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High voltage rails for class D amp?

Discussion in 'General Electronics Discussion' started by eem2am, Apr 15, 2012.

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  1. eem2am


    Aug 3, 2009

    Why do Class D amplifier designers prefer high voltage supply rails?

    Please find attached here two Full-Bridge , Class D amplifier simulations which run in the free simulator called LTspice by

    Each simulation is of a 4KHz sine wave which gives 100W of power in the speaker.

    One of the Class D amplifiers has an 80V rail and the other has a 40V rail.

    Each simulation gives the same output signal (i.e. the same current in the speaker)

    ……so why do engineers prefer higher voltage rails for Class D amplifiers?……….
    ……because these simulations show that you can get just the same speaker signal with a lower rail voltage…… and simply using a lower value of filter inductor.

    (To run these simulations , you will need to make sure that the text files called “sin” and “sin1” are situated in the same folder as the LTspice simulation files.)

    Also, do you know if my representation of the speaker as an 8 Ohm load is correct?…..Does it have an inductance associated with it?…….is this inductance likely to be poorly toleranced.?

    The simulation files are in .txt format..........(i had to do this else they wouldnt upload.....anyway, all you need to do to run them in LTspice is save them as .asc files.)

    The files can be found here ....(the test files sin and sin1 were too big for this forums uploader

    Attached Files:

  2. duke37


    Jan 9, 2011
    The two amplifiers will have the same output if they are set up to have the same gain.
    The 80V amplifier will be able to supply twice the voltage of the 40V amplifier so will have four times the power capability.

    You can get more power out with a low voltage supply if you have a low impedance speaker

    Why do you think 80V is high?
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Without simulation:
    1) Yes, a speaker does have an inductance.
    2) What happens if you turn up the power? The 40V amplifier will at one time reach it's limit whereas the 80 V amplifier can still be cranked up more.

    As long as the output power is within the 40 V amplifier's limit, both amplifiers will simulate and operate equally well.

  4. eem2am


    Aug 3, 2009
    HarolD Kapp:
    Do you know what is the inductance of a typical 100w 12 inch speaker?

    80V is high for a class d input rail, from what ive seen.
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
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