hearing aid

Discussion in 'General Electronics Chat' started by syshanika, Apr 11, 2010.

  1. syshanika

    Thread Starter New Member

    Apr 11, 2010
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    i am designing a hearing aid project with lm 358 as pre amp
    lm386 as power amp ,..and la3607 equalizer for select frequencies
    .i designed it on a pcb ..it works bt has lots of hiss and noise ..wht can i do nw????????need quick help
     
  2. Bychon

    Member

    Mar 12, 2010
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    First, post a schematic. The input configuration of the first stage can be a major noise source. Second, choose a low noise amplifier chip. The 358 comes in at 40nv/radical hz and I'm sure I've seen better. TL072 is 18 nv
     
  3. syshanika

    Thread Starter New Member

    Apr 11, 2010
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    theese are the schematics of pre amp and power amp
     
  4. retched

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    What type of caps are you using in your design?
     
  5. syshanika

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    Apr 11, 2010
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    radial ceremic capacitors and radial electrolitic capacitors
     
  6. SgtWookie

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    Look at an MC33078 opamp. Far greater bandwidth, far lower noise.
     
  7. SgtWookie

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    Split R1 into two separate resistors; one 9.1k and one 1k.

    The 1k resistor goes to +9v, and connects to the 9.1k resistor.
    The 9.1k resistor connects to the electret mic.
    Use a 0.1uF cap from the junction of those two resistors to ground.

    Use metal film resistors instead of carbon or carbon film resistors.

    I do not see a 0.1uF cap across the opamps' supply pins. You must use one.
    [eta]
    At the output from your mic preamp, you have a 4.7uF polarized electrolytic.
    At the input to the amp stage, you have a 1uF nonpolarized electrolytic.
    Get rid of the 4.7uF cap; the 1uF should be enough.
     
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  8. syshanika

    Thread Starter New Member

    Apr 11, 2010
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    thnks
    bt the thing is i have already made whole circuit
    so isn't there ny way i can replace lm358 with another ic with out chaging the circuit
     
  9. Bychon

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    Mar 12, 2010
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    The pre-amp configuration seems ok. It's just that a gain of 100 will put any amp to the test as far as noise goes. I think the next thing to do is search for the lowest noise op-amp you can get. If that fails, design a single, j-fet transistor, low noise pre-amp.

    I did this 20 years ago, but so much has changed since then!
     
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  10. retched

    AAC Fanatic!

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    As long as the pins are the same, you can use a different opamp in its place. If the power requirements are also the same.
     
  11. syshanika

    Thread Starter New Member

    Apr 11, 2010
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    is this means tht an any op amp ic with 8 pins will work or
    should i go through data sheets for details
     
  12. Bychon

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    Mar 12, 2010
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    Did I say TL072? An exact, pin for pin, pop-in replacement?
     
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  13. SgtWookie

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    The filter on the supply to the electret mic will help a great deal, along with using metal film resistors there.

    If you don't do that, the hiss will be greatly amplified.

    The MC33078 is also a drop-in replacement for the LM358. It's 3x faster and 4x quieter than the TL072.
     
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  14. syshanika

    Thread Starter New Member

    Apr 11, 2010
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    ah i'll try this op amp thnks alot
     
  15. Bychon

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    Mar 12, 2010
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    Excellent! I have no objection to wookie finding a better chip. (I'll write it down and use it for my own nefarious purposes.)
     
  16. retched

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    I am also jotting this down... ;)
     
  17. Bychon

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    Mar 12, 2010
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    Hey...ST microelectronics TSH300 has POINT65 nV. I don't see it as a dual, but this makes me want to look at ST Micro some more.
     
  18. syshanika

    Thread Starter New Member

    Apr 11, 2010
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    thnks all of u for the great help
     
  19. Audioguru

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    Dec 20, 2007
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    The preamp is is an inverting opamp with an input resistance of only 1k ohms which loads down the mic signal. It should be non-inverting with a high input resistance then you won't need so much gain and the noise level (hiss) will be less.

    You seem to have too much gain anyway. Your preamp has a gain of about 29 to 286. Since the mic with its biasing resistor is about 2.5k ohms and not zero ohms then the effective input impedance of the LM358 is 3.5k, not 1k. The LM386 power amp has a gain of 200. The total gain is 5800 to 57,200 which is ridiculous.

    With a gain as high as 286 then the old LM358 cuts frequencies above only about 3kHz so everything will sound muffled. People with a hearing loss usually need high frequencies to be boosted, not cut.

    I found an article that describes the poor articulation of speech when consonants aboove 3kHz are cut. Sailing and failing sound the same and a sentence makes no sense. Also map and mat. Pole, coal, goal and sole all sound the same and speech gets very confusing.
     
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  20. syshanika

    Thread Starter New Member

    Apr 11, 2010
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    soo will i be able reduce this disadvantages by using op amp like ne5532 or tl072 in the pre amp section
     
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