question about plate amps

Discussion in 'Archived Threads 2001-2004' started by MichaelAngelo, Mar 5, 2003.

  1. MichaelAngelo

    MichaelAngelo Stunt Coordinator

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  2. Dave Milne

    Dave Milne Supporting Actor

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    Modifying an amp to make it 2 ohm stable is not an easy task. You're basically talking about re-designing the amp. You can start by reducing the rail voltages to the output stage (typically requiring a different power transformer) but you'd probably also need to change some other components to compensate for reduced voltage in the other stages. You could also "parallel-in" additional output stages to handle the higher current. This is pretty straightforward with MOSFETs, but with bipolars, you will need to juggle emitter resistor values to get the outputs to share current and you'd likely need to beef up the driver stages to provide the extra base drive to the outputs. Since most of these mods affect the gain/phase margin, you should really re-analyze the loop stability and be prepared to juggle feedback component values.

    In short, unless you have some amplifier design experience, don't bother.

    Dave
     
  3. Jeff Elliott

    Jeff Elliott Stunt Coordinator

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    Also, the AV-15 voice coils are only about 1.5 ohms and not 2 ohms. At least mine were when I checked them with my multimeter.

    JE
     
  4. Dave Milne

    Dave Milne Supporting Actor

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    Jeff,
    The DC resistance you measured (T/S parameter "Re") is typically about two-thirds of nominal "rated" AC impedance (Znom).

    Actual impedance (Z) varies with frequency, with a big hump due to resonance, and gradual rise at higher frequencies due to voice coil inductance (Le). Znom is sort of an "average".
     
  5. PaulDF

    PaulDF Second Unit

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    Dave, so in other words, if I measure the impedence of a speaker with my multimeter, and get a reading of 5.8 ohms, then the actual average impedence (or Znom) is around 8.7 Ohms? I realize that impedence changes with frequency, but is this sort of a rough estimate of a speakers impedence?
     
  6. Dave Milne

    Dave Milne Supporting Actor

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    A speaker with DC resistance of 5.8 ohms would be considered an "8-ohm" speaker. Znom is more of a rough "grouping" than an exact science. Since 4-ohm and 8-ohm are sort of the "standards" anything with Re of about 2.5 ohms to 3.5 ohms would probably be considered a "4-ohm" speaker. Anything with Re of about 5 to 7 ohms would probably be considered an "8-ohm" speaker.
     

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