RF filter from audiophool to realety. - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: May 13, 2018, 05:49:22 am
content: I am thinking about making a RF / EMI filter for my test bench.I found this on a site for audiophiles and this is often a good reason, not to just trust it. :-)Does this filter work?http://www.lampizator.eu/AC%20FILTER/SILK/SILK.HTML

author: hamster_nz
author_rank: Super Contributor
timestamp: May 13, 2018, 06:12:14 am
content: Quote from: FriedMule on May 13, 2018, 05:49:22 amDefine 'work'You should be able to plug things into it and get power out...I don't like the look of the switched earth wire... I guess it is an attempt to stop ground loops.

author: llkiwi2006
author_rank: Regular Contributor
timestamp: May 13, 2018, 07:49:01 am
content: Putting an inductor in series with the earth connection is not a good idea.Also, why not just buy an off the shelf ac line filter module? This way you don't have to worry about safety.

author: FriedMule
author_rank: Frequent Contributor
timestamp: May 13, 2018, 10:25:32 am
content: My definition of "work" is, do it as promised?Minimize RF and deliver power.Why I don't just bye, because it is a lot more fun to build. :-)(by the way I know a lot more about main power, then I do about electronic. So I think that I am safe)

author: FriedMule
author_rank: Frequent Contributor
timestamp: May 13, 2018, 12:06:55 pm
content: I forget to ask if that circuit would be dangeros , if the plug with phase and null are switched around?

author: rstofer
author_rank: Super Contributor
timestamp: May 13, 2018, 08:42:08 pm
content: Switching the earth ground on all but one outlet seems dubious. It implies that all other components will be tied to safety ground through the audio cable shields. I'm not a fan!The intent of the earth grounds is that all non-current carrying metallic surfaces are to be grounded (or double insulated) and the way I figure it, the outside surface of an RCA jack would have to be tied to earth ground. That's clearly what happens with the BNC connectors on oscilloscopes.I understand why it was done but I wouldn't do it myself. Then again, I don't work on audio.

author: helius
author_rank: Super Contributor
timestamp: May 13, 2018, 09:15:08 pm
content: Ground loops are an issue in all equipment, not just audio, when unbalanced cables are used. If there is a difference of local ground potentials of two pieces of equipment, current will flow through the cable shields of any interconnections between them to equalize that potential. This becomes a ground loop when the difference is a 60 or 50 Hz AC voltage: then, the current imposed through the cable shields is AC, and it is subtracted from the unbalanced signals, which is audible as hum, visible as banding on analog video, and present as a noise source on an oscilloscope.The idea of a "ground lift" is that if one of the interconnected devices' local grounds was allowed to float, it would follow the other device via the cable shield, keeping them at the same potential. It isn't that simple, because some equipment injects current to ground. This means that ground lift switches only work some of the time on some combinations of equipment and not on others. In addition, it makes the lifted device electrically unsafe because it has no permanent safety connection to earth.The best way to avoid ground loops is to tie all equipment safety grounds to a common point, like a single power distributor (a star ground). When this is done, there is no need for any ground lifts. However, if there are RF filters on some of the devices, AC is being actively coupled into the safety grounds. RF filtering actually causes ground loop problems, if the complex impedance of the filters on the equipment differs.In a balanced signal environment, neither signal conductor is grounded, so ground loop currents are isolated from the signal and have very little effect on it. In addition, since the ground reference is not needed to extract the signal, the cable shield can be connected at only one end, which breaks the ground loop. (To learn more about this, search for "pin 1 problems".) Investing in expensive "tweaks" and "fixes" for unbalanced signals instead of switching to more reliable balanced configurations is one of the great audiophile puzzles.

author: FriedMule
author_rank: Frequent Contributor
timestamp: May 13, 2018, 11:05:53 pm
content: It sounds as if the circuit has some "funny" things going on.I am thinking about using a cast aluminium box with an IEC socket, and clean the signal, first incide the enclosiure.Is it possible to make a real filter that "cleans" any noise on an 240VAC main before the transformer?

