What blocking capacitor is needed for Icom 7300?

Discussion in 'Microphones, Speakers & Audio Processing' started by KE0EYJ, Feb 26, 2017.

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  1. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    I wanted to buy a finished Audio-Comm brand XLR plug with the necessary blocking capacitor to run an XLR mic with my Icom 7300, but it seems they are not allowed to ship here, via customs. The Heil ones are way too expensive. Would like to build my own. What sized blocking capacitor is necessary, and where does it go? I suppose I can just solder it on the inside of my mic (the Behringer XM8500).

  2. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    K0YB likes this.
  3. W7UUU

    W7UUU Principal Moderator Lifetime Member 133 Administrator Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    Here ya go:


    N7ANN likes this.
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    Last edited: Feb 26, 2017
    KD2ACO likes this.
  5. W7UUU

    W7UUU Principal Moderator Lifetime Member 133 Administrator Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    I stand corrected. I saw no polarity markings and no mention of polarized in the ad. Old eyes at work I guess.

    KA9JLM likes this.
  6. KK5JY

    KK5JY Ham Member QRZ Page

    But the ad does say Tantalum, and IIRC, that capacitor construction is definitely polarized. If you hook one up backwards, you're going to need eye protection because they fail rather spectacularly. :eek:

    A 3.3uF or 4.7uF electrolytic capacitor is a good choice for DC blocking for a dynamic mic. The (+) goes towards the radio (which supplies positive voltage on the mic lead for condenser mics), and the (-) towards one lead of the mic element. The other mic element lead is grounded.

    1uF sounds a bit small to me, but Bob is a stickler for audio quality, and he may have chosen that value to get a little extra low-bass roll-off from the dynamic mic element for ham use? Dunno. A blocking cap in a microphone audio circuit acts like a high-pass filter, so the larger the value, the less low-end roll-off you tend get.
    KE0EYJ likes this.
  7. KM3F

    KM3F Ham Member QRZ Page

    Basically you ashed what capacitance and voltage level you need.
    The radio supplies about 8 v DC so you logically need a voltage rating of at least 12 vols to have enough tolerance.
    That voltage is supplied by dropping 14 volts through a regulator. If you ground that lead, the internal regulator can be burned out.
    The capacitance value is determined by the audio frequency the mike will produce.
    Since the X sub C of the capacitance is determined by the frequency, that resistance needs to be low enough to pass all the audio without significant attenuation.
    Said another way, if the X sub C is to high the audio LOWS will be attenuated because as the frequency goes lower the resistance goes higher.
    Bottom line is you use a value high enough to not impede the lowest frequency the mike can produce.
    It is also a way to limit too much low end audio if that is desired by adjusting the capacitor value. The voltage rating still need to be 12 or higher.
    I don't think a non polarized capacitor absolutely needs to be used. But do connect by observing normal polarity convention.
    The mike element audio voltage produced is in the very low micro volt range.
    Good luck.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    NP capacitors in the 1uF to 10uF range are easy to find...focus on "NP."

    You can sorta make a 1uF NP capacitor by using two polarized 2uF tantalums in series with opposing polarity (that is, wire them + to + at the junction, so they're - to - at the ends) and that can work, but it depends a bit on the application. An NP cap is easy to find and I'd just do that.
    EVAWILLIAMS likes this.
  9. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    Thanks, all.

    Bob's directions are specific to the point that finding the part here seems confusing. I ended up getting a message back, and ordering a ready-made cable from Audio-Comm, but am a little worried that it will attenuate my lows, from what you are saying. I have no idea what is in it for a capacitor. I sent an e-mail in to the owner of the company. He was able to ship me one, even though E-bay had a block on shipping one here.
  10. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    How does this look to you guys? I don't want to attenuate the lows!


    (You'll note that, along the bottom right of the page, there are several other sizes, if you think those will work better).

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