Could you check this circuit please? 741 Pre Amp(Student / Easy)

Discussion in 'Homework Help' started by Scott Walker, Feb 8, 2016.

  1. Scott Walker

    Thread Starter New Member

    Feb 8, 2016
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    I need to create an inverted 741 pre amplifier circuit on a vero board
    I found this diagram on the internet but it wasnt inverted so I used photoshop to swap pin 2 and 3 so it was correct.

    I have a number of questions:
    Firstly are the polar directions correct on this diagram since the change?
    I don't know how AF in and out are connected, for example would C3 connect to a wire, connect that to AF out and then go to ground?

    Thanks,
    I am an audio student at university but one of the modules is Electronics in which I struggle with sometimes
     
  2. Veracohr

    Well-Known Member

    Jan 3, 2011
    552
    76
    C1 and C3 should be at least 10 times larger unless you're not concerned with low frequencies.

    Try a better opamp than a 741. NE5532 are used a lot in audio circuits and are good.
     
    Last edited: Feb 8, 2016
  3. Scott Walker

    Thread Starter New Member

    Feb 8, 2016
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    Thanks for the reply, I have a set list of components I need to use.

    What about the way this diagram shows AF in and out, how would that be hooked up?

    Cheers
     
  4. Veracohr

    Well-Known Member

    Jan 3, 2011
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    I edited my reply, I was wrong about C3.

    AF in and out connectors will have two contacts: signal and common (ground). All common connections connect together, including the opamp ground on pin 4.
     
  5. tindel

    Active Member

    Sep 16, 2012
    568
    193
    Since this appears to be homework - I won't give answers, but will drop hints.

    Be careful driving a capacitive load with an opamp - it can lead to instability (oscillations). I can think of a couple better ways to vary the output magnitude... although what you have done will work (assuming you figure out how to drive a capacitive load with your opamp). The 741 may be able to drive capacitive loads... I forget and it's been nearly 10 years since I used a 741.

    I disagree that C3 is a good value. C3 is only a good value if the trim pot is dialed in one direction - or you are adding a variable filter.

    Don't forget to decouple your power rails.
     
    Scott Walker likes this.
  6. Scott Walker

    Thread Starter New Member

    Feb 8, 2016
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    So am I right in thinking one connection will go at the start of the circuit, making it's way to pin 2 and the other connection simply going to ground?

    Cheers
     
  7. Veracohr

    Well-Known Member

    Jan 3, 2011
    552
    76
    Say you have a 1/4" TS connector. The tip is the signal and will go to C1, the sleeve goes to ground.
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,542
    1,251
    With C3 where it is, the opamp is putting a constant 7.5Vdc across the pot regardless of the signal amplitude or pot adjustment. While the 741 can do this, there is no good reason for it. I suggest moving C3 so it is in series between 741 pin 6 and the top of VR1, positive end toward pin 6.

    The input and output components form two 80 Hz high-pass filters. To improve the circuit bass response, increase C1 and C3 to 10 uF.

    ak
     
  9. #12

    Expert

    Nov 30, 2010
    16,321
    6,818
    I see another likely problem area. When you dial an op-amp up to a gain of 100, they have a bad tendency to pick up noise. Put this in a metal box to shield the circuits from electronic noise in the air.
     
  10. Scott Walker

    Thread Starter New Member

    Feb 8, 2016
    6
    0
    Thanks everyone for the replies, I will test everything out on a breadboard tomorrow and hope for the best! (Y)
     
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