Input cap polarity for electret mic preamp

Discussion in 'General Electronics Chat' started by tracecom, Nov 25, 2012.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    There seems to be contradictory information about the correct polarity of an electrolytic cap used to couple an electret mic to a preamp. What determines whether the plus or minus side of the cap goes toward the mic?

    Thanks.
     
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
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    The amplifier you're using.

    Post a circuit diagram.
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    Measure the voltage across the capacitor and connect the capacitor so that the positive lead is more positive than the negative lead.

    In most cases of low level audio signals it does not matter.
     
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  4. Ron H

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    Apr 14, 2005
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    If the cap is installed incorrectly and has more than 1.0 to 1.5V across it, it can be destroyed, and most certainly will conduct DC current, screwing up the bias voltages.
    http://electrochem.cwru.edu/encycl/misc/c04-appguide.pdf
    Measure the voltage before you install the cap.
     
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  5. Audioguru

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    Dec 20, 2007
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    Some circuits wrongly show a huge 10uF electrolytic coupling capacitor that passes very low earthquake frequencies down to 0.32Hz into the 50k input resistance of a non-inverting opamp.

    Instead a non-polarized 100nF (0.1uF) film capacitor should be used that passes audio frequencies down to 32Hz.
     
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  6. tracecom

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    And what size cap should be used on an inverting opamp in a circuit such as the attached?

    Thanks.
     
  7. #12

    Expert

    Nov 30, 2010
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    You have a 100k resistor (R2) as the input impedance. The formula is c=1/(2 Pi F 100k)
    Decide your lowest frequency and plug in the numbers.
     
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  8. tracecom

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    So for 32 Hz, the cap should be .049 μF? (Math isn't my strong suit.)
     
  9. Audioguru

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    You have a zener diode feeding power to the electret mic. But the zener diode is not necessary and it produces a lot of noise. A zener diode is used in many noise circuits. Repace the zener diode with a 47uF capacitor to ground.

    The electret mic uses a current of about 0.5mA so the voltage drop across the 11k worth of resistors is 5.5V then the mic gets only 3.3V without the zener diode which is fine.
     
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  10. #12

    Expert

    Nov 30, 2010
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    That's the answer I get. Side effect: using a .047 to .05 uf cap puts you in the range where you can use non-polarized caps. Poly-something is a good choice for audio circuits because of the tiny dielectric absorbtion loss.
     
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  11. Audioguru

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    Dec 20, 2007
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    I usually use 220nF (0.22uF) poly capacitors for coupling audio (into 50k ohms) but I have used values as high as 1uF and they are still pretty small.
     
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