Audio Line-In Aux

Discussion in 'The Projects Forum' started by JMark, Oct 19, 2014.

  1. JMark

    Thread Starter New Member

    Mar 3, 2014
    18
    0
    HI,

    I have a vintage radio (one built-in speaker, stereo through headphones) that uses two TBA820M IC chips as the amp, one for the left channel and one for the right. I added a LINE-IN to this radio by switching the input to each chip (Pin 3) to a 3.5mm stereo input jack. I added one 3.3uf electrolytic capacitor at the jack for each channel. This is to be used with an MP3 player, phone, etc. so the audio source will mostly come from a headphone jack.

    Everything works really nice. There may sometimes be a small audio pop when switching songs or pressing buttons of the audio source device. I was curious if its practical or possible to build a simple muting circuit or filter that can smooth out these occasional unwanted sounds. There is no problem as it is now and everything is working as expected. I just wanted to explore the idea of enhancing this project. I did search the forums and was not able to find anything on this. Thanks!
     
  2. #12

    Expert

    Nov 30, 2010
    16,252
    6,749
    If I had a schematic I might tell you where to add bleed-off resistors to return the input capacitors to ground potential between switching activities. If you're smart and lucky, you can figure out what I mean just from the first sentence.
     
  3. JMark

    Thread Starter New Member

    Mar 3, 2014
    18
    0
    HI, Thanks for the reply! I'll look deeper into what you said but I'm a novice so it may take a while. The service manual/schematic for the radio is available at http://www.imagetavern.com/Satellit 700/Grundig Satellit 700 SM.PDF Attached is a screen shot also. Will the bleed off resistors have any effect on the radio when not receiving the external feed? I actually disabled the radio's muting function so I can tune and scan through the bands without chuffing, something that is commonly done on shortwave radios.
     
    Last edited: Oct 19, 2014
  4. #12

    Expert

    Nov 30, 2010
    16,252
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    This is more theory than a straight answer, but here goes: When you bring in a separate input, it might have a DC level associated with it. I drew this as a 3 volt MP3 player with a 1.5 volt DC level on the output. That 1.5 volts is supposed to be blocked by a capacitor, and it is, but the capacitor charges up whenever you turn the MP3 player on. The principle here is that you let that 1.5 volts leak off the capacitor into the Grundig ground before you switch inputs. Typically a tenth of a second is where you want to set your resistor for. I can''t tell what size the capacitor in the MP3 player is so I can't tell you the resistance. I guessed that 2700 ohms is not going to interfere with the MP3 output quality and might bleed off the click in a tenth of a second.

    If you understand what I'm saying, you can twiddle with different resistors until you get good results. If you don't understand, we're stuck. The best part is that there are a bunch of smart people here that might be able to explain this in ways you can understand.

    And then, I might be completely wrong.
     
  5. JMark

    Thread Starter New Member

    Mar 3, 2014
    18
    0
    HI, I think I understand it at the most basic level. Whenever I do these projects I not only desire that they work correctly, but I like to understand and learn as much as I'm able to along the way so thanks for taking the time to explain this and for the schematic. I will play around with it and report back.
     
  6. #12

    Expert

    Nov 30, 2010
    16,252
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    Look at this one. When you turn this preamp on the output immediately jumps to about 6 volts. If the 390k resistor wasn't there to bleed off the start-up charge, it would click awfully when you first plug it into a power amplifier. Notice that 390k times 4.7 uf = 1.8 seconds, so I could have made that 390 k into a 68k resistor to bleed off the click in about 1/3 second, except it would have eaten about 20% of the output voltage.
     
  7. JMark

    Thread Starter New Member

    Mar 3, 2014
    18
    0
    HI, Fascinating stuff. From your posts I also better understand the function of the input capacitors. I played around with some different resistor values, not a huge difference with the different values but I settled on 24k. The pop has been reduced to a small insignificant split second click. I don't know if its my imagination but the audio seems improved or something seems to just be working better now overall. Thanks so much!!
     
    Last edited: Oct 20, 2014
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