Speaker sensitivity, impedance, and calculating amp power

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.

Hilde45 quoting the great Jim Salk: " 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). " 

This is very interesting information. The implication is that we would benefit from 14 dB to 17 dB of headroom above our average SPL, which corresponds with having from 25 to 50 times more power available.  

Obviously it’s easier to get there with high efficiency speakers, though elsewhere in the passage hilde45 quoted Jim makes some insightful arguments for the lastest drivers having modest efficiency. 

Duke

@audiokinesis Right -- and, FWIW, I think the speakers mentioned here are that way because they're small and yet can do bass pretty well. Other speakers Salk makes are not as insensitive, e.g. the  Song3 Beats. 

@tvad Good suggestion. That may be the way to go. I'm going to hear some Atolls tomorrow and some Quicksilver. Just to hear them -- and they'll play through Dyn Evoke 10s, so we'll see how they perform. Later, when I have the Salks, I'll try out some amplification in my home, and maybe via Reno.