Watts up with that?

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.