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741 op amp dilemma

Discussion in 'General Electronics Discussion' started by lewisluther, Oct 15, 2016.

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  1. lewisluther

    lewisluther

    9
    1
    Jun 20, 2014
    So, I'm trying to build a guitar pedal.
    After much tinkering with the initial buffer op amp (a simple feedback amp), I finally got sound out of it,
    Awful sound.
    Then I realise my battery isnt connected, so it must be attempting to operate passively somehow, hence why it sounded awful.

    Anyway, I attach the battery and it doesn't make even the teeniest sound now.

    Anyone seen anything like this before?
     
  2. Alec_t

    Alec_t

    2,890
    775
    Jul 7, 2015
    Partly due to the 741 being an awful opamp (by today's standards) :) .
    Post a schematic of your amp so that we can spot any likely cause for your woes.
     
    hevans1944 and Arouse1973 like this.
  3. hevans1944

    hevans1944 Hop - AC8NS

    4,536
    2,112
    Jun 21, 2012
    Unfortunately, yes. In the 1970s, early in my career, I once spent an entire working day trying to troubleshoot a complicated op-amp circuit design before discovering the power supply on my workbench was not plugged in.:mad:

    Do please post a schematic diagram as Alec suggested. And tell us what you want your guitar pedal to DO. Is this an effects pedal, a volume control, or what?

    If you are using op-amps, I have to assume some sort of guitar-string pick-up signal conditioning is involved, but this may need to occur after the guitar amplifier input. The shielding and wiring between the guitar pick-up jack and the rest of the real world requires careful thought. The signal is low-level and susceptible to "hum" interference because of the 60 Hz AC world Nikola Tesla conveniently provided for us, long before anyone had electric guitars and amps and monster speakers. If adding effects sounds, it is best to do this after the signal has been amplified to a more suitable level, from millivolts to volts.
     
  4. lewisluther

    lewisluther

    9
    1
    Jun 20, 2014
    Hi, guys.

    Thanks for responding. I've uploaded a diagram here. it is part of a bigger diagram that I modified from someone else's design. that part seems to work ok. it's just this input buffer. I haven't worked out what value R6 should be yet.
    The output from this will then go on to overdrive and EQ circuits.

    What's so bad about the 741 now?
     

    Attached Files:

  5. davenn

    davenn Moderator

    13,654
    1,888
    Sep 5, 2009
    why are you putting 4.5V directly onto the inverting input ??
    .that doesn't look good
     
  6. Alec_t

    Alec_t

    2,890
    775
    Jul 7, 2015
    You appear to have the inverting and non-inverting inputs swapped.
    R6 as shown would provide positive feedback. You want negative feedback for an audio amp.
    R1 and R2 values are rather low, so the battery life will be short.
    The 741 is bad because (a) it is noisy and (b) it can handle inputs within only a narrow amplitude range (~ 3V peak to peak, 1.1V RMS, when the supply voltage is 9V) without causing significant distortion, A guitar pickup output may well exceed that range.
     
  7. lewisluther

    lewisluther

    9
    1
    Jun 20, 2014
    Thank you so much. I swapped the inverting and non-inverting inputs and it's working fine now.

    Do you have any value recommendations for R1&2?
     
  8. lewisluther

    lewisluther

    9
    1
    Jun 20, 2014
    Wait, I was looking at 3&4. I see what you mean now. Thank you. :)
     
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