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yet another laptop noise problem - hdmi
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godathunder View Drop Down
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    Posted: 25 March 2019 at 8:38am
an obvious bodge solution presented itself

laptop direct to tv (class 2) via hdmi

tv to decoder via optical

nasty noises has done goned

still passes 5.1 audio in most formats Im likely to need although Im not going to be able to pass the higher resolution formats (not that makes a lot of difference given the state of my hearing) 

btw, on the subject of higher res formats, does anyone know of a dirt cheap hdmi extractor decodes dsd to dolby digital or similar? 

I picked one of these up which suggests dsd compatibility but doesnt seem to support it in practice
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mini-mad View Drop Down
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Points: 6903 		Post Options Post Options   Thanks (0) Thanks(0)   Quote mini-mad Quote  Post ReplyReply Direct Link To This Post Posted: 06 February 2019 at 11:55pm
Originally posted by fatfreddiescat fatfreddiescat wrote:

In the past I have just hunted down a class 2 laptop psu on fleabay with the same voltage and current specs, chopped the end off my old psu and soldered it to the class 2.

This.
If it sounds like a gorilla is trying to escape, turn it down.
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Djdarch View Drop Down
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Points: 1989 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Djdarch Quote  Post ReplyReply Direct Link To This Post Posted: 06 February 2019 at 9:12pm
what model laptop is it ? dells for example the older laptop PSU's had no earths and work fine on the newer laptops . 
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studio45 View Drop Down
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For a year or so I had an aftermarket, "double-insulated" (no earth pin) supply for my Macbook that worked as a bad neutral detector - if the neutral was even slightly higher impedance than it should be, you'd get a LOVELY 160v AC measurable on the laptop's metal casing, and a detectable "zap" if you touched it while well-grounded. God only knows what it was doing to the DC-in board. At other times, no problem. I thought it was an intermittent fault until I realised it only happened in certain buildings. Oddly though, it never gave a me noise on the audio.
But then, it did only cost £13 delivered, as opposed to £90+ for a real Apple unit. I was about to add a ground wire to it, then it died completely, so I just got another one, and that doesn't do it at all!
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Points: 1414 		Post Options Post Options   Thanks (0) Thanks(0)   Quote djscooby Quote  Post ReplyReply Direct Link To This Post Posted: 05 February 2019 at 9:55pm
I had the issue with this noise through headphone socket through d.i box into desk I tried the naughty remove earth wire the actual PSU didnt work 

I removed the earth in the XLR connections and that cut it out but now use a usb sound card as well now nothing spectacular but it does the job 
Well my wife has kicked me out saying it's over due to my obsession with speakers don't know watt I'm gonna do with no ohm to go to
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fatfreddiescat View Drop Down
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Points: 1083 		Post Options Post Options   Thanks (0) Thanks(0)   Quote fatfreddiescat Quote  Post ReplyReply Direct Link To This Post Posted: 05 February 2019 at 9:55pm
In the past I have just hunted down a class 2 laptop psu on fleabay with the same voltage and current specs, chopped the end off my old psu and soldered it to the class 2.
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godathunder View Drop Down
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Originally posted by APW APW wrote:

When I had my repair business I regularly had mixing decks in with input channels blown due to the DJ removing the laptop psu ground to solve this issue then plugging the laptop into the grounded mixer!!  

As Kevin correctly pointed out the problem with class 1 psu is that they have a higher leakage current than the non-earthed class 2 supplies (3.5mA leakage for class 1 IT equipment, 0.25mA for class 2), and when the earth is disconnected the output from the PSU sits with 115-120Volts leakage on the output (although at low current: @ up-to 3.5mA), when the laptop is then plugged into a grounded piece of equipment the 115-120Volt leakage is then dissipated to ground via the equipment often frying it in the process.    

I see your point

do you feel studio45s suggestion of running a bond to the laptop/psu 0v rail is a sensible one?

I assume that it isnt done because my laptop would no longer be a class 2 device which no doubt has ramifications in compliance

Originally posted by studio45 studio45 wrote:

Can you add a real earth connection - put on a 3-pin plug, add a green/yellow wire - and tie it to the power supply output's 0v side? Unsure of the safety implications of that, but it should ensure that the lowest impedance path for noise is not through the HDMI grounds any more!


