Lend me a hand with my first audio circuit please

Discussion in 'The Projects Forum' started by dihewidd, Dec 21, 2010.

  1. dihewidd

    Thread Starter New Member

    Dec 21, 2010
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    I have all the parts to build this circuit:
    http://hop.concord.org/s1/ext/s1ed1.html

    It's based on the popular 741 op-amp. The goal is to build the simplest, most sensitive, sound intensity meter. We're not interested in LEDs, etc etc. We just want our volt meter to display voltages relative to the sound level in the room. We're open to going in a new direction and dropping the 741 op-amp.

    So far we cannot get any readings from the voltmeter that correspond to ambient sound levels. Instead, we get an oscillating voltage that reads 0.25v and then jumps to 0.55v, and then repeats the cycle.

    The example we followed called for a 9 volt battery which we substituted for a 12 volt 500 milliamp radioshack power adapter. Did we screw ourselves there?
     
  2. #12

    Expert

    Nov 30, 2010
    16,346
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    You tried to give it a voltage gain of 1000. For even a tenth of that you have to put it in a shielded box. Next, a schematic works a lot better on this site than a pictoral. Third, the 741 chip is an antique.

    I'm sleepy now. Somebody else will be right along and help some more.
     
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
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    Throw that 741.
    Get the most recent low noise OPAMP.

    Produce your schematic.
     
  4. dihewidd

    Thread Starter New Member

    Dec 21, 2010
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    0
    Thank you for the feedback. I'm too new to circuits to be able to produce my own schematic. I'm going to need a plan to follow. I'm guessing that I can't just swap out the 741 for the low noise OPAMP without redesigning the circuit.
     
  5. #12

    Expert

    Nov 30, 2010
    16,346
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    A lot of op-amps are built with the same pins in the same places. Get the data sheet for the 741 and go looking for substitutes that will plug in. I use www.mouser .com. They have a search engine that will narrow it down for you. Then get a metal box to put it all in.
     
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    I think your circuit is probably working perfectly fine, but a cheap DIGITAL volt meter will not give you the full picture.

    The digital meter takes 'snap shots' of a voltage over time. It has no way of showing you the voltage levels in between the snapshots. An analog meter might give you a different result than the DVM is giving you.

    Edit: If you must use the 9 volt batteries, get several fresh ones(check them) and put them in parallel to increase their current delivery ability. USE SOME BYPASS CAPS on the IC power pin and maybe even at the battery input point. READ lots of the data sheets on this amp. There are literally hundreds of them out. Some will have helpful circuit design hints.

    and finally; no one is too much of a beginner to design a circuit. Hands on, experimentation is the BEST way to learn in my opinion. You will be more likely to remember the things you learn that way. Design a circuit- draw the schematic, and build it. If it works great, if not, try to find out what went wrong. Everybody, and I do mean everybody, has a first time doing something. If you can get an analog meter, do it. They are very handy sometimes. :)
     
    Last edited: Dec 21, 2010
  7. Audioguru

    New Member

    Dec 20, 2007
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    It looks like every pin on the opamp is the wrong pin in the circuit.
     
  8. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    the picture of the amp is connected 'dead bug' style, but represented as being on a breadboard.

    Flip the op amp over on its back so pins 1 through 4 are on the bottom and run right to left. aka dead bug :)

    Silly graphic artists

    THAT is why your circuit doesn't work dude. Bad graphics!
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    The performance of a 741 opamp is so poor that with the gain of 1000 its output will be full of noise and it will cut audio frequencies above only 800Hz.
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Try using half of a TL082 which you can find at Radio Shack, far better response and far more forgiving.

    I'm about to hit the sack but somebody will come along with a simple design.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    A TL081 dual opamp is much better than an old 741 opamp but you might as well buy a TL072 dual opamp from a real electronics parts distributor for less cost. A TL072 is a TL082 that is selected for low noise.

    Then you can make an amplifier with both opamps so that each one has a gain of 31.6 and the total gain is 1000 and the total bandwidth is 50kHz.

    The 1k resistor that powers the electret mic should be 10k ohms and should be fed from a 1k resistor with a 47uf filter capacitor to ground so that the opamps don't amplify the supply voltage jumping up and down.
     
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