High Gain amp for pickup coil

Discussion in 'The Projects Forum' started by Skyler, Feb 11, 2014.

  1. Skyler

    Thread Starter New Member

    Jun 26, 2013
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    Hi, I'm still a beginner at electronics and I want to build an amplifier for a magnetic pickup coil for "hearing" computer noise from hard drives etc., finding the mains lines, and just to listen to audio frequency electromagnetic waves for fun to see what it sounds like.

    I would like a good gain control knob to listen to very faint signals when I am far away from any interference. I don't care about that fancy automatic gain control, I just would like to hear very faint signals. (i'm sorry I don't have the decibel amount, I don't know how I could find how many DB gain I would like, I just want a device which has the capability of turning up the gain to A LOT.)


    What practical circuit can I build that will achieve this with fairly low noise? I would like it to take voltages in the 5-12 area so standard batteries can power it, and enough power to drive a small 8 ohm speaker with adequate volume like a walkie talkie.
     
  2. Seddie777

    New Member

    Jan 28, 2014
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    A simple op amp amplifier will likely do just fine. You can look here for ideas, http://pdf1.alldatasheetpt.com/datasheet-pdf/view/212589/NSC/LM324.html. A potentiometer will work as a volume control (placed differently depending on the circuit you choose). I'm not very familiar with magnetic pickups, or yours in particular, and what values they operate at, but you will have to do some calculations. One of the first things to consider is the "slew rate" of the op amp. This will depend of what voltage you need the output to be and what frequencies you expect the noises to be. Wikipedia is a good place to learn about slew rate if you haven't heard of it before. Another thing to take into consideration, assuming you choose the op amp route, is what speaker you are powering. Some op amps may not be able to power speakers larger than perhaps an earbud headphone. Again, some calculations are necessary (I know, they never end. It's just like any other kind of project though, measure twice, cut once). If you intend on powering a decent speaker, which it seems you do, you will need a transistor circuit capable of producing the power you need. Hope this helped!:D
     
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I have one with an LM386.

    That will drive a small speaker or headphones easily, and the LM386 datasheet shows what parts values to change to bump up the gain to about 200. That should be plenty, I have to turn mine right down most of the time.

    Have you selected a magnetic pickup yet?
     
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  4. Skyler

    Thread Starter New Member

    Jun 26, 2013
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    Yeah, I just got the one at Radio Shack, and I will also do experimenting with different amount of winds on a homeade one.
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The LM386 is an excellent choice for a simple, basic output stage with a gain of 200. But you probably will still need an additional preamp for an overall gain of 1000 or more for the low levels you are interested in. For example, intercom circuits that use the speaker as a microphone frequently have an opamp or a 1-transistor preamp stage ahead of the 386.

    Try this: Input from the pickup, coupling capacitor, single op-amp stage with a gain of 20, volume control, LM386 with a gain of 200, speaker.

    Another thing about the speaker is its frequency response range. A small speaker might not give you an audible output at power line frequencies (50 or 60 Hz) even though your circuit is delivering them.

    ak
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Small speakers will roll off like 300Hz or even higher.
    You would need a good low range speaker than can go below atleast 40Hz to hear 50Hz
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You might also need a filter (switchable) to cut out mains frequency signals, since those will possibly swamp everything else.
     
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Last edited: Feb 12, 2014
  9. Skyler

    Thread Starter New Member

    Jun 26, 2013
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    Can that be done using Caps and Inductors, or do I need fancy stuff?
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Mine has a cheap little 40 mm 8 ohm speaker, and can hear 50Hz just fine. I can run the pickup over a wall and if a light switch is turned on it will playback the 50Hz buzz as the pickup moves over the wire.

    I'm guessing it's because the mains has plenty of noise on it with a freq content higher than the mains frequency.
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Depends what range of frequency you might want to suppress. A very narrow band filter would involve fancy stuff. With a wide band filter you could be throwing baby out with the bath-water.
     
  12. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    High performance = fancy and lotsa parts, but a few Rs and Cs will probably cover this. Example - add a single RC lowpass network between the preamp and the LM386 with 3 caps and a selector. The corner freqs could be at 10-15 KHz (protect your hearing), 2 KHz (reduce high freq noise more), and 200 Hz (power lines only). Similarly, switch in 2 or 3 caps in the op amp feedback shunt leg for highpass corners. In both cases these are single-pole networks so the effect won't be dramatic, but it will help isolate some signals enough for your hearing to do the rest.

    ak
     
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