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.

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.

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

"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

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

"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

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

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

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

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