Author Topic: Filtering the noise in the mains.  (Read 2543 times)

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Offline ||C|-|E||

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Filtering the noise in the mains.
« on: 02:40, 17 March 16 »
Hello guys!

I have a problem with the mains in my flat that is driving me a bit crazy since long ago. Well, to be honest, the problem is common to the whole area but the neighbours do not care or do not notice it. The thing is that the "earth" in the building is actually not a proper earth but a noisy mess that affects all the earthed analog devices that you connect to it (to the point of blowing up a diode in a earthed amplifier some years ago). My poor Amstrad is not an exception, today I was testing the "new" original monitor I received and everything works great if I power the computer from it. However, since I have an HxC and a Mother X4 with quite a lot of things I need an external power supply as well. Here is were the problems start. If I power the computer from the screen and the other things from the external power supply I can clearly see some noise in the image, noise that is much worse if I power the Amstrad from the external power supply as well. Of course, the noise disappears as soon as I remove the power from the HxC and the Mother X4 and I power the Plus from the screen only. On the other hand, if I power the computer from the screen and the addons from the external power supply but I connect this one to my UPS everything works great again, since the UPS is filtering all the noise. This is what I am doing right now. So, long story short. Could you recommend me a very clean power supply, able to filter all the crap, for my stuff? Having it connected to the UPS is great and all, but I would like to find an alternative approach.

Thanks!  :)

Offline 1024MAK

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Re: Filtering the noise in the mains.
« Reply #1 on: 12:58, 17 March 16 »
The external power supply that you are using, can you give us some more details about it. Does it actually use the earth pin? Make, model, specs?

Earth connections and noise is a very tricky problem to solve. If your problems are being caused by earth loop currents (whereby two power systems both use the earth connection, and both reference the low voltage side to earth) the first thing to try, is to plug both into the same (double outlet) mains socket (same extension lead). You could also try using one of the filtered mains extension leads (but there is a fair chance that this will make no difference  :( ).

You could also try using a double insulated external power supply that does not have an earth connection. Buy a quality one from a suitable nearby shop, then if it makes no difference, you can return it saying that it was not suitable for your use.

Mark
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Re: Filtering the noise in the mains.
« Reply #2 on: 13:28, 17 March 16 »
Thank you!

It is true that I did not give all the details :) So yes, the external power supply I am using is earthed and I tried to plug everything to the same socket using a triple outlet. I did not make any major difference, the only solution I found is to power the computer from the screen and the peripherals from the PSU connected to my UPS. This actually works extremely well, although I feel that the approach is far from perfect. This is the PSU, nothing especial, to be honest:

5v 8a ac/dc power supply adapter with 5.5mm center: Amazon.co.uk: Electronics

Funnily enough, if I power everything (computer and peripherals) from the PSU connected to the UPS and the screen on its on, the noise re-appears.
About removing the earth, it did not help in this case but works very well for my audio amplifiers. I know that it is not OK, but all of them run unearthed  :picard: I could try, indeed, with a good filtered mains extension lead just to see what happens...



Offline arnoldemu

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Re: Filtering the noise in the mains.
« Reply #3 on: 15:39, 17 March 16 »
I have used a power supply from an external parallel port hard disc (from back in the 1990's). This gave patterns on the screen, but a pc power supply didn't.

For mine I'm assuming that the capacitors or something are failing and putting in the noise.
So are you sure your power supply is not failing and old?

Are you seeing this from a new power supply?
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Offline arnoldemu

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Re: Filtering the noise in the mains.
« Reply #4 on: 15:40, 17 March 16 »
Why do you think the ground voltage is noisy in your building? Has it been tested and shown to be this way?

Perhaps the power needs updating to the latest regulations? Maybe you could pay an electrician to do that?


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Re: Filtering the noise in the mains.
« Reply #5 on: 16:17, 17 March 16 »
So yes, this is coming from a new power supply, and actually the noise disappears when you connect it to my UPS, so I would say that the power supply is not guilty. Or, to be honest, is guilty of not being properly filtered, but not of the noise.

