How much gain does the average plate amp have?

Discussion in 'Archived Threads 2001-2004' started by Jonathan M, Dec 27, 2002.

  1. Jonathan M

    Jonathan M Second Unit

    Jul 23, 2002
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    Hi All,

    Just finished my DIY sub + amp, and am not happy at the moment with the amount of gain it supplies to the signal.

    The amp itself has a gain of 23 (Feedback loop is a 22k resistor, with 1k to ground), and the preamp section has a gain of 2.

    How much gain does the average plate amp (Say a 300W into 4 ohm one from PE?). How much of it is in the amp section, and how much is in the pre-amp (gain circuit)??

    If anyone has schematics, that would be great too, although I only need the gain amounts.

    Pics of my sub will follow in the New Year as soon as I get hold of a digital camera!!
  2. Rick Chwiendacz

    Jun 27, 2001
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    That's actually not a terrible amount of gain for a subwoofer amplifier. I'm not sure what the power output of yours is, but it's generally taken to be an axiom that the voltage input to an amplifier from the line-level signal is between 0.1V and 1V RMS.
    Given that you have a total gain of 46, that is sufficient for about 130W into 4 ohms, given a 0.5V RMS input, or 529W into 4 ohms, given a 1V RMS input.
    Seeing as your amp probably falls in the middle of that range, its gain was probably set so that the input voltage of 0.775v (pro/PA audio standard) or so would give the full power output.
    This is definitely a reasonable amount of gain, and it's not unreasonable to assume your preamp or line-level stage can swing 1V RMS. Having said that, I know of one 350W plate amp that has a gain of 69 in the amplifier section itself, and a gain of close to three in the preamp section.
    (check the schematics - I may have missed another stage of preamp gain, as i just looked over it quickly)
    If you want more gain, you can always just replace the feedback resistor with something larger (either in the preamp or the power amplifier), or build a small extra opamp gain stage to splice in at the line-level input of the amplifier.

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