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

"Jim Lesurf" wrote in message
...
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,
I think it might also be worth mentioning that when earth referenced
filters - i.e. caps. etc. from Live and Neutral connected to Earth, that
there is a potential safety issue and that the Earth should never be
disconnected. How many old-timers have disconnected the Earth on a scope
looking to stop loops and found that the chassis floats up to half the mains
volts - albeit limited in current- but still enough to give a bit of a
tingle. IIRC, HP scopes in particular at that :-)

Happy listening

Mike

"Jim Lesurf" wrote in message
...
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


Sorry - pressed the 'Post' key too fast: That should read "IIRC, HP scopes
are particularly good at that"

On Fri, 26 Mar 2010 17:45:55 -0000, Ian Bell wrote:


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.


That's a standard ground loop problem typical of laptops.

James.


--
http://www.jrpmusic.net

James Perrett wrote:
On Fri, 26 Mar 2010 17:45:55 -0000, Ian Bell wrote:


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.


That's a standard ground loop problem typical of laptops.

James.




Yes and no, because it does not go away if I disconnect the serial cable.

Also, if I pull the mains cable out of the laptop PSU so there can be no
ground loop, the whistle is still there and you can hear it change pitch
and disappear as the PSU dies. Not only that, the PSU mains cable is a
double insulated two pole type so there is no ground connection to loop.

I strongly suspect the SMPSU is putting crap straight onto the mains.

Cheers

Ian