Regarding the ground voltage being noisy, I checked it some time ago, when a grounded amplifier I was making lost a diode in quite an explosive way. The story was a bit scary, the amplifier was working beautifully when unearthed (as it is now) and well in the country side, where the earth was a big chunk of metal buried below the house. However, in my flat, connecting it to the ground was causing a strong humming sound and eventually a diode from the bridge blew up and I had to change it. After that, I borrowed a scope and I checked the pipe we are all using as earth (it is a real pipe that goes all over the close and carries the water from the flats) comparing it with a piece of metal I buried in the ground just for this. There was a decent difference of potential between them, and it was actually really noisy in the scope screen (I do not remember the value). I guess that there is an appliance from somebody, or many, causing the problems, but God knows what it is. This pipe is connected with another pipe that goes all over the council  :-X

About calling somebody to fix this. Well, I live in a house that I rent with my girlfriend. This house belongs to a close and it would be actually veeery difficult to convince the other people to have this checked when I am the only one having problems because I tend to use old analog devices. On the other hand, I would need to put it in the hands of my landlady. What did I do at the end? I installed a UPS and I use it to connect anything that is over-sensitive or expensive (Playstation 4 being a good example of temperamental device that is easily affected). A good way to put and end to this would be probably take the ground cable from my flat, disconnect it from the pipe and connect it to another piece of metal buried deep in the garden. Of course, I am not allowed to do this  :D

Offline Bryce

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Re: Filtering the noise in the mains.
« Reply #6 on: 17:22, 17 March 16 »
There are a few things that you are mixing up here. What will matter is whether the PSU is earth referenced (ie: the negative pole of the primary side is connected to earth) or floating. A floating supply doesn't care what the earth is doing, an earth referenced supply does.
Earth is only a reference point. The test you did would of course have shown a voltage, the potential in the ground varies considerably, even over a few meters, so the point you pushed the new pipe into the ground will have a different potential relative to the original ground.
The first question has to be where the noise is coming from. Is it a typical 50Hz? Is it a much higher frequency or really random noise?
As far as I know, the CM14 supply is floating, as your seperate PSU should be too. However, the CM14 has a linear supply and the other PSU is probably a switchmode supply. This is always a bad Idea. I have mentioned it here several times before, but I'll say it again: Using ANY two supplies in parallel is bad practice for many reasons. Always try to supply any connected equipment from a single source with sufficient power to handle the job.

Bryce.
« Last Edit: 17:24, 17 March 16 by Bryce »

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Re: Filtering the noise in the mains.
« Reply #7 on: 17:48, 17 March 16 »
Thank you Bryce! I do not really remember the values of the noise, but it was 50 Hz, that I remember. Where is is coming from, I don´t know  :-X

Regarding the other thing you are pointing, I really agree with you, I was tinkering with the stuff to see if I could isolate the noise source and know where it was coming from. Clearly the screen is providing a very nice clean power output but I do not know if only with 2.4A it will be enough to power the HxC and the expansions. If this is the case, the best solution seems to just use this to power everything and that should really solve the issues...


Offline 1024MAK

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Re: Filtering the noise in the mains.
« Reply #8 on: 22:06, 17 March 16 »
Okay, I think a few things need to be clarified.

The mains supply in the UK (and an lot of places around the world) is 50 Hz AC. Because of the amount of electrical wiring in modern buildings, you can hold a oscilloscope probe in your hand and pick up a strong 50 Hz signal.

Because the mains supply is a 50 Hz AC signal, it is the prime objective of any power supply that has a DC output designed to supply electronics to filter out as much of the 50 Hz waveform as possible. So much so, that a 230V AC voltage is reduced to less than 20mV ripple (or better) in any good power supply.

