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Old 4th April 2004, 03:03 PM   #1
K-amps is offline K-amps  United States
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Default Cable related hum

I installed a 3 prong IEC AC receptacle on a couple of guys Adcom power amps. The Stock had the 2 prong polarized types.

Then grounded the Gdn receptacle to the main chassis (also to the gnd bus between the caps).

Issue is, with two guys who have a cable connection to their pre-amps, when I use these amps, there is significant hum present on the speakers.

The hum goes away when the cable/TV is disconnected. Also with stand alone pre-amps I see no issues so I know the power amp is ok...

Apart from filtering the Coax with 10-100nF caps, is there anything else I need to consider? Also why does the coax dump 60vac into the grounding system?

Thanks, K
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Old 4th April 2004, 03:35 PM   #2
HDTVman is offline HDTVman  United States
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Default Re: Cable related hum

Quote:
Originally posted by K-amps
Then grounded the Gdn receptacle to the main chassis (also to the gnd bus between the caps).

Apart from filtering the Coax with 10-100nF caps, is there anything else I need to consider? Also why does the coax dump 60vac into the grounding system?

Thanks, K
I would not connect the Grd from the receptacle to the power supply Grd. The AC Grd is intended to be a safety ground to prevent the user from being shocked in the event of a short from AC line to case.

The coax is not the one causing the 60vac to show up on the grounding system. The most likely cause is the TV. Most TVs have a network from both sides of the AC line to chassis common, which happens to be the coax cable input, to drain off static electricty for an antenna system. The cable TV system should be grounded where it enters the house. If it is not it has been altered or installed incorrectly, or the AC outlet is wired incorrectly (hot and common reversed).

Later BZ
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Old 4th April 2004, 04:01 PM   #3
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Also, you could use an Isolation transformer of the coax side cable TV box. Or you might buy a bunch of cheater plugs.
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Old 4th April 2004, 04:47 PM   #4
K-amps is offline K-amps  United States
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Default Re: Re: Cable related hum

Thanks Guys,

My observations to your deductions:

Quote:
Originally posted by HDTVman


I would not connect the Grd from the receptacle to the power supply Grd. The AC Grd is intended to be a safety ground to prevent the user from being shocked in the event of a short from AC line to case.
Later BZ
IEC gnd connections:
This is the way the factory Adcom connections are done... All the gnd's are connected to the HQG in a star arrangement. I did it just the way Adcom does it when the factory IEC AC socket is ordered.

Also in a 2 prong polarized set-up, the PSU gnd is connected to chassis, and if a 3 prong is installed, the IEC gnd would also be connected to chassis thereby connecting the IEC gnd to the PSU gnd even if it were not done intentionally (unless the PSU gnd was disconnected and kept floating).

In the way I have it set-up, this would also be a safety precaution in-case the Toroid shorted to the center-tap, now the voltage will be clamped to gnd right?

Hum with Co-ax Cable:
The guy's hum went away after the cable was disconnected, TV was still connected and caused no hum. .... weird aint it?


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Old 4th April 2004, 04:59 PM   #5
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It's funny, but I just moved my Adcoms from the new (electrical) part of the house and liberated the living room to further the cause of hyperaudiophilia. There are no grounding plugs in that room.

Amps (and everything else) are no longer grounded. They have never sounded better. No 60 Hz hum or pops or other turds on the line. Dead silence at high volume levels.

(Although not necessarily safe, its just an observation.)
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Old 5th April 2004, 12:23 PM   #6
John10 is offline John10  United States
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I think K-Amps hum problem may be caused by a ground loop created by the ground difference between house wiring ground and the cable system ground. I had the same problem and solved it by using an isolation transformer as suggested in an earlier post. Iso-Max has one for about $50.00. It connects between the cable and cable box and is probably safer than using cheater plugs.
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Old 5th April 2004, 08:36 PM   #7
K-amps is offline K-amps  United States
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I agree on not using cheater plugs.. defeats the purpose of installing the IEC socket in the first place.

But I am also leaning more in trying out the capacitor in series trick rather spending $50 on the Iso-thingamajigs . (Maybe in the end thats what I will have to do, but lets try the DIY route first. )

Thanks for your comments guys!
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Old 6th April 2004, 10:59 PM   #8
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A lot of electronic devices have emi filters on their inputs that have large caps to safety ground. Occasionally you also have spark arresters that do that too. In effect, they inject noise into your ground reference. All those little currents want to flow to the central ground in your house, and some do it through your amp's ground wiring.

As soon as you decided to couple your signal ground to earth ground, you became vulnerable to such noise from other devices.

That's why you see a small resistor on some input ground circuits in power amps, or a small resistor or thermistor from your amps internal ground to chassis ground. You decouple from the other guys ground noise somewhat. It's also why pros use balanced interconnects, not referencing ground.

The isolation transfomer on the cable device breaks the ground loops too.

There is no free lunch here. If you want to connect your ground to safety ground, fine, but you now have to listen to all the noise injected into safety ground by other devices.
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Old 22nd May 2005, 01:39 PM   #9
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Default Strugling with ground

I have similar problems only with my coax connected.
I get a hum with the cable connected. I used
a couple of radioshack transformers. That works
but it makes the signal weaker and causes
some channels to perform poorly. I bought
a groundloop breaker from partsexpress.
Same problem. The premium channels do
not give a picture.

The cablebox has only a 2 prong AC plug.
I was thinking about grounding tha cable directly to
AC ground. Does this work?

Thanks

Harry
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Old 22nd May 2005, 01:57 PM   #10
K-amps is offline K-amps  United States
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Try the hum blocker on the cable side not audio. Do not try and ground the cable box to the amplifier chassis, you will still get 3-4 v AC across the grounding cable even if you use 8ga wire... best way to filter out the hum from the cable, and simple 6db octave filter set at30kHz or will work. Do a google for values of caps to use, the caps will pass the cable signal and block the AC hum.
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