Edited by godathunder - 05 February 2019 at 9:20pm
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APW View Drop Down
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Points: 1174 		Post Options Post Options   Thanks (0) Thanks(0)   Quote APW Quote  Post ReplyReply Direct Link To This Post Posted: 05 February 2019 at 7:59pm
When I had my repair business I regularly had mixing decks in with input channels blown due to the DJ removing the laptop psu ground to solve this issue then plugging the laptop into the grounded mixer!!  

As Kevin correctly pointed out the problem with class 1 psu is that they have a higher leakage current than the non-earthed class 2 supplies (3.5mA leakage for class 1 IT equipment, 0.25mA for class 2), and when the earth is disconnected the output from the PSU sits with 115-120Volts leakage on the output (although at low current: @ up-to 3.5mA), when the laptop is then plugged into a grounded piece of equipment the 115-120Volt leakage is then dissipated to ground via the equipment often frying it in the process.    


Edited by APW - 05 February 2019 at 8:19pm
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godathunder View Drop Down
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Points: 1833 		Post Options Post Options   Thanks (0) Thanks(0)   Quote godathunder Quote  Post ReplyReply Direct Link To This Post Posted: 05 February 2019 at 7:26pm
thanks for some great suggestions chaps.

unfortunately, as I look at the cost and rather pointless complexity and added points of failure of putting hdmi-usb-isolator-usb-hdmi in the signal path I wonder why Im bothering when I know at heart that toasty nailed it in the 1st post.

if this was being installed somewhere where joe public or dj dave was being put at risk it would be worth the added cost etc but this is for my use only (well, and the kids, but I dont like them). I know the electrical install its being powered from is fine, has a nice low zs and suitably protected (I wired it). the hifi has its own dedicated circuit but the noise persists whether the laptop is powered from this, or an adjacent circuit

it annoys me to do it but I think that cutting off the mains plug and rewiring sans cpc is the best current option though building a linear psu will remain on the project list for the coming months

I think it just galls me that this seems to be such a common problem due to acceptance of cheap and shitty psu design in consumer goods
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APW View Drop Down
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Points: 1174 		Post Options Post Options   Thanks (0) Thanks(0)   Quote APW Quote  Post ReplyReply Direct Link To This Post Posted: 05 February 2019 at 6:33pm
I suspect that heatsink is not up to the job, you need to work out the power being dissipated by the regulators pass element, to do this you need to know what voltage the regulator pass element is dropping and then multiply that by the maximum current it’s going to pass... then multiplying the calculated power by 2.3 (the heatsink in the link is 2.3°k\watt) to give you the temperature rise (in° kelvin) above the ambient temperature.


Is it possible for you to use a USB to HDMI converter for your video and sound, I ask as you can get a USB galvanic isolator?

https://uk.farnell.com/olimex/usb-iso/usb-isolator-1000vdc-for-pc-laptop/dp/1795095

The one in the link above is USB2, I have seen USB3 ones for about £70.00



Edited by APW - 05 February 2019 at 6:42pm
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Danielr View Drop Down
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Points: 209 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Danielr Quote  Post ReplyReply Direct Link To This Post Posted: 05 February 2019 at 5:40pm
Can you send video (assuming that is why you use hdmi) via HDMI. 
But select the output audio device as headphones. Use regular line cable and ground lift as appropriate? 

Not really helping you solve the problem, but maybe working around it? 
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studio45 View Drop Down
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Points: 3864 		Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 05 February 2019 at 5:08pm
You can use a series pass transistor to handle the current, controlled by the regulator - you're still going to end up with a right heavy brick-like unit, though. 
Conceptually, there's switching noise present on the audio because somehow, the path to ground via the HDMI has a lower RF impedance than the neutral connection of the power supply. Why that should be I couldn't say, but that will be the mechanics of what's happening. It all ought to be decoupled in the supply circuitry, but perhaps some caps are either missing or defunct? 
Can you add a real earth connection - put on a 3-pin plug, add a green/yellow wire - and tie it to the power supply output's 0v side? Unsure of the safety implications of that, but it should ensure that the lowest impedance path for noise is not through the HDMI grounds any more!
Studio45 - Repairs & Building Commotion Soundsystem -Mobile PA
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