Ground Reference

Discussion in 'The Projects Forum' started by zain.uet, Feb 19, 2016.

  1. zain.uet

    Thread Starter New Member

    Oct 18, 2015
    Hello Boys
    I have a differential to single ended converter (see attachment). I have applied a differential sound signal. Input signal's and op amp' ground are uncommon. But still op amp is working(not to good as i common the ground). Ideally op amp should not work. so i cant understand how it is still working. Differential to Single Ended Converter.png
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    A differential amp looks for the difference between two signals irregardless of where some ground may be. In a practical circuit there may be a limit on the difference in voltage (called the common mode voltage) but they do not need a hard ground connection to work.

    However, in your case I suspect the 10K resistor R9 is acting as a ground path. Otherwise your op amp is not connected as a differential amp, I suspect it is working mostly as a unity gain buffer.

    What are the input and output signal levels?

    Oh, the 741 is a very bad choice for most work these days. While still commonly used it as severe limitations. In audio work you can hear it hissing along on top of any music playing.

    And welcome to the forums!
  3. crutschow


    Mar 14, 2008
    That circuit is not a differential amp as it responds to common mode signals with a gain of one.

    To make a differential circuit with a gain of one, add 10kΩ resistors in series with both inputs, and add a 10kΩ resistor in the feedback path to the (-) input.
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012

    Use a good quality op amp (not the NTE778A - because I have no idea what that part's specs are based on the datasheet (NTE doesn't make parts, they just put their own name on some commercial part).

    Anyhow, the PIN numbers are for a dual op amp so, pick as you want. A dual op amp can be used with the other amp on that chip to make a virtual ground. (You cannot use V- and ground as the same voltage).

    The common comes out of the left of xlr connector, the differential is the + and - inputs to the op amp.

  5. zain.uet

    Thread Starter New Member

    Oct 18, 2015
    thanks guys