help needed with attenuating audio?

Discussion in 'The Projects Forum' started by rede2go, Oct 1, 2011.

  1. rede2go

    Thread Starter New Member

    Oct 1, 2011
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    Hello, as a newbie i did search through the forum and did not find anything useful in soling my problem.
    I have a old am/fm cassette radio in my old camper and it does not have a
    Aux input. I need to be able to plug a mp3 player or cell phone into it when
    we use it. I soldered a cable to where the tape head went of the board however even turning the player as low as it can go it is still driving it way to hard, it is very distorted my thought was to but a resistor on the end of each leg on the right and left input. Is this a good way to do it? and if not wha is a simple cheap way? my thought was 1/8watt 10kohm resistors.
    Any input would be awesome.
    Thank you all for reading.
    Ron
     
  2. PatM

    Active Member

    Dec 31, 2010
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    72
    Why not tap into the high end of the volume control?
     
  3. rede2go

    Thread Starter New Member

    Oct 1, 2011
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    Im not real sure what you mean? run the output of the mp3 player to the left and right volume control? would that work and or would it interfere with the radio side?
    Thanks I watching and waiting:)
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The tape head outputs are in microvolts, so there is a lot of amplification in addition to noise reduction applied.

    Does the back of the deck have RCA inputs? (Red/White jacks)

    If not, photos of the internals, clear where traces can be followed on both sides, will help.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    You might try something like this:

    [​IMG]

    That will reduce the input volume to ~9.1% maximum.

    A long time ago, you could get AM/FM tuners that would plug into a cassette deck (they were cassette tape shaped) and tune stations with it. They coupled the signal to the tape deck head inductively.

    I don't show decoupling caps in this schematic, but they would be a good idea. 0.1uF/100nF caps between the pot wipers and the tape head will prevent any DC from going through the tape deck heads. Without the caps, you might have DC current through the heads, which will magnetize them, resulting in poor audio quality if you try to use tapes afterwards. Your MP3 player should have caps to block the DC, but you never know.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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  7. rede2go

    Thread Starter New Member

    Oct 1, 2011
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    Thanks all for the responses
    to "thatoneguy" there is no input on the back if there was i would just use that.
    "MR Chips" I did try one of the cassette adapters but the sound was not
    good at all, it might have worked better if the adapter would line up with
    the tape head to make a perfect alignment but i cant stand there all day
    and hold it:)
    "SGTWookie" I like your idea and as far as the cassette possiblly not working, that is ok, As the first thing i did was to remove the wires from the
    board and solder the output of the cable from the mp3 inplace of the tape head. So my plan as of now is to run out and get the few parts and try what you wrote up and give that a try.
    I thank all of you very much and love the power of the web:)
    I will report back on how it goes.
    Again thank you all,
    Ron
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    Before you run out and get some parts, I would try it first with a simple voltage divider. This would save having to buy and install pots. Keep the 100K resistor and try 100 ohms or 1K ohms in place of the pot. Experiment a bit with this to get the right sound level. Try it with one channel first before applying it to both.
     
  9. rede2go

    Thread Starter New Member

    Oct 1, 2011
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    MrChips thats a good idea, I can try it, I have not been to a radio shack in
    25 years i hope they still sell parts?.
    Now i just have to get out of the house and get going.
    Thanks Ron
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Radio Shack still sells some parts. They are actually trying to add a few; nothing like they were in the 70's though.

    However, they'll have some 100k resistors and 5k or 10k pots.

    I suggested the pots so that it would be relatively easy to adjust the volume. I suppose I should have added some ~33 Ohm resistors to simulate headphones, but didn't think of it at the time. Three 100 Ohm resistors in parallel would work for that.
     
  11. rede2go

    Thread Starter New Member

    Oct 1, 2011
    5
    0
    Hello all.
    Ok i have gathered all the items from all of the postings, My questions are
    as follows.
    I can follow basic schematics so i know what to do with the one from
    SgtWookie however the changes via MrChips and the 2nd add on by
    SgtWookie and i am lost. Could you please show a schematic with the
    changes on it so i can try it. I have parts and a hot iron and want to do this. Thank you guys again!
    Ron
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    OK, the original schematic is on the left.
    On the right, the two pots were replaced with fixed resistors. You might do that after you've figured out what resistance works the best using the pots.

    The 33 Ohm resistors I was talking about are also shown.

    [​IMG]
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    Ok, Wookie beats me to it again.

    [​IMG]

    The first circuit is what I would have tried first.

    In my second circuit above, Wookie has added a resistor (R3) to replace the headphones. He has used 33 ohm. Experiment with the values.
     
    Last edited: Oct 2, 2011
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