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Re: Getting matching transformer from telephone

Discussion in 'Electrical Engineering' started by Tomi Holger Engdahl, Dec 29, 2008.

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  1. Ross Herbert

    Ross Herbert Guest

    On Fri, 02 Jan 2009 22:54:04 +0000, Eeyore

    :
    :
    :Ross Herbert wrote:
    :
    :> Since your PC is mains powered and it may not have the required isolation
    :> between the mains side and the sound card input you can do your own thing
    using
    :> an approved 600:600 transformer with 3kV isolation rating to interface the
    :> telephone line to the sound card input.
    :
    :He DOES NOT need a 600 ohm transformer since the input impedance of the sound
    card
    :is not 600 ohms <sigh> !

    Since the application is merely detecting signal "voltage" it hardly matters
    that the secondary impedance of the transformer is 600 ohms and the input
    impedance of the sound card is more like 10Kohms. The only reason one tries to
    match impedances is where one needs to maximise "power transfer" and that
    doesn't apply in this case.


    :
    :It's more likely to be in the tens of kilohms.
    :
    :What he really wants is something like a 10k:10k 'line bridging' transformer.
    In
    :practice, using a '600 ohm' transformer will probably be ok, but being
    incorrectly
    :loaded will degrade the sound quality (freq resonse may be peaky etc).
    :
    :Graham
    :

    Not true!
     
  2. Phil Allison

    Phil Allison Guest

    "Ross Herbert"

    ** Fact is, it will be an advantage as the load across the phone line
    needs to be high as possible so the level there is not affected.

    ** Nonsense.

    Audio and other wide band transformers have rated ( ie optimum) source and
    load impedances.

    This relates *directly* to them both operating efficiently AND exhibiting
    their specified frequency response curve.

    Eg, a 600:600 ohm type operating into 10 kohms may well have a large
    response peak at high frequencies while a 10k:10k type operating into 600
    ohms will likely have serious roll off from a few kHz upwards.

    With transformers that have 1:10 or 1:20 step-up ratios for mic-input to a
    valve stage, both load and source impedances become quite critical just to
    stay within a +/- 2dB corridor across the audio band.



    ...... Phil
     
  3. Thanks Stuart - this fits exactly with my memories of the time. It was a
    while back though ......
    The BBC now has its own digital audio distribution systems, and no
    longer requires the widespread use of BT programme circuits.

    To reiterate for those who have generated all the heat and the
    fabulously inaccurate rantings - The programme circuits were BASEBAND
    AUDIO all the way - amplified en route. As Stuart quotes - carrier is
    possible, but provides worse noise and distortion standards.

    Carrier was not generally used for main UK programme distribution (sorry
    Floyd - you may be right about American practice, but not UK).

    John
     
  4. SoothSayer

    SoothSayer Guest

    Now that was the right, properly worded response to Floyd. Good job.

    I wonder where this places the retarded donkey's assertions, if he ever
    actually made any, other than his rantings. He did claim to have a
    fairly deep knowledge... that he had a schematic.... somewhere.

    It would be nice to see your document posted up in
    alt.binaries.schematics.electronic, if, that is, you have access to it.
     

  5. Fucking environmental retard! Do you do the stupid twit act of
    printing out your e-mails as well, you stupid ****?
     
  6. Indicative of piss poor grasp of modern computer technology, not to
    mention quite lame document handling capacity. You don't know how to add
    a highlight to a doc on screen? Perhaps you need a mini NASCAR scrolling
    bug screen you can scroll with your document, dipshit?

    Bwuahahahahahah!
     
  7. Roy

    Roy Guest

    From: (Archimedes' Lever)
    Not all telephone cable can run ADSL.
    In civilised parts of the world with proper infrastructure it can.
    Graham
        In the right situation, someone might actually consider you
    as credible too. Just not in the reality that we all live in. We call it
    the real world. In it, you and credibility do not homogenize together.
    That is one water your oily ass won't mix with.
    -----------------------
    You truly are deluded, if this is such a real world to you, why do you
    hide to insult and malign other peoples posts....There is no Documented
    Engineering License Granted to an Archimedes' Lever in the Real World.,
    Is it because you are a spineless coward?

