Mains filters
 

In article ,
says...

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

Brand - model?


Texan, circa mid-1970s

Blimey, I remember building one of those from a 'Henrys' kit..
Beefed up with a large torrodal transformer and higher power transistors
following an article I read in a mag. I imagine it sounded horrendous
but it was certainly the loudest thing on our corridor in Hall.

Allegedly designed in the bar at a conference by a bunch of Texas
Instruments engineers. Probably apochrophal though; sadly..

T.

Mains filters
 

Hi,

"Dave Plowman (News)" wrote in message
...
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.

I saw this a lot from active PC speakers. When I worked in a support office
for Matrox everyone had video editing kit and audio gear around the
desks. The GSM phones would come through the PC speakers in a big
way if you left them on the desks.

Regards,

Glenn.

Mains filters
 

In article , Arny Krueger
writes
"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.

It does happen, in fact it happens to the PC speakers that are hooked to
this PC when the mobile starts to ring!. Just uses the aerials to take
RF into the very RF susceptible amp in those...


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


Well TAXIS are FM these days and you wont hear them, you might get a bit
of hum.

Old bill is fast going off to Airwave TETRA and you will not hear him at
all!...

--
Tony Sayer

Mains filters
 

"Tony Gartshore" wrote in message
...
In article ,
says...

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

Brand - model?


Texan, circa mid-1970s

Blimey, I remember building one of those from a 'Henrys' kit..
Beefed up with a large torrodal transformer and higher power transistors
following an article I read in a mag. I imagine it sounded horrendous
but it was certainly the loudest thing on our corridor in Hall.

Allegedly designed in the bar at a conference by a bunch of Texas
Instruments engineers. Probably apochrophal though; sadly..

T.

As I recall, it was originally designed as an article for Practically
Witless ( or was that Practical Wireless ? ) magazine. It *was* designed by
a bunch of Texas engineers at Bedford, but I'm not so sure about the " in
the bar " bit. Several of us built them from Henry's Radio kits, and I think
that for it's day, it represented pretty much state of the art for what any
of us could afford. I seem to remember that its specifications were beyond
anything that we had ever heard. A friend of mine that built one, did that
power upgrade to his, and used it to drive his quad electrostatics, and I'm
sure I recall it sounding superb. I built a 'standard' version, and used it
to drive my EMI 13 x 8's, with 6 x 4 mids ( remember those - they came as a
pair ) rounded off with an Eagle duralumin dome tweeter, and Eagle 3 way
crossover. At the time, I thought it was the dogs ...

Arfa

Mains filters
 

In article , Serge Auckland
writes

"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!

The only thing you need to have done is stick a few hundred PF across
the base emitter junctions of the input stage transistors and thats what
I had to do to lotsa equipment many years ago, when that bloody AM cb
craze was all the go!..

Those junctions are very good demodulators!....

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.

Beg to differ guv!, but RF is a black art and should not be spoken of in
these forums, its much too high for the likes of uk.rec.audio;-).

Takes quite some understanding and its much best left to only those who
"need to know"..know..nuff said;-)..

--
Tony Sayer

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.


Arny
Ask any licensed radio amateur ( at least any one that got his license
before they started giving them away in cornflake packets ) about RF pickup
on speaker leads. It's a well known problem ...

See for instance

http://www.antennex.com/shack/Dec99/beads.htm

look towards the bottom under the heading " Stereo "

Arfa

Mains filters
 

In article , Jim Lesurf jcgl@st-
and.demon.co.uk writes
In article , Rich Wilson
wrote:


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.

Alas, if only it were that easy. :-)

Two examples of snags the designer should have dealt with:

1) Large electolytic caps (as normally used as reservoirs in a linear PSU)
will have self inductance and resistance that means they stop acting as a
capacitor - probably somewhere below 10MHz. Hence for RF 10MHz they may
do nowt.

2) The RF may enter via leads, then radiate around inside the box, neatly
bypassing any filtering. :-)

In each case the RF "might" then be rectified by a device inside the amp,
or combine nonlinearly to down-convert into the audible range.

