A Test. How much Voltage (power) do your speakers need? - Page 49 - diyAudio
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View Poll Results: I measured the test tone at:
2 volts or less 143 37.53%
Between 2-5 volts 130 34.12%
Between 5-10 volts 51 13.39%
Between 10-20 volts 21 5.51%
Over 20 volts. 36 9.45%
Voters: 381. You may not vote on this poll

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Old 8th February 2012, 12:04 AM   #481
Pano is offline Pano  United States
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Originally Posted by revboden View Post
Pano, you seem disgruntled, Don't be.
Thanks Revb. Not disgruntled, just a bit frustrated. You are right about people reading their own biases into it, but I had not expected it.

Yes, I'm glad that Tom is here, he has a lot of good advice. Some of the things that have come up deserve their own threads.
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Old 8th February 2012, 01:13 AM   #482
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Originally Posted by Pano View Post
From the -12dB tone, yes. That's peak, not RMS.
Ah I've been assuming that as the original sine wave was recorded at -12dB as an average level that the output on the amplifier, when measured in an rms voltage was also representative of the rms figure. Hence if you multiplied it by 4 you'd end up with the equivalent requirement for an rms wattage. I may have added to some slight confusion in some of my posts.

Going through the maths though its obvious as the peaks of the -12dB tone actually hit -9dB and if multiplied by 4 we end up with +3 which is impossible.

The -12dB tone might be recorded as an rms value and the DMM might read an rms voltage, and indeed to figure out how much peak voltage you need to reproduce the -9dB peaks you need to multiply it by 1.41. Now you're up at a peak level = -9dB, so to raise that up to a 0dB peak you actually need to multiply 1.41 up by 2.82 = 4 volts. That is 4 volts to reproduce the peaks of a sine wave with peaks hitting 0dB.

Naturally if one starts out with an rms figure of 1 volt for the -12dB, you multiply it up by 4 to arrive at 4 volts, which as explained above = the peak value of a maximum amplitude sine wave. I had assumed that this was the rms value and hence you can convert it directly into an 8 ohm watt figure. You cannot, to do that you have to divide it by 1.41 first.

In other words, if I've just figured this out correctly () the people who are in category 1 and measuring less then 2 volts are actually using no more then 4 watts, and those in category 2 are using between 4 and 25 watts rms, as measured into a resistive 8 ohm load.

If that's correct then it appears that I inadvertently added to some of the confusion.
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Old 8th February 2012, 01:13 AM   #483
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Default deserve their own threads

X2 on that. So meaty.
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Old 8th February 2012, 01:47 AM   #484
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Hi mr push pull

The reason for the null test is it would show you if your amplifier has run out of voltage swing, is large enough for what your listening to.

Usually clip indicators and normally level indicators only show the level as averaged over a longer period of time compared to what is happening cycle by cycle like I am talking about.

Thus, as one might find with an oscilloscope, an amplifier may be instantaneously clipping while showing no signs of that on a level meter or clip indicator or be audible as a familiar flaw (an example of it I cited earlier).

I suggested this test because well before clipping makes a sound that you recognize, it is compressing the dynamics of the signal. If preserving the dynamics is important or desirable, having "enough" is also desirable.

With that null test, if the amp input and output are the same, there is nothing left to hear and no problem. AS soon as an amplifier misbehaves and cannot follow the input signal (like instantaneous clipping) a large difference signal is produced.

The presence of that signal (your amp running out of swing or distorting etc) tells you that for what you were trying to do, your amp is not big enough.

The null test would work with any recording at any level.

5th, sorry you were having trouble with it, the program has been out for a good while.

In the way old days, one could do this with a pair of mic or signal transformers, the input drives one, an attenuator made of a 10k resistor and 2k pot could be used to make it adjustable connected to the output, the out puts are put in serried but out of phase to cancel.

You put in a sine wave or other signal at a low level and adjusted the pot to get the minimum level, the null.
Then you could amplify the difference and do the same thing, instantly ďhearĒ when it isnít following the input signal.

