radio Volume too loud for my PC

Discussion in 'General Electronics Chat' started by mik3ca, Feb 7, 2008.

  1. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    189
    0
    My power amplifier follows this circuit:

    [​IMG]

    I have found something interesting.

    It seems that if the capacitor C2 is high, the volume is TOO LOUD, but the base of the music comes in more clearly. If the capacitor C2 is omitted the volume is OK, but the sound is at best distorted.

    Why do I know it is too loud, because my computer measured it for me.
    My audio output is connected to the microphone input of my computer running Windows 95. When I record sound, all the LED bars light up when C2 is high. when C2 is omitted, all the lights but the top 2 light up. This also applies even if the microphone volume is ridiculously low.

    So, now I think that I need a circuit that can cut the volume down, but can still produce the same (or better quality) output.

    I want to see the LED bars follow the music.

    the LED bars I talk about are the LED bars you see in windows sound volume control applet, not the actual hardware LED's.
     
  2. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Don't use the mic input, try the LINE IN input.
     
  3. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    189
    0
    I don't have a "line-in" input.
    I'm hooking up my radio to my laptop.
    The only audio connections I have are microphone and speaker-out.
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Without C2 then the top of the waveform is severely compressed and is very distorted. I showed a simulation in your other thread.
    With C2 it is a bootstrapped amplifier. The bootstrapping allows the positive swing of the signal and reduces distortion.

    Your computer probably just rectified the top half of the waveform and measured the reduced level of the compressed signal when the amplifier did not have C2.

    A mic input expects the low level from a microphone which is about 5mV to 10mV. Your radio might have a line level output of 100mV to 220mV.

    Simply make an attenuator with two resistors:
    1k in series from the output of this amplifier feeding a 100 ohm resistor to ground. Connect the mic input of the computer to the junction of the resistors and to ground. Then the signal is reduced to 1/11th. If it is still to loud then reduce the 100 ohms to 47 ohms, 33 ohms or to 22 ohms.
     
  5. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Don't use the power amp, instead connect the input to the power amp directly to the PC.

    Or, you could place a 10K pot between the output at C3 and ground, the pot wiper then becomes the output to the PC. Adjust the volume accordingly.

    The MIC input is not the best for these types of signals, they're meant for microphones.
     
  6. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    189
    0
    I was thinking something like that. I guess I will have to try it.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Another thing you could try is reducing the input signal to the amplifier.

    See the attached. There is a 100k audio taper pot added between the signal source and the amplifier. If you connect an audio taper pot incorrectly, it will act more like an on-off switch than a volume control.
     
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