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TAS5825MEVM: Purepath Console Motherboard analog input is distorted. OVF led flashing.

David Johnson5
Prodigy 70 points
Part Number: TAS5825MEVM
Other Parts Discussed in Thread: TAS5825M, TAS5751M, , TAS5751MEVM, PCM3168A

We are running the TAS5825M and TAS5751M amps using the PurePath Console Motherboard (PPCMB Rev G) and comparing them to an amp in an existing product.  We are getting a loud distorted output when using the analog inputs on the PPCMB. 

We started with the TAS5751M and got a distorted output.  We got it cleaned up after a few tries (possibly by hitting the reset tuning button on the PurePath GUI) and were able to make the measurements we needed. 

We are currently trying to do the same with the TAS5825M but cannot get rid of the distortion.  The PPCMB’s OVF led is flashing while using the 5825 but not while using the 5751.  OVF was not flashing while running the 5751 with the same input.   

Any help or advice you can provide would be greatly appreciated. 

Thanks, Dave

  • 0
    TI__Guru 52115 points

    Hi Dave,

    We usually suggest our customers provide the audio data input via optical SPDIF, coax SPDIF or external I2S for performance measurement.  If one of these inputs is used, I think you probably will not see the distortion.

     

    Andy

  • 0 in reply to Andy Liu (BMS)
    Prodigy 70 points

    We do get good results when we test the amp through the digital inputs.  If we use decide to use this amp we will use a digital input. 

    However, we are connecting it to an existing product to compare with our current amp.  This requires an analog connection.  We were able to get the TAS5751MEVM working in this scenario but not the TAS5825MEVM.

    Is it not possible to configure the TAS5825MEVM to use the analog input?

  • 0 in reply to David Johnson5
    Prodigy 70 points

    Thanks for your reply Andy.

    I would like to re-phrase my last question:  We can connect to the analog input.  We have to turn the signal way down or the ADC overflows.  Is it possible to get clean signals at normal levels while using the analog input?  

    Thanks again,

    Dave

  • 0 in reply to David Johnson5
    TI__Genius 14630 points

    If the ADC is overflowing you would have to use a lower input signal, but if you want a similar output level you can try compensating for that difference by modifying the output gain through the PurePath Console.

    Regards,

    Luis

  • 0 in reply to Luis Fernandes de Pina
    Prodigy 70 points

    Thank you Luis.  We have considered this and tried it.  The interesting thing is that another amp module (TAS5751MEVM) works with the same signal (voice clip) at the same level without the ADC overflowing (same PurePath Console).  

    Dave

  • 0 in reply to David Johnson5
    TI__Genius 14630 points

    Can we get a capture of the signal that you are sending to the analog input.

    Regards,

    Luis

  • 0 in reply to Luis Fernandes de Pina
    Prodigy 70 points

    A sine wave over 500mV will cause overflow.  

    Here is the voice file we are using.  

  • 0 in reply to David Johnson5
    TI__Genius 14630 points

    I was able to replicate your results. I will be investigating it further. Thank you for your time.

    Regards,

    Luis

  • 0 in reply to Luis Fernandes de Pina
    TI__Genius 14630 points

    Hello David,

    So for the TAS5825 the ADC on the PurePath Motherboard the PCM3168A gets modified when you change the audio i/o to the analog input by changing the ADC Attentuation(Seen on Table 30 on the PCM3168A datasheet). The TAS5751MEVM uses the default. So if you set this ADC digital attentuation to the default value it no longer overflows on the TAS5825. You can try this using these I2C commands which change the Attenuation level setting from 18.5 dB to 0 dB :

    w 88 59 d7

    w 88 5a d7

    Regards,

    Luis

  • 0 in reply to Luis Fernandes de Pina
    Prodigy 70 points

    Hi Luis,

    It does not overflow now, not even for a large signal.  However the gain does not seem linear.  A lower signal that we set the gain with requires the gain to be set quite high and the signal we sent you is excessively loud.  Perhaps we are looking for a setting somewhere in between.

    The TA5751MEVM overflows at about 2.1v.  The TAS5825MEVM will take much more than that without overloading.

    I think we're on the right track.

    Dave

  • 0 in reply to David Johnson5
    TI__Genius 14630 points

    Hello Dave,

    If you look on the datasheet you can modify this value(the d7) to fit your spec. Do you have any more questions? If not please let me know if you feel I answered your question.

  • 0 in reply to Luis Fernandes de Pina
    Prodigy 70 points

    Luis.

    I'm was just in the middle of doing that as I read your reply.  It looks like ADC attenuation of +6 is close to what we need.  I will proceed with my measurements and mark this as resolved if all works out.

    Thanks again.

    Dave