Hey Ricky,
They did sell them a long time ago at your average Radio Shack. Terms to search for are "RMS Power meter" or "speaker power meter"
Today your chances of finding one are best if you DIY them.
You may also want to see this thread:
https://www.forums.stevehoffman.tv/threads/any-such-thing-as-external-vu-meters.502537/
It is true that for the most part, the accuracy was questionable due to the varying impedance of loudspeakers. Even built-in amp meters assume (incorrectly) that the speakers are perfect 8 Ohms, and extrapolate from the output voltage. The only way to make an accurate watt meter, without significant computing and speaker measurement, is to put some resistor in series with the speaker. Anything that connects in parallel will only offer an approximation.
On the other hand, these meters are usually good enough to tell you how close you are to exhausting your amp's output, so long as your speaker impedance isn't too low for the amp. The limits of amps at high speaker impedance is voltage. They cannot exceed their voltage rails. So if you have a 100 W amp, and you have a meter (inaccurate as it is for Watts), keeping the amp from reaching 100 W on output will keep you from exceeding the voltage rail limits.
"Good enough for jazz" as they say. It is a good experiment though, I think for most music playing you'll be surprised at how little power you use most of the time.
Best,
E
They did sell them a long time ago at your average Radio Shack. Terms to search for are "RMS Power meter" or "speaker power meter"
Today your chances of finding one are best if you DIY them.
You may also want to see this thread:
https://www.forums.stevehoffman.tv/threads/any-such-thing-as-external-vu-meters.502537/
It is true that for the most part, the accuracy was questionable due to the varying impedance of loudspeakers. Even built-in amp meters assume (incorrectly) that the speakers are perfect 8 Ohms, and extrapolate from the output voltage. The only way to make an accurate watt meter, without significant computing and speaker measurement, is to put some resistor in series with the speaker. Anything that connects in parallel will only offer an approximation.
On the other hand, these meters are usually good enough to tell you how close you are to exhausting your amp's output, so long as your speaker impedance isn't too low for the amp. The limits of amps at high speaker impedance is voltage. They cannot exceed their voltage rails. So if you have a 100 W amp, and you have a meter (inaccurate as it is for Watts), keeping the amp from reaching 100 W on output will keep you from exceeding the voltage rail limits.
"Good enough for jazz" as they say. It is a good experiment though, I think for most music playing you'll be surprised at how little power you use most of the time.
Best,
E