Poor frequency response - simplest config!

Hi all,

I'm used to  running much older versions of SigmaStudio - now on to  4.4.  Encountering something really weird.  I have a small audio device I'm planning to EQ with the 1466 DSP.  I measured the device with my measurement rig pre-EQ.  Then I put the ADAU1466 dev board and SigmaStudio into the chain, create the easiest possible config to make sure everything is OK (wire the input directly to the output), compile/link/download and run the test again.  No love!

The first picture is of the config file schematic - I said it was easy!

This shows the before and after:

The green line is before EQ/SigmaStudio/Dev kit, the red is after I plug the dev kit in the path.  It looks like a first order high pass filter at about 10 kHz.

Any clues/tips about what's going on?  I'm baffled  - and I'd hate to have to  just drop  this kit and move on...

  • 0
    •  Super User 
    on Sep 20, 2019 1:16 AM over 1 year ago
    [deleted]
  • Hi Bob,

    When I do as you suggest, I get the red line; when I plug my headphones directly into the test system's output - I get the green line.  It's not the test equipment that's in the way.


    So to clarify:

    test box output -> headphone = green line

    test box output -> input 1 of ADAU1466 dev board -> config shown above (input to output) - > Output 1 of dev board -> headphone = red line

    Not good.  This is a straight pass-through configuration, it should be very close to the green line, as it is in-to-out, no processing.

    Thanks!


    Dan

  • 0
    •  Analog Employees 
    on Sep 24, 2019 4:36 PM over 1 year ago

    Hello Dan,

    So to clarify:

    test box output -> headphone = green line

    test box output -> input 1 of ADAU1466 dev board -> config shown above (input to output) - > Output 1 of dev board -> headphone = red line

    Not good.  This is a straight pass-through configuration, it should be very close to the green line, as it is in-to-out, no processing.

    Can you clarify your tests here? You said Output->headphone-> = Red Line. I would think your answer would be "= I do not hear it or I do hear it".

     I suspect there can be an issue with the wiring. Are you going into a balanced input? I know with my Audio Precision Portable ONE I had to build adaptors to use when going from single ended to balanced inputs because with the input transformer if you leave one end of the transformer unterminated you get a really great high pass filter. No low end and lower level. 

    There are also many settings in the DSP serial ports and routing that can cause no output so to eliminate issues with the project itself I attached a known good project that I personally have tested with an eval board at my desk. I am taking the output from my MAC and I plugged a set of headphones directly into the eval board outputs. Steely Dan sounds just fine. Plenty of low end. 

    Another thought,... Are you driving a really low impedance by any chance? 

    Dave T

    ADAU1466 Basic Stereo Input-Vol-Meter-Mute.zip

  • Hi Dave,


    Driving a 10 kOhm input impedance - so a high impedance.  Single ended inputs and outputs (APx515 and a Crysound 6151).  I hear it - but it is way down and the frequency response is obviously skewed to the high end.

    Thanks!


    Dan

  • 0
    •  Analog Employees 
    on Oct 23, 2020 5:03 PM 1 month ago in reply to DanWiggins

    Hello Dan,

    Sorry this comes so late but I am trying to clean up old things. 

    This is the frequency response I measured with the eval board using an AP2700. This was with it set to 100k load.

    I did not normalize the curve to be at zero. But you get the idea that it is rather flat. The top and bottom scale is +1dBV and -1dBV so it is a fine scale. 

    This graph is with the AP set to 600 ohm load. This is not really designed to drive this low of a load. The scale is +1dBV to -10dBV so this does show a lot of roll off. I would have to look closer at the design of the Op Amp drive circuits. 

    For comparison I took one of the older ADAU1452MINIZ boards to compare to and it looks much better driving a 600 ohm load. So I will pull out the schematics and see what is different between these two boards. 

    The low end roll off is what I would expect driving a 600 ohm load. 

    Dave T