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
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
K-amps said:
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
Re: Re: Cable related hum
Thanks Guys,
My observations to your deductions:
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?
Thanks Guys,
My observations to your deductions:
HDTVman said:
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?
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.)
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.)
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.
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!
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!
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.
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.
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
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
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.
Hi K-amps,
As you have proved empirically, the hum DOES come from the cable ground, which is the worlds largest ground loop. Use 1:1 balun transformers to isolate the cable ground. The better quality ones have lower losses. I had to service all kinds of gear due to ground tracks being blown ... to the video jacks. Hmmmm, not warranty.
The largest amount of destruction I have ever witnessed in a lightning strike occurs every single time when cable is involved. I will not have cable in my house for that reason. Do use the three wire grounded outlets.
-Chris
As you have proved empirically, the hum DOES come from the cable ground, which is the worlds largest ground loop. Use 1:1 balun transformers to isolate the cable ground. The better quality ones have lower losses. I had to service all kinds of gear due to ground tracks being blown ... to the video jacks. Hmmmm, not warranty.
The largest amount of destruction I have ever witnessed in a lightning strike occurs every single time when cable is involved. I will not have cable in my house for that reason. Do use the three wire grounded outlets.
-Chris
http://www.partsexpress.com/pe/show...tnumber=180-075
That's the way to go, this is nothing more than 2 coupling caps 10nF, one for the center conductor and the other screen.
It's wise to not only break up the screen of the coax because even the center conductor is grounded through the input circuit of the rf amp in the tuner. I can't come up with any reason why this could attenuate the rf.
This is a very common problem, hum on audio hum bars on tv caused by a groundloop.
I agree with John10
Björn
That's the way to go, this is nothing more than 2 coupling caps 10nF, one for the center conductor and the other screen.
It's wise to not only break up the screen of the coax because even the center conductor is grounded through the input circuit of the rf amp in the tuner. I can't come up with any reason why this could attenuate the rf.
This is a very common problem, hum on audio hum bars on tv caused by a groundloop.
I agree with John10
Björn
Hi Chris!
I agree, if you by it's better to stay with better brands. A good 1:1 RF transformer shouldn't be lossy.
Anyway the signal at the tv jack is usually between 60-80dbmV so
a couple of db loss will do no harm as long as it doesn't go much below 60dbmV.
I use ceramic disc caps they will do the job.
Bjorn
I agree, if you by it's better to stay with better brands. A good 1:1 RF transformer shouldn't be lossy.
Anyway the signal at the tv jack is usually between 60-80dbmV so
a couple of db loss will do no harm as long as it doesn't go much below 60dbmV.
I use ceramic disc caps they will do the job.
Bjorn
Hi Harry,
You need to totally isolate the signal or you may toast some tuners. There may be a DC path due to an RF input transformer or coil. Any gear between the cable ground and local earth ground is at risk of being totally blasted. Just depends on the ground differential of the next lighting strike.
-Chris
You need to totally isolate the signal or you may toast some tuners. There may be a DC path due to an RF input transformer or coil. Any gear between the cable ground and local earth ground is at risk of being totally blasted. Just depends on the ground differential of the next lighting strike.
-Chris
The tuner is property of the cable company. Of course
the signal can go through my pre amp.
Luckely we are in New York City. Lightning is not frequent
and there are many high buildings around. I think the
problem stems from insufficient grounding. The cable
comes from the roof and services many apartment
buildings. The installation is done pretty poorly.
As a follow up: I did try to ground it by connecting it
to the outside of my metal electrical duckts. That
reduced the hum somewhat but when I listened to the
DVD the hum was there. I disconnected the cable box but
the hum remained. It's able to send the hum through the
ground to the stereo.
Everthing I try to do gives a poorer image quality. I'm wondering
if I have a marginal cable signal? I wonder how I can
verify that? Upstairs strangely enough I do not have a hum.
They are connected through a different cable run. I also have
different gear (I'm using a Maranz HT receiver). Downstairs
I'm using my own pre-amp and Cary tube gear.
I'm wondering if it's possible to buy a cheap DVD player and
modify it so it can work as a Dolby decoder? Then I can put
a filter on the ground of the digital input and I would have clean
sound. If we get a power surge it would only blow the cable
box and maybe the filter.
Harry
the signal can go through my pre amp.
Luckely we are in New York City. Lightning is not frequent
and there are many high buildings around. I think the
problem stems from insufficient grounding. The cable
comes from the roof and services many apartment
buildings. The installation is done pretty poorly.
As a follow up: I did try to ground it by connecting it
to the outside of my metal electrical duckts. That
reduced the hum somewhat but when I listened to the
DVD the hum was there. I disconnected the cable box but
the hum remained. It's able to send the hum through the
ground to the stereo.
Everthing I try to do gives a poorer image quality. I'm wondering
if I have a marginal cable signal? I wonder how I can
verify that? Upstairs strangely enough I do not have a hum.
They are connected through a different cable run. I also have
different gear (I'm using a Maranz HT receiver). Downstairs
I'm using my own pre-amp and Cary tube gear.
I'm wondering if it's possible to buy a cheap DVD player and
modify it so it can work as a Dolby decoder? Then I can put
a filter on the ground of the digital input and I would have clean
sound. If we get a power surge it would only blow the cable
box and maybe the filter.
Harry
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