Watts up with that?


I was concerned that my Belles 30 watt Class A amp (SA-30) was not powerful enough for my Montana XP speakers (seven driver 92db at 2 watts due to 4 Ohm). Using the calculation of voltage squared divided by impedance would give you watts, I hooked up my Wavetek digital multimeter across the speaker posts to read AC volts. The meter has a “max” feature so it keeps displaying the highest voltage reading until reset. My speakers have a very flat impedance curve with a low of 3 and a max of 5 Ohms, so I feel pretty safe using the average of 4 Ohms. Upon playing some music at my average listening levels I got a max voltage reading of 2.13 volts. This calculates to just over 1 watt. I then turned up the volume to much louder than I will usually listen and got a max voltage reading of 3.28 volts after a few songs. So with the volume higher than normal, and at the loudest part on the track, I get just under 3 watts being drawn. I still have a lot of watts left! Are my calculations correct? Is this an OK way to measure power? I was thinking I needed a few hundred watts of available power, but it seems I’ve got all I need at just the 60 watts capability (4 Ohm load) of my current amp. Your thoughts please.
koestner
My non mathematical answer is that 60 watts is plenty but it kinda depends on the amps output trannies if it is not an OTL. I can tell you that my 80 lb integratedb class A Jadis is also rated at 60 watts and seems relatively impervious to load. If it ios a high quality amp I would suggest you are just fine with that much or even less power.
Just an FYI, the Belles SA-30 is a solid state amp.
Yes of course I foorgot as I was writing but your assumption about adequate power is still correct.
I wouldn't count on that measurement being meaningful without having detailed information on the technical characteristics of the meter, which chances are will not be available. One important parameter being how brief a transient it is capable of capturing. Others would include its frequency bandwidth, and some indication of how it handles non-sinusoidal waveforms (e.g., true rms, or peak converted to sinusoidal rms equivalent, etc.).

That said, the power capability you need will vary dramatically as a function of the dynamic range of the music you are listening to (i.e., the DIFFERENCE in volume between the loudest notes and the softest notes). What I would suggest is that you repeat the measurements with material having the widest dynamic range that you would normally listen to. Many classical symphonic recordings, for example, have vastly wider dynamic range than most rock recordings, and therefore require vastly more power during brief peaks, for the same average volume.

Regards,
-- Al
... hooked up my Wavetek digital multimeter across the speaker posts to read AC volts...
I think that Almarg is on the right track in his 1st paragraph. I was about to write almost the same thing myself.
Pull out the spec sheet for your Wavetek DMM & see what it's bandwidth is.
Most, if not all, DMMs do not have the bandwidth to follow an AC signal & so you are getting a heavily RMS'd reading of a fast moving AC signal. I would, like Almarg, not put much confidence in those readings at all.
Your calc technique is correct tho.
Now, if you can get your hands on an oscilloscope & measure the peak voltages & then use V^2/R, you'll get the true pix....
Would you guys think I am off by a factor of two, or possibly much larger?
When ever I had an amplifier with meters I was always amazed at how little watts is being used. Although, not all amplifiers are created equal. I have owned 60 watt amplifiers that sounded more powerful than 200 watt amplifiers. When I owned Dunlavy SCIVs I had incredibly good sound with 60 watt tube monos. The best way to find out if an amplifier is powerful enough is to listen.
Belles does not provide a spec for 4 ohm operation for the SA-30. Are you sure it provides 60 watts into that load?
Not to change that subject too much, but this sounds very similar to the argument you will get when it comes to those who believe Tube S.E.T. Amps are the best. These low powered Tube S.E.T. Amps only put out 2 to 8 watts but will still player loud and proud with the proper efficient speakers.

Also similar with Pure class A amps, for the most part when people listen to music at a reasonable level they are only pulling a few watts anyway.

It's only after you really crank up the music or play movie soundtrack with lots of full range of sound going on will you start eating up watts. Or if you have some really inefficient speakers that can be hard to drive due to their design.

I could be all wrong about thus, so please feel free to correct me if needed.
09-28-12: Koestner
Would you guys think I am off by a factor of two, or possibly much larger?
I have no idea.

IMO, though, Ron (Rrog) correctly stated the bottom line: "The best way to find out if an amplifier is powerful enough is to listen."

