Wow - that really hums

July 13, 2005 9:32 AM

HumFilter: how do I stop the audio connection from my DVD player humming?

I have a power amp with L & R RCA inputs. Connected to the inputs is the master output of a two channel mixer (effectively acting as a pre-amp). Connected to the inputs of the mixer are two CD turntables and a DVD player.

Once I connect the video output from the DVD player to the TV, I get a big audio hum. However, if I take the mixer out of the equation and connect the audio from the DVD player directly to the power amp, I get no hum.

So I'm assuming the hum is from the TV, but why am I getting it when I run the audio through the mixer and not direct to the amp, and how can I fix it (all connections are RCA btw) - is it something to do with grounding?

posted by russmail to technology (9 comments total)

Yes, it has something to do with grounding. It would help if you listed which of your devices have a 2-prong (ungrounded) and which have a 3-prong (grounded) plug.
posted by darkness at 9:39 AM on July 13, 2005

Can't be sure because they're at home and I'm at work, but I think they're all 2 prong. The mixer though runs off a 2 prong transformer if that's any help.
posted by russmail at 9:50 AM on July 13, 2005

Some things to try:

* Try disconnecting any cable/sat/antenna connections you may have.

* Make sure all your equipment is plugged into the same power strip or at least the same wall outlet.
posted by darkness at 9:51 AM on July 13, 2005

You might have the same problem that I did. In my case, a cheap ground loop isolator from Radio Shack fixed the problem.
posted by TurkishGolds at 10:04 AM on July 13, 2005

* Does the hum happen when everything is connected, but the mixer is unplugged from power?
* Your mixer probably has a grounding screw somewhere on the back, it should have a ground symbol near it. Try attaching a wire to it and touching the other end to an exposed screw on the amp, tv, dvd player.
* You might just have to go out and get a ground loop isolator
* $5 says one of your devices is 3 prong
posted by darkness at 10:06 AM on July 13, 2005

A 60-cycle hum (or perhaps 50-cycle outside the USA) is usually the result of a Ground Loop.

Make sure all the equipment is properly grounded, and that it is all plugged into the same circuit.

Furthermore, check your outlets. Small plug-shaped devices (with indicator lights) for this are readily available from hardware/home improvement stores. A faulty ground line could be the culprit.

Ground loop isloators can also help, if you're willing to spend a few $$.

Lastly, a bum power cord to the offending appliance could be the problem. Similarly, a device without a ground plug at all could be using a "floating ground", which isn't a high-quality way to prevent system noise...can't do much about that, except to replace the appliance with a better one...or figure out how to replace its power supply...which isn't impossible, and many audiophiles are doing, nowadays...

Hope that helps.
posted by catkins at 10:18 AM on July 13, 2005

You could always ground the equipment the hard way... find a metal section of the equipment that's hooked to fake ground (like the outside ring of an RCA jack) and hook that to ground on your outlet. Usually, if you don't want to jam wires into the ground pin on your socket, the centre screw on the jack is grounded. Screw a wire to that.

Now all your cheapo equipment is properly grounded! YAY!

(Yes, I did ghetto dance parties before. If you want cheap ground loop isolators, go to your local surplus stereo shop and pick up a car subwoofer isolator. They don't distort the sound enough for me to care, at least...)
posted by shepd at 11:23 AM on July 13, 2005

catkins and shepd: If it's a ground loop, it comes from too much grounding rather than too little.

It's likely that the outer connector of the RCAs connect to the earth/ground plane of each device. If two such devices are connected with 3-prong cords to mains earth, a loop is formed (earth, rca outer, back to earth). As 50 or 60Hz magnetic fields are present everywhere, they will penetrate the loop and cause a current to flow through it. That in turn gives rise to a voltage across the RCA outer conductor, which is heard as hum. As catkins said, if the earth connections of the two devices are on different circuits, the loop gets bigger and the hum gets worse. Nonetheless, you should remove the loop entirely.

Since you don't mention it, this probably isn't so, but a typical example would be if the audio output of the TV was connected to the mixer. That would form a circular path: TV to DVD through video cable, DVD to mixer through audio cable, back to TV by other audio cable. Look for loops like this; they are the most common cause for hum.
posted by springload at 2:09 PM on July 13, 2005

I have a friend who discovered accidently that the cheapo inline headphone volume adjustment dials radio shack sells seems to reduce 90% of this. I'm sure a ground loop isolator is better, but if you happen to have one..
posted by rubin at 3:10 PM on July 13, 2005