ICOM 746 MICROPHONE QUESTION

Discussion in 'Ham Radio Discussions' started by K5FRH, Jun 11, 2019.

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  1. K5FRH

    K5FRH Ham Member QRZ Page

    The wiring diagram for the 746 (original, not Pro) shows the mike audio on pins 1 and 7 with the phantom voltage on pin 2. This makes me assume I could use a non-Icom mike without a blocking capacitor as long as I wire it to pins 1/7.

    I would like to experiment with a non-amplified D-104, Shure SM 57 or 58, and maybe a Turner Plus 2 with the gain turned down.

    I am requesting feedback from other 746 users who may have also experimented with alternative microphones.
    FYI. I would probably be keying with VOX if I use the Shures, but could use PTT on the other mikes.

    Thanks. Rod, K5FRH.
     
  2. K7MH

    K7MH Ham Member QRZ Page

    The phantom power is on the audio line to pin 1. That is why it is called "phantom" power. That's why you need a blocking capacitor about 1 mfd. It's a very small cap and can fit just about anywhere.
    Pin 2 is just plain old power. I think it's 8 Vdc at about 10 ma. Not really sure what it is usually used for.

    You can get some info about Icom rigs and mics on the Heil Sound website.
     
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  3. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    Here is the pre-amp circuit for most Icom hand held mics. A PN2222 will work just fine in this circuit. Note the collector voltage is on pin one but you could use a one mf non polarized cap in line with the mic, as long as the microphone has sufficient output.

    Icom HM-7 Pre-amp1.jpg
     
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  4. KC8VWM

    KC8VWM Ham Member QRZ Page

    Of them mic possibilities, the SM-58 is going to be the best sounding. I have one and it sounds the closest of all when comparing many other mics to my Heil PR-781.

    IMG_20131213_160042_zps85c3cf20.jpg

    If you want to use the phantom power, hook it up to an electret mic capsule for best results.

    [​IMG]


    Incidentally, you might be surprised to hear the quality produced from an electret mic element often exceeds many high dollar studio mics, and yet they can be had for only a $1.

    Typical - common blocking cap / phantom power circuit arrangement:

    blocking caps.png

    Blocking cap is non polarized. I prefer using polyester film caps over using ceramic caps in audio applications. Cap value could be 1 uF but other values are easily substituted instead. I find the value can have an effect on the desired audio frequency range, so feel free to experiment with different values if you like.

    Of course, goes without saying the cap chosen is properly rated for the circuit voltage. DC supply is not critical for the electret mic capsule. Anywhere between 2 - 10 volts works fine and somewhere in the middle is best sort of thing.
     
    Last edited: Jun 12, 2019
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  5. K5FRH

    K5FRH Ham Member QRZ Page

    THANK YOU VERY MUCH FOR ALL OF THE GREAT ANSWERS !
    Just curious if anyone has tried the older Turner Plus 2. Yes, I realize the gain needs to be cranked down, but I just like the looks of the mike. Also, any thoughts on the D-104 without upgrading to the Heil Element.
    Thanks again.
     
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  6. W4NNF

    W4NNF XML Subscriber QRZ Page

  7. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    If you are mildly ambitious you could put a condenser (electret) mic element in the Turner housing and use the circuit that Charles provided. I have a Plus 2 but have never used it on any of my modern rigs.
     
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  8. K5FRH

    K5FRH Ham Member QRZ Page

    I have considered this also. Wondering if there are any suggestions for a condenser/electret element other than the Heil. Thanks.
     
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  9. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

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  10. KC8VWM

    KC8VWM Ham Member QRZ Page

    Electrets come in different prices, flavors and sizes. If you're looking for a brand name, Panasonic make good ones. I prefer one's that are on the larger size because in my estimation they are more sensitive and exhibit some lower end frequency range. Very high quality electrets are often produced for, and used by companies like Motorola inside their radio equipment and speaker microphones. You might notice electrets are also widely used by the TV broadcast industry inside lapel and lavalier microphones often worn by talk show people. You might notice they always sound sharp, crisp and super clean which is exactly what you want to sound like on the air.

    This is an electret demo to demonstrate my point.



    It's important to fill any empty space in the surrounding microphone head / clam shell or whatever is housing the mic element with acoustic insulation material. People often sound like crap because the mic clam shell itself is causing unwanted internal sound reflections. Fiberglass insulation is a good choice (like a D-104 already uses) but avoid acoustic insulation materials that are packed too densely, because they are less sound absorbent. Craft stores sell a white fluffy insulation material for making things like Santa Clause beards or something, which is about the perfect density for "loosely" filling in any empty space.

    I recently modified my FT-817 hand microphone in this way, with a high quality electret capsule made for Motorola and then filled in the empty space inside the mic clam shell with insulation and it now sounds like I am really using a high quality studio microphone! Many people often don't believe I am using a QRP rig because of the way it sounds to them. No one said there's a rule that you always have to sound like you're talking into a plastic solo cup when you're using a low power QRP rig with a stock hand mic right? lol
     
    Last edited: Jun 12, 2019
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