Some additional points:

I found measurements of your amp here. They indicate that it can provide 76 watts into 4 ohms at 1% distortion. However, it is indicated that while the amp operates Class A up to the clipping point into an 8 ohm load, with a 4 ohm load it transitions to Class AB at some unspecified level that apparently is significantly below the clipping point. Conceivably that could have some effect on sound quality at power levels you would be using.

Another way to look at it: Let's call it a 60W amplifier into 4 ohms. Assuming that the 92 db/2W/1m/4 ohm numbers for the speakers are accurate, it can be calculated that at listening distances of say 10 to 12 feet, 60W will result in a sound pressure level of approximately 100 db, neglecting room effects.

Provided that the sound quality of the amplifier is still holding up at that level, 100 db will certainly be loud enough for most listeners with most recordings. It will also certainly not be loud enough for some listeners with some recordings, particularly (as I mentioned earlier) recordings having very wide dynamic range. For instance, I have many classical recordings on labels such as Telarc, Sheffield, Reference Recordings, etc. that at my listening position reach peaks that I've measured at around 105 db, although the average level during those recordings is perhaps in the low 70's. Keep in mind that a 30 db difference between peak volume and average volume means that 1000 times as much power is required for those peaks, compared to the average level of the recording.

Hope that helps. Regards,
-- Al
Hey guys,

I looked into getting an osciliscope. There are these USB ones that can use your laptop to save on the screen and that stuff. They look interesting and only about $100. However, I have no idea how to use one, but I guess I could figure out how to read some AC volts across the speaker terminals. This way I could get a more accurate measurment of peak voltage, I hope.

Al, your 100db calculation... was that for a pair of speakers, or just one? I hope it was just one so then I could add another 3db for the other speaker, also I listen about 9 feet from each speaker. I am going to hopefully audition the MB-200 mono amps from Belles to see if all that extra power does indeed matter or not. Thanks for all the replies so far. I am really enjoying trying to measure the watts used, as long as I can do it accurately.
The 100 db calculation was for the pair of speakers, and reflected the 3 db increase. On the other hand, I should mention that it assumed a 6 db reduction in SPL per doubling of distance, which might be a bit pessimistic (i.e., too large a number), considering the multiplicity of drivers the speakers have, that are spread out over a considerable height.

Assuming the 6 db reduction per doubling of distance is valid, though, which corresponds to 20 times the logarithm of the ratio of two distances, for the 9 foot distance you indicated the calculation works out to about 101 db, for the two speakers.

Also, if and when you perform the oscilloscope measurement, keep in mind that the word "peak" has to be applied with care. Amplifier power and voltage levels are specified on an rms (root mean square) basis, and on the assumption that the waveform is a sine wave. For a sinusoidal waveform, the number of rms watts is calculated based on a voltage equal to 0.707 times the maximum ("peak") voltage that is reached by the waveform. So the word "peak" in that context means something different than the "maximum" power level corresponding to a musical "peak," which refers to rms power and not instantaneous peak power.

In other words, what would be most meaningful is to determine the maximum voltage level that is reached under worst case listening conditions, multiply that number by 0.707, and apply the E^2/R formula to the result. Applying the E^2/R formula to the maximum voltage level that is reached would work in the direction of making the amplifier seem more underpowered than it may actually be, by a factor of about 2.

Regards,
-- Al
Thanks Al, you're really helping a lot. So is everyone else too. It's probably what I expected, I'm fine listening to jazz and vocals at normal to slightly louder volumes, but when I want to play heavy classical pieces to stir me up, I should be aware that this amp does not have large reserves. I still think the oscilloscope is cool and I may get one to play with.
Your speaker isn't a resistor. The REAL part of the complex impedance value is what is doing "work (making music). So be very careful to use the impedance as a resistive load...it is far from that. Speakers are only 5% efficient, so that means the majority of the impedance is imaginary in nature and does not do work.

A reasonable SPL is near 85 dB with 1 watt at 1 meter with a 1 KHz tone. Seems good to me. But, as you increase volume or decrease the frequency, power requirements go up dramatically. To not clip peaks on music (test tones are not dynamic) you aften time need 10 times the average power.

