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Approaching limit of usable mic sensitivity ?

Discussion in 'Recording' started by clintrubber, Oct 21, 2003.

  1. clintrubber

    clintrubber Guest

    - Put this one on techTalk but since it's more mic-related I better put it here -

    The short version:

    What do you think is the lower limit for mic
    sensitivity ?
    Typical is around the order of magnitude of 2 mV/Pa
    but how much lower can you go and still be able
    to bring this tiny signal up to strength with a
    mic-pre ?

    It depends of course on how loud the source is,
    but let's assume this low-sensitivity mic is
    used as a room mic.

    The background:

    Wanting to fiddle around with omni-mics, I remembered
    having an old Akai omni mic around. I had a second
    look at it, its name is ADM-5, seems from an old reel
    to reel (M-8 ?).

    It's non-directional, imped. 50 kOhm, unbalanced jack.
    Inside there's a tiny TX with a ratio of around 1:30
    to boost the signal so the impedance of the element
    itself could be some 56 Ohms.

    The idea was now to see how this mic could be
    of use without that TX of unknown quality - so to
    give the capsule a direct balanced connection,
    be it at a loss at around 30 dB in sensitivity.

    Did a quick & dirty comparison with an SM-57
    and this Akai without the TX gives some 20 dB less
    signal compared to the SM-57 so let's say the
    sensitivity of this Akai-mic is 0.19 mV/Pa.

    Could this still be usable ? Haven't been able
    to hook it up to a mic-pre yet, but I don't believe
    the mic-pre's I use have another 20 dB available.
    I guess this calls for a mic-pre with serious gain,
    but where does this become impractical ?

    I think that removing the internal TX and using another
    TX of known quality and with a turns ratio of less than
    that 1:30 at the other cable end (say 1:5, right before
    the mic-pre) could be a some solution here, right ?

    Curious to your thoughts & experiences,

  2. AudioGaff

    AudioGaff Distinguished Member

    Feb 23, 2001
    Silicon Valley
    I don't know the exact answer to your question, but since your inside messing with the mic any way, you can avoid using so much gain on the mic pre by adding a battery and small opamp circuit to the mic prior to the mic pre.
  3. clintrubber

    clintrubber Guest

    Thanks for the suggestion. This should be a decent (noiseless) gainstage (preferrably a phantom powered one). Fitting in two 9V ceels would be hard, so the INA../SSM-stuff won't do
    I guess. But a discrete circuit on one 9V, yes, could be done.

    Was first thinking of trying a Beyer Dynamic mic-TX before the micpre.
    1:3.15, gives me some 10 dB gain from the TX.

    There's a nice thread on TechTalk about a JFET micpre, full on it does up to 72 dB. That should be enough !


  4. realdynamix

    realdynamix Well-Known Member

    Feb 23, 2001
    :) Peter, I have done the same thing with various HiZ mics, and the only thing I had to do was a direct connection to the element with pins 2 and 3 with pin 1 taking case ground.

    The transformers in any of the consoles I used were fine for getting the sensitivity up. IOW, all I really did was take an unbalanced HiZ mic, with built in transformer, and made it low Z balanced applying the transformer from the console input. These were all mics from older recorders as well. The only other kind was dual impedance types with 2 conductor amphenol connectors attached. The results were the same sensitivity, only allowing use of longer cable runs.

    The only other type of element that would require something quite different, would be a crystal mic of extreme high Z, approaching that of a nonmagnetic phono cartridge.

    Just some experiences.

  5. clintrubber

    clintrubber Guest

    Thanks Rick for the response. Good to hear The Plan (big words for skipping a TX.. :) makes sense.

    > IOW, all I really did was take an unbalanced HiZ mic, with
    >built in transformer, and made it low Z balanced applying the transformer
    > from the console input.

    I understand you removed/skipped the mic-internal TX, right ? (making the line in fact
    carrying a lower signal but at a lower impedance as well).

    BTW, just wanted to add I originally fired this thread at
    (Dead Link Removed)



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