Its almost always caused by unwanted demodulation, and that can also
cause the DC voltages to shift causing further upset!...


The point, though, is that the designer should know about such things, and
know how to deal with them. The normal result being no audible effects to
bother the user. This was hard many years ago, but devices and techniques
developed, so that even 20 years ago, it was largely tractable.

It was..


Slainte,

Jim


--
Tony Sayer

Mains filters
 

In article , Iain Churches
writes

"tony sayer" wrote in message
...

You asked me to post details of the unit which I had seen.
To meet your request I did this, without an opinion or an
interest eith for or against.

Don't suppose you have;)


Since then I have been able to find out the sales figs for
the Scandi countries. I am told the highest sales within the EU
are to the UK. There is a "huge" mark-up. So someone is making
a pile of cash. From your bitterness, I deduce it is not you:-))


Me bitter?, nah!, wish I was as bent as some people who sell/ supply
these things but I can't bull**** like they do and prey on the gullible.

Do you think that the above statement "filtering the mains to make your
TV picture sharper" to would stand scrutiny with sensible double blind
testing, or even the trades descriptions act?..

Good point. In Finland and Sweden we have a "fitness of goods" act
which is much tighter than the trades descriptions act in the UK. Yet
these units are on sale here too.

Look at it this way Tony. If you are planning to market a product you
first need to find out what consumers (think they) need. If they come
beating a path to your door, asking for mega mains cables at E850
then what do you give them? Yes, you've got it....... It would be
churlish to refuse:-).

People seem to think they want mains filters, and the couple of
phone calls that I made this afternoon convinced me that the demand
is certainly there (particularly in the UK!!) so a smart manufacturer
gives the punters what they (think they) want.

Yep guess your right.. Sometimes wonder if I ought do the same thing
con the gullible!....
--
Tony Sayer

Mains filters
 

In article , Jim Lesurf jcgl@st-
and.demon.co.uk writes
In article , Iain Churches
wrote:

Hello Glenn. 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.

Yep perhaps thee was a lot of capacitance at the input of his AM radio
receiver..

Smoke and mirrors came to mind......

May be a very useful device if you happen to have such an RF
detector/demodulator/amp connected to the mains and you want to cut down
the noise it makes without bothering to unplug it. ;-

Jeezzzzz.........

The unit was about 4U in height, and had ten mains outlets on the rear
panel.

I was there for only a few seconds, (car parked on a yellow line) The
dealer has promised me some literature.

The gain, etc, of the device used to produce audible noises from the
'mains' might help.. . :-)

AIUI though, most decent audio equipment isn't actually designed
specifically to demodulate and amplify mains interference. Perhaps he
avoided demoing with audio equipment as the main (pun) result might have
been to warn people not to buy that specific item of equipment. :-)

Oh, boy....

Slainte,

Jim


--
Tony Sayer

Mains filters
 

In article , Iain Churches
writes

"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.

Not a bad idea for more reasons than one!..

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?

All modern gear is suppose to be immune to external RF fields and by and
large its very, very much better then it used to be due top better
design and awareness of the effects of unwanted RF pickup.

Speaker cables are one source of RF into amps but mains leads and input
cables will suffice.....

Iain




--
Tony Sayer

Mains filters
 

In ,
Arfa Daily typed:

As I recall, it was originally designed as an article for Practically
Witless ( or was that Practical Wireless ? ) magazine. It *was*
designed by a bunch of Texas engineers at Bedford, but I'm not so
sure about the " in the bar " bit. Several of us built them from
Henry's Radio kits, and I think that for it's day, it represented
pretty much state of the art for what any of us could afford. I seem
to remember that its specifications were beyond anything that we had
ever heard. A friend of mine that built one, did that power upgrade
to his, and used it to drive his quad electrostatics, and I'm sure I
recall it sounding superb. I built a 'standard' version, and used it
to drive my EMI 13 x 8's, with 6 x 4 mids ( remember those - they
came as a pair ) rounded off with an Eagle duralumin dome tweeter,
and Eagle 3 way crossover. At the time, I thought it was the dogs ...

