Hum/Buzz in my AV setup!

[mkrishnan]

So I know I've been posting a lot about my learning experiences related to getting a new TV... I am *so* close to having everything set up the way that I want, but I am coming up short at the very last step.

There are many places where this setup is not ideal, and I recognize that completely. I'm totally happy to hear suggestions involving investing in new hardware, although at this point, I am primarily focused on getting this working without major new investments.

The basic problem is that I'm experiencing a hiss/buzz (sounds like the kind I'd expect from a grounding problem or an impedance mismatch -- a constant buzzing sound that *is* dependent on the volume setting of the stereo) when I hook both my television and my computer to my stereo.

This is the setup:

Sources:
- ReplayTV DVR
-> connects to the TV via component video + RCA analog stereo audio
- DVD Player
-> connects to the TV via component video + RCA analog stereo audio
- VCR
-> connects to the TV via composite video + RCA analog stereo audio
- iMac G5
-> connects to the TV via VGA
-> I have a long 3.5mm audio cable (headphone jack to headphone jack) that goes from the iMac to a Y-splitter. Right now, 1/2 signal goes to the receiver (3.5mm -> RCA cable) and the other half is unplugged, although intended to go to the TV via 3.5mm cable
- LCD TV (as a pass-through of audio signals only)
-> 3.5mm to dual RCA cable to the receiver -- the TV passes audio through on this port from whatever source is cued.

Destinations:
- LCD TV (Insignia NS-32LCD)
- Receiver (an old but nice pre-5.1 Technics receiver with floorstanding speakers)

Now, here's the problem. Everything works except the audio connections to the receiver. If the TV is totally unplugged from the receiver, then the iMac through the y-splitter works fine. I ruled out an impedance mismatch at the Y-splitter because plugging a cable or headphones to the other port does not cause a problem.

However, if the iMac goes to the receiver and the TV goes to the receiver, even though the iMac does not go to the TV, I get a constant buzz on the iMac's receiver channel, just by virtue of the TV being plugged into the receiver.

So the connections I don't have right now are iMac -> TV and TV -> receiver. As far as I know, I can do iMac -> TV without messing anything else up (I just misplaced a cable ). But as far as I can tell, I *cannot* do TV to receiver without messing everything up.

I know this is long... but can anyone refresh my (minimal) electrical engineering education? Is there an obvious reason why this is not working and cannot work? Is there a grounding fault somewhere I can potentially remediate?

Any help would be soooo appreciated!

 

[mkrishnan]

I just wanted to add that my receiver has a grounding port of some kind on the chassis, but it doesn't have a 3-prong plug. In fact the only thing in the setup that has a 3-prong plug is the iMac. I wonder if it's as simple as that? Although I have to find something I can jury-rig to ground the receiver if that's the issue.... and also I wonder why it bleeds from one receiver input to another?

 

[MattG]

I just wanted to add that my receiver has a grounding port of some kind on the chassis, but it doesn't have a 3-prong plug. In fact the only thing in the setup that has a 3-prong plug is the iMac. I wonder if it's as simple as that? Although I have to find something I can jury-rig to ground the receiver if that's the issue.... and also I wonder why it bleeds from one receiver input to another?

Mohan,

I had a very similar problem. Try buying one of these...I'll bet it'll fix you right up:

http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=180-075

 

[beatsme]

tricky...

it looks like you're connecting the 3.5mm audio out (which is a headphone jack, correct?) on your TV right into your receiver. If it is a headphone jack, then you're connecting a magnetic out to line-level input. That ain't good, and it's probably the cause of your hum.

a question: why connect the audio out on the TV to the receiver at all? It seems to me that a better setup would be to connect only the audio outs on your components to the receiver, which in fact is what I do. If I want to watch regular TV in stereo (which I pretty much never do) I can always turn on the VCR and use that as my cable-TV tuner, since the cable-TV feed runs through the VCR and then to my TV...

 

[mkrishnan]

MattG -- thanks! I will look into that. It makes a lot of sense, though. I've heard of that happening before in other contexts. I think I should be able to verify if the cable lines are causing it by trying the Mac and the TV with the cable TV disconnected. And if that is indeed the issue, that's an easy fix.

beatsme -- thank you too! Mmmm, I do see your point. I did what you suggested, before, sort of. The problem is that I very rarely want the speakers for the TV... in fact, I might just be willing to do without that and solve this that way. It's lame, I guess, but I put my Dolby satellites in a box (the analog dolby never impressed me) and I just have two floorstanding speakers right now, so I'm not so keen on being able to use them for movies -- having them there for the Mac is much more important. The only reason I want this is that every other device purely works with the TV, and this way, everything would be on the same receiver channel except the computer.

I think ultimately I do plan to going to AC3, but I'm not in a huge hurry... perhaps in a year or two, a combo device like the ones Sony makes that will replace my receiver and DVD player with a hi-def DVD and a simple integrated receiver?

As far as explaining the problem, though, I don't think you're right. As far as I know, there's no impedance mismatch between headphone out and line level -- the only difference is that the line level signal is slightly amplified in comparison to the headphone out (see this for instance). So there's no "danger" associated with headphone lines.

Also, to clarify two points: first, this is definitely a constant buzz and has nothing to do with a poor audio quality signal -- i.e. I'm not getting crackles or pops because of distortion in the headphone output circuit. Also, I've used my receiver with the heaadphone output of my iPod and my iBook for several years without this issue, and even now, neither device (the TV or the Mac) seems to cause the buzz on the receiver unless they're *BOTH* plugged in.

But thank you so much for your help!

 

[techgeek]

I think you have an earth loop problem.
The iMac has an earth connection to both the receiver and the TV (via VGA) and the TV and receiver share an earth connection. Since the iMac audio cable is long it will only make matters worse.
If you can, try disconnecting the earth on the iMac audio cable at the input to the receiver.
Have a look here for more info (near the bottom):
http://www.soundonsound.com/sos/nov02/articles/studioinstallation1102.asp?print=yes

You could also try higher quality cables as they tend to be more immune to noise in the first place (but cost a lot).

 

[mkrishnan]

Thanks! I still need to take a half hour and try to see if I can isolate an incident source of the ground loop (e.g. the cable TV signal, perhaps, as the previous poster suggested). I think I just never hooked this many analog devices up to each other before! It's still nice to have everything almost talking to everything else, though. If I have to live with the TV not talking to the receiver, I'm probably quite okay with that, even.