author: bson
author_rank: Unknown
timestamp: May 14, 2018, 03:01:31 am
content: A device like the Furman AC-215A might well do the job. But of course the benefit, if any, depends on what your mains looks like to begin with.https://www.amazon.com/Furman-AC-215A-Conditioner-Auto-Resetting-Protection/dp/B003PJ6NPO

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 14, 2018, 04:39:41 am
content: Said it before and I'll say it again:If you don't have any way to detect an improvement, let alone confirm the manufacturer's technical claims about what the product does --Do you really need itat all?This goes for absolutely anything, not just this. Does your phone provider really have the coverage they claim? (Actually they might. Someone in a claimed coverage area would be very vocally disappointed to see they actually aren't.)Does your car really have the mileage claimed? (If it has a mileage readout on the dash, is it truthful? Ultimately, you can measure average based on logged miles and fuel purchases. And they are held to federal standards, so it's unlikely they'll say anything grossly out of whack with the real thing.)So does the audiophool filter really do anything?It might not. An audiophile isn't going to know the effects of something so abstract. They mostly use amplifiers with iron-core transformers, anyway, which do a good job filtering and absorbing most mains trash to begin with. The filter needn't do anything at all, to still have a filtered environment, even still assuming that the circuit is ultimately sensitive to line noise.But if you want to use that "filter" for something that actually needs filtering -- and whether it even needs it, can be determined in exactly the same way -- how do you know it will work?Now, manufacturers of [real?] electronic filters, provide curves showing their performance. It's important to note: these are made under specific conditions, which are rarely matched by mains wiring and plugged-in appliances. But it's a start at least. The curves depend on the environment the filter is connected into, so one must carefully consider the application, too.So, how do you know? If you've been picking up pops, buzz, spectral lines, etc. that are definitely coming from the line (they go away when filtered properly, or, uh, powered from an inverter maybe -- or a crummy automotive inverter makes things that much worse), that's a start, if not necessarily a good one! If you're doing power quality analysis and directly measuring noise, that's pretty much the end of that. But, if you aren't sure at all, in the first place? Reasonable enough to guess no.Tim

author: helius
author_rank: Super Contributor
timestamp: May 14, 2018, 05:10:43 am
content: I guess it would be interesting to plug a power quality analyzer from Yokogawa or Dranetz into this thing and see what kind of difference it makes.There are many sources of conducted interference and the best filter for one type is different from another.

author: FriedMule
author_rank: Frequent Contributor
timestamp: May 14, 2018, 05:18:13 am
content: T3sl4co1l you are completely correct! :-)To answer your questions: Mr. EEV and others are often fighting with outside and inside noise, while testing or messeure electronic. EEV has build a noise free enclosure with 2 x 9V batteries, but for my purpose I need a lot more power. So therefore a noise filter that cleans main, before it delivers it to my project. So that any noise are from the project itself.

author: rstofer
author_rank: Super Contributor
timestamp: May 14, 2018, 02:54:36 pm
content: Quote from: helius on May 13, 2018, 09:15:08 pmAnd if you unplug the device with the earth ground, all of the remaining equipment becomes ungrounded. As I said, this practice seems dubious (to me).The 'star' ground probably works better and, to some extent, we get this for free when we plug all the devices into a common power strip. The shorter the power cords, the better. Then we get the problem back again if the power requirements exceed the capability of a single branch circuit but that's at least 1200W in the US so I'm not sure if this ever happens.Fiber optics is probably the best answer but the cost of converters (and the signal quality) may put this out of reach.

author: FriedMule
author_rank: Frequent Contributor
timestamp: May 14, 2018, 03:30:01 pm
content: Would the power cords not act as antennas for every signal?I thought that a simple 100hz-3Ghz filter could be perfect for cleaning and then build it into a cast aluminium box?

