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Microphone switch

Discussion in 'General Electronics Discussion' started by davelectronic, Aug 19, 2011.

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  1. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Hi all.
    Ive not asked many questions on things iam doing, but i have one now.
    What this is, well i have had enough of cheap return slide switches in my CB microphones.
    The project i have tried and tested first was a quick set up.
    Its a relay DPDT to replace the slide switch, and a momentary switch to operate the relay, any way iam using a dynamic microphone on a tripod, problem, the audio is to quiet, so ive bought a small kit, a preamp for the microphones output.

    I will leave links to the relay pin out and what was the old switch, and the kit, why a kit, well its quick in this instance, i could build the preamp from a schematic, but this kits is peanuts, so i went with that, as well it was in my local maplins store 5 minutes away, ok iam lazy really.

    On production of the said idea, can any one see a problem combining the preamp with the microphone, oh the mic has a 500 ohm coil, the preamp an out of 1k.
    Reasons why, well the relay/momentary switch will way outlive a cheap mic slide return switch. any feedback appreciated. Dave. PS links below.

    [​IMG]

    [​IMG]

    http://www.maplin.co.uk/k1803-universal-mono-pre-amplifier-2921

    I forgot the microphone, i described it already, the link for it is below, its actually this model.

    http://www.maplin.co.uk/general-purpose-dynamic-microphone-9832
     
    Last edited: Aug 19, 2011
  2. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Relay CB preamp

    I thought there might be issues with a michrophone 500 ohm coil, and the preamps 1K output, in as much as a conflict of impendance mismatch, no replys so i guess its an ok, the microphone and preamp combination. i will try it and see. Dave.
     
  3. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Didn't mean to ignore you, but it was kind of hard to spot the question inside all that information.. ;)
    Where did you find the 1k output spec'? I doubt it has more than 10 ohms. Diagram?
    The Maplin kit is nothing like the Velleman kit by the same kit name btw..
    The Velleman kit also doesn't have a 1k output impedance, even if so stated.

    Impedance matching is only important when connecting a limited energy source like an antenna or a mic to an input.
    Of course also when connecting long cables, to avoid reflections garbling the signal.
    In this case the amp power is "unlimited" and gain can be turned up to compensate for any loss that might occur.
    But if the 741 is good enough for a CB mic preamp - that is something someone else might have an opinion about.
     
  4. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    CB michrophone switch preamp

    Hi Resqueline thanks for getting back to me.
    It is a vellman kit from maplins store near me the spec of the kit i have is the vellman spec, the link at the bottom of my first post shows the spec, but i will try to see if i can put a PDF file link below, thank you for the information, i thought the impedance difference between microphone and preamp might pose a problem. Dave. :)

    http://www.maplin.co.uk/k1803-universal-mono-pre-amplifier-2921

    http://www.esr.co.uk/velleman/products/index_kit.htm

    http://www.vellemanprojects.eu/downloads/0/illustrated/illustrated_assembly_manual_k1803_rev1.pdf

    Essentially its the same kit, my source of the kit was maplins, but ESR electronics have the same kit, both are the same product to a T. I had to get the PDF from ESR electronics, if you compare them, there the same.

    It states output impedance 1K ohm.
    Thanks again. Dave. :) PS my product code is K1803 Vellman kit.

    My input on the original midland CB mic is 1K, so in theory it should be ok ?.
     
    Last edited: Aug 20, 2011
  5. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Ok. Velleman seems to have two versions by the same name, the other having a dual op-amp, 50k input and 10 ohms output impedance.
    The one you last referred to has 2.2k input and max 500 ohms (with RV1 mid-way, going down to 10 ohms with it at either end) output impedance.
    But like I said, the only important thing here is the microphone impedance being equal to or lower than the amp input.
     
  6. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Microphone swictch + preamp

    Thanks for looking into it, yes i new there was a stereo and a mono preamp, the stereo seems to have two op amps in a single package, ive got the single mono one, as my input is fairly low impedance 500 ohms it should be ok, and the 1K out put is the same Independence as the original microphone, iam not looking to blast out, but my first attempt is very quiet, made around a small ABS box, red and green led TX RX and the DPDT relay, a 2.5mm DC input socket, a 1/4" mono jack socket, and the 4 pole line socket, and the momentary switch, and an on/of SPST switch, stop the kids playing with it when iam not about.

    The second build will include the preamp, and lift the audio a little i hope, Thanks again. Dave. :)
     
  7. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    No, not stereo, just a two-stage amplifier (by means of a dual amp package)..
    The output is incorrectly stated as 1k, forget that number..
    It has up to 40dB gain, which is a lot btw..
     
  8. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    preamp

    I thought you meant the two models on the ESR electronics web site in the kits list, as the two schematics are different, mono and stereo, and maplins only has the one kit the mono one.

    I am not aware of any other kit preamp, any way i will go gingerly with the gain increase as its FM so not to over deviate the modulation, the audio only wants lifting a bit, ive had the radio about 2 years it in use 24/7 to keep contact with me mum, she's laid up after a knee replacement operation, in between i talk to some of the locals, but getting some skip coming in now, Europe, and most of the UK and Ireland. The radio came with the audio way low from brand new, i looked at the schematic and turned it up a little under the hood, with the standard mic its fine, but with out a preamp on the dynamic mic its very quiet, the amp should lift its audio to where the standard mic's audio is, i don't wont to blast out, or stress the radios audio circuit, so i will go easy on the gain. Dave. :)
     
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