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About 6 watts

Posted 25th June 2011 at 03:38 PM by Pano

The other day I was wondering just how much power I really need for my rig. Can't be much as I run Altec A5s in a small room - but how much? Would a 10 Watt SE amp be enough?

So I set out to figure out how much power I need.

A few weeks back while doing some crossover work and measuring voltages I was surprised to find that my horns are running about 5mW of power - average. Uhhh, that's not much. OK, but what about the woofers? Here is how I proceeded:
  1. I found a few bass heavy tracks and played them as loud as I would ever care to.
  2. The average volume knob setting for these was noted.
  3. The tracks were opened in GoldWave and low pass filtered at 440Hz, same as the woofer section.
  4. The low passed tracks were analyzed in GoldWave for their RMS value. Average was -15.5dBFS RMS
  5. Using the function generator in GoldWave I made a 40-880Hz, 30 second sweep at -15.5dBFS.
  6. This sweep was played back several times at the same volume setting as before.
  7. Voltage at the speaker terminals was measured and found to be 1.6V RMS over most of the range. The sweep is easier to measure than the musical signal.
  8. Ohm's law says that 1.6 volts into 8 ohms is 1/3 watt. Not much!
  9. But that is just average - what about peaks?
  10. If average is -15.5dB = 1.6V then the peaks will be 15.5dB higher. Or ~9.5V
  11. An amp that can do 9.5V peak would be rated at an RMS voltage of 6.7V (9.5/1.41)
  12. 6.7V means a power rating of 5.6 watts into 8 ohms.
Voila! With 6 watts RMS my system will play as loud as I want it to. Mostly it will not be playing that loud. A 10 watt RMS amp will give me a bit of headroom.

You might want to double check this for errors, but it seems to be a reliable method.
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Total Comments 7


  1. Old Comment
    In an earlier and much younger version of myself I was a performing musician. I was also an electronic technician. As far as guitar and audio amps go, the more power the better, and in certain circumstances that is still true. Now that I have discovered the world of tube amps, especially single-ended ones, my 10 watt se amp and almost 98 db speakers tell the real truth.
    Posted 28th June 2011 at 01:10 AM by scott17 scott17 is offline
  2. Old Comment
    Pano's Avatar
    Yes, it's surprising how little power you actually need in a domestic setting.

    There is a flaw (or oversight)in the calcs above, but so far no one has mentioned it.
    Posted 28th June 2011 at 02:31 PM by Pano Pano is offline
  3. Old Comment
    Radugazon's Avatar
    Dunno where is your flaw, but this reminds me of this article (I guess you remember french ).
    They measured with a special B&K multimeter and saw that even on a very efficient system small amps can't follow the dynamic that had 20 volts peaks.
    Posted 1st July 2011 at 03:22 AM by Radugazon Radugazon is offline
  4. Old Comment
    wintermute's Avatar
    Hmmm I get 1.42W for 9.5V p2p did you forget to divide the 9.5 by 2? or is 9.5V peak 19V p2p which does work at to 5.6W.....

    Ahh I see it now. Your 9.5V is RMS not peak you really need 11.3W

    Posted 1st July 2011 at 03:57 AM by wintermute wintermute is offline
  5. Old Comment
    Pano's Avatar
    Thanks for the comments.

    The main flaw above is the -15dB level. That's common on a lot of rock and pop, but I measured a few loud passages on classical recordings and they are often -20dB (under 500Hz). That means the peaks will be 5dB higher than I calculated.

    So the power requirement goes up if I want to play these as loud as the rock/pop tracks. Who knows, I may want to play them louder. As far as I can figure a 10W RMS amp would limit me a couple of dB on a classical fortississimo. (fff)

    Gazon - I read the page you linked, it's good. The calculator told me that I need about 25W to hit 105dB peak. I use a 25W amp now on the bass, but it's not breathing hard. I was actually surprised at the low levels I'm using. The calculator did confirm that a 1W is all the horns need to hit the target 105dB at the listening position.

    It's always going to be the classical music that needs the most peak power.
    Posted 2nd July 2011 at 03:25 AM by Pano Pano is offline
  6. Old Comment
    Μy 4,5 Watt (estimated) SE is all I want for my OB's with Visaton's. In a large-ish living room, nonetheless.
    Posted 7th November 2012 at 02:06 PM by costis_n costis_n is offline
  7. Old Comment
    bogdan_borko's Avatar
    Nice speakers for a small room
    Posted 5th November 2014 at 01:38 PM by bogdan_borko bogdan_borko is offline

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