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DIY: Mains Line/Noise/EMI Filtering

restorer-john

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#21
but in several places adding a DC blocking filter did fix some transformer hum. I've forgotten the details, but IIRC the magnitude of common-mode offset needed to induce hum in a toroidal transformer was a lot less than I expected (led to me doing a little more research to find out why).
I've got an oldie but a goodie- 'squeaky clean' line filter on my bench feed to run all my test gear- in fact everything (32 outlets) runs through it and it has DC blocking in the form of diodes/caps along with a bunch of other filters all in a heavy steel case with potted filters etc. Pretty sure there are 2 banks of three series connected diodes IIRC when I pulled it apart to replace a few neon indicators many years ago. I think there was a 1.5v differential between input and output voltage. It's riveted together so not fun to dismantle.

Don't have a schematic, but it is very effective at killing toroid hum/buzz and even is effective on some marginally noisy EI transformer primaries.

Our mains waveform here is terrible looking on my DSO, makes me think we should all be locally regenerating our own.
 

DonH56

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#22
I've got an oldie but a goodie- 'squeaky clean' line filter on my bench feed to run all my test gear- in fact everything (32 outlets) runs through it and it has DC blocking in the form of diodes/caps along with a bunch of other filters all in a heavy steel case with potted filters etc. Pretty sure there are 2 banks of three series connected diodes IIRC when I pulled it apart to replace a few neon indicators many years ago. I think there was a 1.5v differential between input and output voltage. It's riveted together so not fun to dismantle.

Don't have a schematic, but it is very effective at killing toroid hum/buzz and even is effective on some marginally noisy EI transformer primaries.

Our mains waveform here is terrible looking on my DSO, makes me think we should all be locally regenerating our own.
I want one! :)

The usual scheme is caps to block DC with diodes across (shunting) them to keep the voltage across them small under transient conditions. That way, you can use large (high-value) caps with modest voltage ratings.

I think the problem with toroids is that it is very difficult to gap them in manufacturing like normal EI transformers making them extremely prone to saturation with small DC offsets applied. Toroids better confine the EM field to reduce noise in packed electronic components but they do have their drawbacks.
 

trl

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#23
"[...] A number of potential sources of DC current injection have been identified, particularly the use of high frequency ballasts within fluorescent lighting loads, and switch mode power supplies for PCs. Additionally there are no limits for DC harmonics defined under EN61000-3-2, and additionally items taking less than 75W do not have any harmonic limits defined at all (exceptions under clause 7). Hence for - some household equipment, such as mobile phone chargers, it is not known whether any DC current component is produced. It is therefore recommended that a selection of these devices are tested to measure and attempt to quantify any DC current components. [...]"
 

graz_lag

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#24
True symmetrically balanced isolation transformer ...

I would like to report some details abt. my most useful upgrade on the main line so far by means of symmetrically balanced isolation transformers.
I decided to develop this upgrade after testing several Schaffner' filters with the intent to reduce the AC noise and electromagnetic interferences. These were so-and-so effective, but I felt the dynamic was also affected in some way. :(

So, after testing the 1st prototype, I have then build four units in total :
transformer #1 for the sources of both Hi-Fi and HT setups
transformer #2 for the Hi-Fi setup's amplifier
transformer #3 for the HT setup's amplifier
transformer #4 for the HT's projector

All are based on transformers with 2x screen magnetic shielding, so that the highest reduction on AC noise, which is the cause of ground loops, can be secured.
Each unit is configured for 230VAC (France) true balanced mode operation, so a 1:1 isolation transformer with a precisely placed center tap on the transformer’s secondary.
The incoming 230VAC (115V on the L terminal and 0V on the N and G ground) is split into halves on the transformer’s output.
The "new" AC line has then 115V on the L terminal and 115V on the N terminal, all that when referenced to the new center-tapped G ground, which remains at 0VAC.
The opposite polarity between L and N is due to completely cancel all common mode noise from the incoming AC line.