Yes, I built my own. Drove to Bedford and bought a kit from the original
manufacturers. Freq response, THD and stuff were leading edge at the time
and it sounded great to me through my Wharfedales (which have only just
expired). My Texan still works and the only problems were worn out volume
controls which I changed around 1995. Design was quite innovative with high
slew-rate op-amps at the front end.

Jo

Mains filters
 

"tony sayer" wrote in message
...
In article , Iain Churches
writes

People seem to think they want mains filters, and the couple of
phone calls that I made this afternoon convinced me that the demand
is certainly there (particularly in the UK!!) so a smart manufacturer
gives the punters what they (think they) want.

Yep guess your right.. Sometimes wonder if I ought do the same thing
con the gullible!....
--



Don't set yoursef up as judge and jury, Tony.
People know what they think they want, so if
you are smart you will get that soldering iron
out, and give it to them:-)

Iain

Mains filters
 

"tony sayer" wrote in message
...
In article , Iain Churches
writes

"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.

Not a bad idea for more reasons than one!..


The first time I came across this was after someone had used a mobile
while a digital backup was being made to Exabite. The streamer failed
to load it the next day. The tape was sent to the workstation manufacturer
who pronounced that it had been rendered unreadable by GSM traffic.

Iain

Mains filters
 

In article , Iain Churches
wrote:

"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.

Precautionary principle. There may be a similar 'ban' in some hospitals,
etc, to avoid the risk of interference with instrumentation.

FWIW The Physics dept which I still give occasional lectures has had a ban
on the use of mobile phones on the premises for some years. Main reason
being the risk that delicate measurements with cobbled-together labgear may
be ruined. Also to stop idiot undergrads having their phone sound off
during a lecture :-) ... or even during an *exam* as I have encountered on
one occasion when invigilating.

Following the above incident I took to including in my preamble when chief
invigilator a warning that any phone that sounded during the exam would be
removed from the student and flung out the window as hard and as far as
possible. Perhaps this should become part of the exam rubric... :-)


Is coupling via speaker cables the only way that mobile phones can
affect a domestic system?

In principle, it can enter via any wiring or any gaps in metal casework.

Do manufacturers take mobile phones into consideration, and how?

I assume it would vary with the resources of the makers in question.
However it is easy enough these days to simply try using a mobile right
next to the unit and seeing what effect (if any) it has upon it. There are,
I think, some EU 'rules' about this nowdays - which prompted some makers to
fit filters and then encourage users to snip them out as they 'degraded
performance'. sigh However as I'm not in the biz anymore, I don't know
the current details I'm afraid.

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 article , Jo
wrote:
In , Arny Krueger
typed:


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

A snag here is that some designers feel that an output choke increases the
HF o/p impedance and degrades the sound. Classic example being the older
Naim power amp designs which just used a 0.22 Ohm resistor...

.... and then may require about 10microH of lead inductance to be stable.
:-)

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 article , tony sayer

wrote:



Well TAXIS are FM these days and you wont hear them, you might get a bit
of hum.

Old bill is fast going off to Airwave TETRA and you will not hear him at
all!...

You may do,,, ;- The TETRA mobiles are pulsed, so may produce AM
envelope demod of the data pulses if they find enough nonlinearity in a
suitable place. However unless you are worried about a police raid, the
powers reaching your audio gear should be well below what you get when
someone uses a mobile phone in the room.

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
 

"Iain Churches" wrote in message
...

"tony sayer" wrote in message
...
In article , Iain Churches
writes

"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.

Not a bad idea for more reasons than one!..


The first time I came across this was after someone had used a mobile
while a digital backup was being made to Exabite. The streamer failed
to load it the next day. The tape was sent to the workstation
manufacturer
who pronounced that it had been rendered unreadable by GSM traffic.