    Yeah, that's got to be it., because you're so illustrious, upright &
    eductaed., in your very own private little real world...

    Surely you can Back Up your little Trolls with your Real Name and
    Title.What's up? Scared you might get burned?

    Roy Q.T.
    [have tools, will travel]
     
  8. Roy

    Roy Guest

    From: (Archimedes' Lever)
    Stuart wrote:
            Eeyore <>
    wrote:
    I have never seen a transformer in ANY phone including the carbon mic
    type which also 'draws power from the line' to power the mic as you say.
    How do the clowns think they determine ON and OFF HOOK ?
    Graham
    Oh dear, you need educating.
    http://www.telephonesuk.co.uk/images/232_cct.jpg
    What do you think "Coil,Induction No20" is?
    I'm afraid I wasn't around in 1935. 73 yr old technology is somewhat
    irrelevant.
    The definition of on/off hook supervision has not changed, and that
    makes it exceedingly relevant.
    These days a phone is a chip and a few passives (that *aren't* big
    inductors).
    Not necessarily. Some do have transformers.
    Such as ?
        Is it really so goddamned hard to accept the fact that such
    items do exist?
        Probably just as hard it is to accept the fact that you are
    a self denying, US hating retard.
    ---------
    You don't speak for the US, Troll - He just hates You! but most of US
    here just Loathe you., you usenet scoundrel.

    Roy Q.T.
    [have tools, will travel]
     

  9. Post yours, asswipe.

    Maybe one day, you will understand the concept of a nym.

    I doubt it, however, since dopes like you are the reason that civil
    folks like me remains anonymous. I piss too many of you totally retarded
    fucks off.

    So no, RoyFag, you will not ever be getting my real name, despite
    claiming that you would "be revealing it".

    If you do ever post my real name, you will instantly get charged for
    every threat you ever made. You will place the lock on the gate with
    that stupid move. So go ahead, Roytard. Pull more of your retarded
    horseshit.
     
  10. More than a twit like you ever could.
    No, Roy. He hates you too.
    You do not speak for any other poster here, RoyFag. You cannot even
    back up your own bullshit, so I am sure that you have no grasp on anyone
    else's position. Most of them certainly do think of you as a stupid,
    know nothing dope, however.


    So, RoyFag. You have no clue about the donkey's hatred of the US, so
    you have no place telling anyone what you think he does or does not hate.

    You jump into threads 100% uninformed on a regular basis, Roy. That is
    why we KNOW that you are NOT an engineer, you stupid ****.
     
  11. Roy

    Roy Guest

    You're day will come you Trolling Bastard!
     
  12. Paul B

    Paul B Guest


    ** That is entirely false.

    What the OP *actually asked* was this:

    " If I strip down some landline phones I 've got here, then
    will there be a matching transformer in each one? "

    Chances are high there are no iron core transformers ( hybrid or
    other) in his phones at all - cos they are too modern.

    Secondly, his purpose requires a transformer with high voltage
    safety isolation from the phone line - which the common
    600:600 ohms phone line types all have and others do not.

    Thirdly, his purpose requires a transformer with very high CMRR
    - cos that is why he is getting humming noises at the moment
    when making recordings with his passive ( ie non-transformer )
    adaptor device.

    Have you ever done this ??

    I can assure you it ain't as simple as you suspect to get a hum
    free and distortion free result.

    Not the least of the problems is that portable voice recorders
    have high sensitivity mic inputs with automatic gain control
    circuits - which cannot be defeated by the user. This results
    in annoying gain pumping and regular bursts of overload
    distortion PLUS increasing background noise & hum whenever
    there is a pause in the conversation.

    The trick is to use just the right amount of resistive divider
    attenuation before and after the 1:1 transformer to eliminate
    this - plus cap couple the input side to stop pulling the
    line low.

    ..... Phil
    [/QUOTE]


    Hi Phil and others. I am the original poster. Thank you to everyone
    who has kindly contributed to this discussion.