It is this dynamic power requirement that demands attention. When music moves from 1 watt to two watts average, for instance, you need an amp ten time bigger than the last one! A rule of thumb is every 3dB average SPL increase needs twice the power as the previous level. Most music will NEVER see a 30 dB dynamic range for this very reason. No amp can manage it. With digital you could do it, but should you?

If you listen to "normal" SPL around 83 dB and 93 dB peaks (where I listen on most music, and with typical 10dB dynamic range) with 92 dB SPL rated speakers it looks like you should have decent headroom with 30 watt continuous amps as they usually provide more than the instantaneously.

Rower, thanks for your comment, but I disagree with some of your statements:
Speakers are only 5% efficient, so that means the majority of the impedance is imaginary in nature and does not do work.
Much of the inefficiency reflects real (resistive) impedance, that consumes power but converts most of it into heat, rather than sound.
When music moves from 1 watt to two watts average, for instance, you need an amp ten time bigger than the last one! A rule of thumb is every 3dB average SPL increase needs twice the power as the previous level.
This statement is self-contradictory. An increase from 1 watt to 2 watts IS a 3db increase (as is an increase from 10 watts to 20 watts), and requires twice as much amplifier power (as the second sentence indicates), not an amp that is ten times bigger.
Most music will NEVER see a 30 dB dynamic range for this very reason. No amp can manage it.
I could show you waveform diagrams on my computer of the Sheffield Lab recording of Prokofiev's "Romeo and Juliet," which clearly depict a difference in volume between the loudest notes and the softest notes of approximately 55 db. That corresponds to a power ratio of 316,000 times. At my listening position, the softest notes are around 50 db, and the loudest are about 105 db. My 65W amp and 98 db speakers have no problem at all dealing with that. MANY other symphonic recordings in my collection EASILY exceed 30 db of dynamic range.

The 10 db typical dynamic range you refer to is probably typical of (or even greater than) the dynamic range of the majority of rock recordings that are released these days, but does not apply to a lot of other kinds of material.

Regards,
-- Al
Speakers are only 5% efficient, so that means the majority of the impedance is imaginary in nature and does not do work.

Everybody knows that speakers play on one wall better than on another. It is because 90 degree rotation eliminates imaginary (parasitic) impedance.
The dynamic range reference is from the AVERAGE SPL, not the minimum. So if your avevrage SPL is 85dB, your peak dynamic range will be 115dB, not a value most systems can manage. Music has to be recorded to be at a comfortable volume WITH the expected dynamic range. If you keep turning down the volume to increase the dynamic range...sooner than later you can't enjoy the music.

...This statement is self-contradictory. An increase from 1 watt to 2 watts IS a 3db increase (as is an increase from 10 watts to 20 watts), and requires twice as much amplifier power (as the second sentence indicates), not an amp that is ten times bigger....
You're right, it should be twice, not ten. Every 3dB is twice the power.

The "real" part of the impedance component is the entire vector sum of the X-over and the driver both. The entire speaker is measured, not just a part of it. Most energy going into a speaker never makes a peep. "Real" power vectors are not reflected, that's why you want them to be large (but they aren't)...they do work. Some of that precious little energy is wasted as heat as you say, but the majority is imaginary in vector.

And no, you can't remove imaginary components of an impedance curve by physicaly moving your physicaly.
And no, you can't remove imaginary components of an impedance curve by physicaly moving your physicaly.

I'm sure I can. For instance when I rotate my incandescent bulb 90 degree counter-clockwise they become dim but when I turn them 90 degree clockwise they're lit again. I'm sure it is this imaginary impedance taking over.
10-01-12: Rower30
The dynamic range reference is from the AVERAGE SPL, not the minimum. So if your avevrage SPL is 85dB, your peak dynamic range will be 115dB, not a value most systems can manage.
I had indicated that the average SPL of the recordings I referred to were "perhaps in the low 70's." I doubt that anyone would want to play them at an average level of 85 db, with peaks of 115-120 db. That is simply too loud. As a point of reference, as noted in this thread 8 hours is the limit of permissible continuous exposure to 85 db, beyond which hearing damage can be expected to occur.
Some of that precious little energy is wasted as heat as you say, but the majority is imaginary in vector.
What do you base that on?

