Mains filters
 

On Thu, 16 Mar 2006 10:39:13 +0200, "Iain Churches"
wrote:

A couple of days ago, I popped in for 30 secs to
an audio demo at a local dealership to take him some audition
material. He was demonstrating a mains filter unit. He had a
tunable RF gadget with a small inbuilt speaker which he plugged
into a wall socket. Radio transmissions and very loud hash could
clearly be heard. Then he plugged the gadget into one of the
filtered outlets. Silence.

So you CAN use the power cable as a RF aerial. Does your audio
equipment do so? Bad design if it does, surely?

Mains filters
 

"Arfa Daily" wrote in message

"Rich Wilson" wrote in message
...

"Glenn Richards" wrote
in message ...
Rich Wilson wrote:

If you're going to upgrade your power cable you really
ought to do it all the way back to the substation,
otherwise it's a bit pointless...

Well that was my initial thought... but apparently not.

In a conversation I had with a friend, we determined
that power cables are good at carrying low frequencies,
and poor at carrying higher frequencies. So RFI picked
up at the substation won't make it to your house, but
interference sources in your house will cause RFI to
reach your kit.

What I want to know is how exactly the RFI gets through
all the smoothing capacitors and so on in the rectifier.
I've got a little headphone amp here with enough
capacitance to not notice, say, a 2-second cutout in its
power supply, so I fail to see how any audible frequency
could get through.
Electrolytic capacitors generally have very poor
characteristics at high frequencies.

Unh, they used to be this way, maybe 20-30 years ago.

This is why you will
often find them bypassed with a small value polyester or
other film capacitor.

See former comments about 20-30 years ago. These days the capacitors may
still go in, but they are there for marketing, not technical purposes.

If you think about it, its trivial to eliminate inductance, other than that
due to lead length, from an electrolytic.

A large electrolytic across a
supply as a smoothing cap will, as you say, happily
supply the amplifier's needs for a couple of seconds, but
as far as RF goes, it may as well not be there.

Ignores the fact that the major trap for RF in a power supply is the power
transformer.

Also, you're confusing RF with audio. A signal is not really
considered to be RF until it reaches 60 or 70 kHz or so.
RF is of course, not directly audible anyway, even if it
gets into your amp, but if it had enough level, it could
conceivably drive early stages into non linearity, in
which case, its effect may become audible.

The most likely way that RF enters power amps is thorugh the input
terminals. Most of the time there is a simple small series resistor and
parallel cap at the input that deals with this issue.

Also, power amps should be able to defend themselves. The amp itself should
have a low-pass response characteristic that extends up to many megahertz.
Above that, the simple input filter should suffice.

In contrast, although you might consider impulse
interference from say a drill motor, to be an audible
frequency, the very fast rise times to high peak
amplitudes, will again bypass any electrolytics, as
though they weren't there, and may then become audible as
' interference '

This discussion about large electrolytics with excess inductance is old,
obsolete news.

Mains filters
 

"Jim Lesurf" wrote in message

In article
, Glenn
Richards wrote:


You can't hear much above 20kHz.

Varies from one person to another, but what you say is
broadly correct so far as we can tell.

But signals at frequencies above this will affect what
you do hear, harmonics, interaction etc.

What evidence do you have for the above? There have been
some claims to this effect, but tests I've seen don't
show much support for the idea.

Why do you think there's been so much work done on
DVD-A, SACD etc, using sample rates of 96kHz or even
192kHz, rather than 44.1?

Various reasons. :-)

Say the word: marketing.

Also lets tell the truth - DVD-A is a failure in the marketplace. Some
record industry executives who hitched their wagons to the DVD-A star were
already fired over a year ago.

SACD is still solderiering bravely but with no market growth. This is a
testimony to Sony's stubborness and deep pockets.

In professional realms - insistance on high sample rates are a sign of an
amateur. It reminds me of the old saying "Amateur soliders worry about
strategy, professionals worry about logistics".

e.g. a concern (a la. Peter Craven) for avoiding unwanted
*in band* time dispersion effects. Moving the bandwidth
up gives more elbow room for ensuring a well-controlled
response in band.

e.g. Making 24-bit-sample recordings makes it easier to
avoid running out of dynamic range by accident, but this
may not matter to the final result.

The major barriers to the as-made, as-used dyanamic range of recordings are
at the ends of the record/playback process. The distribution media is in the
middle.

e.g. Marketing and the need to replace (patents expired)
audio CD with a new format (with new patents) and get
people to re-buy the same material yet again. :-)

The last time through, a low-cost reduced-sound-quality format won the
reissue race: MP3. The moral of the story is that the much-demeaned CD
format was actually overkill.

Mains filters
 

In article , Iain Churches
wrote:

"Jim Lesurf" wrote in message
...

I should also be 'grumpy' at this point. We had water coming through a
ceiling on Monday due to snow melt getting in via a flat roof.
Currently looking out at occasional showers of rain/snow/sleet and
seeing if the large tub behind one speaker is going to get any more
contents...

Had some plastic sheeting tacked over the roof as a temporary measure
this morning, so keep yer fingers crossed that it helps!

Fingers tightly crossed, Jim. We know all about melting snow here. The
very few houses (built in the sixties) that had flat roofs have all been
converted to sloping (22 degrees from the centre)

I don't know the quantity of snow on your roof. Fresh snow here has very
little moisture, and is too powdery to make snowballs, but as it nears
melting point, its weights increases dramatically. This too can be a
problem.

