Amplifier current draw/wattage

01-17-12: Charles1dad
I don`t want to measure speakers, but want to know the power output of the amplifier at different volumes. IOW how much power is used at say SPL of 70db vs 85db(how hard is the amp working) measured in watts.

Charles, that can be calculated (to a reasonable approximation). Caveats are that the calculation assumes that speaker efficiency (or, alternatively, voltage sensitivity and impedance) is known accurately, and that the calculation neglects the contribution of room reflections to SPL.

Your Total Eclipse II's are spec'd at 94 db/1 watt/1 meter. For a box type speaker (as opposed to a planar) subtract 6 db for each doubling of distance. Add 3 db to approximately reflect the presence of two speakers.

So at a listening distance of 4 meters (about 13 feet), 1 watt per channel would result in an SPL of 94+3-6-6 = 85 db.

That figure can be adjusted to reflect different power levels based on the db relationship of the ratio of two powers, which is 10logarithm(P1/P2). From that it can be calculated that a 15 db reduction in SPL corresponds to reducing the power level by a factor of 31.6. So a 70 db SPL at 4 meters would correspond to (1 watt/31.6) = 0.032 watts.

The 8 watt maximum power rating of your Franks would correspond to an SPL at 4 meters of 85 + 10log(8/1) = 94 db.

For other listening distances, adjust up or down based on 20 times the logarithm of the distance ratio. For instance, the SPL at 3 meters would be higher than the SPL at 4 meters by 20log(4/3) = 2.5 db.

Best regards,
-- Al

Another, more obtuse path to the same data would use an SPL meter, test tone disk and a good DVM or scope....scope preff'd. The voltage at the speaker is related to *power* to the speaker.
OR
You could also measure watts used...of the amp, and calculate backwards using probable efficiency of amp. Hi/Lo estimates will yield a range of powers.

Al's method relys on knowing the sensitivty of the speaker. Not always reliable or even measured the same way.

BTW, Charles, amps need a load to generate power. So, in that sense they are entwined. Don't run any tube gear without a load or you'll get another lesson in electronics....and the bill to prove it.

Hi Everyone,
For me, this thread has been quite helpful, thank you !
Elizabeth, I believe I could have described my problem
with better wording, but I do understand and agree with
your explanation.
Ngjockey, you raise a good point about impedance matching;
another variable I should examine closer.
Koestner, I don't know why I didn't think of getting
a wattmeter myself.
So, hopefully I'll be fine with a normal 20-A supply,
and listening to the music at semi-sane levels.

Hello AL,
Thanks for your response. Al I already knew the calculation and realized that at my listening distance= 9feet and usual sound levels=70-85db(depends on recording) I`m only using small fractions of a watt! I was just curious to know if the inexpensive device could measure and confirm with an actual meter readout.
Best Regards,