That would say that a speaker whose impedance has small phase angles across the audible frequency range would approach 100% efficiency. My understanding is that most speakers do not have as much as 10% efficiency. You cited a figure of 5%. And it is rare for a speaker to have phase angles that exceed or even approach 45 degrees across broad parts of the spectrum (although that can occur across narrow ranges of frequencies). Meaning that their impedance is mostly resistive. That can be seen in the measurements that are provided by John Atkinson in Stereophile's speaker reviews.

As an additional point of reference, note in this Wikipedia writeup that the acoustic power radiated by a jackhammer is all of about 1 watt! If the 50 or 100 electrical watts or thereabouts that may be sent into a speaker at times were reduced to that kind of acoustic power level (or less) primarily as a result of non-resistive impedance characteristics (as opposed to conversion to heat), it would say that the impedance would have to be almost entirely either inductive or capacitive (i.e., having phase angles approaching +90 or -90 degrees) across nearly the entire frequency range. Which, frankly, is nonsensical, as well as being completely inconsistent with JA's measurements.

Kijanki, LOL :-)

Regards,
-- Al
As a followup to my previous post, see Figure 1 here for an example of a speaker having small phase angles (i.e., having an impedance that is pretty much resistive) across most of the spectrum. Note that the phase angle never goes below approximately -12 degrees, and only goes above approximately +20 degrees in the extreme upper treble, reaching a maximum of around +38 degrees at 20 kHz.

Yet its sensitivity is only moderately high, at 91 db/1W/1m. So since little of the inefficiency occurring in the conversion of electrical power to acoustical power can be attributed to inductance or capacitance, it follows that the great majority of the lost power is being dissipated in the speaker as heat.

Regards,
-- Al
... My understanding is that most speakers do not have as much as 10% efficiency. You cited a figure of 5%....

How can an almost pure resistive load be 5%, or even 10% efficient? THAT does not make sense. By definition a resistor consumes the entire load placed across it. Speakers don't so they aren't resistive loads...not anywhere near. Where did the other 90% to 95% go? It's the imaginary component of the speaker fighting the electroniocs, that's where. You have HUGE winds of wire in a magnet and you think that that is mostly resistive when it is driven?
By definition a resistor consumes the entire load placed across it.
Yes. And it converts it into heat.
Speakers don't so they aren't resistive loads...not anywhere near.
Speakers do consume most of the power that goes into them, they just don't convert most of that power into sound. The power that is not converted into sound is mostly dissipated as heat.

The faulty logic of your statement is addressed further in the final paragraph of this post. First, though, see the plot I previously linked to, of the impedance phase angle vs. frequency for a particular speaker. See any plot of impedance phase angle vs. frequency for any other speaker. While most speakers will not have phase angles that are as close to being resistive as the one I linked to, as I previously stated, "it is rare for a speaker to have phase angles that exceed or even approach 45 degrees across broad parts of the spectrum (although that can occur across narrow ranges of frequencies). Meaning that their impedance is mostly resistive."
You have HUGE winds of wire in a magnet and you think that that is mostly resistive when it is driven?
In the example I linked to, the inductance of the tweeter voice coil is undoubtedly the reason for the rise in impedance phase angle in the upper treble region. As frequency decreases, a given amount of inductance becomes progressively less significant. The crossover network further complicates matters. The bottom line, for a given speaker, is the measured impedance phase angle. How can you claim that the impedance is essentially inductive or capacitive, when the measurements indicate otherwise?
How can an almost pure resistive load be 5%, or even 10% efficient? THAT does not make sense. By definition a resistor consumes the entire load placed across it. Speakers don't so they aren't resistive loads...not anywhere near. Where did the other 90% to 95% go?
I suspect that you'll agree with me that the impedance of an incandescent light bulb is essentially resistive, at least at 60 Hz and other low frequencies. And I suspect that you'll agree that the great majority of the power supplied to it is NOT converted into light, and its efficiency is therefore very low (roughly 10% or so per this Wikipedia writeup). Where do you think the rest of the power supplied to it goes? Ever touched a 100W light bulb that has been on for a few minutes or more?

