Hi,

Just to let people know that I have just put up a couple of new pages that
explain the basics of how mains RFI filters work. The aim being to allow
readers to decide which type of filter - if any! :-) - might be most
appropriate if they have a problem with RFI.

The first page is at

http://www.audiomisc.co.uk/mains/filters1.html

and that has a link icon at the end to take you to the second page that
deals with ferrite and VDR devices and their use.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Jim Lesurf wrote:
Hi,

Just to let people know that I have just put up a couple of new pages that
explain the basics of how mains RFI filters work. The aim being to allow
readers to decide which type of filter - if any! :-) - might be most
appropriate if they have a problem with RFI.

The first page is at

http://www.audiomisc.co.uk/mains/filters1.html

and that has a link icon at the end to take you to the second page that
deals with ferrite and VDR devices and their use.

Slainte,

Jim



From endless searching through catalogues, it seems to me that most
commercially available mains filters are aimed at trying to stop
equipment *transmitting* crap onto the mains rather that stopping it
getting in to equipment though I guess reciprocity applies to at least
some degree.

Cheers

ian

Ian Bell wrote:
Jim Lesurf wrote:
Hi,

Just to let people know that I have just put up a couple of new pages
that
explain the basics of how mains RFI filters work. The aim being to allow
readers to decide which type of filter - if any! :-) - might be most
appropriate if they have a problem with RFI.

The first page is at

http://www.audiomisc.co.uk/mains/filters1.html

and that has a link icon at the end to take you to the second page that
deals with ferrite and VDR devices and their use.

Slainte,

Jim



From endless searching through catalogues, it seems to me that most
commercially available mains filters are aimed at trying to stop
equipment *transmitting* crap onto the mains rather that stopping it
getting in to equipment though I guess reciprocity applies to at least
some degree.

Cheers

ian


I should also have said that my experience with linear supplies is that
there is a huge amount of crap on the mains from laptop SMPSUs and the
like that falls squarely in the audio band. RFI filters are not
effective in the audio band of course.

Cheers

Ian

"Ian Bell" wrote

I should also have said that my experience with linear supplies is that
there is a huge amount of crap on the mains from laptop SMPSUs and the
like that falls squarely in the audio band. RFI filters are not effective
in the audio band of course.


I agree that there's plenty of audio-band crap on the mains, but not from
SMPSs, which generally work at frequencies well above the audio band. AF
crap is either harmonic distortion of the mains waveform, or comes from
commutator motors and the like.

AF crap shouldn't be a problem with a linear PSU. The transformer will
remove common-mode noise, whilst the filtering on the rectified output
(which you need anyway to remove mains-frequency ripple) will eliminate the
rest.

David.

David wrote:

I agree that there's plenty of audio-band crap on the
mains, but not from SMPSs, which generally work at
frequencies well above the audio band. AF crap is either
harmonic distortion of the mains waveform, or comes from
commutator motors and the like.

Just because SMPS "work at frequencies above the audio band"
doesn't mean they don't also produce audio-frequency noise.

A SMPS with a fixed switching frequency can easily employ a
mains input filter that targets that frequency, although it
isn't so easy to deal with all of its harmonics.

It's likely that its PFC boost converter, if it has one, has
a different problem. It tries to ensure that current drawn
by the supply is in synchronous proportion to the mains
input voltage. This is problematic in the face of the large
following capacitance which is trying to drag the current
out of phase. It's errors appear in terms of phase and
amplitude, so harmonic distortion is a likely outcome. A
large number of mains harmonics fall within the audio band.

Also to take into consideration is how the SMPS is
regulated, and what it does when under or over-loaded. It
could be that it operates in bursts, and the frequency of
bursts could be within the audio spectrum.

AF crap shouldn't be a problem with a linear PSU. The
transformer will remove common-mode noise, whilst the
filtering on the rectified output (which you need anyway
to remove mains-frequency ripple) will eliminate the rest.

Sounds fair enough. Why is it a problem for Ian Bell,
though, I wonder?

It's a problem for my AVO valve tester, which only rectifies
the grid voltage, and does no smoothing at all. There's a
simple European standard for mains harmonic distortion from
consumer equipment. It allows a fair amount, IIRC. They
don't appear to have taken my AVO into consideration.

Ian

Ian Iveson wrote:
David wrote:

I agree that there's plenty of audio-band crap on the
mains, but not from SMPSs, which generally work at
frequencies well above the audio band. AF crap is either
harmonic distortion of the mains waveform, or comes from
commutator motors and the like.

Just because SMPS "work at frequencies above the audio band"
doesn't mean they don't also produce audio-frequency noise.

A SMPS with a fixed switching frequency can easily employ a
mains input filter that targets that frequency, although it
isn't so easy to deal with all of its harmonics.

It's likely that its PFC boost converter, if it has one, has
a different problem. It tries to ensure that current drawn
by the supply is in synchronous proportion to the mains
input voltage. This is problematic in the face of the large
following capacitance which is trying to drag the current
out of phase. It's errors appear in terms of phase and
amplitude, so harmonic distortion is a likely outcome. A
large number of mains harmonics fall within the audio band.

