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no image Tuesday, January 02, 2018 by Joaquin Valdez

Do op-amp really need a split power rail?

I recently built a amplifier out of a lm3886 and it works fine but I was curious if a op-amp needed  a spilt power supply for good audio quality all I found was that without the spilt power rail the audio quality could be distorted and if so how bad would the distortion be?


Also I’m useing 2 one watt 1kohm resistor in series to create a ground point is there a better way to create a ground point?

Comments

  • by  Richard Gabric

    If you built up the LM3886 with a single supply, then you will be aware that coupling capacitors are needed in places where they are not needed in a split supply design.

    As long as the positive power supply pin is higher than the negative,  and the total supply  gives sufficient head room for the signals being amplified, and the inputs are biased to reflect that, then any supply within the maximum power supply voltage range may be applied to the power supply pins. 

    Texas Instruments have a good application note on op-amps, which includes a good section on power supplies and biasing of single supply circuits.

    http://web.mit.edu/6.101/www/reference/op_amps_everyone.pdf

    A voltage divider may work as long as the currents are small and equal, which will be the case where they are biasing the input of an op-amp to half rail, for example.

    Sometimes a voltage divider with a voltage follower is used to drive your "ground", which within the output current drive capability of the op-amp, keeps the power supplies split equally (or at any other voltage you choose)

    The disadvantage of single supply audio power amps can be that the coupling capacitors have to charge up on power up, which can drive the speaker quite hard momentarily,  "anti thump" circuits are often needed to avoid this problem.


    Cheers,

    Richard

    • by  Aubrey Kagan

      Sometimes a voltage divider with a voltage follower is used to drive your "ground", which within the output current drive capability of the op-amp, keeps the power supplies split equally (or at any other voltage you choose)

      TI make a device just for this purpose the  TLE2426. Just heed Richard’s advice about drive capacity
  • by  Aubrey Kagan

    Sometimes a voltage divider with a voltage follower is used to drive your "ground", which within the output current drive capability of the op-amp, keeps the power supplies split equally (or at any other voltage you choose)

    TI make a device just for this purpose the  TLE2426. Just heed Richard’s advice about drive capacity
  • by  Rick Curl (edited)
    Hi Joaquin-

    Richard is absolutely correct.  Let me just add a couple of things. One reason a split supply makes sense is that audio signals must be able to go both positive and negative in relation to ground.   Having dual supplies ensures that with an input voltage of zero, the voltage at most points along the signal chain will also be zero.

    Some op amps are optimized for single supply operation, and others work better with dual supplies (although it is often possible to use a "single supply" op amp on split supplies).

    -Rick  

  • by  David Ashton (edited)

    Looking at the datasheet of the LM3866 here, and specifically the reference schematic on page 2, if you wanted to use single supply you'd have to:

    - use a capacitor on the output line (pin 3) to the RL - the + of the cap to the LM3866 pin 3.  Note that Rf1 would still ahve to be connected directly to pin 3, not after the capacitor.  Use 1000uF or more, depending on what frequency response you want.

    - use a capacitor from your source to the point labelled "Input" on the diagram, + to the input point,  10 to 50 uF should be OK again depending on your desired frequency response.

    - use your voltage divider as per your diagram to generate the ground point, make sure you use big capacitors (I'd say 200 uF or more) to keep it stable.   If you have a scope, set it up to look at your pseudo-ground with reference to the negative rail and see if there is much variation on it when in use, if so you need bigger caps / smaller resistors.

    - you could also use a power opamp (even another LM3866) to provide the ground if you find the resistors don't do a good enough job.

    - I'd agree with the above comments in that a split supply is far preferable.  You only have a one-winding transformer?  Just use half wave rectification (one leg of the winding grounded, the other with two diodes of opposite polarity going to your rails) and use BIG caps for smoothing.  The extra cost of the caps will be compensated by not having to buy an output cap.  Here's a diagram.

    - the other thing you can think about is using a bridge amplifier.  There's a good article here, and a circuit using the TDA7266 here which provides 4 amplifiers in bridge configuration especially designed for single supply  stereo amps in TVs, Portable radios, etc.   There's a datasheet for the TDA2030 here which gives split and single supply circuits (page 8),  and there is a discussion on the difference between them here which might be informative for you.  All these use circuitry very similar to the LM3866 (but they are lower power).

    Hope this helps // David


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