a stereo aux return.
Half-normalled—When no patch cables are inserted the signal is passed from the top (output) to the bottom (input) jacks. If a cable is inserted in the top jack, this signal flow is not interrupted. When a patch cable is inserted in the bottom jack, however, the automatic signal pass-through connection is broken. This setup is especially useful when the signal output needs to be sent to two places, including the normalled input as well as another patch point.
Example: Out of the multitrack into the console tape return and split (by patch cable) into an effects unit (or used for a separate cue mix).
Open—A condition in which there are no electrical connections between patch jacks unless patch cables are used manually to make connections. While this may be the most flexible and direct of the connection schemes, it also requires the most work (in making the connections) as well as a significant number of cables.
Example: The output of a novelty effects processor (which doesn’t get used often enough to be normalled to the console) is on the top row, and the input for that processor is on the bottom. A normalled, half-normalled, or parallel setup would create a feedback situation.
Parallel—When jacks are wired in parallel, they are always connected regardless of any inserted patch cables. Often, this technique is used to connect more than two jacks, allowing a single signal to be split and repeated at multiple jacks (for this reason these types of connections are called mults).
Example: The output of a mic pre (being used to record a vocal) is patched into a mult so that it can be split and sent to four places simultaneously: an input of a 2-inch tape machine, an input to a DAW, a monitor mixer, and a reverb unit.
Be aware that mults are best used to split a single signal and send it to multiple places rather than to merge multiple signals into a single one as this may cause shifts in level and/or distortion of the signal. Also, a mult should not contain both the input and output of any single device, as that could cause a feedback loop.
Earlier I explained the importance of the third-prong safety ground with A/C power. Not only is this a safety feature, it is also important in removing, or avoiding, hum. Besides ground, there are numerous additional factors in the power supply that also have an impact on audio quality and the longevity of studio gear. Occurrences of power surges, spikes, brownouts, line noise, voltage fluctuations, and even power losses, are all quite common. Fortunately there are many devices on the market to combat these problems.
Power conditioners, regulators, and surge protectors come in many configurations and can often serve several functions at once. To simplify, I have listed the various functions below:
Conditioning—uses filters to remove line noise and improve the quality of the power signal by making it more closely resemble a sine wave. This can improve audio quality and protect equipment.
Regulation—adjusts the incoming voltage level to keep it within an acceptable range. This is easier on the equipment and keeps it working in a more consistent manner. Be aware, however, that a good regulator is quite expensive. While there are cheaper ones out there, many switch between voltage ranges in a manner that, while keeping voltage in range, causes it to jump in a fashion that can be bad for audio quality and be hard on your gear.
Suppression—removes momentary spikes or surges that can damage equipment.
Breaker—cuts the power if an unsafe condition occurs.
Voltmeter—displays the voltage being supplied from the utility (and making it to the metering unit).
Ammeter—displays the amount of current (in amps) drawn by connected studio gear. This can help determine how close you are to overloading the circuit.
UPS (Uninterruptible Power Supply)—A UPS is basically a backup power supply. These units keep the studio up and running in the case of brownout or power failure, at least for the brief time required to shut down computers and other gear in an orderly manner; many also condition the power and allow a further measure of protection from spikes and both voltage overs and unders.
A balance of power
A balanced power supply system can be used to further reduce hum and noise in a studio’s power. By tapping into a power transformer at the center and two ends, two 60V lines are created. While these are in phase, and sum back to a full 120V, their connection to the center-tap ground is out of phase. This causes hum and noise to be cancelled. Manufacturers of these systems claim the loss of noise can be in the range of 16 to 20dB. This added quality does come at a price, however. Equipping a project studio with a 20-amp balanced power unit costs around $2,500. This does include some surge protection, but not a UPS system or any specialized wiring, cabling, or outlets.
Notes on Digital Audio Connections
Before moving on, I would be remiss not to make a few final points about digital audio cabling.
Cabling for the digital audio standards was outlined in TCRM 4, but now that analog cabling has been considered more closely there are a few things to note. Though S/PDIF cables use RCA connectors, they are not really interchangeable with cables intended for use with line level analog audio. They may look the same, but these two cable types actually differ in several ways, including impedance and gauge. S/PDIF cables use cabling more traditionally used for the higher bandwidth and frequency of video signals rather than audio. Similarly, AES/EBU cables do use dual XLR connectors, like balanced analog line or microphone cables, but are also not entirely compatible with their analog counterparts.
Using traditional analog audio cables for digital audio signals can lead to data transfer errors creating audible clicks, noise, incorrect sample rates, or even complete inability for a digital audio system to function.
TCRM 8 will begin a three-part series on microphones. In those next three installments I will outline the different types, patterns and attributes of various microphones as well as explore their usage.
John Shirley is a recording engineer, composer, programmer and producer. He’s also on faculty in the Sound Recording Technology Program at the University of Massachusetts Lowell. He hates ground loops… but loves the safety ground!
Check out his wacky electronic music CD, Sonic Ninjutsu, at http://www.cycling74.com/c74music/009