Audio amp hum/whine

Discussion in 'General Electronics Chat' started by Veracohr, Jun 29, 2015.

  1. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
    552
    76
    I built a little audio amp pretty much for something to do, and I'm having one problem with it.

    When the input is left floating (ie. not connected), there is a whine (or hum, but higher). When the input is connected to an audio device (or I ground the input pin), the whine stops. It's at about 900Hz, but changes a little over time and can also change by adjusting a pot (see below).

    I think it might be the power supply ripple coming through, because it's around the same peak-to-peak value as the maximum that the supply's datasheet specifies (150mV p-p max). But that seems odd because I expected the circuit to reduce the ripple at least somewhat even without a regulator, according to simulation. It's a wall-wart switchmode supply.

    The schematic is attached. R19 (shown with the value "{ra}") is adjustable from 3.6k - 4.6k (to optimize distortion), and adjusting it affects the frequency of the whine over about an octave.

    What might be causing this?

    amp.png
     
  2. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    Start with this one:
    Connect R17 and R18, but not to the input. Add a filter cap at the center point and add a hundred k from the center voltage over to the input pin.

    There is also the idea of not operating the amplifier with nothing plugged in to the input.
    Old story about Fender amplifiers: If it hums when the volume control is turned to zero, don't play it with the volume control turned to zero. :D

    You could try a couple of nanofarads across R3 to place a gain roll-off at 17KHz.
    Add a nanofarad right across the power supply pins of U1...
    or isolate the power supply to U1 with a few hundred ohms and filter with a hundred uf plus a nanoF.
     
    Hypatia's Protege likes this.
  3. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    2,791
    1,236
    Indeed!:D

    @Veracohr: In general, 'linear' and 'near linear' amplifiers tend to exhibit instability under 'zero' excitation conditions -- Double-ditto when the input is 'left floating' or otherwise terminated with excessively high impedance...

    Best Regards
    HP:)
     
    Last edited: Jun 29, 2015
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Mr Practical 12 is correct the problem is almost certainly arising at the input to the high impedance TL071.

    How long is the connection between R17/R18 and the op amp?
    Check the layout to avoid pickup.
    And yes if that does not work then you will have to stabilise the artificial ground they create at their junction, better with two capacitors than one as #12 suggests.
     
    #12 likes this.
  5. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
    552
    76
    If this is what you mean, it didn't work and also significantly reduces the gain in simulation because of the size of the input cap. I assume one would want a low filter cutoff so I used a larger filter cap.

    input.png


    I've never had problems operating commercially made amplifiers with nothing plugged in.


    This worked. 100 ohms plus 100uF. I didn't want to use too high of a resistor so as not to drop the opamp supply too much. Thanks!


    I also tried substituting a NE5532 (much lower input impedance) and it had the same problem.

    A couple millimeters.

    Since I also want to learn in addition to just having a working amp, what's going on that is causing this? The fact that adjusting the pot can change the frequency makes me doubt that it is just the power supply ripple, since I would expect that to be a steady frequency. Also, I connected an identical amp on a breadboard (the previous one is soldered on stripboard), and the buzz was MUCH louder. The only difference is that the output NPN transistors have much higher beta than the ones soldered (same model).
     
  6. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    Somebody else can try to figure out why the second stage affects the first stage.
     
  7. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
    552
    76
    I meant I tried the other opamp before trying your solution (and before starting the thread).
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,540
    1,251
    The schematic in post #6 has a problem. The DC bias network on the + input is not right. R17, R19, and C9 form a virtual ground network, and it's junction goes to the + input through R11 as shown. But the right side of C4 goes directly to the + input, *not* the DC bias point. Also, what is Vx and what is the purpose of R3 and C3?

    ak
     
  9. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    @AnalogKid
    1) That's what I've been trying to tell him.
    2)Vx is a feedback from the final output so gain = 1+ 39k/4.7k
    give or take a bit for that 115k providing a DC path for the bias current of the inverting input.
     
    Last edited: Jun 30, 2015
Loading...