In the UK, there are typically three mains earth systems. In older properties, the main earth terminal is connected to the steel wire armoured cable of the main electrical supply cable. Or the metal mains water pipe may instead be the main earth (water pipes are no longer permitted to be the main earth). Alternately, a proper separate earth rod (or a number of linked earth rods or earth grids) may be used.

In more modern properties, at the point the electrical supply cable first terminates in a building (normally where the electric meter is located), the consumer/customer earth is connected to the power company neutral cable conductor (shared earth and neutral).

Regardless of what is providing the main earth, all copper/metal mains water pipes and all copper/metal gas pipes have to be cross-bonded to the mains earth terminal (normally next to the electrical meter or next to/in the fuse box or MCB "consumer unit").

In most cases, the earth wiring will have some level of 50 Hz AC voltage detectable, because of inductive and capacitive coupling from the live conductors in the same cable runs.

DC Power Supply Systems
Unless properly designed to be connected in parallel, you should never connect two DC power supply systems in parallel. For a 5V DC supply, by this I mean connecting both the positive connections together. Either use a power supply that is suitable for the total load (all devices that need power), or separate the DC power network into two sections and supply each section with it's own power supply.

Can you show us a picture of the symptoms of the interference that you are getting?

The cause could actually be stray switching signals from your external power supply mixing with the internal signals used in the monitor. The two frequencies mix and the result is unwanted interference.

Mark
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Re: Filtering the noise in the mains.
« Reply #9 on: 23:33, 17 March 16 »
Thank you for all this useful info! I will try to show a picture, but what I basically have is a band on noise like hum, 3 cm wide that moves from top to bottom of the screen scrolling very smoothly. The band itself is composed of many small horizontal lines that move as well. When a whole big band disappears (and it takes a few seconds to go all over the screen) another one substitutes it in the top of the screen and scrolls down as well. That is basically it. Oh! and the color of the lines is grey.

What I am going to do now is to try to power everything from the screen, that should solve the problem if the PSU in the screen is powerful enough to drive the Plus with the HxC and Mother X4. Otherwise, I will have to power everything from a external power supply that does not interfere with the signals in the screen, I guess. In any case, it should be possible to power the HxC from the external power supply if the computer and the boards are powered from the same PSU, right? it is a completely separate device.

Thank you again!
« Last Edit: 23:36, 17 March 16 by ||C|-|E|| »

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Re: Filtering the noise in the mains.
« Reply #10 on: 01:16, 18 March 16 »
So, I found a way to almost eliminate the problem :) . I can power everything from the external PSU and keep the power cable from the screen disconnected if I connect the external PSU to my UPS and I physically separate it from the screen like a couple of meters too. This is still not completely perfect (noise visible when the screen is black) but almost, to be honest. Maybe is time to buy a decent PSU from RS components...at the moment what I am doing is just to power the HxC from the external power supply and the rest from the screen.
« Last Edit: 01:21, 18 March 16 by ||C|-|E|| »

Offline 1024MAK

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Re: Filtering the noise in the mains.
« Reply #11 on: 01:57, 18 March 16 »
Slightly off-topic, but relevant if you are thinking of buying, or have bought an inexpensive PSU. On another forum, a user has been buying "modern" switch mode power supplies (SMPSU) designed with both +5V and +12V outputs. He wanted them to power dual 3.5" floppy drives.

He has bought at least three, all from different suppliers. Two via eBay and one from an online shop in Germany. The prices ranged from a few pounds to I think £22 incl. shipping for the SMPSU from Germany.

He has opened the cases of all of them to see the quality of the circuit board and the components used. One of the cheap units did not have sufficient clearance between the PCB copper tracks of the primary (mains) circuit and the secondary (low voltage output circuitry) as I would have liked (this is a possible safety issue). The other two were okay in this respect.

But all three had no interference suppression components (inductor/capacitor filters) on the primary (mains) side of the circuit. And the filtering on the secondary DC outputs was basic (just a single filter capacitor per output, instead of the better capacitor/inductor/capacitor arrangement). Further, the cheaper units appeared to have only a basic control circuit (a discrete transistor design).

