Electronics > Beginners
RF filter from audiophool to realety.
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bd139:
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.
FriedMule:

--- Quote from: rstofer on May 14, 2018, 03:58:51 pm ---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.aspx

They 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.

--- End quote ---

Thanks for the tip, I'll try to find one for the Danish system.


--- Quote from: rstofer on May 14, 2018, 03:58:51 pm ---
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.


--- End quote ---

Forgive 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 pm ---
I think T3sl4co1l's reply above about sums it up.  If you can't measure it, it doesn't exist.

--- End quote ---

This 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 pm ---
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.pdf

Power 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...

--- End quote ---

I 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.
floobydust:
I recommend against building a discrete mains EMI filter, especially from a napkin sketch, with a ground lift switch  :palm:
High 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.
BrianHG:
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 a mains isolation transformer with no FG connection can 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.
floobydust:
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.
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