Dear All,
I have an issue that leaves me puzzled. I have a totally DIY system, amps, pre, etc. The system is completely silent. One of the source is my Home theater amp for the front channel. When I attach the amp, the system is still completely silent.
I have a very strong hum when I attach the TV to the amp via HDMI. Note that the TV is NOT the source of audio, but it is used only as a monitor. Both TV and Home Theater amp (Denon 3808) do not have grounding.
An even more nastly noise I get if I connect the Macbook as a source via HDMI. in this case I can hear the hard drive moving.
Any solutions ? I could not find any HDMI galvanic isolator in the market.
Thanks for help.
D.
I have an issue that leaves me puzzled. I have a totally DIY system, amps, pre, etc. The system is completely silent. One of the source is my Home theater amp for the front channel. When I attach the amp, the system is still completely silent.
I have a very strong hum when I attach the TV to the amp via HDMI. Note that the TV is NOT the source of audio, but it is used only as a monitor. Both TV and Home Theater amp (Denon 3808) do not have grounding.
An even more nastly noise I get if I connect the Macbook as a source via HDMI. in this case I can hear the hard drive moving.
Any solutions ? I could not find any HDMI galvanic isolator in the market.
Thanks for help.
D.
Have you tried grounding the TV and amp? I suspect that could improve matters, currently they will be picking up switching and hum directly from the mains through their PSUs, so that significant current flows to earth along your audio cable from the home-theatre unit.
A non-earthed piece of equipment with a SMPS will have significant capacitive coupling to both switching waveform and mains waveform
and should be properly earthed to conduct this away if possible.
Some MacBook PSU's have no earth connection, some do...
A non-earthed piece of equipment with a SMPS will have significant capacitive coupling to both switching waveform and mains waveform
and should be properly earthed to conduct this away if possible.
Some MacBook PSU's have no earth connection, some do...
You are lucky because grounding the TV chassis is not a good idea unless the IEC socket on the TV has 3 pins: phase, neutral and earth.
You grounded the shell of an HDMI connector; the HDMI cable braid is connected to the HDMI socket on the TV, which is connected to the ground return (ground plane) on the PCB. The PCB ground plane is AC coupled to the chassis, usually by means of 0.1 and 0.01uF SMD paralleled capacitors, at each point where the PCB is screwed to the chassis.
The IEC/mains RF filters will help a lot; also, run the MacBook on batteries - that should eliminate the noise completely.
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Some questions first. Do you have a cable TV box of any kind? The ground reference on those is often a long ways from your real ground, which is a problem.
With the TV and Denon connected, and speaker connected to the Denon, disconnect every other device one at a time observing the change in hum. There may be one device, source or amp, that's part of the problem, regardless if it's quiet without the TV.
Also make sure the TV and audio system components are all connected to the same circuit, preferably the same outlet or outlet strip. Doing that often cures ground loops immediately.
HDMI is a 19-wire interconnect. An isolation transformer set would be difficult and costly. However, you can get ground isolators in the HDSDI world, which is basically a single 75 ohm coax with BNC connector for everything on HDMI. There are conversion boxes. The isolators still aren't cheap (the good ones) and you'll have to bump from HDMI to SDI and back to use one, but it may be the only practical solution if you can't plug everything in together.
Any other connector outer terminal or shell is also ground. The TV has one or more F connectors for antenna/cable. The outer of that is at chassis ground too. Any RCA analog audio or video connector outer is also at chassis ground.
That's not what's happening, the ground loop is entirely in the analog domain. A ground loop is simply a difference in ground potential between two or more devices that causes an AC current to flow between them, ultimately modulating the ground reference of an analog audio component causing the AC signal to appear on the output.
The task is to eliminate that current by breaking the ground loop or forcing all ground references down to the same voltage with external ground wiring. Sometimes not having the 3rd pin grounded cures the problem too, though it's technically a possible safety risk. In practice, the risk is extremely small, but nobody here is suggesting you do something unsafe, other than to make a temporary diagnostic test.
+1
Helps in a lot of cases when video is involved and sorts out a lot hum problems with subwoofers in HT systems.
Another way of looking at the matter is that in a world of more and more devices that are in some way earthed, unbalanced connections suck.
