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tas5706b fault modes (and sound?)

Anders Frederiksen
Prodigy 20 points
Other Parts Discussed in Thread: TAS5706B, TAS5756M, TAS5760M

I was trying to fix a Bosch radio for a friend. No schematics or spare parts available (shame on Bosch! ;-). The radio uses a tas5706b in connection with an ADC. All supplies seem OK. ADC input is OK and output looks like decent I2S. SPI signals look OK and are present when volume is changed. But the speakers are dead silent. I can see a small sine wave (probably the modulation freq. from the PWM - at around 380khz (as I remember it). I'm considering trying to change the IC (a pain with the power pad - and I'm afraid to burn the new chip before getting the power pad soldered (only have hot air for one side)). However, I'm a little puzzled that it would burn at all. No signs of thermal overload on the PCB. And according to the datasheet the chip is pretty well protected against almost any probably error... So I'm a bit puzzled why/how it would burn in a system with absolutely no direct external connections!

So the question: What kind of fault modes have you seen out there?

And secondly: Have anyone listened to a tas5706b on a decent stereo setup? Or maybe other similar digital input parts? The features of ie. tas5706b are excellent for an active system - but my previous experiences with Ti PWM amps have not been great. Might be the "lowest possible cost consumer device implementations" that’s not optimal (naturally) - but I'm still a bit reluctant to put in a lot of resources in trying it out.... ;-)


Thanks in advance for your response! :-D

  • 0
    TI__Mastermind 34675 points

    Hello Anders,

    It's challenge to fix or debug issues without the proper tools. Let me see if I can provide some guidance for you in you debug. Here are some suggestions.

    • Measure the PWM frequency and duty cycle at pins 4 , 46, 50 and 64 to ensure all outputs are switching.
    • Measure on the device pin or before the inductor for the LC filter.
    • The PWM frequency should be ~384kHz on PWM outputs.
    • If the duty cycle is 50% on all output it can mean that device is muted (check mute pin 21, it should be high) or there is no audio signal coming from the I2S input. 
    • If the PWM output is modulating (not ~50%), There maybe audio signal coming out, which you can measure after the inductor for the LC filter going to the speaker.
    • Check that the speaker is working. You can you use a 9V battery across the speaker terminals as quick check or hook it to another audio source. 

    Let me know if this info is useful and if you have any questions related to TAS5706B you need answered. All the best with your debug and I hope we have an opportunity improve your perception of our Class D amplifiers because they are really awesome Amplifiers!

  • 0 in reply to Damian Lewis
    Prodigy 20 points

    Hello Damien,

    Thanks so much for a swift and good reply.

    I (re-) checked all your points - and everything was (still) OK. But I noticed that the I2S data was fairly static. Turns out that the ADC in front was faulty. Again, very strange in a more or less closed system! But it does make sense that the seemingly very well engineered TAS5706B indeed was OK. So I'm very happy your reply did stop me from trying to exchange it! :-D

    I guess I should look for a demo board or something and give the ti class-D parts another chance in the HiFi area. Any recommendations? I get 14 parts in a quick search - and it's not obvious to me which one can be expected to be the best sounding....

    Best regards,

     Anders

  • 0 in reply to Anders Frederiksen
    TI__Mastermind 34675 points

    Hello Anders,

    I'm glad you were able to find the failing component. I don't know your system, but I'm assuming the ADC receives audio inputs from the external system via line-in jack or RCA. Too large input signals can damage the ADC.

    I would recommend TAS5756M and TAS5756M digital audio processing (EQ, DRC, Mixing, etc.) is needed and TAS5760M if you don't need digital audio processing.

    Thanks for the feedback on your debug progress. Let me know your experience with our mid power amps I recommended. All the best with your audio experiences!