Using ADAU 1701 stereo inputs as mono balanced input


I am curious if the ADAU 1701 can be used to digitally accept a balanced input without a dedicated line receiver.

In my mind, I envision it as follows:

XLR/TRS input connected directly to left/right inputs of ADAU1701:

Pin 1 (ground) -> Ground

Pin 2 (positive polarity) -> Left Input

Pin 3 (negative polarity) -> Right Input

In SigmaStudio: Invert polarity of Right input, sum Left and Right into mono signal

Would this work, or would a hardware line receiver be necessary?

  • 0
    •  Analog Employees 
    on Jul 8, 2020 7:07 PM 28 days ago

    Hello Harooner,

    The basic idea will work but there are some issues and tradeoffs. 

    If the sending device is electronically balanced then it has two ground referenced outputs inverted from each other. So therefore you can accept them as two single ended signals then invert and add in the DSP. The problem will be if you connect something that is using a transformer output. Then the output + and -  are only referenced to each other and not ground. So the DSP input will be looking at the signal with respect to ground and that will be ugly. No low end and lots of garbage. So if you are certain that there will never be a transfomered output device connected then that is fine. 

    The other issue is with common mode rejection. You are forcing the ADC to convert the common mode garbage, the RF and power noise etc., and passing it on to the DSP. The single pole anti-alias filter will take care of some of it but there is a risk of some of this noise getting into the ADC and aliasing down into the audio band. So that is a concern I would have. This again depends on your application if this is super important or not. Having a good well designed differential amplifier on the input will give you the best CMRR. 

    Those are my first thoughts. If anyone else reading this has other thoughts please chime in. Perhaps others have done this before. 

    Dave T