Two options:
1. If you insist on having your DIY gear being IEC Class I appliances, use balanced connections.
2. If you insist on having unbalanced connections, learn how to (safely) build you stuff as IEC Class II appliances, like established hi-fi manufacturers traditionally do.
In this case, it's kind of a "damned if you don't, damned if you do" situation. The problem comes about as follows:
1. One of the DIY components presumably is built as a safety earthed (Class I) device. In any case there is one earth connection there.
2. The TV power supply has some mains-side filter capacitors that would connect to protective earth if that were present, but the thing is floating and presumably has no antenna connection (or network cable shield) to drain to either. Result, TV audio ground is floating around near half the mains supply voltage, not to mention all kinds of power supply hash.
3. The potential difference between these two results in a current flowing over unbalanced connection shield, which has a finite resistance. All the way through Denon receiver, preamp and power amp, I suppose.
The situation for the Macbook power supply presumably is similar to the TV.
Now even if the TV did have an earth connection of its own, you would still have a ground loop there. The old "plug everything into the same power strip" trick may reduce the disagreement in ground levels to an acceptable level then but the problem still isn't 100% solved.
I would suggest these measures that may help cope with the situation:
1. In DIY power amps you may run across the problem of too much capacitive coupling to the mains in the transformer (easily 500-900 pF, orientation dependent). A Class II design above a certain power level (let's say 20-30 VA transformer or so) pretty much requires a shield winding in the transformer. Otherwise hum on open inputs is quite likely as amplifier ground floats at part of mains voltage but its surroundings do not.
If your transformer is devoid of such luxury, what you can do is using a 3-conductor mains cable to bring in the protective earth connection, which then is used only for a safety capacitor (4.7-10 nF Y2, typ) going to secondary-side ground. (Build the device to Class II standards otherwise.) This connection is quite low impedance compared to transformer leakage capacitance so largely eliminates the hum, but high impedance compared to our unbalanced connection shields so would still be unproblematic in terms of ground loops.
2. When using SMPS, also bring in PE and use it to connect mains filter components, then again use a safety capacitor (4.7-22 nF Y2, typ) from secondary-side ground to PE. Some supplies may already include the latter for EMI reasons.
I would wish that a bit more attention in power supply design were directed towards the mains systems in use worldwide and the best connections and filter topologies for the system at hand. Lots of floating SMPS designs use a topology that is only well-suited for "technical" (balanced) mains that is used in relatively few places (like Norway or on ships). Many more people have unbalanced mains, not uncommonly (but not always) with PE available, sometimes with defined polarity as well.
Two options:
1. If you insist on having your DIY gear being IEC Class I appliances, use balanced connections.
2. If you insist on having unbalanced connections, learn how to (safely) build you stuff as IEC Class II appliances, like established hi-fi manufacturers traditionally do.
In this case, it's kind of a "damned if you don't, damned if you do" situation. The problem comes about as follows:
1. One of the DIY components presumably is built as a safety earthed (Class I) device. In any case there is one earth connection there.
2. The TV power supply has some mains-side filter capacitors that would connect to protective earth if that were present, but the thing is floating and presumably has no antenna connection (or network cable shield) to drain to either. Result, TV audio ground is floating around near half the mains supply voltage, not to mention all kinds of power supply hash.
3. The potential difference between these two results in a current flowing over unbalanced connection shield, which has a finite resistance. All the way through Denon receiver, preamp and power amp, I suppose.
The situation for the Macbook power supply presumably is similar to the TV.
Now even if the TV did have an earth connection of its own, you would still have a ground loop there. The old "plug everything into the same power strip" trick may reduce the disagreement in ground levels to an acceptable level then but the problem still isn't 100% solved.
I would suggest these measures that may help cope with the situation:
1. In DIY power amps you may run across the problem of too much capacitive coupling to the mains in the transformer (easily 500-900 pF, orientation dependent). A Class II design above a certain power level (let's say 20-30 VA transformer or so) pretty much requires a shield winding in the transformer. Otherwise hum on open inputs is quite likely as amplifier ground floats at part of mains voltage but its surroundings do not.
If your transformer is devoid of such luxury, what you can do is using a 3-conductor mains cable to bring in the protective earth connection, which then is used only for a safety capacitor (4.7-10 nF Y2, typ) going to secondary-side ground. (Build the device to Class II standards otherwise.) This connection is quite low impedance compared to transformer leakage capacitance so largely eliminates the hum, but high impedance compared to our unbalanced connection shields so would still be unproblematic in terms of ground loops.
