Question about jack output

Discussion in 'General Electronics Chat' started by cumesoftware, Jun 11, 2007.

  1. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
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    Anyone knows what might be the standard RMS or peak voltage of a typical jack audio output? I need this to determine the gain using a TDA2003 amplifier (I won't be using gain 100, the full gain of the amp, but maybe 20 to 30).

    P.S.: The last amplifier project I made was with a LM386 with gain 20, but the output was still bouncing to the rails (fortunately it happened only when the volume was set to maximum). I used bass "boost" (tecnically it is a treble atenuation), so the effect was less noticeable. The output was from a s*ny diskman stated as a 12mW output.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    I believe the level for an audio signal is 1.25 VRMS.
     
  3. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
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    I have been looking for this and confirmed that for some amps with 25mm jacks. But for a 15mm stereo jack (phones), is the output the same?
    Also, my diskman gave me a different, much smaller value. With a cap charging on the output (since the output is polarized), I could read 200mV sometimes (being this the peak value). I don't know, but the 1,25V RMS seems a tad big to me. I think with those values, the LM386 amp (used like the one in the projects section) would saturate. It only happened with volume set to the maximum in the diskman and in the amp as well.

    Also, for a positive waveform like the one present in the LM386 amplifier (input and output as well), what should be the RMS factor?
     
  4. beenthere

    Retired Moderator

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    My understanding is that is the 0 dB level, and that it is the same for every audio connector.

    The gain of a power amp is such that a 0 dB level signal should drive it to the maximum output level. That is why level controls are attenuators.
     
  5. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
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    Isn't the Decibel unit a relative unit? So 0dB means no gain (and no atenuation), as far as I can understeand. The only ablsolute units I see derived from the decibel unit are dBu (or dBv), dBV, dBmV and dBmW. To which are you refering?
     
  6. beenthere

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  7. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
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    Saw that on wikipedia. The confusion is, 1dBV = 1V RMS, 1dBu = 0.775V RMS. So should I consider dBu or dBV?
     
  8. beenthere

    Retired Moderator

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    For audio signals, the VU scale is the one to use.
     
  9. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
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    Thanks. That's what I want to know.

    So, refering to the VU scale, if we have 1dBu = 0.775V RMS, the peak value is 1.25V?
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    Were it only that simple. The VU scale is not absolute, unless you happen to be measuring a steady sine wave input. My best recollection of the source of the 1.25 volt figure I gave you came from an answer to a question I asked many years ago. "About 1.25 volts" was the answer. Being a technician, it seemed close enough.

    Perhaps this reference will give you a better idea of audio input metering - http://www.tape.com/cgi-bin/SoftCar..._or_not_db.html?L+cassette+nqna2923+995498793.
     
  11. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
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    Thanks. Thats a good reference.

    I saw that balanced and unbalanced connections (jacks) use different definitions, being 1.23V for balanced and 0.316V for unbalanced (peak voltages I presume). My connection is unbalanced stereo, and the value I got is more close to that.

    Helped me a lot. Thanks!
     
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