Why am I waffling on about this you may ask. Well, these inexpensive SMPSUs are a known source of stray interference.

Further, most CRT displays don't play nice if there are either magnetic or electromagnetic devices near to them (including other CRTs). So keep loud speakers and power supplies at least 6 inches away from them.

Now, the PSU that you are using is new to me. So I do not have enough information to make any determinations. But I wanted to post this information, in case it helps.

Mark
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Re: Filtering the noise in the mains.
« Reply #12 on: 02:16, 18 March 16 »
Thank you! If I buy it will be a good unit from RS components, those are quite expensive but nice. However, if you have a particular suggestion I am more than happy to follow it and buy that one  :)

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Re: Filtering the noise in the mains.
« Reply #13 on: 13:47, 19 March 16 »
I was checking and there are some apparently decent linear power supplies around that I could probably use. For example, this one looks very fine:

Hifi linear DC 5V output power 25W USB/amp/DAC/external power supply | eBay

What do you think of it or something similar?  :)

Offline chinnyhill10

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Re: Filtering the noise in the mains.
« Reply #14 on: 17:00, 19 March 16 »
Thank you! If I buy it will be a good unit from RS components, those are quite expensive but nice. However, if you have a particular suggestion I am more than happy to follow it and buy that one  :)


Just get the RS supply. If that doesn't work your issues are far more serious.
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Offline chinnyhill10

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Re: Filtering the noise in the mains.
« Reply #15 on: 17:03, 19 March 16 »
Slightly off-topic, but relevant if you are thinking of buying, or have bought an inexpensive PSU. On another forum, a user has been buying "modern" switch mode power supplies (SMPSU) designed with both +5V and +12V outputs. He wanted them to power dual 3.5" floppy drives.

He has bought at least three, all from different suppliers. Two via eBay and one from an online shop in Germany. The prices ranged from a few pounds to I think £22 incl. shipping for the SMPSU from Germany.

He has opened the cases of all of them to see the quality of the circuit board and the components used. One of the cheap units did not have sufficient clearance between the PCB copper tracks of the primary (mains) circuit and the secondary (low voltage output circuitry) as I would have liked (this is a possible safety issue). The other two were okay in this respect.

But all three had no interference suppression components (inductor/capacitor filters) on the primary (mains) side of the circuit. And the filtering on the secondary DC outputs was basic (just a single filter capacitor per output, instead of the better capacitor/inductor/capacitor arrangement). Further, the cheaper units appeared to have only a basic control circuit (a discrete transistor design).



Most of the no name Chinese PSU's are absolute rubbish which is why I recommend the RS supplies which come with data sheets and proper test for noise etc.


I did a video a few weeks back about 9v supplies for Spectrums. The 2 Chinese supplies were badly designed and one had recycled components inside it with wires still hanging off!


[youtube]http://www.youtube.com/watch?v=G9v30Wa7c7E[/youtube]

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Offline Bryce

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Re: Filtering the noise in the mains.
« Reply #16 on: 13:21, 20 March 16 »
I was checking and there are some apparently decent linear power supplies around that I could probably use. For example, this one looks very fine:

Hifi linear DC 5V output power 25W USB/amp/DAC/external power supply | eBay

What do you think of it or something similar?  :)

That's actually a very nice supply indeed. I like the case. I assume it's regulated, but the angle of the pictures doesn't show how they did it.

Bryce.

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Re: Filtering the noise in the mains.
« Reply #17 on: 23:53, 20 March 16 »
Thank you! :) One thing I really like about that linear psu is that I know them much much better than the switching ones. I have made and repaired a few of those, always for audio amplifiers, and it is always nice to use something that you can actually understand. Besides, it will probably not interfere with the screen and it looks very nice. It is regulated, yes, but I do not know the details. I will try to find more info about it!

