Audio Hum Solutions (07-09-99)

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What's this stuff I hear about "noise/hum" on the ground from the Sound Guys. I was just speaking to a Sound Guy in LA yesterday (they call him the Hum Buster) and he was telling me something that scared me; to solve the "hum" they disconnect the equipment ground wire from the electrical system and connect it to a separate ground rod.

Answer No. 1

This is the same problem that instrument techs and engineers in the industrial side of the business do and design. This is supported and encouraged by the Honeywell's, Foxbough's, other instrument companies. Check their web sites and see what they show for grounding... watch out on how they show the common and ground coming from the transformer... they don't and leave the reader to believe figure out that it is a high resistance ground.

Harmonic current that circulates via the ground path and inductive/capacitive couple to the signal circuit creates the hum. This is the reason that thermocouples and other instrument signal have only one end of the shield grounded. Petrochemical plants will put in a separate "quite ground triad" for instrument/computer ground reference. This is somewhat addressed in the section of the NEC that allows the ground cable to be run with the non-grounded conductors and terminated at the power system ground. However Foxbough show on its web site that the final solution is to run the instrument ground wire a different path to a different ground rod on the ground triad. This is wrong in that it allows higher impedance during a ground fault when the grounding conductor is not closed bundled to its ungrounded conductors.

Reference Chapters Chapter 3 (3.9) and Chapter 4 of William D. Stevenson Jr. book "Elements of Power System Analysis" and Westinghouse's Electrical Transmission and Distribution Reference Book, and Dr. Mack Grady, U. of Texas on Harmonics. Since he is a personal friend of my from the Power Distribution Conference (I am on the Executive Planning Committee and Chairman of the Industrial Section of the Conference) and Mack is world expert on harmonics. Mack wrote the IEEE Harmonic Standard now in use. These references should help you if additional questions come up. This is a good topic to publish and clear up for both the commercial and industrial sides of the business.

Follow-up calls to our local Graybar and GE Supply offices revealed that the above products are not currently stocked locally, but are stocked at the factory and readily available. List prices are in the $150 - $170 range! Cost to industrial accounts is $70 - $90. Lower prices may be available to some companies through special preferred supplier contract arrangements. I assume that prices will drop significantly when the supply/demand increases with Code implementation in 2002.

— John D. Rowland Sr. PE,

Answer No. 2

The first thing to do is to put all of the audio equipment onto the same transformer, which should be isolated rated (Square D is an excellent source of this stuff) and run isolated grounds and isolated grounded outlets for all equipment. This puts the entire audio system onto the same ground plane and helps to prevent the inducing of hum into the system in the first place. This can be a pain in a large building. The techniques around this is if various equipment is on different power sources (remote speaker clusters on remote amp racks) then the signal is fed via fiber or isolation transformers so that the 2 grounds are separated and will prevent potentials (voltage).

In arenas and other venues now, it is common to have a separate disconnect and power source just for audio to assist with this. Video trucks will try to do the same, but will run into problems when connecting to other systems (house audio, band audio, etc) and the use of a good microphone splitter with proper transformers helps alleviate all of this.

About balanced systems, this is not the same thing as 530-G. In the audio low voltage world, you can have balanced and unbalanced signals, unbalanced is a + signal and uses the ground or shield for return (typical of home audio systems, uses only 2 wires) where a balanced will have a + and - for the signal and the ground or shield is separate for noise attenuation. Another practice in the audio world is to only terminate the shield and the source end, which would help to eliminate the connection of potentials. If it is an existing system, isolation audio transformers, ground lifts, etc, are all that you have. You try to everything onto the same ground plane or isolate it from the rest.

P.S. Sources to look up some of this stuff would be the Audio Engineering Society (a little too techno for me), but are located at There are also numerous books; I would have to go digging for some info.

— Bill Ellis, IESNA,

Answer No. 3

Mike there is a solution, which is allowed for in the 1999 edition of the NEC 250-6 or 250-21 in the 1996 edition in my opinion. Typically noise is not considered an "objectionable current" relatively speaking so other standards would be required for this application. IEEE emerald or green books, FIPS 94, "Ground Loop Causes and Cures" - Moore Ind. and other pubs written on the subject. I have had personal experience with the problem on several occasions and have seen some installers and designers do the same thing. There are several solutions (alternatives) to the situation. The solution would be based on several factors such as frequency, location, voltage and design. Alternative grounding techniques and / or filtering are typical solutions. Without more information an accurate solution would not be possible. However we can definitely agree that his present solution would not be acceptable.

Mike - I assumed that the sound guy has used the proper equipment to diagnose his problem. If not, he must us a Pink Noise Generator, Analyzer & Equalizer in that order. This test procedure usually filters out any audio noise he may have. Removing the ground just opens his ground loop and does not fully solve his problem. An equalizer is a series of LCR filters, Tank Circuits and Isolators.