This old way works real time but I donít imagine many folks have a pair of mic or signal transformers lying around and I figured software might be easier.

The cool part about the old way is it doesnít care about ground convention and allows you to see or rather hear real time when you have run out of gas well before you would be able to hear it from your speakers and it works with any signal at any level with any program material..

So, if one considers the instant an amplifier is no longer able to do what itís told is an indicator itís not powerful enough or otherwise unable then this seems like a good bet and doesn't require an oscilloscope.
Best,
Tom
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Old 8th February 2012, 03:02 AM   #485
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Hi Tom,

if only determining if clipping occurs is what you're after, simply using the sound board as a scope may be even simpler. one can even record and do post-analysis. not to mention real-time analysis.
an advantage of recording real music through the sound card instead of using sines is that one can capture behavior with a real, reactive load with the added bonus of not risking to burn a driver with high power sines. I guess I'm among the very few that sent some 100W of pure sine into a $1000 woofer (it is alive and well, thank you).
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Old 8th February 2012, 10:30 AM   #486
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Originally Posted by 5th element View Post
In other words, if I've just figured this out correctly () the people who are in category 1 and measuring less then 2 volts are actually using no more then 4 watts, and those in category 2 are using between 4 and 25 watts rms, as measured into a resistive 8 ohm load.
Correct! The test tone is 12dB RMS below FULL SCALE. That makes it 9dB below the highest possible sine wave value. A little confusing, I know. But I believe that full scale should always be the reference.
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Old 8th February 2012, 03:44 PM   #487
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Default My measured value.

Thanks Pano for an educational and informative thread.

My measured value is.
1.9 V -12dB at 120Hz.
Loudspeaker sensitivity of 92dB (2.83V/1m).
Room 4x5m.

If I understand correctly. this means??
My max value is 4x1.9 = 7.6 V
My RMS W value of 8ohm is 1.9 Vx2, 83 = 5.38 V -- SQR 5.38 / 8 = 3.6 W.

If my speakers has its lowest impedance 3.5 ohms at 120Hz. My maximum power peak is aprox. 16.5 W (SQR 7,6/3,5).?????

Thanks Pelle
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Old 8th February 2012, 04:34 PM   #488
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Hi Push pull
A problem with a sound card based oscilloscope and the file comparator software that uses a sound card that I had suggested earlier is like I also said, that what youíre looking for can contain or predominantly be information above what the sound card can capture.

For audio work, one wants an oscilloscope with at least a decade greater BW than what youíre looking at.
For example, a sound card can capture a 20KHz sine wave, but if your amp was clipped to a full square wave at 20KHz, all the 16/44 sound card would see / capture is a sine 20KHz wave.

Now, most of the audio is well below that and so a sound card based method would be of value (and is why I suggested it).

I mention the old way too because 5th had problems with the software but the idea is you compare the input to output, the amp gain is only scaling, it does not change the wave shape and THAT is what youíre looking for (the output NOT matching the input, such as at instantaneous clipping).

I mention the transformer based approach because that is real time and an ancient way to get the difference between an input and output but mention it last because if people donít have test equipment lying around, they probably donít have a pair of signal transformers lying around.

Unfortunately, to examine features in signals that happen very quickly and are over just as fast, takes more than ones ears or a simple meter.
Best,
Tom
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Old 8th February 2012, 05:34 PM   #489
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Originally Posted by 5th element View Post
Going through the maths though its obvious as the peaks of the -12dB tone actually hit -9dB and if multiplied by 4 we end up with +3 which is impossible.

In other words, if I've just figured this out correctly () the people who are in category 1 and measuring less then 2 volts are actually using no more then 4 watts, and those in category 2 are using between 4 and 25 watts rms, as measured into a resistive 8 ohm load.

If that's correct then it appears that I inadvertently added to some of the confusion.
So you need 25 watts, or 50 watts, what's 3 dB amongst friends .
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Old 8th February 2012, 06:11 PM   #490
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e friend (he's not a member here) just did the measurement: he actually listens at about 200W maximum with 90dB speakers. wow!
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