I paid from 120 EUR to 250 EUR including shipment (2015) for each transformer from a French manufacturer, plus : the aluminum enclosures, the IEC sockets, wirings, circuit breakers on the outgoing, silent blocks, minor stuffs here and there, say from 300 EUR to 450 EUR depending the size each finished unit ... including the fancy voltage indicators ...
I have tested the integration of filters (Schaffner #2070) but I prefer with no filter.

Manufacturers like Balanced Power Technologies, Bryston, Keces, ect., sell these units for thousands of $ ... :facepalm:

The same principle can be applied to any incoming VAC, i.e. in the case of 120VAC one gets +60VAC and -60VAC on the outgoing section.

Easy, funny, but most of all, a "straight-to-the-problem" solution with an incredible value for money ! :p

PS The most difficult task is the sourcing for the transformers, as these "symmetrically balanced isolation" ones are not very common within the manufacturer standard catalogs.
 
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trl

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#25
Some pictures will really help. The price you paid is a really low indeed, I paid about 100 EUR myself for my isolation transformer (250 VA, 150 Watts). However, we could both purchase few 1000 EUR power cables anytime. :)

Is there's a way for you to safely measure your mains input, in front of the isolation transformer, then measured again after the transformer? This will help in understanding what this isolation transformer resolves for your audio setup. In my case, I have a good AC main at home, so my isolation transformer did absolutely nothing to my AC sinewave (same THD, same sine etc.), instead did resolved a tiny hum-noise caused my a ground-loop (same results could be achieved in a cheaper way).

Also, RMA and ARTA with your audio devices connected directly to AC mains, then connected to isolation transformer might also help in understanding something:
- 1) Why would you need isolated balanced power for?
- 2) Any measurable improvements on the AC sinewave?
- 3) Any measurable improvements on the Hi-Fi audio devices?

Please be aware that loads for more than 50% after the isolation transformer will increase THD.
From what I know and I was able to measure myself, the real benefits of an isolation transformers are:
- reducing the possibility of getting electroshocked and die when touching only one AC wire (touching both wires will shock us anyway)
- removal of ground loops, if exists (in conjunction with ground lifting safely)
- removing DC-components from the AC mains, if exists
And low-lights, from my point of view. would be:
- added THD when output load approaches its nominal output power (sinewave will start bending and look more like DC, also some spikes could appear and transformer will start buzzing)
- coil noise under some circumstances

Also few of my thoughts here: https://www.superbestaudiofriends.o...tors-ups-yadayadayada.282/page-16#post-234579. I don't expect people to take my words for granted, mostly because my AC mains looks really good, so I don't need to improve anything, so if find a way to safely measure your mains quality and how much of your Hi-Fi noise is really related to your AC mains, then please post back.

Note: If you never measured AC sinewaves directly before, please don't try now! By safely measuring the AC lines I mean to use a scope with a differential probe without getting electroshocks into your body. Also, a safer way to do that would be using a step-down transformer connected to a regular scope could also help (cold be 230V-to-120V or simply a 230V-to-12V transformer).
 

DonH56

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#26
I have thought about doing something similar over the years but was always nervous about the safety aspects, and without UL certification or some such my insurance would take a pass if anything happened and they knew it was in there. The good news is that, for better or worse, except for the occasional ground loop I have never really seen problems with mains leakage or such that was not either more than adequately rejected by the components or overwhelmed by other noise and distortion sources in my system.

Technically, the few times I tried it, I saw the same problems @trl describes with transformer linearity. It worked great on preamps and low-level stuff, especially when I applied it to my TT and preamp, but the large current spikes demanded by power amplifiers would cause havoc with the output unless I oversized the transformer by at least a factor of two (I was lucky to find a big power transformer so was able to put an industrial 2 kVA unit on my old 100 W/ch stereo amplifier).
 

trl

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#27
My home power lines are pretty clear and perfectly sinewaves, so my tests will be non-conclusive, I need to find a home with much noisy and ****** power line. :) The only thing I can replicate would be the hum noise when ground-loops are happening (easy to reproduce that), but other than that...nope, sorry.

I've also measured the noise between ground and null wires and when a vacuum cleaner or other appliances are connect in the same outlet or even the same circuit, then the noise increases about ten times (it becomes a problem only in case of ground loops).
 

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