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Old 2nd June 2015, 08:14 AM   #1
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Default Mains filtering for a digital equipment.

Hi !
i would like to understand more the requirements for mains noise filtering in digital equipment.
To explain a little better my situation i have a nice usb to spdif converter without any particular filter on the mains input.
The mains voltage goes directly to a toroidal voltage transformer (that i know is not the best filter in the world) and then to a normal power supply powering the pcb.
I really would like to add something upstream the transformer to suppress some noise that can be in the mains (i live in an industrial area and sound is better during holidays with factories closed).
So i am quite sure that there must be some noise in the mains.
And i cannot wait for holidays to enjoy decent sound ...
I woud much prefer a kit solution and made with passive parts.
Thanks a lot for any advice.
Kind regards, gino

Last edited by ginetto61; 2nd June 2015 at 08:19 AM.
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Old 2nd June 2015, 10:19 AM   #2
AndrewT is offline AndrewT  Scotland
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You can buy mains filters.
They come in metal boxes that need to be electrically connected to the equipment Chassis.
They are also available as IEC input sockets with an integrated filter.

The filters should work both ways.
They attenuate cable born interference coming in from the Mains and they should attenuate interference created in the equipment and reducing what your equipment could put back into the Mains.
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Old 2nd June 2015, 10:25 AM   #3
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Quote:
Originally Posted by AndrewT View Post
You can buy mains filters.
They come in metal boxes that need to be electrically connected to the equipment Chassis. They are also available as IEC input sockets with an integrated filter.
The filters should work both ways.
They attenuate cable born interference coming in from the Mains and they should attenuate interference created in the equipment and reducing what your equipment could put back into the Mains.
Hi and thanks a lot for the very helpful advice
I have one general question on them. I see that they are specified for high currents, like from 7A up.
Given that the current in my unit is very very low, around 0.1A, will they be equally effective also at this low current ?
And do you think that, as i think, they could be beneficial for overall sound in a digital equipment ?
Thanks a lot, gino
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Old 2nd June 2015, 10:29 AM   #4
AndrewT is offline AndrewT  Scotland
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I keep a small stock of 1A, 3A and 6A IEC socket filters.
Just look at the retailer catalogues for the current rating you require.

BTW, the attenuating capability goes down as current rating goes up.
i.e. you get more attenuation of interference by using a correctly rated filter than you get from a too high rated filter.
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Old 2nd June 2015, 11:34 AM   #5
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Quote:
Originally Posted by AndrewT View Post
I keep a small stock of 1A, 3A and 6A IEC socket filters. Just look at the retailer catalogues for the current rating you require.
BTW, the attenuating capability goes down as current rating goes up.
i.e. you get more attenuation of interference by using a correctly rated filter than you get from a too high rated filter
Hi and thanks a lot for this very important advice. This reflects what i heard already. My idea would be to purchase some service manual of very good dvd players or similar and look what they use for the mains input filter.
I understand that also the diodes after the transformer are extremely critical
And i guess some kind of filtering should be used also there.
But i see this more tricky to carry-out on my unit.
Instead adding something before the transformer could be done more easily.
Thanks a lot again for confirming a very important point.
Regards, gino
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Old 2nd June 2015, 11:36 AM   #6
AndrewT is offline AndrewT  Scotland
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I don't agree that the rectifier diodes are critical.

Are the PSUs in modern digital equipment, SMPS direct off the mains line, or via an isolating transformer to usable LV?
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Old 2nd June 2015, 05:50 PM   #7
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Quote:
Originally Posted by AndrewT View Post
I don't agree that the rectifier diodes are critical.
Are the PSUs in modern digital equipment, SMPS direct off the mains line, or via an isolating transformer to usable LV?
Actually i do not know. I have here this unit below, a usb to spdif converter, of which i would like to upgrade the power supply providing a DC clean power instead of connecting it to the mains

Click the image to open in full size.

The unit has almost no mains noise filtering and toroids are famous to let all the garbage through, while instead EI type are much better at blocking HF noise, so bad for digital.
the idea is to bypass transformer and diodes and to feed clean DC from an external good quality PS directly to the blue caps before the main voltage regulator.
In this way there will be another regulation stage before the one on the board, and noise reaching the circuit should be already very low.
In this way i hope to reduce the noise entering the unit from the power supply.
I have already a quite silent DC power supply to test.
Thanks, gino

Last edited by ginetto61; 2nd June 2015 at 06:00 PM.
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Old 2nd June 2015, 10:28 PM   #8
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This post might be worth considering:
UCD180 questions
Medical grade mains filters do not use Y caps to ground.

As far as low noise DC regulators there are many different to choose from, depends on your current/voltage needs. Example Analog Devices ADM7150 is capable of 800mA with ~1.0 V noise. Texas Instruments TPS7A4700 1A, ~4 V. Both can be sourced quite cheap. DIYINHK got kits for both but can't comment on quality.
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Old 3rd June 2015, 10:50 AM   #9
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Quote:
Originally Posted by henryspencer View Post
This post might be worth considering:
UCD180 questions
Medical grade mains filters do not use Y caps to ground.
Hi and thanks a lot for the very interesting link. I will study it.

Quote:
As far as low noise DC regulators there are many different to choose from, depends on your current/voltage needs. Example Analog Devices ADM7150 is capable of 800mA with ~1.0 V noise. Texas Instruments TPS7A4700 1A, ~4 V. Both can be sourced quite cheap. DIYINHK got kits for both but can't comment on quality.
Wow ... these are impressive figures indeed.
But i wonder if the comment about regulators being much less effective on HF noise applies also to these high end parts.
I have come to the conclusion that HF noise is the evil of digital.
The grainy, hazy, harsh sound could be due to some HF noise reaching the chips like clock and dac ...

In the meantime i have decide to replace the power supply.
I have bought a 40VA pcb mount transformer that i hope will have primary and secondaries separated (better filtering of mains noise), some schottky diodes and new Nichicon FG caps ...
I will build a bridge with the schottky follow this nice procedure

Click the image to open in full size.

I have one question.
Given the more VA of this trans, is it ok to use just one of the two secondaries and leave the other open ?
The total consumption of the unit will be less than 10VA i guess.
Thanks a lot again, gino
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Old 3rd June 2015, 11:57 AM   #10
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What part did you decide to replace?
It seems to have double regulated psu. First the 4 big caps followed by the first regulator(LT1963) that probably makes about 5V, then all separate parts have separate 3.3V regulator.
Then near the chip many local filter caps that supposed to remove the leftover HF noise.
Not sure you can make much improvement without seriously modifying the whole pcb.
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