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Theme: LM7171 in High-End and Hi-Fi audio applications question.

Alexey KLimenko
Prodigy 70 points
Other Parts Discussed in Thread: LM6171, LM6172, LM7171, LM4562, LMH6601, THS4031

Dear Sirs,

As we know, LM6171 (LM6172), as well as LM7171, are used in high-quality audio applications (but quite rarely).

We are thinking to use LM7171 in such audio applications as:

1. Just after DAC.

2. As active Low Pole Filter, etc.

3. Driver to control next amplification stages (including to control vacuum tube, etc.).

 

Our questions are related to actual harmonics spectrum of LM7171 in different modes to find a way how to reduce highest order and odd harmonics as much as possible.

And we would like to ask you to help us with any available additional info.

The question # 1. Do you have any another (and more detailed) measurements of harmonics spectrum of LM7171 in addition to published datasheet?

The question # 2. What is the dependence of harmonics spectrum of LM7171 from signal volume, load resistance, frequency, etc?

And the final issue:

As we know, many of operational amplifiers, advertised as Low Distortion or as High-Fidelity Audio, in fact are very much degraded in THD when operating with small level signals.

With some input constant reference voltage usually we can expected to avoid (or decrease dependently of IC internal circuit) so called "step type" distortions (or switching artifacts) when output polarity change.

Hence the question # 3. Do we can improve (and how much) an harmonics spectrum of specifically LM7171 (and lower its THD) on low signals (and on the big signals also), if we are going to imply some input constant reference voltage (to avoid output polarity change when operating with AC signal)?

Thank you beforehand for any help.

Sincerely,

Alexey Klimenko

SE SRTRI

  • 0
    TI__Mastermind 22825 points

    Hello Alexey,

    The only distortion data available on the LM7171 is what you find in the datasheet. Sorry.

    I am not personally aware of what you have identified as "distortion increase with low amplitude". Generally, distortion in dBc reduces with less swing! May be you can provide more details?

    Other than that, do the following to get the lowest distortion:

    • Av=+2 (lowest closed loop gain setting)
    • Light load
    • Minimize output swing
    • Operate with either output source or sink (not both) as in single supply DC coupled load returned to ground, if possible, to reduce / eliminate crossover distortion
    • Minimize parasitics
    • Minimize the supply decoupling return current passing near the sensitive input nodes using clever board routing
    • Apply these PC board techniques: http://m.eet.com/media/1134294/15200-476907.pdf

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Prodigy 70 points

    Hello Hooman,

    Thank you for the good advices and for the interesting article link!

    Regarding your question: “I am not personally aware of what you have identified as "distortion increase with low amplitude". Generally, distortion in dBc reduces with less swing! May be you can provide more details?” please find below my explanation.

    In relation statement “Generally, distortion in dBc reduces with less swing” - for the second harmonic, usually it is.

    But we are interested in higher odd harmonics elimination first of all.

    With lower signal, for instance 5-th (or some higher order) harmonic can be also lower in its absolute value. But its dBc (as addition of fundamental signal power,) can be much higher (no doubt audible and very unpleasant for human hearing).

    As an example, please look at LM4562 official datasheet (High-Fidelity Audio Operational Amplifier).

    On the page 6 – figure 4 and on the page 9 – figure 25 for instance.

    So, THD and IMD are more than dramatically increased (!) with low fundamental signal amplitude for LM4562. And I can suppose that relative total distortion level is increased due to higher order harmonics enlargement. And higher order odd harmonics serve as a killer for sound, as all we know.

    That is why we are thinking about LM7171 with its higher quiescent current (and possible lower crossover distortion) for High-End audio application.

    P.S.: could you please be so kind as to explain in a few more details technique “Operate with either output source or sink (not both) as in single supply DC coupled load returned to ground, if possible, to reduce / eliminate crossover distortion” or send me some pictures - examples of it.

    Regards,

    Alexey

  • 0 in reply to Alexey KLimenko
    TI__Mastermind 22825 points

    Hello Alexey,

    Regarding your comments on low power distortion getting worse based on the LM4562 Figure 4 and Figure 25:

    These plots at “low x-axis (output voltage)” are mostly a measure of the output RMS noise (not distortion). In this low output power region, where the plot is a straight line, as you lower power level there is a fixed amount of output noise such that the y-axis increases (divide a constant by a smaller number) since this noise power is a bigger percentage of the fundamental power. The rise is not indicative of higher distortion but a constant noise power with ever decreasing fundamental power. So, I don’t agree with your assessment that THD (dBc) gets worse at lower power. It is noise that dominates at the lower power levels.

    Regarding my other comment about operating in either Sink or Source mode (and not both):

    The odd order harmonics could be related to cross-over distortion. So, if your output stage is only sourcing (as with a DC coupled single 5V supply where the load returns to ground), output current is always flowing out of the LM7171 (always sourcing) and there is no cross-over distortion. For a discussion of this, please see the LMH6601 datasheet and the discussion around Figure 49.

     

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Prodigy 70 points

    Hello Hooman,

    Thank you so much for the clarification!

    Regards,

    Alexey

  • 0 in reply to Alexey KLimenko
    TI__Expert 3030 points

    Alexey,

    I wanted to chime in here real quick. Several years ago I took some Audio Precision measurements with the THS4031. I have seen several people use this part for audio applications for its very low noise and very high speed performance (relative to audio needs anyways). I am attaching a simple summary of this testing.

    8764.THS4031 Audio Precision Testing.pdf

    Cheers,

    Randy

  • 0 in reply to Randy S
    TI__Mastermind 22825 points

    Hello Randy and Alexey,

    I've found this great PowerPoint presentation on THD+N considerations and ways of improving distortion for your reference:

    5305.Audio TI THD+N versus Frequency Presentation 7_3_13.ppt

    Here is a good discussion as well:

    http://www.planetanalog.com/author.asp?section_id=3086&doc_id=560118

    Regards,

    Hooman