We've all experienced it—the errant hum, buzz or noise getting into our signal chain and making our lives difficult. There are no 'magic wands' to wave in every situation, so we're going to look at several possible causes and solutions, as well as going over best practices to prevent noise in the studio. The practice of eliminating noise in the studio is an art in itself—we'll cover the most common situations and, hopefully, give you the knowledge you need to understand how best to solve your particular issues.
Do You Have The Power?
One of the most common causes of hum or other noise is improper grounding, including using ungrounded outlets for gear that requires a ground by way of a 3 prong to 2 prong 'cheater' adapter. Please do not use these—they are a potential hazard and, as they say, it's always fun until someone gets and eye poked out or, in this case, electrocuted. If the wiring in your studio was installed before 1969, it's likely that you only have these 2 prong, ungrounded outlets. There is only ONE safe way to use these outlets with grounded equipment—you need to hire an electrician to rewire the outlets with a ground. Yes, it's expensive, but it's worth every penny considering the alternative. Contrary to some claims, using a GFCI (Ground Fault Circuit Interrupter) or RCBO (Residual-current Circuit Breaker with Overcurrent protection) is NOT an alternative. These devices, if fitted into an outlet without a ground, still do not provide the necessary ground wire and, while they give some measure of protection, won't solve any hum or buzzing problems you may experience.
Along with ungrounded outlets, improperly grounded or mis-wired outlets are another major cause of noise. You can use an outlet tester to verify proper wiring and that a possible ground exists in your studio power outlets, however this does not test for a relatively common situation known as bootleg ground and it's far more dangerous variation, RPBG (Reverse Polarity Bootlet Ground). A bootleg ground is when an older, 2 prong outlet has been converted to a 3 prong outlet, but without an actual ground wire—the ground is tied to neutral instead. An RPGB is that same situation, but where hot and neutral wires have been reversed, creating a extremely hazardous situation.
In either bootleg ground situation, your typical outlet tester will show a properly wired outlet and so you need to test the outlets with both a ground impedance tester (aka Wiring Circuit Tester) and a non-contact voltage tester (NCVT). The ground impedance tester will indicate if you have a ground fault (newer models give you even more info about your wiring and its integrity) and the NCVT will indicate an RPBG situation. To test using the NCVT put the probe end of the unit into the ground circuit hold of the receptacle—it will indicate a 'negative' (typically a red light) in a situation where RPBG exists. Using the ground impedance/wiring circuit tester is usually as simple as plugging it into your outlet—its panel indicators will show any fault situations.
Lastly, the difference in ground potential can be a major cause of hum and noise. The audio signal grounds between two pieces of gear are dependent on the AC outlets ground (and sometimes the gear itself can be a factor) and differences in the ground potential can cause the audio signal ground to 'loop' through to the other piece of gear and back again, causing the classic 'hum' you've all heard.
When pieces of gear are connected to different circuits that have ground wires with differing impedances all signal grounds will go through the circuit with the lowest impedance. Common causes are ground wires that have different gauge wires in the circuit, bad 'ties' (where ground wires are tied together using wire caps and such) and so on. Again, a certified electrician should be called in to solve this type of situation.
In cases where you have the option, connecting all your gear to the same circuit can bypass any problems that would occur when connecting them to different circuits. This can be a quick and easy fix for ground loop noise and one that I recommend people start with when experiencing hum in the studio.
I realize this is a lot of info to digest, but proper studio electrical wiring is the best place to start to eliminate pesky noise and protect you from hazardous situations.
Go To Ground
Along with the ground potential in the electrical circuit situation above, you may come across situations where that audio gear or other factors are causing a difference in ground potential. If you know for certain that your power circuit ground is good, you'll need to trace down which piece of gear is the likely suspect for the problem.
Unfortunately there is still plenty of gear out there that does not follow the proper practice of tying the ground wire in a balanced connection to chassis ground, rather than signal ground (aka 'The Pin 1 Problem). You can typically consult the schematic of the piece of gear to determine if the ground/shield wire is connected to signal ground or chassis ground. If the gear ties it to signal ground, you may have to resort to using custom cables when connecting to that device that have the ground wire disconnected at one end of the cable. What this does is eliminates the connection loop between the pieces of gear. You also have the option, in some cases, of tying the ground wire of the interconnect cables to chassis ground via a screw on the case of the gear or to a specific chassis ground wire terminal on gear that supplies one. While more involved and time consuming, it can do wonders for eliminating hum and noise.
Custom cables should be consider the least desirable option when trying to eliminate noise, but one that can be useful when necessary.
Audio ground lift boxes are another possible solution—and I stress 'audio' because we want to make a clear differentiation between audio ground lift and power ground lift. The latter should never, ever be used for the reasons discussed earlier in this article. Audio ground lifts essentially do the same thing as the custom cables mentioned above—they disconnect the audio ground wire at one end.