author: rstofer
author_rank: Super Contributor
timestamp: May 14, 2018, 03:58:51 pm
content: They do make shielded power cords and even recommend cutting off the computer end to break out the shield as a 4th conductor. At the equipment end, I would probably leave it unterminated.http://www.slt.co/products/ShieldedPowerCords/ShieldedPowerCords-6Foot-12Foot.aspxThey may even make twisted cable but I didn't go look for it. Of course, you could use any suitable cable as a power cord and Belden makes just about every conceivable cable.3 GHz? I thought the discussion was around audio, sorry about previous references to audio. When I look at the OP, it mentions only a filter for the workbench.I think T3sl4co1l's reply above about sums it up. If you can't measure it, it doesn't exist.We did use LC filters on the power coming into screen rooms, back in the day. It was just a big, heavy, sealed box that hung on the side of the structure. Power went in on one side and came out on the other. I have no idea what was on the inside other than it generically being an LC filter.In concept, they were a lot like these commercial units except they had enclosures over the connections suitable for conduit.http://www.kemet.com/Lists/ProductCatalog/Attachments/718/KEM_EMI0002_FLLD4-TH.pdfPower line conducted interference is only one source and may be the easiest to deal with. Radiated interference is much harder to deal with and that's why we had screen rooms. And no fluorescent lights...

author: bd139
author_rank: Super Contributor
timestamp: May 14, 2018, 06:13:41 pm
content: I wouldn’t bother with any of that crap. If you’ve got a problem those clip on ferrites are pretty damn good. Stuff them just before the cable entry to whatever is picking up the RFI and problem solved.

author: FriedMule
author_rank: Frequent Contributor
timestamp: May 14, 2018, 06:40:08 pm
content: Quote from: rstofer on May 14, 2018, 03:58:51 pmThanks for the tip, I'll try to find one for the Danish system.Quote from: rstofer on May 14, 2018, 03:58:51 pmForgive me for being so bad ad writing my question, It is wary easy to read it as about audio.Quote from: rstofer on May 14, 2018, 03:58:51 pmThis is properly a truth with modifications:-)My scope can't measure 2.4 Ghz but that does not mean that mobile phones does not exists or that they can not effect electronic in some bad way, that a later scope can detect.Quote from: rstofer on May 14, 2018, 03:58:51 pmI hope to take care of it all by filtering the input and shield it by a cast allu box.My hope is to make a main in, a filter that cleans the power, so it does not effect my project, a sort of clean power without hum, noise from light, phones and so on.

author: floobydust
author_rank: Super Contributor
timestamp: May 14, 2018, 07:23:22 pm
content: I recommend against building a discrete mains EMI filter, especially from a napkin sketch, with a ground lift switchHigh voltage and high energy, with home-made magnetics... you have to be careful about insulation, spacings and fire if an arc happens.Commercial two-stage mains filters are the same as the napkin sketch. Just go and buy one for ~$30.Some have a DM then CM, others CM then DM chokes, some are two CM chokes. Several different architectures.It depends on whether you need to filter incoming or outgoing EMI, and if DM or CM noise is your worst enemy.I find AM radio-band interference (from power line hash, radio transmitters, SMPS) upsets audio gear the most, and a ferrite clip-on does nothing for DM noise or CM low-band at a few MHz.I recommend something like Delta Electronics DS/DW/DR series for electronics labs, recording studios, metrology.They are cheaper than Wurth or Schurter. Digikey is more expensive than Mouser so shop around. Aliexpress has some DIY power line filters for 1/2 the price but they are just a microwave oven CM choke and a few caps, very basic filtering, many with audio foolery words and placebo inductors that do very little.Of course, this all assumes you have a good ground. If you have a long cable run to the outlet, everybody wrongly assumes ground is ground at a wall outlet, but the impedance of 5m of #14 wire means an EMI filter can make things worse, due to the Y-caps. It's better in a commercial building with metal-conduit wiring.

author: BrianHG
author_rank: Super Contributor
timestamp: May 14, 2018, 07:37:53 pm
content: These filters dont improve sound. If you have audible interference in your sound, and it is coming from the AC, they may remove that problem. But, once again, the audio quality probably not change. An exception is in the case of an optical isolated DAC unit, when using amains isolation transformer with no FG connectioncan help some sound quality, however, this is solving an issue with your amp's RCA audio input GND impedance and the quality of your RCA cables GND and the DAC's GND wiring on the RCA connectors. If your DAC isn't completely GND isolated and the mains isolation transformer is completely floating at all points possible, this improvement is nullified.Note that some of the better quality older DACs which would not pass UL today are internally isolated to this level.