Edit: I found some pictures. It seems a very simple power supply, with a linear voltage regulator. I use these all the time  :) It is not earthed, though. Less noise  :laugh:

[attachimg=1]

[attachimg=2]
« Last Edit: 00:00, 21 March 16 by ||C|-|E|| »

Offline chinnyhill10

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Re: Filtering the noise in the mains.
« Reply #18 on: 02:19, 21 March 16 »
I use one of these:


ECP-20-5U | 5V dc, 1 Output, 2.1 x 5.5 x 9.5 mm Centre Positive Switch Mode,


As you can see the specs are extremely good and there's plenty of juice there for a CPC + an HxC and anything else you want to add. With a 100,000 hour MTBF, it will outlive your CPC. And being RS, those specs on the data sheet can be relied on.
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Offline Bryce

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Re: Filtering the noise in the mains.
« Reply #19 on: 11:12, 21 March 16 »
Thank you! :) One thing I really like about that linear psu is that I know them much much better than the switching ones. I have made and repaired a few of those, always for audio amplifiers, and it is always nice to use something that you can actually understand. Besides, it will probably not interfere with the screen and it looks very nice. It is regulated, yes, but I do not know the details. I will try to find more info about it!

Edit: I found some pictures. It seems a very simple power supply, with a linear voltage regulator. I use these all the time  :) It is not earthed, though. Less noise  :laugh:


It looks very well made. Pity they didn't add an earth, but that's not a gamestopper, you could add one yourself. As far as repairability goes, you can't get much better than that, and it definitely won't have any switching noise :)

Bryce.

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Re: Filtering the noise in the mains.
« Reply #20 on: 11:53, 21 March 16 »
I agree Bryce, I think that I will go for that one. Not having earth it is the easy way to avoid noise in a cheap PSU that is intended for audio systems, I guess  :D . I have used the trick myself many times because it simplifies making the whole system more silent  :-X Actually, I would say that the ones I made are very similar to that one but without the fancy case and the display. The last one is mounted in a valve headphone amplifier and it works very well. It is not that I am using it , it was for a present. Anyway, I will add a earth to this one, just in case, the connectors are already there.

@Chinnyhill10, thank you for your suggestion as well! That one is definitely a nice switching PSU, the only thing that prevents me from taking it is the switching nature of the device, something that I would really like to avoid  :(

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Re: Filtering the noise in the mains.
« Reply #21 on: 12:55, 21 March 16 »
.

@Chinnyhill10, thank you for your suggestion as well! That one is definitely a nice switching PSU, the only thing that prevents me from taking it is the switching nature of the device, something that I would really like to avoid  :(


It's is a world apart from the cheap switching supplies on Ebay.
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Re: Filtering the noise in the mains.
« Reply #22 on: 03:07, 06 April 16 »
So, I finally received the linear psu today and the first thing I did was to open it to check what it was actually like in the pictures and to ground it :) . Grounding was very easy because all the chassis is made of metal and you have lots of places to choose. I decided to use one of the screws to hold an electric connector and crimp the ground cable to it. The rest of the things were, luckily, as expected: it has the transformer, a diode bridge with the required capacitors, the voltage regulators and a little display to show the voltage. I think that I would had used thicker cables if I was the one making it, but anyway,  the computer works very well, it does not interfere with the CRT anymore and with 5A it has enough juice for everything I can throw at it. Moreover, it should be very easy to repair it if something happens  :) .

Sorry for the extreme bad quality of the picture, I had to take it with my crappy phone  :picard:

[attachimg=1]

Offline Bryce

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Re: Filtering the noise in the mains.
« Reply #23 on: 10:56, 06 April 16 »
The wires look thick enough. For 5V at 5A they only need to be about 0.5mm² and they look like they are probably 0.75mm².

Bryce.

Offline 1024MAK

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Re: Filtering the noise in the mains.
« Reply #24 on: 18:33, 07 April 16 »
the computer works very well, it does not interfere with the CRT anymore and with 5A it has enough juice for everything I can throw at it.
Glad that you are now sorted  :D .
Mark
Looking forward to summer in Somerset :-)