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Old 14th March 2001, 04:43 AM   #1
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I'm getting tired of this. When I run my amps without being connected to the decoder, they are fine. But when I connect them to the decoder, I get very audible hum. I also get a lot of noise from my TV.

What's the solution? I found some shielded wire. Aluminum-polyester shield 14AWG two conductor. Do you think if I use this for interconnects it would solve the noise? I don't think it would solve the hum though.

Thanks,
pixie
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Old 14th March 2001, 07:31 AM   #2
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It sounds like a) the PSU of the decoder is not properly filtered/regulated or b) the RF shielding is defective. Other than that, using shielded wires is not a bad idea.
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Old 14th March 2001, 01:56 PM   #3
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Default Ground loops?

It is possible that it creates a ground loop. All the cables are grounded and all the amp boards are grounded forming a loop from the outlet ground(or neutral if you are not running a 3 prong plug) to decoder to amp(through the low level cables) to ground. All the equipment I have has external grounds to tie all the componants together with a wire seperate from the signal wires. I havn't had to use them yet, but that is what they are there for.

Isn't hum normaly caused by ground loops. I doubt the decoder has a ground loop all by it'self I would guess it is created by the combination of equipment.
Wade
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Old 14th March 2001, 09:29 PM   #4
grataku is offline grataku  United States
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Pixie,
it's got to be a ground loop. The cure is going to be painful. You have to go through your wiring and isolate the cause of the problem.
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Old 15th March 2001, 12:22 AM   #5
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I tried grounding the cases together. That didn't work. I'm not even sure if it's hum now. There seems to be a lot of other frequencies. It may just be noise.

I have a cheap decoder. It could be it's power supply. Maybe I can replace it. I'm almost willing to try anything to get rid of the noise in my system. It drives me nuts.

I even thought about ripping out the pre-amps from decoder and building new ones. I don't need the amps in the decoder anymore. Maybe I'll cut the power to those and see what happens. Hey, that's a great idea... I love thinking and typing at the same time.

pixie
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Old 15th March 2001, 02:51 AM   #6
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Pixie,
Before you rebuild your entire house, let's see if we can track this down in a more orderly manner. Some thoughts:
1) I am assuming the noise is in both channels, yes?
2) Turn off the decoder (but leave it hooked up). Does the noise stop?
3) Try 'cheater plugs' on the AC cables to lift the ground for the decoder, the amp, and both at the same time.
4) Flip the cheater plugs over (i.e. reversing the hot and neutral blades on the incoming AC).
5) Aka 3a & 4a...take a multimeter and measure voltage (if any) from the chassis of the decoder and the amp to a known ground. Generally speaking, you'll reorient the plugs (with cheaters attached) to the lower of the two voltages.
6) Check all cables. You may have lost the ground in one or both. This seems obvious, but I can't count the number of times I've run across problems caused by broken or poorly connected (often corroded) grounds. Try different cables.
7) With amp on and decoder off, try unplugging the decoder from the wall. I have a piece of equipment from a reputable company, in good working order, that causes a ground loop *just by being plugged into the wall.* A cheater plug on this one piece of equipment solves the problem for the whole system. (I NEVER remove gound pins. I like 'em. Leave them intact for when you move and find that the mysterious ground loop is gone, and you can have everything plugged in normally.)
8) Are you *sure* the ground pin in your AC outlet is actually a functioning ground? File this in the 'ya can't get good help any more' category. Not all electricians are Einstein in their spare time. In my experience, about ten percent of them are idiots who do things like leave the ground lug unconnected when putting in an outlet--or fail to connect the ground/neutral in the breaker box, etc. Another possibility is that the connection has come loose because it wasn't tight. Measure between AC hot (should be the smaller of the two blades on the right), AC neutral (the larger blade on the left), and the ground pin. There should be a reading between AC hot and either neutral or ground. There should be no reading between AC neutral and ground. If you prefer (and it's safer...), they sell little gizmos that plug into the outlets and light up LEDs to tell you what is/isn't hooked up properly in your outlet.
9) Although it seems that it would work, running a lead from chassis to chassis doesn't usually do any good. Signal ground and chassis ground are usually one and the same. The major exception to this is an amplifier topology that floats the output, and sometimes the input with it. Slone doesn't seem like the type. Anyway, hooking a signal cable between most pieces of equipment *usually* grounds them together. Yes, it'd be nice if you could cause the nasty AC to flow through one wire, and the good AC through another, but--assuming that you don't have a high impedance connection (i.e. poor signal cable)--it's not likely that another cable will help.
Good luck.