Iain




When I was presenting regular live broadcasts on ILR, the station required
that studio guests had their mobiles switched off. Not only interviewees and
muso's but the folk who legitimately get past the glass for the 'thrill' of
on-air sightseeing ;-) Its enough describing the studio to them in music
breaks without the mating calls of mobiles (Analogue desk then). At least
we had no problems with that kind of interference and of course observers
obeyed the red light.... except for the one not so memorable school kid who
broke wind loudly...

Mike

Mains filters
 

"Mike Gilmour" wrote in message
...

"Iain Churches" wrote in message
...

"tony sayer" wrote in message
...
In article , Iain Churches
writes

"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.

Not a bad idea for more reasons than one!..


The first time I came across this was after someone had used a mobile
while a digital backup was being made to Exabite. The streamer failed
to load it the next day. The tape was sent to the workstation
manufacturer
who pronounced that it had been rendered unreadable by GSM traffic.

Iain




When I was presenting regular live broadcasts on ILR, the station required
that studio guests had their mobiles switched off. Not only interviewees
and muso's but the folk who legitimately get past the glass for the
'thrill' of on-air sightseeing ;-) Its enough describing the studio to
them in music breaks without the mating calls of mobiles (Analogue desk
then). At least we had no problems with that kind of interference and of
course observers obeyed the red light.... except for the one not so
memorable school kid who broke wind loudly...

Mike

You weren't in " The Tripods " by any chance ?

Arfa

Mains filters
 

"Arfa Daily" wrote in message
...

"Mike Gilmour" wrote in message
...

"Iain Churches" wrote in message
...

"tony sayer" wrote in message
...
In article , Iain Churches
writes

"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.

Not a bad idea for more reasons than one!..


The first time I came across this was after someone had used a mobile
while a digital backup was being made to Exabite. The streamer failed
to load it the next day. The tape was sent to the workstation
manufacturer
who pronounced that it had been rendered unreadable by GSM traffic.

Iain




When I was presenting regular live broadcasts on ILR, the station
required that studio guests had their mobiles switched off. Not only
interviewees and muso's but the folk who legitimately get past the glass
for the 'thrill' of on-air sightseeing ;-) Its enough describing the
studio to them in music breaks without the mating calls of mobiles
(Analogue desk then). At least we had no problems with that kind of
interference and of course observers obeyed the red light.... except for
the one not so memorable school kid who broke wind loudly...

Mike

You weren't in " The Tripods " by any chance ?

Arfa


Sorry thats not me. The part of Jack in 'The Tripods' was played by
another Michael Gilmour. I've got the perfect face for radio ;-)

Mike

Mains filters
 

In article , Jim Lesurf jcgl@st-
and.demon.co.uk writes
In article , tony sayer

wrote:



Well TAXIS are FM these days and you wont hear them, you might get a bit
of hum.

Old bill is fast going off to Airwave TETRA and you will not hear him at
all!...

You may do,,, ;- The TETRA mobiles are pulsed, so may produce AM
envelope demod of the data pulses if they find enough nonlinearity in a
suitable place. However unless you are worried about a police raid, the
powers reaching your audio gear should be well below what you get when
someone uses a mobile phone in the room.

If U reckon U can decode the TETRA old bills using Jim, then there would
be a lot of people very interested in that!....

--
Tony Sayer

Mains filters
 

In ,
Jim Lesurf typed:
In article , Jo
wrote:
In , Arny Krueger
typed:


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

A snag here is that some designers feel that an output choke
increases the HF o/p impedance and degrades the sound. Classic
example being the older Naim power amp designs which just used a 0.22
Ohm resistor...

... and then may require about 10microH of lead inductance to be
stable. :-)


Yes, this brings us back into the topic of "but can humans really hear the
difference ?" that occupies many threads. I wonder how much extra impedance
is added to the output circuit at audio frequencies by the addition of RF
chokes ? The formula for inductive reactance is simple and for a 100uH choke
I get:

XL=1.3K at 2MHz and 13 Ohms at 20KHz which would seem to be a significant
contribution to the impedance of an amp output circuit...

