A Test. How much Voltage (power) do your speakers need? - Page 11 - diyAudio
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View Poll Results: I measured the test tone at:
2 volts or less 141 37.50%
Between 2-5 volts 129 34.31%
Between 5-10 volts 49 13.03%
Between 10-20 volts 21 5.59%
Over 20 volts. 36 9.57%
Voters: 376. You may not vote on this poll

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Old 27th January 2012, 11:31 AM   #101
brig001 is offline brig001  United Kingdom
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It definitely works. For my test (details in the full range section), I recorded 0.75V with my 98dB @ 1W speakers. Squared gives 0.5625W. If you work it out long hand, 0.75V @ 8 ohms gives 0.0703W (V^2 / R) at -12dB. Each 3dB is double power, so 0.1406W at -9dB, 0.28125W at -6dB and 0.5625W at -3dB (biggest sine wave possible).

So what happened there, well to work it out Pano's way, I squared the voltage. To do it long hand, I squared the voltage, divided by 8 (speaker impedance) to work out power, then multiplied by eight (3 doubles) to get from -12dB to -3dB. Cool, wish I had thought of that.

Brian.
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Old 27th January 2012, 12:21 PM   #102
kvholio is offline kvholio  Netherlands
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I also did the test.
"normal" listening-level gave me 0.5v,
Loud (at the treshold of being annoyingly loud) gave me 2-3v.
This was on ~88dB/2.83v/1m loudspeakers, 4 ohms impedance.
(3*3)/4=2.25 times 8 gives me 18 watts.
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Old 27th January 2012, 01:43 PM   #103
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Ok I get it now. Whew!
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Old 27th January 2012, 02:49 PM   #104
Pano is offline Pano  United States
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And we have a winner, ladies and gentleman! Please give a nice round of applause to Brian - Brig001.

He explained it very well and succinctly, but I'll go over it again. Mooly was getting there, too.
  1. You set your playback level with a digital source. In this case I asked you to set the loudest level you ever use.
  2. You measure the -12dB (below full scale) test tone and get a voltage
  3. Maximum sine level on digital is -3dB, or 9dB higher than the test tone
  4. 9dB is a voltage gain of 2.83 Does that number look familiar?
  5. Multiply your measured voltage by 2.83 to reach the maximum possible sine level
  6. You now have the highest RMS voltage a sine wave will ever play on your system at that volume setting.
  7. Find the power of that sine wave.
  8. Measured voltage X 2.83 squared/8 Voila! That's how much power an amp needs to supply at 8 ohms for your music not to clip.
Let's look at that with example numbers:
  • Your measured voltage is 5 volts.
  • 5x2.83=14.15 (the 9dB offset)
  • 14.15x14.15=200.22
  • 200.22/8=25 watts
  • Or... 5x5=25 (watts) Voila!
Why divide by 8? Because most amplifiers state a power rating into 8 ohms. They may also give other ratings, but you'll always see an 8 ohm rating. It's just a matter of convenience for you to know the standard power rating of the amp you need. 5W, 25W, 250W.

Your speakers may not be exactly 8 ohms, but the voltage you measured can be tracked back to an amplifier's 8 ohm rating.
If you have 4 ohm speakers you might want to go by the amplifier's 4 ohm rating. Simply take your measured voltage, square it, then double that to find the 4 ohm power rating you need.

If an amplifier is honestly rated*, for example to AES or IEC standards, then it can output a sine wave at the rated power into the specified load. 8 ohms, 4 ohms, whatever. Peak voltage will be 3dB higher, or twice the power. The amp may be able to supply a little more voltage, but it's going to soon start distorting and clipping.

This little test is simple, it lets you measure a voltage, then multiply it to get a reference level. E.G. "I measured 3V on the test tone -OK, I need at least a 9 watt amp to play that loud".

*Yes, we all know that the marketing department writes the specs, but you are smart enough to see thru that. Look for IEC ratings, or better yet, measure for yourself!
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Old 27th January 2012, 02:54 PM   #105
Pano is offline Pano  United States
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Quote:
Originally Posted by ra7 View Post
And 6.5" Tannoy dual concentrics in a Voight pipe gave me 4.0V. That would mean a peak of 16V is required. By your equation, that means a 250W amp for clean peaks. Hmmm... kinda high.
You went a step too far. All you needed to do is 4x4=16. That's the power rating of the amp you need. (see my post above) You made the mistake of squaring that again to get to your result. Good effort, but one step too far.
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Old 27th January 2012, 03:08 PM   #106
Pano is offline Pano  United States
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Like Ra7, you are making it too complicated.
Quote:
Originally Posted by back View Post
since i already know my speakers lowest impedance(have measured it)
it would be.
7x7=49/2.7(ohm)=18.14AX49Volt=888.86watts.
If you measured 7V, then your maximum sine level would be 19.8V (9dB higher). You need that much voltage at the maximum sine wave level. Peak will be 3dB higher.

If you want to know how many watts that is into your 2.7 ohm load, do this:
19.8x19.8/2.7=145 You need an amp that can supply about 150 watts RMS into a 3 ohm load. See?
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Old 27th January 2012, 03:09 PM   #107
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Don't you need to use the same digital source to play the test tone as you used to play the high volume music?

It's important that no volume adjustments are made between playing the two tracks.
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Old 27th January 2012, 03:29 PM   #108
johnr66 is offline johnr66  United States
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2.5v rms into 4 ohm load.

I had it cranked up listening to some rock. I may not listen to it at this level for long because it is hard on the ears. The speakers are quite efficient in the 93 db range.

This DIY chipamp will do about 10v rms into 4 ohm resistors continuous, both channels driven before clipping.

I will have to try my other listening system. Fun test!
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Old 27th January 2012, 03:41 PM   #109
back is offline back  Greece
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Quote:
Originally Posted by Pano View Post
Like Ra7, you are making it too complicated.

If you measured 7V, then your maximum sine level would be 19.8V (9dB higher). You need that much voltage at the maximum sine wave level. Peak will be 3dB higher.

If you want to know how many watts that is into your 2.7 ohm load, do this:
19.8x19.8/2.7=145 You need an amp that can supply about 150 watts RMS into a 3 ohm load. See?

your way is wrong.

unless if an 150watt rms amp can give 890 watt peak power.
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Old 27th January 2012, 03:56 PM   #110
ra7 is offline ra7  United States
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ok, I got it from Brian's explanation. I was put off by your first post that said peak volts will be 4X the measured value. Anyway, all is well. So, my 25W amp is just enough.
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