Grey
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Old 15th March 2001, 06:12 AM   #7
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Some success.
The noise is comming from all channels.
The center amp is my new one and is the most sensitive.

I started turning things off. Didn't change.
I switched off the power strip. Powering off everything except the center amp. Still there!

Then I started unplugging audio connections. It went away as soon as I unplugged the VCR. It's a fairly good and new JVC S-VHS. Actually I think it's still their best.

It's in a plastic case. So, I took it appart and connected a wire to a good ground and ran it outside. I connected it to my new amp because it's the only thing with earth ground. That cut the noise in half.

Here's a strange thing though. I put the top back on my new amp and that cut the noise down more. It was sitting away from everything else. So, I can't see why this happened. Oh well, I guess the top belongs on anyway.

Oh, unless I'm prepared to cut traces, I can't cut power to the amps in my decoder. Wow it's a mess in there. It appears the power is AC to the amp board, the amp board does AC-DC and gives it back to the decoder then the decoder passes it to the pre-amps. I can't figure out the signal path though. The pre-amps have a bundle of wires going to the amps. So, I thought I'd remove them. But, now the pre-amps don't have any amplification. I can't figure out why the pre-amps are dependent on the apparently down stream amps. I'm guessing the amps actually provide amplification for both the power outputs and pre-amp outputs. I'm not surprised to see this junk in a $400 decoder... I need a better one.

pixie
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Old 15th March 2001, 02:01 PM   #8
Eric is offline Eric  United States
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Question Ground loop from CATV?

Hey Pixie: I'll bet the ground loop is caused by your cable feed to the television. It is then travelling from component to component until it finds a component with a 3-prong grounded plug. The hum will affect everything between the point where the CATV signal enters to where the 3-prong grounded plug hits the outlet.

Several devices will cure this:
1) a typical power conditioner. I have a panamax Max1000+ that solves this problem for me
2) a M.A.G.I.C. box - available from Audio Advisor, but they cost $100
3) try two 75ohm-to-300ohm converters back to back.
4) a "cheater plug" on everything with a ground.

Good luck tracking down the problem!
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Old 16th March 2001, 02:27 AM   #9
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Finally a real sounding solution. This is driving me crazy. Everything I ground takes a little away. But, I'll never get to zero this way.

BTW, I'm assuming a cheater plug is one that allows a three prong plug to connect to a two prong outlet. Thus isolating the earth ground. I can do that by removing the earth ground connection inside my new amp.

Where can I find this power conditioner? Would anyone recommend the MAGIC box before even trying a power conditioner? Is it that much better?

Thanks,
pixie
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Old 16th March 2001, 02:49 AM   #10
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Pixie,
Yes, 'cheater plug' is one name for a three prong to two prong adapter.
You can build a simple power conditioner quite cheaply. I favor using an isolation transformer (i.e. 1:1 turns ratio--120VAC in/120VAC out with sufficient current capability) with caps (.1uF 600VAC Sprague Orange Drops work well here) across both input and output leads. You can get much more elaborate if you wish, but that will do for a start.
I'm not sure I'd undo the ground in the amp if it's the only ground in the system. It's nice to have a real ground for safety reasons.
Try a cheater on your VCR first (they're cheap), and see if that does the trick. Then consider using an isolation transformer or power conditioner for the VCR since it seems to be the offending item from what you said. Fiddling with the incoming power for an amp gets expensive fast because they draw so much more current.

Grey
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