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OPA1656: problem

Gerald
Genius 9570 points
Part Number: OPA1656
Other Parts Discussed in Thread: OPA1642

Hi Experts,

One of our customers recently bought several OPA1656 directly from us. He replaced five OPA1642s with them in an audio active crossover, convinced that they would be equivalent (after reading their datasheets), and that this would improve performance. He did measurements to verify that it was stable. However, when connecting the crossover, there was no audio signal. What could happen? 
I hope you can help us. Thank you.
Best regards,
Gerald
 
  • 0
    TI__Guru 55466 points

    Hi Gerald,

    Would it be possible to post a detailed schematic?  

    The OPA1656 (gain bandwidth product, GBW=53-MHz) and OPA1642 (GBW = 11-MHz) have a different bandwidth and we would need analyze the complete audio active crossover circuit to analyze stability and determine the issue.

    Thank you and Regards,

    Luis

  • 0 in reply to Luis Chioye
    TI__Genius 9570 points

    Hi Luis,

    I already requested for it with the customer and he might be able to provide it to us next week. I will send update here as soon as I received it. Thank you.

    Best regards,
    Gerald

  • 0 in reply to Gerald
    TI__Guru 55466 points

    HI Gerald,

    Thank you, will wait for the Customer's update.

    Best Regards,

    Luis

  • 0 in reply to Luis Chioye
    TI__Genius 9570 points

    Hi Luis,

    This is the crossover circuit schematic. Originally this had LM4558, which the customer later changed to OPA1642 with very good results (He had to adapt the DIP and SOIC mounts). But this didn't work with the opa1656. 
    He is looking through his papers for information on the values of capacitors and resistors.
    He is extremely grateful for your kind help. 
    Best regards,
    Gerald
  • 0 in reply to Gerald
    TI__Mastermind 23935 points

    Hey Gerald, 

    Thank you for the schematic. If you are able to provide component values we can analyze this circuit in TINA Ti for stability and recommend solutions to use the OPA1656. I was unable to find the product information for the LM4558. Do you have a link for this product to look at the product datasheet?

    Best Regards,

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    TI__Genius 9570 points

    Hi Chris ,

    The customer is looking for the values of capacitors and resistors.
    All the best,
    Gerald
  • 0 in reply to Gerald
    TI__Mastermind 23935 points

    Hey Gerald, 

    Thank you for providing the datasheet. We suspect that the circuit is marginally stable. The GBW of the LM4558 is 5 MHz which isn't much different from the OPA1642 (GBW = 11-MHz). Since the OPA1656 GBW is much larger, this is the reason we suspect the issue is possibly due to stability. 

    A possible solution would be to lower the positive feedback capacitors and increase the resistors such that the op amp isn't driving large capacitive loads. By lowering the capacitor values and increasing the resistor values the same frequency response for the filtering could be achieved. The trade off would be noise vs stability. Below, I have highlighted the components that could be adjusted if this the issue. This is just a theory. In the meantime we will wait for the customers feedback on the component values in order to check the stability of each op amp below using the OPA1656. 

    Best Regards, 

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    TI__Genius 9570 points

    Hi Chris,

    The customer is working on the circuit following your suggestions. Please refer below for customer feedback:

    "I've just seen that five of the electrolytic capacitors appear "inflated", indicating that they have been damaged. I am checking the power supply, because obviously there is a problem there.
    I'll keep you posted. 
    Thank you thank you very much!
    "
    Best regards,
    Gerald
  • 0 in reply to Gerald
    TI__Mastermind 23935 points

    Gerald, 

    Thank you for the update. 

    Best Regards,

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    TI__Genius 9570 points

    Hi Chris,

    Please be informed of the customer feedback below:

    Thank you very much for your kindness. I have fixed the power supply and incorporated the interesting suggestions you made in your previous post, and opa 1656 works very well.
    I thank you again, and salute you!
    BRs,
    Gerald