Audio Input Attenuator Pot

Discussion in 'General Electronics Chat' started by tracecom, Dec 13, 2012.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    In a circuit such as the one attached, what difference does the value of VR1 make? Would a 5k pot perform any differently from a 50k or 100k?

    Thanks.
     
  2. Audioguru

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    Dec 20, 2007
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    Does your input signal have enough "muscle" to drive 5k ohms without distortion?
     
  3. tracecom

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    I am connecting the audio output from my TV to an FM transmitter so that I can listen to the TV with the FM function on my MP3 player and not disturb my wife. I have not measured the output from the TV, but the audio received on the FM signal is quite distorted. Using a lower level audio source (another MP3 player), the audio on the FM signal sounds good. So, it seems that the audio output from the TV is quite high.

    Thanks.
     
    Last edited: Dec 14, 2012
  4. tracecom

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    @ AG
    The output from the TV is about 1.6 Vpp (maximum.) Here is the schematic of the transmitter. (I know you don't like the BA1404, but that's what it is.)

    What value would you recommend for an attenuation pot?

    Thanks.
     
  5. MrChips

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    Oct 2, 2009
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    The TV output is probably designed to drive a low impedance headset.
    Hence any low resistance voltage divider or pot should work.
     
  6. tracecom

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    The stereo outputs from the TV that I am measuring are a couple (R and L) of RCA jacks, which are not affected by the volume control or the mute switch on the remote. I don't actually know what their intended purpose is.
     
  7. MrChips

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    Ok. RCA jacks must mean LINE OUT.
    Try any size pot. 5k should work.
     
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  8. Audioguru

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    The input pre-emphasis treble boost RC circuits in the lousy old obsolete BA1404 FM stereo transmitter circuit are calculated for European or Australian FM radios.
    In North America C1 and C4 should be 1500pF.

    If a volume control pot is used at the input then the pre-emphasis works best if the pot resistance is 5k ohms or less. The inputs to the transmitter IC are usually fed from the very low resistance of a headphones output that has its own volume controls.
     
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  9. tracecom

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    Do they need to be mylar film or will ceramic work just as well?
     
  10. Audioguru

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    I always use a 5% polyester film capacitor for passing or filtering audio. EPCOS brand capacitors are small, inexpensive and are packaged in a blue plastic box.

    A ceramic capacitor has a horrible tolerance that also changes with temperature changes and causes distortion because the value also changes with voltage (signal) changes. A ceramic capacitor is also microphonic (picks up and produces background sounds).
     
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