Also to take into consideration is how the SMPS is
regulated, and what it does when under or over-loaded. It
could be that it operates in bursts, and the frequency of
bursts could be within the audio spectrum.

AF crap shouldn't be a problem with a linear PSU. The
transformer will remove common-mode noise, whilst the
filtering on the rectified output (which you need anyway
to remove mains-frequency ripple) will eliminate the rest.

Sounds fair enough. Why is it a problem for Ian Bell,
though, I wonder?


Indeed, it is probably not anything to do with the linear supply itself.

Here's the scenario. I am measuring the noise from a mic pre. Its input
is balanced and shorted and anyway the gain control is turned down to
zero so there's no chance of pick up at the input. Output is unbalanced
with a source impedance of about 800R feeding a battery operated Lindos
MS1 which is connected to the laptop via a serial link.

The Lindos has a built in speaker so you can actually hear any noise
present. With the laptop running on batteries the output noise is close
to -80dBu as expected and sounds like hiss in the speaker. Turn on the
laptop PSU though and the reading rises to -40dBu and you can hear a
high pitched whistle in the speaker.

Cheers

Ian

In article , Ian Bell
wrote:
Ian Bell wrote:
Jim Lesurf wrote:
Hi,

Just to let people know that I have just put up a couple of new pages
that explain the basics of how mains RFI filters work.

From endless searching through catalogues, it seems to me that most
commercially available mains filters are aimed at trying to stop
equipment *transmitting* crap onto the mains rather that stopping it
getting in to equipment

I'd agree that dealing with problems by 'silencing' the source is - where
possible - probably better that having to protect each individual 'victim'.


though I guess reciprocity applies to at least some degree.

You sometimes have to be careful with reciprocity when people start playing
with ferrite devices. :-)

An advantage of fitting suppression at (or near) the source is that it
helps cut down on the number of possible routes by which interference might
then distribute itself.

Alas the problem in practice may sometimes be from sources outwith your
control or even being able to locate them.

I should also have said that my experience with linear supplies is that
there is a huge amount of crap on the mains from laptop SMPSUs and the
like that falls squarely in the audio band. RFI filters are not
effective in the audio band of course.

Yes, although that does depend to some extent on the type of filter and
RFI. So for example the use of ferrites as common mode chokes can work down
to relatively low frequencies as they discriminate in a way that lets them
pass the wanted mains (or loudspeaker signal) more easily. But that doesn't
help much if the interference isn't common mode. You may also need to use a
loop of ferrite as a 'cm choke' rather than a simple clip-on chunk so as to
get a large enough inductance.

Snag here is the absence of useful measured data on the details of how much
any given commercial filter rejects.

I didn't comment on approaches like 'coating the wires with ferrite' as I
think some cable makers say they do. That struck me as a perhaps dubious,
and probably pointless exercise. Too little ferrite and too easily
saturated by the mains power, I suspect.

Must admit I also wonder about using VDRs as it occurs to me that in some
cases they may *draw* current to them which otherwise may have gone
elsewhere.

Unfortunately, I've not found a lot of reliable measurements on these
matters that show what tends to happen, and how often, in domestic UK
situations. I did find some data on wall socket impedances, but with no
guide to how consistent this was from one house to another!

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Personally, I'd love to shove some crap into the mains for a few days to
clobber these power line adaptors so the idiots scap them and I can listen
to short waves again.

Brian

--
Brian Gaff -
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
Blind user, so no pictures please!
"Ian Bell" wrote in message
...
Jim Lesurf wrote:
Hi,

Just to let people know that I have just put up a couple of new pages
that
explain the basics of how mains RFI filters work. The aim being to allow
readers to decide which type of filter - if any! :-) - might be most
appropriate if they have a problem with RFI.

The first page is at

http://www.audiomisc.co.uk/mains/filters1.html

and that has a link icon at the end to take you to the second page that
deals with ferrite and VDR devices and their use.

Slainte,

Jim



From endless searching through catalogues, it seems to me that most
commercially available mains filters are aimed at trying to stop equipment
*transmitting* crap onto the mains rather that stopping it getting in to
equipment though I guess reciprocity applies to at least some degree.

Cheers

ian

In article ,
Brian Gaff wrote:
Personally, I'd love to shove some crap into the mains for a few days to
clobber these power line adaptors so the idiots scap them and I can
listen to short waves again.

Could you have a decent external aerial fitted? I have one feeding my Quad
AM3, and SW reception is pretty good.

--
*Marathon runners with bad footwear suffer the agony of defeat*

Dave Plowman London SW
To e-mail, change noise into sound.

BadJokeYou Should Be Working For Toyota!/BadJoke

On Thu, 25 Mar 2010 11:51:42 UTC, Jim Lesurf
wrote:

Hi,

Just to let people know that I have just put up a couple of new pages that
explain the basics of how mains RFI filters work. The aim being to allow
readers to decide which type of filter - if any! :-) - might be most
appropriate if they have a problem with RFI.

The first page is at

http://www.audiomisc.co.uk/mains/filters1.html

and that has a link icon at the end to take you to the second page that
deals with ferrite and VDR devices and their use.

Slainte,

Jim



--
Regards,
Galen
------
There are only 10 kinds of people in the world. Those who understand
binary and those who don't.