I would highly suggest that he use this equipment since it will give him an accurate picture of the cause and effect, it is simple and easy way if he has not already tried. If this fails he should install some type of filter for his grounding conductor or use another type of grounding technique such as an AC-DC isolator.

I hope this helps - inform the technician that this is not the first time that this event has occurred and it can be solved, just be tenacious.

— David R. Carpenter,

Answer No. 4

Hum is a common problem with regards to the installation of sound systems. The method you described is sometimes referred to (incorrectly in my opinion) as "Sudo-Balancing."

It is a result of a "ground Loop" where in something is not properly grounded with regards to everything else in the "system."

Sharing grounds with equipment that is not part of your "audio" system, such as refrigerators, air conditioners, and other appliances and equipment most commonly causes it.

The ideal and best way to "power" up a sound system is to give it it's own "dedicated" Power source which is separate from any other power in the building.

This allows it to have it's own "grounding" (scheme) system separate from all Non Audio related equipment, and prevents that "other" equipment from introducing" the ground loops into your sound system.

One way to resolve this type of problem is to put everything on the same "grounding scheme, which can be accomplished by either grounding everything or lifting the ground from any "offending" piece of equipment. My strong suggestion is once again, that a "Balanced system" be used, up to and including an UPS which is solely dedicated for the entire audio system.

The deleting or "lifting" of the "ground" is very common through out the Sound Industry, and can prove to be very dangerous if you should ever experience a short in the unit. Unfortunately it is a practice that many audio techs adhere to, and may be difficult to break them of. My input on this matter may even offend some, and say I am all wet, but when it comes to safety, using a balanced system is the proper way (up to and including a UPS).

I am an electronics technician by trade, and have made a living for over 16 years by installing and maintaining Pro Audio Sound Systems and Recording Studios here in the US. I hold several degrees on Design and Estimating of P.A systems as well.

I think a good source for those that question my comments is to have them contact NSCA (National Association of Sound Contractors) and NAB (National Association of Broadcasters). The NSCA holds classes and seminars on all levels of audio design and installation.

I would suggest that some of these courses be considered, as there is much to R&D work better known as Design and Estimating of P.A. Systems. I hope my input has been of help, if you desire further specifics I will give you titles of books to study.

— Roberto Clemente,

Answer No. 5

Your audio "HUM" is caused by ground loops there are several ways to solve this depending on your install. Solving ground loops on DC or automobile installs is as easy as this. Common point - Return ALL of your amps/stereos/and CD players to the same point. This will solve the problem most of the time.

Solving this on Home is just as easy go to "Radio Shack" (not my first choice) and buy a ground loop isolator. This is just really a pair of 1-1 isolation transformers that go in line with the incoming audio (you will lose some audio but it's safe.) This should solve your problems.

— Scott,

Answer No. 6

The dilemma is that solving "noise" problems is an art within itself. Since it doesn't come up every day, we all have limited practical experience. This has spawned an industry for those who are now specialize in solving noise problems. I noted several types of audio hum buster gear on the Internet for sale. All references cited at the end of the article are available on the Internet.

The bottom line here is that a perfect "quiet" ground does not exist. Your "sound guys" are referring to the type of noisy ground (ground loops; current running the equipment grounding conductor, metal within the building, and grounding electrode conductor) and other problems they get in their specific area of work. Computer guys have the same problem in their line of work and so forth.

However, audio studios have their own set of unique problems. Here is a quote from one of the references below--Audio Noise and AC Systems by Martin Glasband. If you want to fully understand the problems and solutions of audio hum you must read all his article.

"Use of any of today's standard 120-volt single-phase AC systems mean potential problems for audio equipment. Ideally, the correct type of AC power for audio gear in the United States is 120-volt, two-phase (balanced) power. Commercial use of two-phase (equi-potential) AC isn't new. In fact, it is probably the first type of AC power ever put into widespread use in this country (Chicago in the early 1900s). Nevertheless, its commercial use is all but dead these days. Properly grounded 120-volt two-phase wiring systems aren't mentioned at all in the National Electrical Code (NEC). However, this unfortunate fact didn't deter a few innovative engineers and studio execs from employing a 120-volt equi-potential AC system at the Zoo Studios, Studio City, CA. Soon after its opening, a studio musician plugged in his Marshall amp and declared it was broken because there was no hum. Much to his surprise, his amp worked just fine. He mentioned to the studio engineer that this was the first studio he'd ever worked in where his amp simply made no noise at all. It should also be noted that it was not necessary to drive a single ground rod. That's pretty clean AC".

The basics of all noise problems on the grounding system boils down to what is objectionable current. With the exception of hospital systems, the definition is vague at best. The standard electrical grounding system throughout the building isn't designed to have current constantly flowing through it--and yet it does, you cannot stop it. The reason a ground will not and never be perfectly noise free is that the grounding electrode conductor is nothing more than a long wire from point A to point B. And the longer the wire the more noise it will pick up. If you have electricity you will have electromagnetic lines cutting through the air and anytime one of the lines cross any conductor in the ground system; it will produce an unwanted voltage. And then you have direct feeds from faulty equipment, ill-designed, and poor installation.