2. When using SMPS, also bring in PE and use it to connect mains filter components, then again use a safety capacitor (4.7-22 nF Y2, typ) from secondary-side ground to PE. Some supplies may already include the latter for EMI reasons.
I would wish that a bit more attention in power supply design were directed towards the mains systems in use worldwide and the best connections and filter topologies for the system at hand. Lots of floating SMPS designs use a topology that is only well-suited for "technical" (balanced) mains that is used in relatively few places (like Norway or on ships). Many more people have unbalanced mains, not uncommonly (but not always) with PE available, sometimes with defined polarity as well.
Yep. Right there is your best troubleshooting advice.
It's the best place to start.In my work we have kilometers of audio and video cables strung around. HDMI, DVI, HD-SDI, Ethernet, balanced and unbalanced audio at mic and line levels. Chasing down hum and buzz can be a real pain. You have to be systematic and patient.
Dear All,
First of all, thanks for the valuable inputs and the time you devoted to my silly issue.
Long story short: the tip to ground the antenna was the winner one. It suppressed almost completely the noise.
I want to give more details on my kind of complex system.
Let's look it from speakers to source:
1) The amps are monoblocks balanced class I. (DIY)
2) The pre I am presently using is just a balanced input selector with MUSES volume control. The only source that is single ended is the home theater, that is connected through transformers to obtain a balanced output from an unbalanced input. The pre is also class I.
3) I have the following sources:
A) Two balanced DACs. Class I
B) Denon Home theater. Class II unbalanced
4) The TV is an LG OLED. (Class II)
5) The laptop (ClassII) feeds the DACs via USB and the video to the Denon.
Now the hum was definitely an issue between the TV and the Denon, because with all disconnected I still had hum in the headphones if only these two components were disconnectd.
Let's be quantitative. I measure with my portable Fluke around 1mV AC when the system is silent. The hum, brings the noise to 4 mV AC. Grounding the HDMI shell brings the hum to 2.5 mV AC. Grounding both HDMI shell and antenna (at the wall) brings the hum to 1.1 mv AC. I think we can be happy.
I found that the laptop had a secondary problem that I also solved without really understanding it.
The hum did not change when the laptop was on battery and when the laptop was attached to power.
The DACs were both connected to the same USB port. Giving each DAC his own port remove the noise. I think I should invest in I2S galvanic isolation inside the DACs to avoid this problem in the future.
Regards,
Davide
D.
First of all, thanks for the valuable inputs and the time you devoted to my silly issue.
Long story short: the tip to ground the antenna was the winner one. It suppressed almost completely the noise.
I want to give more details on my kind of complex system.
Let's look it from speakers to source:
1) The amps are monoblocks balanced class I. (DIY)
2) The pre I am presently using is just a balanced input selector with MUSES volume control. The only source that is single ended is the home theater, that is connected through transformers to obtain a balanced output from an unbalanced input. The pre is also class I.
3) I have the following sources:
A) Two balanced DACs. Class I
B) Denon Home theater. Class II unbalanced
4) The TV is an LG OLED. (Class II)
5) The laptop (ClassII) feeds the DACs via USB and the video to the Denon.
Now the hum was definitely an issue between the TV and the Denon, because with all disconnected I still had hum in the headphones if only these two components were disconnectd.
Let's be quantitative. I measure with my portable Fluke around 1mV AC when the system is silent. The hum, brings the noise to 4 mV AC. Grounding the HDMI shell brings the hum to 2.5 mV AC. Grounding both HDMI shell and antenna (at the wall) brings the hum to 1.1 mv AC. I think we can be happy.
I found that the laptop had a secondary problem that I also solved without really understanding it.
The hum did not change when the laptop was on battery and when the laptop was attached to power.
The DACs were both connected to the same USB port. Giving each DAC his own port remove the noise. I think I should invest in I2S galvanic isolation inside the DACs to avoid this problem in the future.
Regards,
Davide
D.
One important bit of information I forgot to add: the Denon is an 100V model, so it is connected to main through an isolation step-up transformer. It probably would matter if the device would be class I, but it's not.
The amps and the pre have substantial filtering on the main cable.
Regards,
Davide
The amps and the pre have substantial filtering on the main cable.
Regards,
Davide
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