HUM FAQ, by Bill Lummus (Beta version)
Does your system hum? Here is some basic information and a troubleshooting algorithm
How do I figure out what is causing the problem?
1. Disconnect everything from the amp except the power cord and the speakers. Do you still have the hum? If so, see number 2
2. Disconnect everything from the preamp but connect the preamp to the amp and plug them both into the same outlet. Do you have the hum now? If so, see number 2
3. One by one connect each source to the preamp. When you hear the hum you’ve localized the problem. What is most common is that when you connect a source that has any connection to the video system (like a DVD player connected to the TV or a cable box) then the hum will appear. Go to 1.
NOTE: optical interconnects are immune to ground loops and EMI/RFI.
1. Ground loops
Most common in the home theater environment is a ground loop. What is a ground loop? Simply put, it is when the ground currents have more than one path from the component to earth ground. This is most severely and most commonly cause by having two separate grounds. One main electrical house ground and one for the CATV, antenna, satellite, etc… This can result in a safety hazard at worst and some nasty current flow and noise at best. Usually this is manifested as a low pitched 60 hz hum.
Tuner box----------------------CATV/SAT ground
You can see that if the two grounding points are at different potentials you will get current flow through the system and noise.
Systems with a properly unified ground can also have ground loop hum simply because the impedance of the different roads to ground are different.
Different equipment can have dramatically different sensitivities to ground loop current. Very powerful amps, amps with high input sensitivities, amps with high input impedances, and systems with very sensitive speakers will all be very sensitive to ground loop hum. Other types of systems may not be so sensitive. This is why a system which has never hummed before can hum when a component is added or changed. It may be that a ground loop was created, or it may be that an existing ground loop is now noticeably audible.
A. The best solution is to unify the grounds in your system. First and foremost be sure your CATV/SAT/ANT is properly grounded to your main house ground. You can find your main house ground next to your utility meter. It’s a copper rod driven into the ground and it has a large gauge copper wire connected to it. You can run a wire from the splitter for your video source to this grounding rod. All the ingredients are available at your local hardware store, Home Depot, or Lowes. Frequently this will solve the hum. If you don’t feel like doing this yourself, call you cable or satellite installer. It’s code- they are supposed to do it correctly.
It is important for safety reasons to have all the grounds bonded together. Say, for example, a bolt of lightning struck the earth near your service entrance. This is going to cause a large voltage surge at the ground. As long as everything is connected and bonded to that ground- no problem. There will be no difference in voltage and no current over the systems in your house. But if you have a CATV or satellite dish ground somewhere else you have a problem. That ground is going to be at a dramatically different potential than your main house ground. Guess what those grounds are connected through… your equipment! Now you have a surge of up to 6000 volts traveling from one ground site to another through your house and equipment. Equipment damage is likely and you or your family could be injured or killed. “Cheater plugs” are OK for troubleshooting and testing, but should not be used for a long term solution.
B. Now that everything is grounded properly we can proceed to other possibilities. Remember that ground loops can propagate even if everything is grounded properly. All that is needed is more than one path from your equipment to the grounding point. One solution is to plug everything into the same outlet. If you demand on having more than one outlet for your system, you may need one of the ground loop isolators. These are isolation transformers which break the ground loop without affecting the sonics. There are several available. Probably the most famous are from Jensen transformer and they have excellent white papers about proper grounding and system hum available on their website.
C. It can happen sometimes that a particular component (audio or video) will dump a lot of noise on the safety ground. If you connect everything back one by one and, for example, when the TV is brought into the loop the hum appears. But you disconnect the CATV/SAT from the wall and the hum persists then this is probably the case. See B above for solutions.
2. You could be having a problem with EMI/RFI, you could have a power problem, or you could have a defective component.
EMI/RFI usually has more of a hiss quality than a deep hum.
A. Use shorting plugs on every unused input. EMI can enter your system over unused RCA or XLR inputs. You can buy shorting plugs at various location, or you can make your own by soldering the center pin to the grounding pin on cheap RCA connectors. If you need to short unused XLR plugs, check with the manufacturer. DO NOT SHORT THE OUTPUTS- you will damage the equipment.
B. Be sure to use shielded interconnects. Unshielded interconnects are very susceptible to ambient noise.
C. If you do all this and it still doesn’t help, next is to troubleshoot power problems. Unfortunately, there is no easy way to do this. A lot of household appliances dump noise on the power lines. Most notorious for this are halogen lights and dimmer switches- both commonly used in HT construction. You can take the equipment to another part of your house and hook it up to see if the problem persists or you can just carry it down to you local HIFI shop and have them plug it in, If the hum goes away with the equipment in another location- you have a problem with your power source. Power conditioners may or may not help. You may need an isolation transformer, balanced power, or a power regenerator.
D. If the hum is there no matter where you plug it in, you could have a defective component. Take it back to where you bought it or send it back to the manufacturer to be tested. You could also have a very sensitive system and just be hearing the noise floor of your equipment. This is a more common problem with 100+ dB sensitivity speakers.
If you still can’t figure it out, post a message.
This is a beta version, please send me a private message with suggestion for improvement, do not post them here!