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Hifi Amplifier power question?

Discussion in 'Miscellaneous [BG]' started by Electricblue, Dec 21, 2012.

  1. Electricblue


    Feb 1, 2011
    So I connected my Realistic APM500 to my hifi system.

    I have 4 120w MAX / 8 ohm Vintage pioneer speakers connected to my cheapo 2 x 150w RMS / 4 ohms power amp.

    When I have the system running loud enough to feel in your chest (about 1/3 to 1/2 volume on amp) the meter says it's only pulling about 4 watts a side (steady volume) and sometimes reaching peaks of 11 watts (loud parts)

    What's going on? :eek:

    EDIT: The APM500 is a speaker power meter.
  2. Power meters on amps are notoriously inaccurate. They really do not read "power"....sorry.
  3. Electricblue


    Feb 1, 2011
    No worries, haha. Any idea what my amp would actually be putting out turned up half way? It would have a good hot phono signal from my DIY pre-amp. I'm looking for a proper HiFi amplifier and I'm trying to work out how much power I actually need. Even though I like headroom and a nice strong output section which can produce strong peaks when it needs to...
  4. The position of your volume knob is irrelevant, a hot input signal at 1/2 on the knob can drive the amp to its full volume just as much as a weaker signal at 3/4 on the knob.

    For home audio, "how much power do I need" can open a real can of worms. The single-ended tube guys swear they only need 5 watts. Other folks gotta have their 1200 watt subwoofer.

    A good HONEST 100 watts per channel amplifier, into moderately efficient speakers, will get VERY loud with good authoritative bass. The speakers are a major part of this equation though.

    Your amp which is 150 watts into 4 ohms is probably putting out at most 80 watts into the 8 ohm speakers you are using.

    Also, the speakers you are using may need to have their crossover caps replaced (won't affect bass but the mids and highs will get muffled with older crossover caps that are not up to spec). If the foam surrounds on your speakers are decaying,you will lose bass.
  5. Electricblue


    Feb 1, 2011
    I know where you're coming from on the home audio thing, I don't like their forums much. There's so much misinformation around.

    All four speakers have good condition surrounds and crossovers, all checked before I rigged them up :bassist:
  6. megafiddle


    May 25, 2011
    I am not recommending this, but you could turn the volume up carefully until it just begins
    to clip (distort). At that point, see if the meter reads full power. That would be about the
    simplest way to check the power meter accuracy.
    And again, I don't recommend as I don't know the "health" of the components.

    And anyway, the meter actually responds to signal voltage and will only be accurate for a
    specific load. You may find the load impedance for accurate readings specified somewhere.

    Volume perception is logarithmic. A 2x increase in volume requires a 10x increase in power.

    Since most stereo speakers are 8 ohm, the meters might very well be calibrated for 8 ohm loads.
    If that's the case the actual power for an 11 watt reading would be 22 watts.
    (I assume you have 4 ohm per channel, two 8 ohm speakers each side)

    That 22 watts is more than 1/2 full perceived volume. In other words, you would not be able
    to double that 22 watt peak in perceived volume; that would take 220 watts.

    So those low wattage levels can be a lot louder than you might expect. But it does sound like it's
    reading lower than actual. About 1/2 actual, if my guesses are correct.
  7. I knew I would learn something useful before I even opened this thread. Good stuff to know.