Speaker sensitivity, impedance, and calculating amp power

Glad you didn't delete the thread. I wanted to know this too

@daledeee1 This conversation with Jim Salk has a graph, perhaps of the kind you're asking about? Of course, you're asking about getting graphs for *any* speaker we want to buy. Salk's graph is not in this excerpt but you can find it here:

https://www.innerfidelity.com/content/beauty-sight-and-sound-salk-wow1-mini-monitor-page-2

This part of the Inner Fidelity conversation bears a bit on this topic:
"Q: Tying in with the above - desktop users don't typically have space for a massive amp. They tend to use compact units with relatively low power ratings, with Class D implementations being more and more common. Any comment on amplifier matching with these speakers?

A: Today, the trend in speaker design is to develop smaller and smaller speakers that play deeper and deeper. The problem is, the laws of physics dictate that the resulting speakers will be somewhat insensitive and will require more power to drive. But since that is what speaker manufacturers want to produce, most of the R&D for cutting-edge drivers these days is focused on insensitive drivers that play deeper in smaller cabinets. So if you want to use the cutting edge drivers (which we do), the models you develop will tend to be lower sensitivity. They will require more power to drive, but will be more accurate and exhibit higher sound quality.

At the same time, amps are becoming more powerful to address this need. This is especially true of Class D amplifiers. These provide a lot of power at a very reasonable price and are very efficient as well. While some people feel that Class D amps are not quite ready for prime time and can be a bit analytical, they are certainly accurate and provide a lot of performance for the money. Plus, they run cool which is another advantage.

As for the small class D amps, these are quite attractive cost-wise, but don't work all that well with low sensitivity speakers. Years ago, some friends did some interesting experiments at the Rocky Mountain Audiofest. They set up both average power meters (RMS) and peak-reading meters and monitored the power levels while playing music. While the RMS meter registered 5 - 8 watts, the peak-reading meters hit peaks of 200 - 250 watts during instantaneous transients (drum hits and the like). So while, on average, 5 - 8 watts is sufficient, if you want the cleanest possible performance, the more watts you have, the better. So while these small "T-amps" are quite inexpensive, many of the lower power amps won't have the muscle required to control the woofer in the bass region. So I'd pass on trying to marry them up." (Jim Salk)

There are two kinds of audiophiles when it comes to amps and this subject; the good enough type, and the overkill type. The good enough type look at the rating and usually stay within that range, i.e. 50-100wpc as good enough. The overkill type, such as myself will use amps that are seemingly far more powerful than necessary.

I have seen no preference emerge as to any given simple formula that will assure satisfaction. I disagree that you must stay with very efficient speakers; the sonic characteristics vary so widely in terms of frequency extension, tonality, etc. That simple formulas are not helpful either in regard to speakers or amps.

Actual comparisons reveal one’s priorities quickly. :)

Thanks, Doug. I was listening to Paul McGowan on why overkill buys you the benefits of "linearity" in the speakers' response on a video yesterday, but he was careful to point out that *how* you listen makes a huge difference as to whether it will matter. For critical listening, it can make a genuine difference and it's best to know that before purchasing. That's why I'm researching these questions.

And of course the character of the amp's sound is critical. But to make a careful appraisal in the listening, I want to ensure that power levels  are sufficient to do the speakers justice, with perhaps a little to spare. Of course, if I cannot tell, I cannot tell, and then saving money would be my priority, at least to shepard it toward elsewhere in the system.

The larger questions I've raised in a couple threads on this forum — on sensitivity, on impedance, on crossover controls — are seeking a better theoretical understanding so that I can make some good guesses as to what to audition.

If, for example, I know that a 4 ohm/88db sensitivity speaker would benefit from an amp of 200 watts more than 100 watts, I should try to arrange *that* as my auditioning lineup. After all, dealers are patient — they'll give me a chance to try out different combinations — but only to a point. So, in order to try out what might be something I want to actually buy, I need to do as much homework, first. And I want to try my hardest to buy from a dealer, if they carry the gear I actually prefer. I'm convinced that brick and mortar listening opportunities are a value, and if they treat me reasonably and offer reasonable prices, I want to buy with them.

daledeee1 2-20-2020
How can an amp have different current output with the same "wattage" as a different brand?

With a very few exceptions, nearly all solid state amps are designed such that for a given input voltage their output voltage remains essentially constant regardless of the load impedance, as long as the amp is operated within the limits of its voltage, current, power, and thermal capabilities. And for a given output voltage from the amp, per Ohm’s Law the current drawn by a given load impedance will increase as that impedance decreases. And correspondingly the power delivered into that load will increase as that impedance decreases, since the power delivered into a resistive load corresponds to the voltage applied to it multiplied by the current it draws when that voltage is applied to it. (Speaker impedances are not purely resistive, of course, but I won’t get into that for purposes of this explanation).

So as an example if we consider two amps rated at say 100 watts into an 8 ohm resistive load, one amp may be able to deliver 200 watts into a 4 ohm resistive load, and 400 watts into a 2 ohm resistive load, if it is able to provide the correspondingly increased amounts of current. The other amp might be able to deliver only 150 watts into 4 ohms, and 200 watts into 2 ohms. So the second amp has less current capability than the first amp, even though they are both rated at 100 watts into 8 ohms. (This assumes, of course, that both ratings are accurate, and it also assumes that the 100 watt/8 ohm rating of the first amp has not been understated to create a false impression that the amp can double the maximum amount of power it can deliver into halved load impedances).

Tube amps are a different story altogether, though. While most solid state amps have output impedances that are near zero, which enables them to maintain essentially constant voltage into varying load impedances (within some limits), tube amps have output impedances which are significant relative to speaker impedances, and which vary widely among different tube amps. As a result, depending mainly on their output impedance, and assuming they are operated within their capabilities, different tube amps tend to fall at various points along a spectrum whose end points are maintaining constant voltage and maintaining constant power into varying load impedances. Usually not at either of the end points of that spectrum, but somewhere in between.

Finally, as you probably realize the majority of speakers these days are designed to be driven by solid state amps having near zero output impedances, while others are designed such that they are best driven by tube amps, while others are happy with either type.

Regards,
-- Al

P.S: @dwmaggie, thank you for the kind words.