Regards,
-- Al
Here are the "real" OSHA numbers for continuous noise, not random, like music, exposure. 85 dB levels with music is far from continuous and is more than fine. My "ear" says the Fletcher Munstrom (spelling) flattens out at about 80dB, where the low end seems linear. The human ear is lousy below about 80 dB for linearity of broad spectrum sounds. We hear around 1-4 KHz at 75 dB OK. That's as nature intended. True, you can "flatten" the sound by EQing to 75dB (remember loudness controls), just don't turn it up much with that EQ active!

http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=standards&p_id=9735
TABLE G-16 - PERMISSIBLE NOISE EXPOSURES (1)
______________________________________________________________
|
Duration per day, hours | Sound level dBA slow response
____________________________|_________________________________
|
8...........................| 90
6...........................| 92
4...........................| 95
3...........................| 97
2...........................| 100
1 1/2 ......................| 102
1...........................| 105
1/2 ........................| 110
1/4 or less................| 115

And, Go here;
http://www.stereophile.com/content/dynaudio-confidence-c4-loudspeaker-measurements

Most of a speakers "power" is dissipated at the lower frequencies below 200 Hz and more than 4/5th total isn't uncommon. Here, the graph of C4's clearly shows significant phase departures from "zero" that approach +30 to -50 degrees. It isn't near zero till about 200 Hz in this speaker. Since the majority of the load is mostly reactive below 200 Hz this is a terrible "resistive" load per your conclusion. It is NOT a resistive filament. No where near. It does produce a very reactive signature to the majority of the signal power being applied. This is what your amplifier is fighting against. The numbers at the frequency of the power dissipation peaks show that this isn't as bright of an idea as lighting a light bulb.

As far as the jackhammer example, It's amazing that a device that is intended to crush rocks loses just 1 watt to sound. Sound SPL at a given volume has a measurable power in air that is always the same no matter what produces it. 1KHz at 100 dB SPL is ALWAYS the same power level in-air. It doesn't know who it's mommy is. The energy "used" to launch it varies tremendously in wasted effort, true. The SPL of a jackhammer's frequency range is pretty low and at a pretty high 120 dB SPL so it is amazing that it has a 1 watt sound launch energy value. And the actual "sound" efficiency would be the power going in to make the SPL level coming out (here we'd ignore the jackhammers real job and look at it as a noise maker) right?. How much power is going into a jackhammer relative to the lost energy due to the "sound" escaping from the intended process? Most of the energy is going into the rocks, so it's a pretty lousy speaker for making 120 dB of noise...as it should be.
Since the majority of the load is mostly reactive below 200 Hz this is a terrible "resistive" load per your conclusion. It is NOT a resistive filament. No where near. It does produce a very reactive signature to the majority of the signal power being applied.
Rower, the point of contention was whether the 90 or 95% inefficiency of a speaker is mostly the result of power dissipated in the speaker as heat, or is mostly the result of reactive impedance characteristics.

In the severe example you provided, the impedance phase angle only exceeds 45 degrees by a very small amount in a very narrow range of frequencies, between approximately 60 and 70 Hz. Throughout most of the spectrum, including the bass region, its phase angle is much less than 45 degrees. While that kind of impedance characteristic could perhaps be legitimately referred to as "very reactive" in a relative sense, relative to many other speakers, it is not by any means "mostly reactive below 200 Hz." An impedance is not "mostly reactive" unless the phase angle exceeds 45 degrees. And there is no way that the depicted impedance phase angle characteristic, or just about any other reasonably conceivable impedance phase angle characteristic, would account for the major portion of a 90 to 95% inefficiency.

Yes, I certainly agree that the speaker will be a difficult load for many amplifiers to drive, with good sonic results. But that wasn't the issue.

Regards,
-- Al
Al,

I'd like to see the weighted impact of energy distribution across the spectrum to conclude it's a "small" effect. I'm not saying you're wrong, but the laws of physics say the WEIGHTED effects in the frequency range of highest energy can be pretty severe. The lower in frequency you go, the more of the total power that is distributed, and hence, the reactance will really bug an amplifier.

True, the phase angle has to be greater than +/- 45 degrees to me mostly reactive.

True too, where a speaker is mostly resistive the power is tossed as heat. It just seesm to me that 75% of the power is applied across the lower ~200 Hz, where the speaker is most unsettled in impedance. So it isn't necessarilly the "range" of the frequency but the percentage of power supplied AT that frequency. Still, this could be much less than the minds eye thinks looking at the graphs. Stuff can get away from you when we make assumptions (we all know about assumptions).