    Please don't overlook some details I posted at:
    http://groups.google.com/group/uk.telecom/msg/4513d4edcdc47c3e

    You are quite right about high sensitivity mic inputs and automatic
    gain controls. As you can imagine, when the phone's handset is
    picked up and used, the mic input is faced with something approaching
    line levels and I get severe overload of my recorder. Then when it
    goes quiet, the AGC kicks in. It's as you describe.

    I have tried to "listen through" any such automatic gain as best as I
    could and I feel there is still detect much more hum than I would
    expect on the line.

    In truth, I really need a more suitable flash recorder but they are
    harder to find than I expected.

    Currently I use the Olympus WS-331M.
    <http://www.olympus.co.uk/consumer/2581_WS-331M.htm>

    Cost is important to me so I don't want to fork out for a digital
    field recorder like this low-end one especially as some reviews have
    not been kind to it. <http://www.dv247.com/invt/41901/>.

    Prices in the flash-memory recorder market are tumbling such that now
    personal video recorders (like the Archos AV400) are about the same
    price and offer some additional very useful function. Although i
    don't know if that sort of model has suitable input and output
    sockets.

    On the other hand, I don't want some MP3 player with a recorder
    thrown in as an afterthought because ones I have seen can have
    surprisingly limited bit rates and low quality. Anyway, many
    probably don't have a line level input, so I will once again most
    likely get problems with a mic input and AGC.

    I take your (and other posters') points about a phone probably not
    having a suitable transformer. My headache from trying to make sense
    of some of the more technical aspects in this thread suggests it
    might be easier in the end to go and buy some 600:600 transformers.

    I'm not too clear about this "CMRR" you mention. I know next to
    nothing about "common mode rejection ratio" although I did come
    across the concept when looking at balanced inputs with chunky XLR
    connectors. I googled it and got the following PDF but in the end I
    couldn't really understand anything in it about CMRR. Do I really
    need to consider CMRR if I am going to be making a recording from a
    phone line?

    <http://www.jensentransformers.com/an/Audio Transformers%
    20Chapter.pdf>

    One new line of thinking I would like to bounce off you and the other
    kind contributors to this thread is whether it is possible in my case
    to use a simple mixer (like those a pop group might use). If I can
    avoid those 3-wire balanced inputs on such a mixer and use just the
    2-wire inputs then would the mixer's op amps and other electronics
    have a suitable isolating effect? The price of such a mixer seems
    very competitive. I am thinking of a unit similar to this one:

    Behringer XENYX 502 (£32) http://www.dv247.com/invt/31558/

    In turn, that leads me to consider if I should attempt to get a
    separate a analogue-to-digital converter which goes into the PC via
    the USB. That way I hope to eliminate earth loop hum and other
    spurious noises which I mentioned in my first post.

    Maybe something like the Behringer UCA202 for £20
    http://www.dv247.com/invt/32730/? To be honest I have no idea if
    this would work but it seems to be good value.

    I'm sorry to be darting about with various options but I didn't know
    it was going to be so complicated!

    Any views and useful info from anyone would be most welcome.

    Andy.
     
  13. Ross Herbert

    Ross Herbert Guest

    On Sat, 03 Jan 2009 22:51:18 -0900, (Floyd L. Davidson) wrote:

    :>"Ross Herbert"
    :>
    :>>
    :>> Since the application is merely detecting signal "voltage" it hardly
    :>> matters
    :>> that the secondary impedance of the transformer is 600 ohms and the input
    :>> impedance of the sound card is more like 10Kohms.
    :>
    :>** Fact is, it will be an advantage as the load across the phone line
    :>needs to be high as possible so the level there is not affected.
    :
    :No, it merely needs to be "high enough", not "high as
    :possible". If the bridging device has ten times the
    :circuit impedance, there will be a 0.3 dB drop in levels
    :eek:n the circuit, and a 30 dB loss across the bridging
    :device.
    :
    :That is considered satisfactory for virually all
    :applications. In this instance it would probably be
    :satisfactory if the bridging impedance where half that.
    :
    :Regardless, you 10k Ohm value is absurd.
    :
    :>> The only reason one tries to
    :>> match impedances is where one needs to maximise "power transfer" and that
    :>> doesn't apply in this case.
    :>
    :>** Nonsense.
    :>
    :>Audio and other wide band transformers have rated ( ie optimum) source and
    :>load impedances.
    :>
    :>This relates *directly* to them both operating efficiently AND exhibiting
    :>their specified frequency response curve.
    :>
    :>Eg, a 600:600 ohm type operating into 10 kohms may well have a large
    :>response peak at high frequencies while a 10k:10k type operating into 600
    :>ohms will likely have serious roll off from a few kHz upwards.
    :
    :So read what you just wrote, and tell us how that affects a telecom
    :circuit that is specified at 400-2800 kHz?

