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repairing an electret microphone

Discussion in 'Electronic Repair' started by Peter, Apr 8, 2009.

  1. Peter

    Peter Guest

    To replace a wire which broke in the swivel
    of the boom, I reconstructed the boom of an
    Altec Lansing headset. This included
    resoldering the wires to the back of the
    electret capsule. The speakers still work
    but the mic does not after this repair.

    Found a similar failure after soldering a
    new electret onto the wires of an inexpensive
    Creative/Telex desktop mic.

    Is the electret mic particularly heat sensitive?
    If so, what technique is recommended? Already
    I was careful to apply minimal heating.

    Thanks, ... Peter Easthope
     
  2. Arfa Daily

    Arfa Daily Guest

    I've never found them to be particularly heat sensitive within common sense
    limits, but note that they are polarity sensitive, as they contain a FET
    preamp which is phantom powered via the output terminal ...

    Arfa
     
  3. Arfa Daily

    Arfa Daily Guest


    I'm using the term "phantom powered" loosely Dave, in that the audio out is
    floating on the DC in, because there are only two connections, one of which
    is the FET drain terminal, the series DC 'feed' resistor therefore actually
    being the drain load resistor. The other terminal (also the capsule case) is
    of course ground or FET source. Where there is this type of power / signal
    setup, it is often considered generically, to be 'phantom powered' ...
    Excuse my ignorance, but what's "AB power" ?

    Arfa
     
  4. Arfa Daily

    Arfa Daily Guest


    OK, but seems that all of this is tending to look at 'whole' mics as in
    something that a newsreader would clip to himself, whereas the original
    poster was talking about just the capsule inside, which was also what I was
    referring to. As far as the polarity of electret mics varying, I can't
    remember ever seeing a capsule where the case wasn't a negative-side ground,
    and over the years, I have dealt with and replaced many in cordless phones
    and similar.

    I appreciate that to a sound engineer, the term "phantom power" has a
    slightly special meaning in terms of voltage level etc, but that still
    doesn't change the fact that generically, any system where DC power is
    supplied to an active signal source, using only the wires that are carrying
    the signal rather than any additional power carrying wire, are considered to
    be 'phantom' powered, irrespective of the voltage involved. TV antenna
    amplifiers for instance, are often described as being phantom powered, as
    also are satellite LNBs.

    However, all that said, I do take your point that it could give rise to
    confusion between a sound engineer reading it, and an electronics engineer,
    who might better understand the overall concept. Perhaps it would be better
    to call the electret capsule 'line powered' ...

    Arfa
     
  5. Arfa Daily

    Arfa Daily Guest


    No Dave, they aren't. The term "phantom power" is used for many situations
    where an active device needs powering and only the signal cable is
    available. Antenna amplifiers, satellite LNBs, cable operators' distribution
    amplifiers and so on, are all routinely described as being "phantom
    powered". There are plenty of web references to the technique of phantom
    powering in these applications. Long ago, when I worked in the early days of
    cable TV, all of our line amplifiers were powered in this way, and it was
    always referred to as phantom powering, both by all of the high-end network
    engineering bods, and also our in-house lecturers, responsible for training
    of all of the company's engineers.

    In fact I would go as far as to say that the technique has been around and
    called that for a very long time, and the 'hi-jacking' of the term by sound
    engineering to try to mean something very specific, is actually the
    questionable use of the phrase.


    How so ? What do you perceive as being the difference ? If DC is travelling
    one way, and signal the other on a single cable, they must be mutually
    invisible (or made so by appropriate circuitry techniques at the active
    device, and what it's feeding at the far end of its cable). I have never
    seen any distinction made before. As far as I am aware, "Line powered" is
    just a slightly more technically descriptive way of expressing "phantom
    power".

    Arfa
     
  6. Peter

    Peter Guest

    Arfa wrote, "... they are polarity sensitive, as they
    contain a FET preamp ..."

    On the Altec, I had the polarity wrong. Should have
    trusted the mark I applied before removing the wires.

    The headset works as well as ever now.