....BUT if a decent common mode type bifilar wound filter choke is used the
effective inductance in the speaker circuit would be much less than this and
so any degradation of the audio signal would be negligible....or would it
:-)

Jo

Mains filters
 

In article , tony sayer

wrote:
In article , Jim Lesurf jcgl@st-
and.demon.co.uk writes


Well TAXIS are FM these days and you wont hear them, you might get a
bit of hum.

Old bill is fast going off to Airwave TETRA and you will not hear him
at all!...

You may do,,, ;- The TETRA mobiles are pulsed, so may produce AM
envelope demod of the data pulses if they find enough nonlinearity in a
suitable place.

If U reckon U can decode the TETRA old bills using Jim, then there would
be a lot of people very interested in that!....

Afraid I can't as I don't have the details of the encoding. But that isn't
required simply to detect the pulse power envelope. :-)

Indeed, you may recall some 'health scares' about TETRA that were fuelled
by its pulse profile - although those who panicked at the time had the
details wrong so were worrying about nothing much. If interested, have
a look at

http://www.st-and.demon.co.uk/temp/tetra.html

Note that although the base stations probably don't normally pulse, the
mobiles probably do, both to save energy and to reduce slot conflicts.
The point of the above page was to show that 'health' worries about
the *base stations* was probably groundless. Locally, we had all kinds
of stories about 'Cows not giving milk', etc, when TETRA bases were
installed - in fact before they came into actual acive use. :-)

A distinction to be made is one that (surprisingly) in my experience
often escapes people in the military, etc, who deal with encrypted or
spread spectrum comms. The payload data may be unreadable, but the actual
signals are easily detected, and often easily indentified in terms of class
of system. Alas, it seems easy for people to assume that the information
being encrypted as being the same as the actual transmission being
undetectable. Tain't so.

For the above reason you could, if you wished, easily detect when the Bill
are transmitting and even say where from... Technically, not particularly
difficult. Indeed, I've done this sort of thing for other (legal) reasons
in the past... :-) Knowing what they are saying would be more
difficult.

For similar reasons, locating, jamming, etc, of mobiles is much easier than
actually listening to the message payload/content.

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
 

Serge Auckland wrote:

He had a Ferrograph F307 amplifier...

Wow! I've got one of those sat up in the loft... picked it up for about
a fiver at a car boot sale many years ago (came with an AM/FM tuner and
a set of rather ropey speakers).

Sounded lovely, until the PSU blew up... smoothing caps went and took
out one of the bridge rectifiers. Managed to salvage a couple of much
newer caps, same value, similar size, from a dead Cyrus amp (was
terminal, beyond economic repair).

Apart from the crackly volume pots it's still working fine. Lovely bit
of kit, that was.

--
Glenn Richards Tel: (01453) 845735
Squirrel Solutions http://www.squirrelsolutions.co.uk/

IT consultancy, hardware and software support, broadband installation

Mains filters
 

Stewart Pinkerton wrote:

If your opinion is that filtering won't make a difference, state your
opinion, avoid personal insults, and let civilised debate commence.
It won't, if your gear has decent RFI rejection, as it certainly
should. Moral of the tale - avoid 'high end' gear from back street
companies with no real engineering ability.

I'd hardly call Arcam a "back street company"...

[usual tirade of personal insults snipped]

Anyway, been doing some testing today with interesting results. Will
write up a summary and post when I've got a bit more time to spare...

--
Glenn Richards Tel: (01453) 845735
Squirrel Solutions http://www.squirrelsolutions.co.uk/

IT consultancy, hardware and software support, broadband installation

Mains filters
 

"Jim Lesurf" wrote in message
...
In article , Iain Churches
wrote:

Hello Glenn. 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.

May be a very useful device if you happen to have such an RF
detector/demodulator/amp connected to the mains and you want to cut down
the noise it makes without bothering to unplug it. ;-

The unit was about 4U in height, and had ten mains outlets on the rear
panel.

