Mic Output Impedance/Preamp Inpit Impedance

Discussion in 'Pro Audio' started by Haolemon, Mar 14, 2006.

  1. Haolemon

    Haolemon Guest

    I can't recall what the optimal ratio for this is. The question has
    come up in connection with the LoZ switch on a Fearn VT-2 and the use
    of certain transformerless mics. The ones I interested in are:

    Gefell UMT70S 150 ohms
    Gefell M300 100 ohms
    Schoeps CMC6 50 ohms

    I will certainly see what sounds best, but wanted to know the
    theoretical answer as well.

    Haolemon, Mar 14, 2006
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  2. Haolemon

    Paul Stamler Guest

    The Gefell mics will all want to see an input impedance of at least 1k.

    The Schoeps may have some issues, though, because the Fearn's input
    transformer may get some upper-frequency peaks if it's driven by that low an
    impedance. I don't know how Schoepses perform into lower impedances.

    Paul Stamler, Mar 14, 2006
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  3. Haolemon

    RD Jones Guest

    There is an old guideline of 10:1 (or higher) ratio of source
    impedance to load but with modern transformerless electronics
    it has become a trivial issue.
    Also be aware that many manufacturers published "rated"
    impedance may not be either the actual source measurement
    or intended load.

    RD Jones, Mar 14, 2006
  4. Haolemon

    David Satz Guest

    Haolemon, this is a bit tricky because with many preamps it isn't a
    question of an optimal ratio--while in other cases the situation might,
    conversely, be looked at in such terms.

    Microphones are designed to act as voltage sources, not power
    amplifiers, so to avoid adverse loading effects (reduced headroom,
    unnecessary reduction of sensitivity, etc.) they should be loaded with
    an impedance that is at least five (preferably ten or more) times their
    own output impedance. For transformerless condenser microphones such as
    your Schoeps CMC series, this is a simple matter. Their output
    impedance is very low and is essentially resistive in character, so
    nearly any existing preamp input will have high enough impedance so as
    not to present a "problem load" to the microphone as such. Even
    condenser microphones with output transformers (at least the ones from
    the more serious manufacturers) are relatively uncritical as to
    loading; 1 kOhm or higher will generally be just fine.

    With some other types of microphone, e.g. ribbons, the rated output
    impedance may not be maintained at all audio frequencies. As an
    example, the Royer SF-1 has a 300 Ohm output impedance across most of
    the audio band, but near its resonant frequency (which is low), that
    impedance zooms up above 1 kOhm. So that can be one aspect of the
    situation to think about--though not with the particular mikes which
    you listed.

    Where the situation becomes more complicated yet again is that you may
    also have to consider the particular preamp input and the limits which
    it may impose on the situation. Some preamps have input transformers
    with rather large turns ratios, e.g. 1:15 or even 1:20. These preamps
    have a tendency to vary in their frequency response depending on the
    particular impedance which is driving that input. In extreme cases
    there can be severe peaks or rolloffs in the top octave or two (or in
    some cases, an octave or so above the audio band). When a sharp peak is
    the result, this can even lead to oscillation or parasitic ringing. Not
    coincidentally, this type of input circuit is also more vulnerable to
    response variations when long microphone cables are used.

    For that type of preamp, there is a more or less narrow range of
    microphone-and-cable impedances in which the response will be
    acceptably flat. It is probably most likely (note the necessary weasel
    words!) that this will be about 150 Ohms, but it's not possible to say
    in general what that range will be for all such preamps since each one
    has a different input circuit. You have to find this out specifically,
    e.g. by measuring with a test head or sending signal from a
    low-impedance generator through a variety of series resistors.

    For preamps in which the input transformer has a more moderate voltage
    step-up ratio, this variability is correspondingly less and the range
    of driving impedances that produces an acceptable frequency response
    will be correspondingly broader. For transformerless preamps with input
    impedances that are high enough, such as the Millennia Media HV-3
    series or the Sonosax SX-M2, this set of issues essentially doesn't
    exist at all. (That's one reason I like them.)

    Of course, given the irrationality and ignorance in this field, some
    people actually prefer preamps that have unpredictable interactions
    with microphone and cable impedance. When these interactions produce
    audible variations in frequency response, the owners congratulate
    themselves on having found a preamp "so sensitive that it can resolve
    the difference between two cables"! Of course, from an engineering
    perspective, one might equally well say that the preamp is vulnerable
    to cable characteristics that wouldn't have to have audible side
    effects at all, if the input circuit had been designed properly.

    --best regards
    David Satz, Mar 14, 2006
  5. Haolemon

    Mark Guest

    Of course, given the irrationality and ignorance in this field, some

    Very well said!!!!!

    Mark, Mar 15, 2006
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