The problem certainly manifested its symptom (leak in via the ceiling -
just 1ft behind an ESL63!) when the snow we'd had melted. However this is
due to the 'melt' being trapped as is gradually emerges from the snow, plus
the rain we were getting that was helping to melt the snow...

However (fingers crossed) the symptom has not re-presented since then, so
am awaiting the 'quote' from the roofing firm...

I used to climb on the roof each summer and check, but in the last 5 years
or so I've not been well enough, and checking this got forgotten.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Mains filters
 

In ,
Arny Krueger typed:

The most likely way that RF enters power amps is thorugh the input
terminals. Most of the time there is a simple small series resistor
and parallel cap at the input that deals with this issue.


RF can also enter via the speaker leads if this isn't taken care of in the
design. A while back I had an amp that would pick up VHF taxi and police
transmissions quite clearly. It even picked up Radio Moscow on occasions
when global MF radio propagation conditions were favourable. Home made
longitudinal chokes on the speaker leads fixed most of this problem.

Jo

Mains filters
 

"Jo" wrote in
message

In ,
Arny Krueger typed:

The most likely way that RF enters power amps is thorugh
the input terminals. Most of the time there is a simple
small series resistor and parallel cap at the input that
deals with this issue.

RF can also enter via the speaker leads if this isn't
taken care of in the design.

I keep hearing about this, but have never seen the problem up front and
personal. I have had RF problems but they were in the front end, not the
back end.

A while back I had an amp
that would pick up VHF taxi and police transmissions
quite clearly.

Brand - model?

It even picked up Radio Moscow on
occasions when global MF radio propagation conditions
were favourable. Home made longitudinal chokes on the
speaker leads fixed most of this problem.

Most power amps already have chokes in series with their output terminals.

Mains filters
 

"Jim Lesurf" wrote in message
...


FWIW If you are worried about 'RF' then you should also worry about
interference which is directly radiated into units. e.g a mobile phone in
the same room as the audio system, coupling in via the speaker leads. A
mains filter will have no effect on this. Hence it is the kind of thing
the
designer should have considered...

Hello Jim. That was to be my next question. In some digital production
suites, the use of mobile phones is strictly forbidden, and the receptionist
takes them from clients as they come in the door.

Is coupling via speaker cables the only way that mobile phones can
affect a domestic system? Do manufacturers take mobile phones into
consideration, and how?

Iain

Mains filters
 

"Arny Krueger" wrote in message
...
"Jo" wrote in
message

In ,
Arny Krueger typed:

The most likely way that RF enters power amps is thorugh
the input terminals. Most of the time there is a simple
small series resistor and parallel cap at the input that
deals with this issue.

RF can also enter via the speaker leads if this isn't
taken care of in the design.

I keep hearing about this, but have never seen the problem up front and
personal. I have had RF problems but they were in the front end, not the
back end.

A while back I had an amp
that would pick up VHF taxi and police transmissions
quite clearly.

Brand - model?

It even picked up Radio Moscow on
occasions when global MF radio propagation conditions
were favourable. Home made longitudinal chokes on the
speaker leads fixed most of this problem.

Most power amps already have chokes in series with their output terminals.

In 1972 I had a friend who lived in South London, half way between Crystal
Palace and Croydon. In those days, there was 405 line TV, which had AM
sound. He had a Ferrograph F307 amplifier which picked up TV sound through
the 'speaker leads and/or through the mains lead, probably both. TV sound
came through at normal listening level regardless of volume control position
and source selected, and indeed, even whether there was a source plugged in
or not. Shorting the inputs made no difference.

We tried ferrite filters in the 'speaker leads, in the mains leads,
capacitors across the mains, and across the 'speakers. Putting the whole
amplifier in a screened box, using coax for the 'speaker leads, (of course,
the leads inside the speakers remained unsceened) nothing worked. After a
few weeks of not being able to use his audio system, my friend moved!

I used to have a home-built power amplifier which ocasionally picked up
Radio Moscow. Couldn't find a reason until one day I put a 200MHz scope on
it. The amp was hooting at around 20MHz, never saw it on my 5M scope.

There are lots of reasons why rf interferes, and getting rid of it is the
very devil.

S.

Mains filters
 

In article ,
Iain Churches wrote:
Hello Jim. That was to be my next question. In some digital production
suites, the use of mobile phones is strictly forbidden, and the
receptionist takes them from clients as they come in the door.

Not just those. I was using a new Calrec analogue desk a few years ago and
my mobile (clipped to my waist) broke through loud and clear on to it when
simply making its mating call to the base station. Also get this if within
a few feet of some mics.

--
*When the going gets tough, the tough take a coffee break *

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

Mains filters
 

In ,
Arny Krueger typed:

I keep hearing about this, but have never seen the problem up front
and personal. I have had RF problems but they were in the front end,
not the back end.

A while back I had an amp
that would pick up VHF taxi and police transmissions
quite clearly.

Brand - model?


Texan, circa mid-1970s

It even picked up Radio Moscow on
occasions when global MF radio propagation conditions
were favourable. Home made longitudinal chokes on the
speaker leads fixed most of this problem.

Most power amps already have chokes in series with their output
terminals.
As they should. The Texan didn't.

Jo