Hum from decoder

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
 
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
 
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
 
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
 
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
 
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!
 
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
 
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
 
REmoving Hum

Pixie:

Here is the URL for the M.A.G.I.C. box. I personally don't have one, but lots of people over at the Home Theater Forum swear by them. They go for about $100

The Panamax line conditioner can be found here. It conditions AC wall power, as well as one CATV line. I use one of these with pretty good success. It also allows me to control power on/off to all of my components thru my preamp. These go for about $250

One other method I forgot about was to use a CATV signal amplifier, around 10-12dB. The process of injecting power into the signal also works to remove the hum. The one I used before I got from Radio Shack for about $15. May want to start with this one first - its one of the cheaper solutions.

By using these devices, you don't have to lose your ground connection from your equipment.

Let us know how it works out. Don't give up, many others have made their hum go away before!
 
Pixie,
Forgot to say--in a pinch, an extension cord makes a dandy stand-in for a cheater plug. Try to scrounge up an old one that's got both blades the same size. This will give you the option of flipping the plug over to see if hum is lower one way than the other.
Incidentally, all my cheapter plugs are older ones that have two same-size blades. If you buy a new one and find that it's got a larger blade...take a file to it. You're not destroying some major, expensive piece of equipment, and I doubt you're going to lose sleep over 'voiding the warranty' on a $1.98 cheater plug.

Grey
 
OK. I talked to Randy Slone too. He told me several ways to get rid of hum problems in his design. The one major thing I did was run a huge wire from the center tap of the power transformer to the high quality ground. That cut it down to a tolerable level. I still haven't figured out a way to reconnect the earth ground. If I do, it doubles the noise. I'm going to try a 5 ohm resistor between my HQG and earth ground. We'll see what that does. That's something Nelson Pass suggests.

I tried disconnecting the cable TV. That's not the problem. It's somewhere else in the VCR. If I just connect the VCR output directly to the amp the noise is the same. So, it's just the VCR reacting with my amp. I removed the grounding wire between the VCR and the amp. That actually cut the noise down.

Any more suggestions?

Thanks,
pixie
 
Pixie,
Does the VCR hum going straight into your McIntosh? I'm assuming that you would have mentioned it before if it were so, but I just wanted to double check.
Also, you might want to use something more than just a 1/4 or 1/2W resistor for that ground. You'd be surprised at some of the voltages that can run through a circuit in the ground loop.

Grey
 
I think I have a simliar problem. When I run my VCR into my technics receiver, I get hum. However, this only occurs when the preamp is switched to something other than the VCR input. For instance, if the preamp is on CD, I get hum. If I unplug the cables from the VCR inputs it goes away. Like I said, there is no hum when the actual VCR input is selected on the preamp.

Could this be caused by the CATV connection to the VCR?
 
Finally...

OK. I found the problem, I think.

The ground input of my amplifier was shorted to the case. It wasn't connected directly to the HQG.

I'm not sure why it was only a problem when the VCR was in the path, but now the only hum is comming from the transformer.

Does anyone have suggestions for how to get rid of the physical vibrations from the toriod transformer? Not likely, but I thought I'd ask.

Thanks,
pixie