author: floobydust
author_rank: Super Contributor
timestamp: May 14, 2018, 08:20:26 pm
content: It's easy to see RF with a scope or spectrum analyzer when you probe around audio pre-amps, power amplifiers, mixing consoles etc.It's in the air, on the rails and in the audio signal.The less EMI getting into equipment via the power (line) transformer, the better. Mains EMI filters can "improve sound".It largely depends on the equipment's power transformer- stray capacitance and core bandwidth determine how much power line hash gets through and into equipment.DACs and other small gear with modern split-bobbin transformers are not as sensitive to CM noise, there's very little inter-winding capacitance.I find audio power amplifiers with toroidal power transformers are the worst for passing-through mains RFI, 100's nF of capacitance there.Neumann U47 microphone power supply, does pass mains RFI due to its windings being stacked and the singer and recording engineer can hear it like night and day. It's a "grit" that you can hear, demodulated AM radio superimposed on the audio. Sometimes I'm lucky enough to hear the radio station's announcer and then know right away what is distorting the signal. It comes in on mains, and rarely on a poorly shielded cheap mic cable.Another example, a turntable synchronous motor has 50nF stray capacitance from mains windings to the metal chassis. So mains RF gets coupled to the chassis and into the phono preamp.It's not all audiophoolery.But many manufacturers exploit the psychoacoustics aspect and charge a fortune for special cables, special filters and utter bullshit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 14, 2018, 10:02:50 pm
content: Quote from: FriedMule on May 14, 2018, 06:40:08 pmI covered that! Then you've been "picking up pops, buzz, spectral lines, etc. ... that's a start, if not necessarily a good one!""Spooky action at a distance" is a hallmark of invisible, unmeasurable (with what instruments you happen to have at hand) RF. If it's varying with proximity (wave your hands over it, touch things -- assuming it's generally safe to touch), that's a classic sign.GSM phone beeping in amplified speakers is a painfully common example... however this is not usually introduced on power lines, because of the very high frequency.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 14, 2018, 10:15:45 pm
content: Quote from: FriedMule on May 14, 2018, 03:30:01 pmThis is neither possible nor practical, because the filter must exhibit its characteristic impedance near 100Hz, and 50Hz is much too close to this. The consequence: excessive series inductance, severely impairing output voltage under load, inrush current, voltage distortion, etc. On the upside, input power factor will be pretty good (harmonic current drawn by the load will be filtered out).You can make the characteristic impedance very low (so that load-side regulation is acceptable), but now you're drawing a lot of reactive power from the mains, which needs to be compensated out. You need huge, high quality inductors, and they're still going to get hot.Mains bandwidth (what's intended, and practical to carry on the network) extends to the low kHz, then becomes increasingly mushy beyond there (especially over distances). For this and the above reason, filtering below 1kHz is impractical, and may not even be desirable, but something cutting in the 10-100kHz range is quite practical, even common.But again, if you're just doing this for kicks, at the very least you're missing the elephant in the room -- the room itself. You need a shielded room to block ambient radiation. Much more practical to filter signals and power at the device level, and let the room and cables be noisy!Tim

author: FriedMule
author_rank: Frequent Contributor
timestamp: May 15, 2018, 11:09:24 am
content: It is amazing all what I learn from you!!If we say that I would like to build an amplifier I understand that I can not (within reason) build a perfect filter but it is posible to filter a range of hz'si.e annoeng noise that comes from the room, radio, calling mobile and power cables.My knowlede are not good enough to decide the best action out from your great advices.The sollution of bying are probberly the best, but you know "it is fun to build it yourself", I just have to be sure that it is not dangerous and properly shielded!I would like to end up with (random order) a soft start, noisefilter, transformer and a audio circuit.What would you do and how (floobydust schematic 1, 2, 3 or other?), if you had to make a great filter for your own similar project?