Why do my new speakers hum? - AVS Forum
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post #1 of 11 Old 02-18-2002, 10:30 PM - Thread Starter
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Just picked up a pair of Paradigm Monitor 7s and hooked them up to Denon 3802 receiver. When i put my ear to the tweeter of either speaker, i can hear a hum. It's hardly noticable, and i can only hear it if my ear is 10-15cm away from the tweeter. But it bothers me.. it's not supposed to be there, is it? Why is there sound coming out at all if nothing is playing? The receiver is hooked up to my Audigy sound card via Radio Shack gold-plated RCA cable. I'm using a mono-mono 1/8-RCA connector to connect the cable to my sound card.

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post #2 of 11 Old 02-18-2002, 10:39 PM
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Why do my new speakers hum?
Because they don't know the words! :>)

Seriously, though, unless they have standard sized RCA plug outputs designed for connection to the line level inputs of external hifi/HT components, computer sound cards are NOTORIOUS for introducing hum into an HT system. Typically the output at the minijack is simply not electrically compatible with the line level inputs on most receivers/preamps/pre-pros, having usually been designed for use with either headphones or computer-grade "powered speakers". You are actually pretty fortunate that the hum is so close to inaudible. On my system, the hum was so loud, I abandoned the experiment while using a sound card that only had mini-plug outputs.

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post #3 of 11 Old 02-19-2002, 05:13 AM
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There are lots of reasons why a computer audio card might introduce hum into a system. The most common one is powering the computer from a different AC outlet so that it has a slightly different gound potential than your audio equipment. As a result, current flows on the audio cables between them, causing hum.

If you don't already, try powering them from the same outlet to see if that reduces the hum. Another way to avoid ground currents between equiment is to use optical digital audio interconnects. You might consider geting a sound card that provides an optical digital output.

I'd suggest posting your problem, including your brand of audio card, in the Home Theater Computer forum. I'm sure people there will be able to suggest other improvements.

I hope this helps a little.

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post #4 of 11 Old 02-19-2002, 12:38 PM
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sometimes hum is created by the computer, I've found that sometimes there will be a faint hum from my PC and that turning down the sliders on the mixer in Windows can eliminate the noise. Usually, I'll leave "Wave Volume" turned up but turn down "Synthesizer" and "CD" volumes and this seems to eliminate the hum.

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post #5 of 11 Old 02-19-2002, 06:47 PM - Thread Starter
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I tried unplugging the receiver from my PC, and guess what... Yep, it's still there. Oh and i just realised that it's only hard to hear if the volume is low, like it was when i was listening last night. If i crank the volume up, i can hear the humming pretty damn well. Also got new wires - 12ga Home Depot with gold-plated bananas on each end. No difference. Dammit, what can it be?

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post #6 of 11 Old 02-19-2002, 10:49 PM
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Do you have a cable TV or antenna feed hooked up to the system anywhere, such as connected to a VCR which is then connected to your receiver? If so, does the hum go away when you disconnect it (physically detach the wire, not just turn off the cable box, or VCR or whatever)? If the answer is "yes" (the hum disappears when you disconnect the cable feed) then you need a ground circuit isolator between the cable feed and whatever component it is connected to.

Prices for such devices vary, as does their effectiveness in different systems. I happen to like the job the Mondial Antenna Ground Isolation Circuit (aka "MAGIC" box) does, but it is fairly pricey at about $100 retail. Other such devices from Jensen and other companies sell for a lot less, but I do not have any first hand experience with them. There are also a number of low cost DIY "fixes" that I tried, but in my system they didn't really work very well. (IOW the hum was still noticeable with them in place.)

Let us know if the cable feed is the culprit! It is one of the most common sources of hum in HTs. (The other major one being ground differentials introduced by having components in the system plugged into the house wiriing on different circuits which are not grounded the same.)

Good luck -

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post #7 of 11 Old 02-20-2002, 12:39 AM - Thread Starter
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Do you have a cable TV or antenna feed hooked up to the system anywhere
Well there is a TV cable connected to my cable modem, which is then connected to my PC, which is connected to my receiver. But i just unplugged it and the hum is still there.

Now, i just realised that Windows is outputting a ton of it too. The barely audible hum is always there, whether the receiver is hooked up to anything or not. But i noticed that when i unmute Windows volume, it goes up, and i mean way up. With the receiver volume all the way up, i can hear it 3 meters away. Ok i'm guessing this is a problem with my sound card? I believe the solution to this is to get that Hoontech bracket so that i don't have to use the 1/8?

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post #8 of 11 Old 02-20-2002, 03:48 AM
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Diabolic,

From what you've said, you have multiple sources of hum, but you keep leaving out critical pieces of information, so it's hard to be sure.

All of your a/v equipment, including your computer, subwoofer, TV, etc, must get its power from the same outlet. Otherwise you are likely to have hum. Sometimes you can get away with using separate outlets, but it depends on how power is distributed in your home and what else might be plugged into each circuit.

To localize the source(s) of the hum, you must unplug all other electronics from your receiver. There should be no hum. If there is, then you probably have a power supply problem in the receiver. Most likely a filter capacitor has failed.

Then start connecting one thing at a time. When the hum starts, that's your culprit. Make sure each addition is not connected to anything else when you plug it in, since it could be that secondary connection (like your cable modem) that is the real culprit. You have to be methodical about this, otherwise you'll just confuse yourself (and us).

If you cannot power the computer and its CRT from the same outlet with everything else, then you should use an optical connection.

If you hear hum from the PC when using only its local speakers and it is not connected to anything else, especially not the cable modem, then there is something wrong in your PC: most likely a filter capacitor has failed in the power supply.

I hope this helps a little.

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post #9 of 11 Old 02-20-2002, 09:38 AM
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Is ground loop hum likely to be heard at the tweeter? I had the impression that the hum you'd hear from ground loops would be generated by drivers that handle the lower frequencies (e.g. 60 Hz, 120HZ) corresponding to AC's 60Hz frequency (and harmonics).
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post #10 of 11 Old 02-20-2002, 03:10 PM
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Will,

You're right, of course. I was focusing on the word "hum". Personally, I'd describe a continuous tone coming from a tweeter as being a "whistle".

So, Diabolic, the question is now: exactly which speaker driver is the sound coming from? The tweeter is the smallest driver. The ground-loop hum that I've been concentrating on would be coming from the largest driver.

Can you tell us roughly what frequency it is?
60Hz? 600Hz? 6KHz?

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post #11 of 11 Old 02-21-2002, 03:34 PM
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I get just a tiny bit of noise from my tweeters as well. If I put my ear pretty much right over the tweeter, I can hear a faint buzz. I can't hear it from an inch or so further away. It's so quiet, I don't worry about it. If I had to speculate I'd guess that it might be noise that's inherent in the amplifier (doubt it) or might have something to do with one of the broadcast video boxes (DSS & VCR) connected to the pre-pro.
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