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hearing aids revisited

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by MrDEB, Nov 30, 2009.

  1. MrDEB

    MrDEB Active Member

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    I tried simulating the schematic Audio GFuru posted

    Looks like a better AGC circuit BUT valuers of C3, R7, R5, C2
     

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  2. MrDEB

    MrDEB Active Member

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    Here is just the AGC section comparision

    values for C3, R7, R5, C2??
     

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  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Why don't you understand that the inputs of an opamp must be biased properly or it won't work?
    You copied an AGC circuit that uses a positive and a negative supply so that the inputs of the opamps are biased at 0V which is half the total supply voltage. Then the inputs (and outputs) can swing up and down with the audio signal.

    But you are using only a positive supply. Each and every opamp (and maybe the Jfet) must also be biased at half the supply voltage but yours are biased at 0V which is the negative supply voltage. Then the input transistors of your opamps have no input bias voltage so they are turned off.
     
  4. dave

    Dave New Member

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  5. MrDEB

    MrDEB Active Member

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    I didn't realize I had to use a split supply.
    Will make corrections.
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You do not need to use a split supply if you bias the opamps at half the supply voltage.
     
  7. MrDEB

    MrDEB Active Member

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    Would a split supply be easier?

    Here is same but w/ split supply. Changed the op amps from OPA227 to OPA364
    Might go with TLo74 as I have several on hand but total noise is alot more.
     

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  8. MrDEB

    MrDEB Active Member

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    I discovered a new feature of the TINA software. I can play a wav file. The program has several different wav files, music, drums as well as speech.
    Using the transient response I can load different files. Trying to figure out if it is better to embed the wav file or not.
     
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    There are so many things wrong with this new schematic that I am suprised that a simulation does anything.
     

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  10. MrDEB

    MrDEB Active Member

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    Pretty sure I have the schematoics right?

    The peack detector is the one you posted and the BP section dosn't require a Voffset if using a Bipolar supply.
    I got the wave file working but not sure if its right as it sounds the same reguardless of pot settings.
     

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  11. MrDEB

    MrDEB Active Member

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    found missing connection Rf (R4 in Tina) to op amp output but the BP filter looks correct?
     
  12. MrDEB

    MrDEB Active Member

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    Had this printout and tried out

    I found this under a bunch of circuit printouts and was curious.
    Simulated and LOOKS like the out put audio lever stays constant
    Your opinion
    Audio limiter circuit schematic
    I subbed an OPA134 and 9v split supply.
    Added the pot (maybe not correct?)
    Going to copy/paste onto the BP filter.
     

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  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You copied the Wien Bridge Sine-wave Oscillator circuit that I got from Texas Instruments. It doesn't have an input, it has a +2.5VDC reference voltage for the Jfet. The Jfet stabilizes the output amplitude. You didn't even copy the circuit correctly.
    But you don't want an audio oscillator, you want an audio AGC circuit which is completely different.

    Your highpass filter is missing the 13.7k resistor which is an important part of the highpass filter and it provides the offset voltage (with your split polarity supply the offset is 0V).
     

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  14. MrDEB

    MrDEB Active Member

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    things were acting funny yesterday

    using the multimeter probe I wasn't getting correct supply voltages. Added poweer connections to all the op amps.
    Switched to TL074 op ams as I have on hand. If you feel this circuit is correct I plan on etching a circuit board w/ mics and see how it works.
     

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  15. MrDEB

    MrDEB Active Member

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    Going back to the OPA4134 (quad 134 op amp)
    The noise, distortion etc is lower and going w/ all surface mount
     
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Now your transistor T3 is an NPN with a negative voltage on its collector which is wrong. Use a PNP transistor instead.
    Your bandpass filter looks good if you like the tinny and muffled sound of a telephone (what did you say??).
     
  17. MrDEB

    MrDEB Active Member

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    "Your bandpass filter looks good if you like the tinny and muffled sound of a telephone (what did you say??). "
    The filter is the speech bandpass filter. Any suggestions on improving?
    I was, am hopping the amp would take care of the volume?The last op amp is supposed to be a headphone amp
    will change the transistor and post results
     

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  18. MrDEB

    MrDEB Active Member

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  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A bandwidth of 300Hz to 2800hz is for spoken vowels by women or little kids. But speech is much more than just AEIOU vowels. It has many consonants with frequencies up to 14kHz like S, Sh, T, Th, C, F, H, K, P and many more.
     

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  20. MrDEB

    MrDEB Active Member

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    300hz to 7khz bandpass filters
    Will do some researchThanks
     
  21. MrDEB

    MrDEB Active Member

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    downloaded TI-FILTERPRO software

    input for a filter from 300hz to 7khz w/ center frequency of 3khz
    Gassiun, etc. is kinda greek.
    Any suggestions of what to input for this app
    Here is the design report
    Maybe you can make heads or tails of this report?
    the software asks for filter type, frequency , etc.
     

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