Some of the types of noise we deal with are:

  1. Common Mode Noise--this is noise that exists between the power conductors, hot and neutral, with respect to the ground conductor.
  2. EMI - this is electromagnetic coupling between conductors.
  3. RFI - this is radio frequency interference.
  4. Normal mode noise - this is noise that exists between the power (hot) wire and neutral conductor.
  5. Inter system ground noise. Inter system ground noise can exist between the ground wires supplying interconnected devices.

Don't violate code! Your sound guy "cut the Gordian Knot" by adding the extra earth ground reference in an attempt to quickly solve the problem. By adding an extra ground he solved some of the effects of the problem but did solve not the problem itself. As you indicated, the extra isolated ground is against code. A building is to have one and only one grounding system. He can drive an extra rod if he so desires but he must tie it into the whole building grounding system. He didn't do this because the "noise" problem would come right back. His system works because the impedance of the earth between his isolated ground and the building ground attenuates the noise.

I solve general noise problems by the following but in reference to audio hum from studios, refer to the references cited below.

  1. If at all possible I run isolated receptacles and grounds. A typical equipment grounding conductor can go through a dozen or more poor splices and connections before it gets back to the building grounding system.
  2. Measure the current on the grounding electrode conductor and from there to the individual equipment grounding conductors--a sheriff's dept. in a local town had 15 amps of current on the grounding electrode conductor. Find where the current is coming from and solve that problem.
  3. Tie all individual grounds into the building grounding system. This is, of course, code--but it is rarely done. Try to verify that all conduit and grounding systems are secure and tight.
  4. I install the best, lowest impedance grounding electrode system I have time and money to do. Code is rarely enforced on ground resistance--I have yet to see an inspector follow the 25 ohms or less reference. In the country, most homes, including wood structure commercial buildings, have only one rod. I install a minimum of eight rods with Cad-Weld connections. And if there is a steel-cased well, I tie into that. But it has been shown that even a steel well as a ground will not stop audio hum.
  5. Since it is rarely possible to completely eliminate all noise sources I plug the equipment that generates noise into an Isotel filter before it is plugged into the circuit. Noise sensitive equipment also goes into an Isotel filter. When budget permits, noise sensitive equipment gets a standby power supply for filtering.
  6. If traced to one piece of 2-condcutor equipment, change it over to a 3-conductor. If said equipment has a large power supply and is giving noise, change it over to a symmetrical power supply. Sometimes noise comes from unexpected places. When working for Lockheed Missile and Space, I had to solve the ripple and noise problem being generated from one large piece of gear. The ripple was so large I could see it on the oscilloscope. The problem was that a mouse had crawled into the power supply and fried himself. What was left of him was a giving a high resistance ground from capacitor terminal to frame.

References: All available on the Internet:

Audio Noise and AC systems By Martin Glasband
Lifting the "Grounding Enigma" by Martin Glasband
The How's and Why's of Isolated Grounding--Thomas M. Gruzs
Liebert Corporation, Columbus, Ohio Equitech Technical Support Bulletin Audio Wiring and Grounding

— Rex Cauldwell,

Answer No. 7

When you have two active devices are on difference power (110v +) phases (transformer secondary) phase in the power panel. Sound systems want to be on the same leg of the power transformer.

Primarily finding out the power routing from the electrician and rerouting the power feeds or have the electrician pull a new feed to the external sound equipment or use isolation (1:1) audio transformers on audio circuits between two different devises (my preferred method). The ground lift and stake is only used on a rental setup when the equipment would be shelved later in the day or week. (Non permanent installations).

— John Lasher,

Mike Holt's Final Comment

Answer No. 6 states "Don't violate the Code! ... A building is to have one and only one grounding system. He can drive an extra rod if he so desires but he must tie it into the whole building grounding system."

I thought and taught for many years that the NEC contained this requirement, but it doesn't (never did as far as I know). As a matter of fact the NEC [250-54] recognizes the use of a supplement grounding electrodes, which is often required by CNC machine instructions, RF grounding grids, etc. The key is that the earth itself cannot be used for equipment grounding, but a supplement electrode can be installed without bonding it to the building grounding electrode system.

1999 NEC Section 250-54. Supplementary Grounding Electrodes

Supplementary grounding electrodes shall be permitted to be connected to the equipment grounding conductors specified in Section 250-118, but the earth shall not be used as the sole equipment grounding conductor.

If after reading this, you would like to make any updates, comments, corrects, etc. please let me know. I want to keep this question and answer updated to the current standard.

IMPORTANT UPDATE: My latest and up-to-date grounding and bonding book Bonding and Grounding (textbook), 2017 NEC is available by calling our office toll free at 888.632.2633 or clicking here.

God Bless, Mike Holt