amplifier hum question
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nysulli

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so i've had some amplifier hum for some time now, audible from the tweeters on my mains and center channel, not really noticable on my rears, i've tried the easy stuff, disconnecting the cable etc.

the other day i took home a demo rotel 900 series line conditioner ($200) to see if it could clear up the problem, which it didn't, and it also had a very minor effect on the picture quality and detail in the audio, some inprovement, but it was less then going from a radioshack rca to a kimber pbj, or biwiring the mains (psb image 6t)

from previous posts, it sounds like i may simply have some dc hum, what cheap way if any is there to clear this up, 200 is really the max i can afford at the moment, i'd rather not spend that much, I only took home the rotel for a demo because it was the only line conditioner i could demo
 
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ooheadsoo

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How loud is the hum? On most equipment, some level of hum is unavoidable, or normal. Some components have more hum than others. In specific situation, for example, I got rid of my hum by switching to a battery powered dac.
 
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pbalcer

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Sometimes the hum is a result of, so called, common mode currents.
Try do disconnect the audio cables and make sure all audio components don't touch each other. If there is no hum after separating the components and the hum appears after connecting "ground" paths only - you may be facing CMC.

In short, CMC happens when different pieces of equipment have different levels of "ground" - this causes current flow from one component to another through "ground" paths (shielding of cables, etc.). The solution is usually to break galvanic connection between the pieces of equipment (using fibre-optic cables, etc.) - this may not always be practical or do-able. Another solution is to connect "ground" of all components together using short wide copper tape (flexible - easy to work with) and then ground it well.

You migh find more info on CMC and how to get rid of it on news groups devoted to ham/amateur or shortwave radio. CMC is a common problem there and there is a lot of info available on how to get rid of it.

Of course, there may be other reasons why the hum is present. Sometimes the design of the power supply is "sub-optimal", sometimes a component within the equipment is "fried", sometimes there is no good contact on the connectors, etc....

Edit: common mode currents = "CMC".
 
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Melchior

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And occasionally it's as easy as using shielded power cables and making sure all interconnected equipment is plugged into a single power tap/surge protector.
 
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nysulli

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well, i've gone after the common mode current issue in the past, without any luck in many of the ways you mentioned, with a grounding wire between each piece of equipment, and having all the equipment plugged into on power strip

unfortunitly the cord for my reciever is not disconnectable, and its only a 2 prong wire, so making a better power cord is out there.

but the more and more i think about it, something is loose inside the reciever, i can hear a very faint click as i turn the volume knob, but none of this is anything i can hear from the listening position, or at very low volumes, i have to turn it up pretty high to hear it without anything playing, this weekend i'll probably open up the amp to see if theres a something obviously loose, but given my financial state as a student, i won't be messing around with it to much, cause i can't afford to spend 1000 bucks on a replacement if i screw it up

thanks for the ideas guys, keep em coming
 
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bundee1

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Do you have your cable box or satellite receiver plugged into your stereo? If so that could be the problem. Buy a couple of antenna connectors ,75ohm to something else, I cant remember, do a search because I had the problem as well and someone else named the parts I needed to get rid of it..
 
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ooheadsoo

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Hum that you can't hear from a few inches away may very well be normal. I don't have any common ground issues and I had that hiss with various equipment. It's annoying for the hiss sensitive (tubes, anyone?,) but unfortunately normal.
 
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dvw

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Here's another possibility. If the amplifier just developed the sympton even without anything connected to it, (sometimes it's off and on) it could be your power supply electrolytic capacitor is shot. You'll need to recap.

I had an amplifier that did that once. After recap, hum is gone.
 
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dgardner

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Quote:

Originally Posted by bundee1
Do you have your cable box or satellite receiver plugged into your stereo? If so that could be the problem. Buy a couple of antenna connectors ,75ohm to something else, I cant remember, do a search because I had the problem as well and someone else named the parts I needed to get rid of it..


Another common form of this problem is to have cable/sat hooked to the TV directly and then have cables running to and from the TV to the AV Receiver. This will almost always result in form of slightly audible hum. Parts Express sells ground breakers for Cable tv hookups.
 
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