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Problem with chained LM3915

Discussion in 'General Electronics Chat' started by fezder, Nov 1, 2015.

  1. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Good, it works.
    Your frequency is extremely low at 8.69Hz so your 'scope shows the peak detector output voltage ramping down between half-wave pulses.
    Try removing Q3 and/or R14 to remove the Automatic Gain Control compression. The AGC allows a signal 10 times higher to activate the same LEDs as the lower level.
    Do not exceed a peak input voltage of 2.1V which is only 0.276W RMS in an 8 ohm speaker. The value of R9 must be increased to allow higher input levels.
     
  2. fezder

    fezder Well-Known Member

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    thanks, i was wondering whether you had interest in this subject left.
    Ah so that meaning AGC had. How that portion works? higher input signal=more gain? at least so it seems as q3's base is tied to opamps output
    No worries, i don't think i'll exceed input voltage, thanks for warning

    I tested taking q3/r14 out circuit, and it doesn't effect output waveform, which is good
    but, i tried changing r9 from 50k to 100k and output from peak detector drops so it doesn't show peak anymore, even at new cutoff which is half from earlier
    all shots have Q3 removed
    r9=50k (9.8hz)
    NewFile12.png
    r9=100k (9.8hz)
    NewFile13.png
    r9=100k (4.8hz)
    NewFile14.png
    and even at audible frequencies like 60hz or so peak doesn't rise in r9=100k situation
     

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