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  1. #1
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    replacing speaker with aux cable??

    hi fellas.
    I want to replace output of a ampifier(lm386) which is a speaker with an aux cable, so I can use amp circuit to have more sensitive sound recorder. (20x gain)
    is it this simple or im wrong?
    can this cause any damage at 2nd circuit (my phone aux cable input)?

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  2. #2
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    Re: replacing speaker with aux cable??

    Hi,

    You can use it to
    * increase gain

    But it also has some drawbacks:
    * increased noise
    * increased distortion

    can this cause any damage at 2nd circuit
    You get increased voltage .. and it may drive a lot of current...
    So there is a possibility that your "2nd circuit" (whatever this is) may not withstand this.

    btw: the output capacitor is calculated for the speaker impedance. Thus the cutoff frequency may become very low.

    Klaus
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    Re: replacing speaker with aux cable??

    You should use a low noise audio opamp, not a noisy LM386 power amp.



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    Re: replacing speaker with aux cable??

    I want to replace output of a ampifier(lm386) which is a speaker with an aux cable, so I can use amp circuit to have more sensitive sound recorder.
    Surely you can; but the aux output follows some standard for compatibility. Aux output is designed to be fed into a power amplifier.

    In this particular case, the speaker is a low power one and connecting the aux output to the speaker may work but you cannot connect that directly into ANY aux input without checking the voltage levels.

    Aux signals are what is called at line level; they are usually output from the preamplifier after equalisation, mixer etc and are designed to be fed directly into a power amplifier.


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    Re: replacing speaker with aux cable??

    Quote Originally Posted by KlausST View Post
    Hi,

    You can use it to
    * increase gain

    But it also has some drawbacks:
    * increased noise
    * increased distortion


    You get increased voltage .. and it may drive a lot of current...
    So there is a possibility that your "2nd circuit" (whatever this is) may not withstand this.

    btw: the output capacitor is calculated for the speaker impedance. Thus the cutoff frequency may become very low.

    Klaus
    Quote Originally Posted by c_mitra View Post
    Surely you can; but the aux output follows some standard for compatibility. Aux output is designed to be fed into a power amplifier.

    In this particular case, the speaker is a low power one and connecting the aux output to the speaker may work but you cannot connect that directly into ANY aux input without checking the voltage levels.

    Aux signals are what is called at line level; they are usually output from the preamplifier after equalisation, mixer etc and are designed to be fed directly into a power amplifier.

    thanks for the reply! I almost finished the circuit, and I've added a blinking LED with two transistors. all of the components are being fed through 78lo05 and I found that theres no output sound except the hush and the beep pulse of the led :/ so this means I should remove led circuit? (the whole package could look nice together,ps there's no more space for extra component )
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    Last edited by Javid.zare.s; 6th February 2019 at 10:39.



  6. #6
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    Re: replacing speaker with aux cable??

    Quote Originally Posted by Javid.zare.s View Post
    I found that theres no output sound except the hush and the beep pulse of the led
    Then the audio amplifier circuit does not work.
    The extremely high value (250uF) of the output coupling capacitor is calculated to charge quickly when feeding an 8 ohm speaker but will take all day to charge into the 100k input of a power amplifier. Did you re-calculate the value of the output coupling capacitor so that it is suitable to feed whatever is the input resistance of your power amplifier?

    Your schematic shows a grounded volume control at the input of the LM386 amplifier which is correct only if the input signal is also grounded with no DC voltage on it. Measure if there is any DC voltage on your signal source and block it with a properly calculated coupling capacitor if there is any DC. If the mic is an electret type then it has DC on it and you must block its DC with a coupling capacitor. Use 0.33uF for voices or use 1uF for music.

    You did not show the schematic of your blinker so we do not know if it is overloading your voltage regulator. You do not even say what is the voltage that is feeding the voltage regulator. A worn out 9V battery that drops to 6V will not work. Does the input to the regulator have a capacitor to ground as shown on its datasheet?



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  7. #7
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    Re: replacing speaker with aux cable??

    Quote Originally Posted by Audioguru View Post
    Then the audio amplifier circuit does not work.
    The extremely high value (250uF) of the output coupling capacitor is calculated to charge quickly when feeding an 8 ohm speaker but will take all day to charge into the 100k input of a power amplifier. Did you re-calculate the value of the output coupling capacitor so that it is suitable to feed whatever is the input resistance of your power amplifier?

    Your schematic shows a grounded volume control at the input of the LM386 amplifier which is correct only if the input signal is also grounded with no DC voltage on it. Measure if there is any DC voltage on your signal source and block it with a properly calculated coupling capacitor if there is any DC. If the mic is an electret type then it has DC on it and you must block its DC with a coupling capacitor. Use 0.33uF for voices or use 1uF for music.

    You did not show the schematic of your blinker so we do not know if it is overloading your voltage regulator. You do not even say what is the voltage that is feeding the voltage regulator. A worn out 9V battery that drops to 6V will not work. Does the input to the regulator have a capacitor to ground as shown on its datasheet?
    ive changed input configuration, the 10 mf should be replaced with 0.33mf?
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    and this is blinking led circuit
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  8. #8
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    Re: replacing speaker with aux cable??

    You say 10mf which is 10 milli-farads but we say 10uF which is 10 micro-Farads. Yours is 1000 times more.
    Before you showed the amplifier pin 3 input properly connected to 0V with a 10k volume control but now you add an input capacitor to block the DC that powers the electret mic. The missing capacitor is why the amplifier did not work.

    Why was the input capacitor the very high value of 10uF? The value must be calculated by the lowest frequency you want, not guessed. Since you do not show a volume control at the input of the amplifier then the capacitor value can feed the 50k input resistance of the amplifier shown on its datasheet and produce the lowest frequency of 10Hz (an earthquake frequency) if a 330nF (0.33uf) input capacitor is used. Use a 100nF (0.1uF) film capacitor instead.

    I was right. The LED blinker circuit is missing an important current-limiting resistor so each blink causes the 9V battery voltage to fall flat on its face and cause noises in the sound. Also the LED and BC547 might burn out.
    Also the voltage regulator is missing an important 330nF (0.33uF) ceramic high frequency capacitor shown on its datasheet, plus another 220uF capacitor parallel with the battery to smooth the 9V.
    Also the 10k resistor that powers the mic should be fed from a 1k/47uF filter.


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