Making a mic, several questions

Discussion in 'The Projects Forum' started by torea, Jul 18, 2012.

  1. torea

    Thread Starter New Member

    Jul 3, 2012
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    Hey all, I've started a new project. I'm making a small microphone, and decided to stick with the basic schematic. Keep it simple, ya know?

    Here's the schematic. It's also attached for those who don't want to load another site - http://www.linkwitzlab.com/images/photos/mic-amp.jpg


    So, here are my questions:

    1) The OPA134 is rated at 18V absolute max. If I'm reading this schematic correctly, the two 9Vs are wired in series which would send 18V into the opamp. Wouldn't this be risky? I'm new to ICs, so I'm not sure if I'm missing something.

    2) I'd like to add an LED on-light for this, because I hate accidentally leaving this stuff on. Would the LED go between the battery and the opamp? Also, if that's where it goes, the resistor should be 1600 ohms correct?


    Thanks for the help! If anyone has suggestions for audio opamp tutorials (starting from the beginning!) that would be great too! I've been reading Rod Elliot's page but more would be great for the learning process.
     
  2. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    You're misunderstanding the supply voltage limit. It's a split supply of +18V and -18V. These amps are typically run on a ±15V supply so your ±9V is no stress.

    A power on indicator should be connected across the + and - power rails so it will see the full 18V. A blue high brightness LED would be more efficient than other colors and should be operated at 2mA or less. Be sure it's visible when the supply drops to 14V (dead battery voltage).
     
    Last edited: Jul 18, 2012
  3. torea

    Thread Starter New Member

    Jul 3, 2012
    16
    0
    Ah! My bad. I figured it would be OK, since the designer seems very well regarded. But I wanted to make sure =)

    Thanks for the help!
     
    Last edited: Jul 18, 2012
  4. torea

    Thread Starter New Member

    Jul 3, 2012
    16
    0
    I built this yesterday and it runs great! Thanks for the tip on the LED, that works well too and is much better than no on/off light.

    As stated in the schematic, the mic has an output impedance of 200ohms. If I wanted to change that, say to match to an effects pedal, would it be as simple as changing that (those) resistor(s)?

    Also, on the 2.2uf cap: That's a coupling cap, correct? In the description, the creator says to use tantalum. Would a non-polar film cap work? As I'm told, film caps are better for audio than tantalum or electrolytic whenever possible.


    Thanks!
     
  5. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    I only have the schematic. Apparently you have some additional information. The answers in sequence are: No, the output impedance is probably close to 600Ω; sort of, it's also a filter; maybe, but why would you want to change the design when your engineering skills are less than those of the designer.

    I'm just a lowly technician so I don't understand all that's going on in that circuit. :(
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,453
    3,371
    While a 2.2μF electrolytic coupling capacitor will work fine, a non-polar cap is a better choice in this situation.
     
  7. torea

    Thread Starter New Member

    Jul 3, 2012
    16
    0

    Good point, I was just asking. Read some stuff about impedance matching, got me curious.


    Good to know. Thanks!
     
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