    In this poor picture, the original capsule from the
    Creative/Telex desktop mic is on the left.

    http://carnot.yi.org/Mics.jpg

    The replacement, on the right, has no silver
    trace from a solder pad to the capsule. Half of
    the head of the capsule is a brown Bakelite color
    typical of a PCB. The other half is black. Is that a
    conductive layer to connect one pad to the capsule?

    In any case, I can buy another mic and try again.

    Thanks everyone for the help, ... Peter E.
     
  7. If the lines were designed to be phantomed, as many long-distance
    telephone circuits were, the phantom should meet the same spec as the
    main pairs.
     
  8. As a slightly quirky aside: starquad cable uses the 'phantom' for the
    main circuit with the side pairs short-circuited. The 'phantom' in this
    particular case gives better immunity to localised inteference than the
    side pairs would on their own.
     
  9. Tim Phipps

    Tim Phipps Guest

    IIRC the capsule in the Realistic (Tandy/RadioShack own brand) PZM
    microphone had the +ve side connected to the case.
     
  10. Arfa Daily

    Arfa Daily Guest

    Now *that's* below the belt even for you ...

    Arfa
     
  11. Arfa Daily

    Arfa Daily Guest

    Fair enuff. Still, Tandy / Radio Hack says it all ... :)

    Arfa
     
  12. Sorry to spoil your rant, but I used a phantom system to save on copper
    when recording stereo from a remote position; I installed it last year.

    It runs two high grade line-level audio circuits over dirt cheap 4-core
    burglar alarm cable and the phantom can either be used for a third mic
    or for talkback.


    There's always one awkward so-and-so, isn't there.... :)
     
  13. Tim Phipps

    Tim Phipps Guest

    Agreed, it was one of the few decent things you could buy in Tandy.
    Although it came with an unbalanced jack fitted, its output was actually
    balanced so all one had to do was cut the plug off and fit an XLR instead :)
     
  14. Over a 100 metre run, the phantom was quite good enough for a
    full-bandwidth (20c/s - 20Kc/s) mic circuit. As it is usually the
    terminating equipment that determines the quality on 'short' lines, I
    would not expect much worse over a kilometre or two.

    The crosstalk of the phantom to the outer pairs was around 60dB at
    10Kc/s whereas the pair-to-pair crosstalk was better than 80dB (both
    worsened at 6dB/octave). That might vary with line balance and is also
    affected by the physical arrangement of the transformers, which all had
    to be mounted in the same box and did not have any screening pots.

    It would even have been possible to run a 100v P.A. line as talkback on
    the phantom, because the breakthrough would only occur when the side
    pairs weren't carrying live programme.

    It was the cheapest four-core flexible cable in the Radiospares
    catalogue and it was twisted with a constant physical relationship
    between the cores. Telephone twised pair would have been fine for a
    fixed installation, but this was for occasional 'outside broadcast' type
    of use where a a lot of bending could occur, so a flexible type was
    preferred.

    Normally cost of the terminating equipment would make such an
    arrangement uneconomical for 'short' lines of a few kilometres (or even
    less in this particular case), but the mics already had built-in preamps
    which delivered 0dBm level and the transformers were part of a surplus
    job lot which I obtained cheaply many years ago.

    Given the choice between buying expensive mic cables or buying burglar
    alarm cable and sticking some spare transformers in a box, I decided to
    give phantoming a try - and was pleasantly surprised by the results.
     
  15. The test run was only 100 metres, but runs up to a couple of kilometeres
    may be used in future. (See my other reply for the reason this was
    economical in this particular case)

    No, although there is no engineering reason why a telephone circuit
    could not be used.


    This is where someone with a good wide working knowledge of electronics
    can achieve the 'impossible' while the graduate engineer is still saying
    it can't be done.
     
  16. For the difference in price, it is worth the extra effort of being
    gentle with it. I don't let anyone else do the rigging.


    That sort of routing might work at DC, but I wouldn't trust it above
    50c/s. If the phantom is 'official' the telephone people ought to
    ensure that it is always run on two adjacent pairs; I have even heard of
    a long route where the phantom signal was on a separate pair for part of
    the route and only phantomed on the congested bits.

    My phantom is always in the same cable, so I don't need to worry about
    the pairs getting separated.
     
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