    Should that not be Hz?

    If so, voice frequency circuits were all 300 - 3400Hz in my day.

    Phil is just being a bit too specific. My remarks were applicable specifically
    to the VF telephony freq range, which is the area of interest to the OP.

    :
    :>With transformers that have 1:10 or 1:20 step-up ratios for mic-input to a
    :>valve stage, both load and source impedances become quite critical just to
    :>stay within a +/- 2dB corridor across the audio band.
    :
    :At perhaps 15-20 kHz. So just who cares, in this case?

    Exactly!
     
  14. Phil Allison

    Phil Allison Guest

    "Paul Bullshit ARTIST "


    ** Hey pal.

    YOU already know what to do ( ie use a proper 600:600 ohms ISOLATION
    transformer or a device incorporating same) ) and are just objecting to a
    price of a few pounds.

    Make you a fucking PITA wanker.

    **** OFF.


    ** Who cares ?

    YOU already know what to do ( ie use a proper 600:600 ohms ISOLATION
    transformer or a device incorporating same) ) and are just objecting to a
    price of a few pounds.

    Make you a fucking PITA wanker.

    **** OFF.


    ** I bet this fuckhead is up to something that is highly illegal !!!!

    Secretly recording a phone conversations that YOU are NOT one party to is
    a crime in most places - including the UK.

    Plus, even then, the recording MUST NOT BE used for ANY purpose other
    than protecting your own legitimate interests.



    ...... Phil
     
  15. Eeyore

    Eeyore Guest

    Untrue. A non-optimally loaded audio transformer will not have a flat frequency
    response. Nor is it about power transfer.

    Graham
     
  16. Eeyore

    Eeyore Guest

    It's the standard 'bridging' value.

    Graham
     
  17. Eeyore

    Eeyore Guest

    So, your argument is "Let's do it wrong because it really doesn't matter".
    Pathetic.

    Graham
     
  18. tony sayer

    tony sayer Guest


    No its just him off his meds as usual. He's in uk.rec.audio where the
    same hatred of us..........


    *********************** !!!fecking pommie arseholes!!!...........

    ...can be found and every other vile*********

    ***************pommy or American- shithed- arsehole- who steals the eats
    the earths Oxygen to breathe!..etc....


    *********and they all are know nothing shitheads arseholes --insert
    expletive of your choice--;!

    So no your not any different at all!...

    fortunately there aren't -that- many about like him thankfully;)...
     
  19. Roy

    Roy Guest

    I know the difference between zzip and zzzap, and it's more than
    swapping or adding trannies., to eliminate harmonic distortion it will
    need a few extra detailed components, unless this guy [Paul] is
    designing a prototype, he's just better off getting a preamp - forget
    all that breaking down the specs jazz.

    Roy Q.T.
    [have tools, will travel]
     
  20. Ross Herbert

    Ross Herbert Guest

    On Sun, 04 Jan 2009 20:36:09 -0900, (Floyd L. Davidson) wrote:

    :
    :> voice frequency circuits were all 300 - 3400Hz in my day.
    :
    :The PSTN is specified from 400 to 2800 Hz, with 24 dB SNR.
    :
    :Individual channels on various carrier systems, and some
    :private line voice circuits are specified with more
    :bandwidth.
    :

    In Australia PSTN is specified for 300 - 3400 Hz bandwidth.
     
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