op amp buffer

Discussion in 'Burnt Fingers DIY Effects' started by poiureza, Apr 22, 2013.

  1. poiureza

    poiureza Tele-Holic

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    So I wanted to get my feet wet with op amps and I built this very simple IC buffer from Jack Orman below.

    [​IMG]
    http://www.muzique.com/lab/buffers.htm

    I used a 9V battery and one side of a TL072 instead of the TL071 here.

    Of course things didn't work out as expected :rolleyes:
    I triple checked the few connections and rebuilt the circuit several times with identical results

    The buffer should be unity gain but my ears tell me that output is less than half the input and I have a serious treble rolloff. And a lot of noise to boot

    I'm completely new to this and thus completely at a loss here.
    The only idea I have is that these op amps normally require Vcc+ and Vcc- on Pin7 and Pin4 while it's +9V and 0V here. So the op amp "ground" would be a floating ground @4.5V (or am I talking nonsense ???) and the output is delivered with respect to this floating ground instead of 0V (I'm not sure my writing makes any sense...).
    I connected all "grounds" toghether (thus Pin4, input jack ground, output jack ground and battery negative lead). Could that be the problem ?

    I would greatly appreciate any insight.


    PS : I also made the inverting buffer circuit below with similar results. Vr comes from a voltage divider (figure 1 in the muzique.com link where I ommited the cap to ground since it's a battery). I didn't include the 5pF cap.
    Output is slightly higher but still far from unity gain. Treble loss and noise are same as previously

    [​IMG]
     
  2. Lostinthe50s

    Lostinthe50s Tele-Afflicted

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    As you suspect, there is no negative swing from the opamp. You need to lift the ground to 4.5v. Use two 1k resistors in series between power pos and neg. "neg" now becomes -4.5v for the chip neg and the junction of the two 1k res becomes ground for all of the rest of the circuit.
     
  3. Jubal81

    Jubal81 Tele-Meister

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    You're also using a dual opamp and you can't leave the other side unattached. You need to make the second half into a buffer and tie the input and output to ground.

    Also, if you followed the schematic, the reference voltage at the input is correct for use with a single (non-split) supply.

    If you want to adjust the gain, use a trimmer resister in the feedback loop and you can adjust it to unity gain.
     
  4. poiureza

    poiureza Tele-Holic

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    Thanks both.

    I checked the board layout at beavisaudio and mine is exactly like that (save the LED), so it should work with a single +9V supply.

    [​IMG]

    Anyway, I'll try the series resistors between power pins 4 & 7 and see what happens.

    Jubal81, I thought that both sides of a dual op amp were totally separated. Not sure what you mean by making it a second buffer but I tied the 3 unused lugs to ground on my first attempt. I let them float on the following ones. I didn't notice any change at all.
    I never really understood the need for grounding unused leads actually.
     
  5. FenderLover

    FenderLover Poster Extraordinaire

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    Look at Jack's Super Buffer here:
    http://www.muzique.com/lab/superbuff.htm

    The two 1M resistors you have (R1 and R2) are R8 and R9 in this schematic.
    Add R6 and C4 to your circuit. You'll be surprised.

    I use this buffer with two op amps in parallel (rather than 4 shown) to drive multiple outputs.

    Keep in mind that the two 1M resistors are electrically in parallel, so your input impedance is half of what you think.

    You don't have to make a second circuit out of the ofther side, just ground the inputs if you feel you need to. The reason is that when the chip is powered, both sides are active. With 'nothing to do' on one side, there could be oscillation the makes the chip go cookoo.
     
  6. tjk3052

    tjk3052 Tele-Holic

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    Post the pin voltages and maybe a pic of what you have going
     
  7. Jubal81

    Jubal81 Tele-Meister

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  8. poiureza

    poiureza Tele-Holic

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    Thanks all, I finally managed to get it to work : I simply have a row on my breadboard that seems to be worn out (or close to). Time to upgrade ...

    Thanks for that interesting link Jubal, I didn't realize that both sides are powered and that the second op amp is not "off" when unused.
    I notice a quite large difference in noise level between inverting and non-inverting buffer circuits. Maybe that's due to the non-terminated second op amp. I'll check that.


    Fenderlover, I have no idea what this super buffer is about. I don't see the advantage of putting several stages in parallel ???
     
  9. FenderLover

    FenderLover Poster Extraordinaire

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    The article describes a little of its application, basically for long cable runs to overcome capacitance. It's kind of a special application, but I parallel two in order to conveniently use both halves, and it certainly doesn't hurt as a buffer driving multiple loads. Driving a balun transformer for electrical isolation can be an excessive load for some op amps to handle solo.
     
  10. poiureza

    poiureza Tele-Holic

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    OK thanks
     
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