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[enquiry]ALC and AGC

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by elnino86, Dec 12, 2008.

  1. elnino86

    elnino86 New Member

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    Helllo,

    After days on research, the term Automatic Level Control(ALC) and Automatic Gain Control(AGC) confused me.What is the different between this term?As far as i concern,both of this term nearly the same.


    TDA 2822M - linear amplifier
    NE 570 - (Compandor) non-linear amplifier

    Do i get this correct?If yes,i would not be able to coect this ICs together right?


    Thanks.
     
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Effectively the same thing.

    Yes, of course you could - the TDA2822 is an audio power amplifier (I think?) - you would use that to feed a speaker, and the compander IC could be used to feed it's input to keep the volume level constant as the input signal changes.

    Really it depends what you're trying to do?.
     
  3. elnino86

    elnino86 New Member

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    Hey,thanks.:) BTW,Im designing a hearing aid system.
     
    Last edited: Dec 12, 2008
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The Compandor Cookbook I gave you has an Automatic Level Control schematic.
    Its output can feed the input of the TDA2822M power amplifier.

    Won't ALC be dangerous? Loud sounds and soft sounds would sound the same. SPLAT!
    Hit by a car, truck or airplane.
     
  6. lasvegasmale31

    lasvegasmale31 New Member

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    I am building a spectrum analyzer based upon the LM 3915 ic and using rectified input from the audio source (line out from car stereo). I was wondering if there was a way to use an AGC circuit to even out the input to the circuit so at various volume levels, the spectrum analyzer operated within the scale. I hope I am asking the question well enough so you know what result I want.
     
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    An LM3915 does not need an input rectifier. A rectifier would cutoff all low input levels. It needs to have a peak detector circuit so that your vision can see the peaks. Without a peak detector then the LEDs are just a dim blur.

    If you want to use an ALC circuit then it must be slow acting so that the peaks still show but the average level is slowly adjusted.
     
  8. lasvegasmale31

    lasvegasmale31 New Member

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    Sorry Guru, I mis-stated, it is a peak detector.

    Is there such a circuit that you know of that I could use to perform this averaging function while still performing the "ALC" I am looking for?

    I don't mean to ask you to do my work for me, but am having poor results searching google for the proper information. Maybe I am not phrasing my search words correctly.
     
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The NE570, NE571 and NE572 compandor ICs have a rectifier capacitor whose value determines the speed of its ALC. A big capacitor will cause a reaction time that is slow enough for what you want.
     
  10. elnino86

    elnino86 New Member

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    First of all,thanks for the info.:)

    Im intend to amplify only soft sound(reduce dynamic range) while ensuring that the loud sound are not too loud, reduce distortion, and unify conversational speech.

    Since ALC circuit wont cope with my criteria,obviously i need compression circuit right?Do i need DS1666 as a volume control if im intend to use NE 570?
     
  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You need an ALC circuit which will expand low levels and compress loud levels.

    I don't know anybody with a hearing aid. I think the volume is not adjustable, I think the volume is adjusted only when it is new to the user then it is not changed again.
    You can use a trimpot for your demonstration.
     
  12. elnino86

    elnino86 New Member

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    Thanks. As you wrote earlier, loud sounds and soft sounds would sound the same.Im starting to get confuse here.Can you explain a little bit?Sorry.Im learning from the best.
     
    Last edited: Dec 13, 2008
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Deaf people usually have a loss of high frequency sounds. Equalization to boost high frequencies is used and maybe some extra gain at all frequencies if the hearing loss is at all frequencies.

    I don't think ALC is used in hearing aids. Maybe compression is useful so that the hearing aid does not distort loud sounds.
     
  14. Mikebits

    Mikebits Well-Known Member

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    Not always.
    When I lost my hearing in my left ear the response was 70 dB loss across the spectrum, with a little peak around 1KHz.
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Is there a hearing aid that is capable of boosting sounds 70dB?
     
  16. Mikebits

    Mikebits Well-Known Member

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    I have no idea. Lucky for me, my hearing returned after a good dose of prednisone which had its own side effects...
     
  17. johngordon77

    johngordon77 New Member

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    Hello, I need help with an auto level controll circuit. I'm using the NE572 compander. I've designed according to the data sheet available from phillips semiconductors. The problem is with bandwidth. I'm building a chromatic tuner for fourth year electrical engineering design course. I'm using the auto level controll to hold my signal level from a mic for acoustic instruments long enough so the rest of my circuit can work with it. I'm using an electret mic. The problem I'm having is that the gain is constan fro, 100Hz to 1kHZ, anything above or below that and the gain is halved. I need to go from 27.5Hz to about 4.5kHz. The circuit I used is straight off the data sheet, and I'm using a supply of 6V. I also seem to be having some problems with the electret mic. I'm running it off a 6V supply from a 10k resistor. If anyone can help, i would be forever greatfull.
     
  18. elnino86

    elnino86 New Member

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    Hi johngordon,

    to polarize the electret mic,you need to know what type of ecm do you used. From 6V supply and 10K resistor,i think you polarize your mic with 0.6mA.Is that enough for your mic?

    Regarding to NE572,if you using NE572 it is quite diiferent from NE571.As stated in the cookbook,

    1.There is no internal op amp
    2.The attack and release time are programmed separately.

    Im not really sure about the bandwidth here.
     
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The NE572 is designed for line level signals, not mic levels that are much too low.
    I used the NE572 for expanding and compressing hi-fi line level audio and the bandwidth was perfect.

    With a mic level input the circuit is operating all the time with max gain and maybe then the bandwidth is reduced.
    A preamp is needed for a mic.

    An electret mic powered from a 10k resistor from +6V has a current of about 0.4mA, not 0.6mA. The current would be 0.6mA only if the mic is shorted.
     
  20. elnino86

    elnino86 New Member

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    Hai Audioguru,how the current can reduce to 0.4mA?

    How to set a threshold voltage for NE571?It not stated in the cookbook.:(
     
  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    An electret mic has a FET transistor. Its current is about 0.5mA when it has 4V across it (8V supply feeding a 10k resistor in series) and is about 0.4mA when it has 2V across it (6V supply feeding a 10k resistor in series).

    The NE571 is designed for a signal of 0.9V RMS where it does not expand nor compress. Higher levels are compressed and lower levels are expanded.
    Use a preamp for more sensitivity or use an attenuator for less sensitivity.
     

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