I was there for only a few seconds, (car parked on a yellow line) The
dealer has promised me some literature.

The gain, etc, of the device used to produce audible noises from the
'mains' might help.. . :-)

AIUI though, most decent audio equipment isn't actually designed
specifically to demodulate and amplify mains interference. Perhaps he
avoided demoing with audio equipment as the main (pun) result might have
been to warn people not to buy that specific item of equipment. :-)



I think we had better not pursue this line, Jim. The power amp
demonstrated was a Krell which was being compared with a
Conrad Johnson. He phoned me this morning to say he took two
orders for the CJ.

Regards to all
Iain

Mains filters
 

Thus spake Jim Lesurf:
Snipped
Indeed, you may recall some 'health scares' about TETRA that were
fuelled by its pulse profile - although those who panicked at the
time had the details wrong so were worrying about nothing much. If
interested, have
a look at

http://www.st-and.demon.co.uk/temp/tetra.html

Note that although the base stations probably don't normally pulse,
the mobiles probably do, both to save energy and to reduce slot
conflicts.
The point of the above page was to show that 'health' worries about
the *base stations* was probably groundless. Locally, we had all kinds
of stories about 'Cows not giving milk', etc, when TETRA bases were
installed - in fact before they came into actual acive use. :-)

A friend was using Tetra radios as a constable on his beat in Skegness more
or less from launch. He reported no personal health issues but related
several anecdotal stories of scrambled brains. IIRC, the poorest aspect was
the pass-through onto the mobile network wasn't brilliant therefore was
easier to use his mobile.

A distinction to be made is one that (surprisingly) in my experience
often escapes people in the military, etc, who deal with encrypted or
spread spectrum comms. The payload data may be unreadable, but the
actual signals are easily detected, and often easily indentified in
terms of class of system. Alas, it seems easy for people to assume
that the information being encrypted as being the same as the actual
transmission being undetectable. Tain't so.

For the above reason you could, if you wished, easily detect when the
Bill are transmitting and even say where from... Technically, not
particularly difficult. Indeed, I've done this sort of thing for
other (legal) reasons in the past... :-) Knowing what they are
saying would be more difficult.

For similar reasons, locating, jamming, etc, of mobiles is much
easier than actually listening to the message payload/content.

The current use of the radio spectrum of allocating certain frequencies to
particular purposes makes it a no-brainer to know if that band is in use.
However, if UWB/SDR/Cognitive Radio ever reaches the ether, it will be
virtually impossible or very difficult to pull out military, emergency,
mobile, TV etc, etc from one another, ever before one can even contemplate
decrypting anything. If CR ever happens, it will be incremental, probably
with holes all through the spectrum for non-hopping radio. I can't see a
future humble radio hopping from DC to light either - just too expensive for
the intended purpose. Propagation characteristics are hardly even from DC to
light either. It will certainly make the whole spectrum a hell of a lot more
efficient, once regulators & the military can grasp the concept of loosing
fixed bands but retaining priority of use, security etc

--
Basically, I hate people who preface nearly every sentence with the word
'basically'!

Mains filters
 

Thus spake Arny Krueger:
Snipped
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.

Yup. The "you've never had it so good!" era of CDs (yes, I know this is
disputed by a few) maybe a thing of the past where we all have to put up
with 128k MP3s as the de facto standard & take a reduction in quality that
99% of consumers will be perfectly happy with. As for DVD-A or SACD, I've
never seen them in a retail outlet - not that I frequent them that often as
I prefer not to get ripped off. As for Blu-Ray or equivalents - region
coding for music anyone?

My Rotel will play HDCD discs but finding anything I want to actually listen
too is the challenge. I guess at the licensing fee being to high - better to
make a little money often than too much rarely. Goes to show that there's
still room for capitalists with brains to understand their chosen market.

As for the lost opportunity that digitised music could have brought to the
music industry...