How Much Amplifier Power is Needed to Drive My speakers? Do the Math!

mhardy6647

heh... some of us use impedance matching devices.




...besides...

Since very, very few loudspeakers present flat and unvarying impedance loads (as a function of frequency) to an amplifier (Some Magneplanars* have been pretty close) -- close enough... is close enough. Corollary: The perfect is the enemy of the good.


* The Magneplanar MG1.6/QR illustrates this.
https://www.stereophile.com/content/magnepan-magneplanar-mg16qr-loudspeaker-measurements

Its impedance (fig.1) is basically that of a 4.5 ohm resistor, modified by the crossover, which results in the peak at 600Hz. Any good 4-ohm-rated amplifier should be able to drive the Maggie without difficulty. There are no cabinet resonances—the MG1.6/QR doesn't have a cabinet.

mhardy6647

... here's a more perfect example of a resistor-style load in the real world of loudspeakers (also a Magnepan product)...



https://www.stereophile.com/content/magnepan-lrs-loudspeaker-measurements

... but most loudspeakers feel more like this to the poor amplifier...



https://www.stereophile.com/content/polk-lsiiim703-loudspeaker-measurements

The electrical impedance (fig.1, solid trace) remained between 4 and 7 ohms over most of the audioband, but dropped to 3 ohms at 125Hz. There is also an awkward combination of 4 ohms and –47° phase angle at 90Hz, a frequency where music can have considerable energy. Polk specifies the LSiM703 as being "compatible with 8 ohm outputs," but I suggest that a good 4 ohm–rated amplifier or receiver would work best with this speaker.

maxward

Late to this thread, but it’s a good one. I’m a little reminded of a button one of my former coworkers would occasionally wear: We have charts and graphs to back us up, so f*** off. And speaking of math, remember this wise quote, “Fifty percent of the game is half mental.” (Y. Berra)

shs

Would someone want to comment on Class D amplifiers in these respects? There is much less power "wasted" in the transistors in those amps.

mhardy6647

Class D (pulse width modulation) amplifiers (which aren't a new thing at all, BTW) are extremely electrically efficient. Basically, they use transistors as what they are good for -- switches. The "problem" with them is that they generate large amount of high frequency (as in ultrasonic, all the way into radio frequency) noise, which must be dealt with by
filtering of various kinds... and which can cause interference with other (analog) signals.

This being said, even the really cheap ones today sound pretty good, and the better ones can sound quite good.

They probably are the future of hifi as more and more territories demand ever-more energy-efficient electronic components.


https://www.maximintegrated.com/en/design/technical-documents/app-notes/3/3977.html

mhardy6647

Early PWM audio amp (ca. 1965) offered as a kit by Sir (eventually!) Clive Sinclair



http://www.vk6fh.com/vk6fh/sinclair x10.htm

EDIT: and the X-20 in 1967



source:
(page 72 of the magazine, which is page 64 of the PDF)

mhardy6647

anonymouse wrote: »

Clive Sinclair was like the Elon Musk of his time.

Yeah... kinda... he aimed a lot lower, though.

Remember the Timex/Sinclair $99 computer?
I do.

daddyjt

I remember when I used to worship at the altar of stats...

To illustrate my point on why stats don’t tell anywhere near the whole story, take the photo below. The amp on the left is rated at 380wpc @ 8Ω. The one on the right is rated at 100wpc @ 8Ω. Guess which one sounds better...?

gp4jesus

Reminded me of a NAD*/Rotel** THX shootout, both ~$600 (Stereophile Nov ‘92).
100 WPC vs 120 WPC RMS.
NAD’s dynamics’ specs 370WPC/8ohms in stereo
Rotel’s 😢.

I don’t have the all reviewers “sound” notes, missing the NAD’s entirely. I’m not anti-NAD - all my first system was NAD. I still have a working 3020.

When it came to the bottom line (sound quality): IIRC their strong points were
NAD - grace under pressure*, but never great
Rotel - finesse until just past its spec’s limits
*never too fussy regardless of load

Something to think about. Tony

PS: If you find the above article on-line, feel free to correct me