It's All About Balance
Once you've conquered any problems with your power (properly wired and grounded circuits) the absolute best way to avoid noise and hum is to use balanced connections whenever and wherever possible. Balanced cables and connections are designed to eliminate noise and are your best friend in the studio. If your audio gear has balanced connections, it's worth the extra cable expense to use them.
When your gear has only unbalanced connections such as RCA or TS (Tip-Sleeve) 1/4” connectors, it's best to use isolation transformers between it and a balanced input. Using isolation transformers is another option instead of the custom cable assemblies mentioned above. In all cases pin 1 on the balanced end should be tied to chassis ground.
Direct boxes are another tool in the arsenal to eliminate noise—better ones even come with an isolation transformer and nearly all include an audio ground lift switch. You can use them between a high impedance, line level, unbalanced output to connect to a low-impedance, balanced input (think guitar or keyboard output to a mic input on your console or audio interface).
You Gotta Keep'em Separated
RFI (Radio Frequency Interference) and EMI (electromagnetic interference) are two other forms of noise induced in audio systems. RFI comes from everywhere—cordless and cell phones, wireless mics, radio transmitters, garage door openers and the list goes on. EMI noise sources are typically power supplies, monitors, computers, appliances and the like. These can be some of the most difficult to track down and eliminate but, by using these tips and techniques, your chances of experiencing them will go down significantly.
Keep your audio cables and power cables away from each other. If they have to cross, make the cross at a 90 degree angle. The further away you can separate them, the better.
Keep your audio cables away from power distribution boxes and power adapters, as both can be sources of EMI.
Put appliances on a different circuit than your audio gear. Have you ever had your refrigerators compressor turn on and hear a hum in your monitors? That's EMI—the refrigerator is putting spurious noise in the power lines, which then leaks into your audio chain.
Turn off cordless phones or, if that's not possible, keep them as far away as possible from audio gear.
Light dimmers in the studio—again, putting them on a separate circuit from your audio gear can help immensely.
Other RFI problems, such as hearing your local radio station in your audio system, are much more difficult to combat, your chances of experiencing this when following all the info in this article is lowered significantly.
When faced with EMI/RFI problems, it typically boils down to making sure you do not have power line noise sources on the same circuit as your audio gear and either eliminating or relocating devices and noise sources as far as possible away from your audio lines and hardware. Try moving things around and see if the noise goes away.
Bring Up Your Shields
One situation I see very often is people using low quality cables in their studios. These are not only more prone to noise induction, but also can degrade your signal quality. Several studios I've worked on over the years have completely eliminated noise and significantly improved their audio quality simply by upgrading their cables.
Audio cables typically consists of an inner conductor(s), a dielectric material, a shield and an outer jacket. The difference between low- and high-quality cables comes down to the quality of each component.
Shield—There are 4 standard shield types (from lowest to highest quality): stranded, spiral, braided and foil. Stranded shielding is simply strands of wire run along the cable underneath the jacket. The problem with stranded shielding is that is does not give full coverage—any EMI/RFI noise sources can penetrate the shield and be induced directly into the inner conductor. Spiral shield is better, but flexing of the cable can cause gaps in the shield and, again, noise sources can penetrate through the gaps. Braided sheiding is typically the best in terms of coverage and flexibility, while foil provides the best in terms of coverage, but continual flexing of the cable can cause breaks or gaps in the foil, so it's usually only used in situations where the cable is installed and never moved.
Dielectric—This is the material between the shield and the inner conductor(s) which acts as an electrical insulator between the shield and inner conductor(s), to prevent electrical conduction between the two. Typical dielectrics are made from PVC (polyvinyl chloride), PE (polyethylene) and PTFE (Polytetrafluoroethylene, aka Teflon). The ideal dielectric would completely isolate the shield from the conductor. The differences in the dielectric rarely come down to materials, but rather to their quality and consistency. Good quality dielectric materials will be the same thickness through the entire cable and be of high quality. Low quality dielectrics typically have defects, contaminants and other anomalies that cause variations in its electrical properties along the cable length.
Conductor—The debate rages on about silver vs. copper vs. oxygen-free vs. whatever material gimmick they come up with next to separate you from your hard earned cash. The reality is that, once you get to a certain point, the differences are minimal at best. Higher quality cables will use a thicker conductor (typically between 28 and 22 gauge) and some will use two twisted pair conductors for additional noise rejection.
The shielding is likely the most important factor in cable choices—braided shielding is what you should aim for if you are going to be moving cables around, while a braid/foil combination is best for installed cabling.
One last factor that can play a part in elimination of noise is cleaning your cable and hardware connectors. Over time these can become corroded and oxidized, creating additional resistance and capacitance, which influence some of the situations discussed earlier. Use of a contact cleaner, like CAIG DeoxIT, will improve electrical conduction. Check out your cables—are the connectors bright and shiny or dull? If they're dull—bust out the contact cleaner!
Every section in this article merely scratches the surface of the technical and situational aspects of what they cover but, hopefully, I've given you enough information to put you on the right track to a hum, buzz and noise free studio.