Two things drive me truly crazy: 4:3 video stretched out to fill a 16:9 screen, and hearing hum or buzz in the audio portion of a program. I remember being on a tour of the multi-million-dollar Kodak Cinesite facility when it first opened, sitting down in their private screening room…and hearing hum in the audio. Which could have been fixed for as little as $20. I’d like to share a few ways you can fix it yourself.
In most cases, buzz and hum in the audio is caused by one of three things:
- Too many ground connections, such as the power cords grounding two different pieces of equipment, which also happen to be wired to each other via a third connection: the ground or shield on an audio cable. This can cause a ground reference “loop” - down one power cord, up the other, and across the shield to back where it started - which in turn causes the hum. This is particularly common when hooking up gear (from tape decks to powered speakers) to computers, which have notoriously bad grounding systems.
- Insufficient ground connections. The ground connection is vital to drain off noise and interference; bad ground connections on either audio or power connectors can cause a “ground lift” (as opposed to loop) condition, which often results in buzz.
- Audio cables with insufficient shielding to keep noise out.
The most common way to cure the first problem is to cut or otherwise disable the grounding pin on the power cords. Bad idea: That ground pin is what’s trying to drain away the noise in the first place, and what is providing a path for electricity to go down should a short circuit occur (otherwise, that electricity may go through you - I’ve been shocked to the point of bleeding thanks to improperly grounded equipment).
The better solution is to use what’s known as a Ground Loop Isolator: It’s a transformer that conducts your audio signal, but which does not have any wired connection between input and output, thus breaking the loop. I have a number of Radio Shack Ground Loop Isolators in my toolchest, including many modified to have 1/4” connectors on the ends (as seen here). For more serious applications, I like the Ebtech Hum Eliminator line; I also use their Line Level Shifters to interconnect professional (+4 dB reference) and consumer (-10 dB reference) gear.
The second problem is, ironically, most often caused by people who cut or otherwise disabled the grounding pins on their power cords! A broken shield in an audio cable can also cause ground lift buzz. Look at these culprits first; if they don’t fix it, try adding an extra grounding strap to your gear by running a thick copper wire from a screw on its chassis to the middle screw on a power outlet (which should be grounded). It is possible to confuse ground lift and ground loop problems; also try isolation transformers for a cure.
The third problem can be cured by using better audio cable, and being more careful how you route the cable:
- Avoid power transformers (either inside gear, or plugged into the wall), power outlets, and power cables. Get audio cables as far away from these power sources and connections as you can - even a few inches helps.
- If you must run power and audio close to each other, try to run audio cables at right angles across power cables rather than parallel to them.
- Shorter runs are also better, as are balanced connections (XLR or tip-ring-sleeve connectors versus RCA or tip-sleeve) and higher signal levels (such as +4 instead of –10) to cancel or drown out the noise.
As for the cable itself, I like Canare Star Quad (click here to download a PDF from Canare explaining its advantages); just switching to this cable alone has cured some of my induced-noise problems in the past.
If you want to learn more about how to wire together your equipment in ways that will help eliminate buzz and hum, one of the audio industry’s long-time favorite references has been Rane Corporation’s RaneNote 101: It contains a lot of solid advice, plus a series of wiring diagrams showing the correct way to create a wide variety of audio cables. Print it out, and keep it near your soldering iron. (If you’re truly an audio geek – or want to become one – also read RaneNote 151 on Grounding and Shielding Audio Devices; a list of all RaneNotes can be found here.)
But in the meantime, run down to Radio Shack and get a couple of Ground Loop Isolators – they’ll make you a hero, solving problems even multi-million dollar facilities couldn’t figure out.
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And if you’re really hard-core about eliminating hum from your studio, there’s the “balanced power” route. Equi=Tech (http://www.equitech.com/) is the leader in this field; go to http://www.epanorama.net/documents/groundloop/balanced_power.html to read a good introductory article on what balanced power is and how it can help.
Posted by Chris Meyer on 02/26 at 03:37 PM