How To Measure "Current" In An Amplifier?

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.

Gdnrbob 9-26-16
I wasn’t quoting you saying High DF was bad, only referring to Almarg’s response that a high DF would wreak havoc on tube amps.

Hi Bob,

No, I didn’t say that. Or at least I didn’t mean that :-)

I suspect you are referring to this statement:

I’ll mention also that once damping factor gets above say a few tens (in other words, away from tube amp territory) I would not expect much if any correlation between damping factor and performance into low impedances.

Tube amps often have damping factors in the single digits (i.e., less than 10), and in some cases even less than 1. The highest damping factor I can recall ever seeing for a tube amp was in the vicinity of 25 or so. While of course solid state amps commonly have damping factors in the hundreds, and sometimes even above 1000 (especially in the case of many class D amps).

I read a Wikipedia page explaining it, and it sounded like a high damping factor allowed an amp to control a speaker cone better than a low one.
So, what makes having a high damping factor bad?

Speakers vary widely in the amount of damping that is optimal, not only in terms of "control" but with respect to the tonal effects resulting from the interaction of their effective output impedance (which is inversely proportional to damping factor, as Erik indicated) with the speaker’s variations of impedance over the frequency range.

The majority of speakers are of course designed these days with the expectation that they will be used with solid state amplifiers, and those speakers will perform well with amps having relatively high damping factors. However once the damping factor gets above a few tens (arguably as little as 20, but 50 or 75 so would seem to be a safe ballpark) further increases in damping factor will **in themselves** make little or no difference in terms of both control and tonality. I say that despite beliefs to the contrary that are prevalent among many audiophiles, and claims to the contrary that can be found in a lot of marketing literature.

Many other speakers, though, are equally happy or even more happy with tube amps having low damping factors. It depends on the intention of the designer, as Ralph has said in a number of past threads. And some speakers are equally happy with tube and solid state amps, and with high or low damping factors. In those cases the intrinsic sonic character of the particular amp can be determinative of which is most optimal.

So how can a high damping factor be bad?

1)It may be a poor match for the particular speaker, the result being too much damping and "control," and/or tonal balances that were not intended by the designer.

As Erik indicated, a low damping factor, corresponding to a high output impedance, will result in output voltages from an amp that may vary significantly as a function of a speaker’s impedance variations over the frequency range. However some speakers will sound best when that variation **is** significant, i.e., when the variation of amp output as a function of load impedance variations results in power being held closer to constant than voltage. See Ralph’s paper on that subject.

2)High damping factors (and also very low total harmonic distortion) are accomplished in some designs by means of heavy-handed application of feedback, which can result in objectionable amounts of transient intermodulation distortion (that is not normally specified, and for which measurement standards do not exist), and/or increases in certain harmonic distortion components that are particularly offensive (even though **total** harmonic distortion may be reduced).

Hope that clarifies more than it confuses :-)

Best regards,
-- Al