electric mic over a long distance

Discussion in 'General Electronics Chat' started by bug13, Aug 13, 2014.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Hi guys

    What would the effect on connecting a electric mic over a long distance (15M) to the input of an amplifier, with a shielded cable.
    • Is that going to pick up a lot of noise?
    • If it does, how can I fix it?
    • Will that be any other problems associate with this?

    [​IMG]

    Thanks!
     
  2. Metalmann

    Active Member

    Dec 8, 2012
    700
    223
  3. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    I assume you mean an electret mic which is considered low impedance. You can go hundreds of feet with it with no problem.
     
    bug13 likes this.
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Watch the quality of the mic cable. A cable that has high capacitance will reduce the high frequencies. You can measure the cable wiht a capacitance meter if you have one.

    Also some cheap cables are noisy and will make crackling sounds when flexed and stood on. You'll have to check that the hard way. :)
     
    bug13 likes this.
  5. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    How much of capacitance would be considered high?
     
  6. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Hi Lestraveled, do you mind explaining why low impedance mic can go hundreds of fleet? So can I assume that a high impedance mic can't go very long?
     
  7. to3metalcan

    Member

    Jul 20, 2014
    228
    23
    The mic + cable start to form a lowpass filter, like so:

    [​IMG]

    Where R is the impedance of the mic and C is the capacitance of the cable. The longer the cable is, the bigger the "plates" of the capacitor become, which means the filter cuts off lower and lower. If the mic impedance is low, this effect is usually negligible, or at least inoffensive for most applications. But if R was already big, it doesn't take that much C to start getting audibly low cutoff frequency!
     
    anhnha and bug13 like this.
  8. to3metalcan

    Member

    Jul 20, 2014
    228
    23
    Y'know, RB, I know this is true from personal experience, but I don't actually understand why. Care to comment?
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    I can't remember exact values it's been a long time since I worked on pro-audio equipment.

    If you google some specs for expensive high quality audio coax the pF/foot spec should be listed (or pF/metre more likely these days). Then cheap cable it won't be listed because they are embrassed.

    I remember doing one job for a commercial recording studio where I went back later with the capacitance meter as a favour and measured a heap of coax cables, some were terrible for capacitance. The studio sound engineer knew the bad cables too, from their shitty sound. They were still fine for some tasks.


    No idea really. I think it moves the bare braid wires (or conductor wires?)relative to each other, making them touch or scrape each other which makes a noise signal. It is worse in real old beatup cables so they might have more conductor breaks and are more affected by cable movement.
     
    to3metalcan likes this.
  10. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,128
    266
    I read somewhere that microphonic behavior in cables is caused by triboelectric effects in the insulating materials, they generate minute voltages when flexed or struck.
     
  11. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    That, and also when you have a phantom voltage to supply a condenser mic on the far end, then as you step on it you squish the dielectric so the capacitance changes which induces change in voltage.
     
    bug13 and Sensacell like this.
  12. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the low impedance of the electret is because of the fet amplifier in the mic cartrege. low impedance signals have lower voltages, but have higher current than high impedance signals, like from a piezoelectric mic.
     
  13. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Just another question, I measured the resistance of a couple electret mic, it reads ~800ohm and ~1k ohm, it that still consider low impedance? Or did I do it the wrong way?
     
  14. to3metalcan

    Member

    Jul 20, 2014
    228
    23
    Bug, that would be about average impedance for an older dynamic microphone, but it doesn't really apply here: the electret mic has a built-in active FET (that's why electrets require a voltage.) Its impedance is not measurable the normal way. When the transistor is energized and running, the effective output impedance is quite low.
     
    bug13 likes this.
  15. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,282
    1,242
    The impedance is determined by the bias (load) resistor. So with long cables you want it low but within spec for your microphone. This will make the nominal output lower, but the high frequency roll-off will be less pronounced. Not all cable is created equal so pick some with low capacitance.
     
    bug13 likes this.
  16. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    The amp connected to the mic only have 16kHz sample rate, does it mean I only need to worry the max frequency up to 8KHz? Instead of 20KHz normally?
     
  17. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,282
    1,242
    Yes, In general you want the sample rate at least 2X the desired frequency response.
     
Loading...