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OPA564: Composite Audio Amplifier

Josh Lang
Prodigy 30 points
Part Number: OPA564
Other Parts Discussed in Thread: OPA1612, , BUF634A, OPA1656, BUF634, THS6012, TPA6120A2

It's more recently become fashionable in the audio industry to use a composite design in power amplifiers, especially headphone amplifiers. In many case, a super high performance audio opamp (say OPA1612) is followed by a high-current amplifier (say OPA564). These two amplifiers are designed in a feed-forward / feed-back type network. This gives the best of all worlds:  exceptionally low THD, low noise, and high current.

Does T/I have a reference design for this type of audio amplifier? Or maybe a technical article on designing with composite amplifiers to achieve all-around top performance? 

  • 0
    TI__Guru 55466 points

    Hi Josh,

    We do not have a reference design for a composite audio amplifier specific to the OPA1612 and OPA564.  However, a device commonly used as an output buffer, on composite amplifier configurations is the BUF634A. 

    The BUF634A can be connected as an open-loop buffer and use with a low-noise, low-distortion amplifier at the input, closing the feedback loop around the input audio amplifier. For example, below is a suggested composite amplifier circuit using the OPA1656 and BUF634A to drive headphones and small speakers providing low noise and low distortion.

    You can find more information about the OPA1656 and BUF634A in the datasheets below:

    OPA1656:

    https://www.ti.com/lit/ds/symlink/opa1656.pdf

    BUF634A:

     https://www.ti.com/lit/ds/symlink/buf634a.pdf

    Thank you and Regards,

    Luis

     

     

     

  • 0 in reply to Luis Chioye
    Prodigy 30 points

    Thanks Luis. The BUF634 is a single-ended device. Does T/I have an app note for a similar composite driving the non-inverting input of a high-current audio opamp, like the OPA564? If not, would I treat the opamp composite feedback loop similar to the BUF single-ended design? 

  • 0
    Guru 48395 points

    Fashionable indeed Josh, I did work through this in a series of Audio Express articles, full listing here - The BUF634A has been in the lit for awhile, I think the parts shown here would work considerably better, at one point AudioXpress said they were going to make all of these available as pdf without subscription, but I don't think they have. 

    6685.Summary artilce list for AX series.xlsx

    here is article 16 using the OPA1612 and THS6012 model, but use the TPA6120A2, same die but smaller package (but no TINA model?) 

    Very Low Distortion Composite with OPA1612 and THS6012 AX#16 Dec 24_2020.docx

    And I think maybe the last TINA file I had for this, I vaguely recall having to work on the noise model for the THS6012 as well here, 

    Initial composite OPA1611 plus THS6012 compensated.TSC

  • 0 in reply to Michael Steffes
    TI__Genius 12955 points

    Hi Josh,

    Can you clarify; by single-ended, you mean single-ended supply, correct?  Or, you are looking for a differential output (similar to a bridge-tied load configuration)?

    Regards,
    Mike

  • 0 in reply to Michael Hartshorne
    Prodigy 30 points

    The BUF634 has a single-ended input. It's not an opamp with two inputs (inv and non-inv). My question was about using one input of an opamp with the feedforward approach. I think it should work fine, as the other input of the power opamp would be in the feedback loop.

  • 0 in reply to Michael Steffes
    Prodigy 30 points

    Thank you, Michael, for these links. Very helpful!

  • 0 in reply to Josh Lang
    TI__Genius 12955 points

    Ah I see what you mean.    Yes the OPA564 in can work in a composite amp loop, it actually has decent THD before the composite loop gain improvement, which would even be further improved by more loop gain.  

    Michael's article "Very Low Distortion Composite..." gives some good feedback about configuring THS6012 in a composite loop - the similar principle can apply to the OPA564.  We don't have any collateral directly featuring the OPA564, though. 

    How much output current do you need?  I am only familiar with headphone use cases up to ~200 mA.  

    Regards,
    Mike