How To Measure "Current" In An Amplifier?

Erik, saw your last post as I was posting. It would seem to me that we are in agreement then:-)

OK, but even with that feedback, unlike many ss amps, the vast majority of tube amps can't double down/halve up with varying speaker impedance and the corresponding varying sensitivity, and I would assume therein deviate from linear frequency response and ergo(?) distort timber, no?

No. What happens, and this is the part where we see that tube amplifier power is more expensive, is that the amplifier makes its full power into the lower impedances, and makes less than that (for example, 1/2 power into 8 ohms as opposed to 4 ohms) into higher impedances.

With a solid state amp, we are used to seeing it double power as impedance is halved. The difference here is that a tube amp is going to make its normal power into the **lowest** impedance, while the transistor amp is going to make its normal power into the **8 ohm** impedance.

I can see how this could be really confusing, so the other way to look at it is that in both cases the amplifier has to act like a voltage amplifier, which is to say that it will be constant power into all frequencies presented. Since the tube amp can't double its power as impedance is halved, instead it cuts its power in half as impedance is doubled.

Either way you get flat frequency response.

The real question is, 'Is flat frequency response important to the human ear?' which is another question entirely and the answer is 'sometimes'. The problem is that the ear converts any form of distortion into some form of tonality. The brain has tipping points where if there is enough distortion it will favor that sort of tonality over actual frequency response (whether flat or not). This is why some amps can sound bright and others not even though on the bench they measure exactly the same frequency response. The brightness can be caused by a tiny amount of higher ordered harmonic distortion or by IMD.

This is why speaker/amplifier matching is still a thing, even though speakers are **supposedly** voltage-driven! IOW you still have to take it home and see how it sounds, even after all the measurements!

@atmasphere  "Regarding your amplifier, the specs don’t add up as presented so there is more to the story. The 45 amp figure seems to refer to output. Converted to RMS as Al did (so it makes more sense) the 15.9amps does not gel with 220 watts. Into 8 ohms at 220 watts the current is 5.24 amps. The 15.9 amp figure makes more sense if the load impedance is slightly less than 3 ohms. At this point the amp would be making a bit over 660 watts. Does that sound right to you? "

I agree at face value the numbers don’t add up. I have sent a question to Musical Fidelity, but wouldn’t hold my breath for a response.

To your point "IOW you still have to take it home and see how it sounds, even after all the measurements!"

Yes, I wholeheartedly agree! This is kind of what I was trying to convey to Erik earlier in this post. What remains so puzzling, amazing and interesting to me after so many years in this hobby and "knowing" so much LESS than so many members in this thread is how after all the reading is said and done, and the actual listening begins, how the ears belie so much of the experience one gathers during the reading of these threads.

As the Grateful Dead would say, "if you get confused, listen to the music play".

Atmasphere, Yes, confusing indeed.
Your saying that tube amps halve their power as impedance is doubled? Isn't that what ss amps do?

Your saying that tube amps halve their power as impedance is doubled? Isn't that what ss amps do?

Yes, it is.

Maybe the way to look at this is the 8 ohm power. With a tube amp that is 35 watts, it will also be 35 watts into 4 ohms, and if the impedance of the load is double will then be 17 watts.

With a solid state amp that makes 35 watts into 8 ohms, its output into 4 ohms might be 70 watts, so 35 watts into 8.

This is what I meant about tube amplifier power being more expensive.