When JA measured the amp, the amp did not meet its specified power output of 75 watts. I think that many people are under the assumption that when they buy an amplifier, the amp does meet its stated output power and does so at the distortion levels advertised. The amplifier under review could only muster 61 watts of output power into a 4 ohm load from the 4 ohm tap. The kicker is that it could only reach 61 watts into 4 ohms at 1% distortion which JA further defines as driving the amp into clipping. The output power into 8 ohms from the 4 ohm tap was only 43 watts at 1% distortion. When JA drove the amp into 3% distortion, the output power increased to 46.5 watts into 8 ohms and 70 watts into 4 ohms from the 4 ohm tap. So even if you relax the distortion levels to 3%, the amp still falls short of its advertised power output which I find surprising for a number of reasons. When the output power was measured into an 8 ohm load from the 8 ohm tap, the power came close to meeting the specified power. The amp put out 70 watts from the 8 ohm tap into an 8 ohm load, but again it was at 1% distortion.
So here's the kicker: How does an amp that measures poorly in terms of distortion for anything approaching it's specified power output sound so good? Objectivists who love many zeros after the decimal point in their distortion measurements would surely cringe. Frankly, I also have a little problem with a manufacturer saying an amplifier puts out 75 watts a channel when it doesn't and not clearly specifying the distortion levels that will be required in order to approach the rated output power. I think it's called truth in advertising. So here we have a $9K amplifier that is specified as 75 watts per channel at 0.6% THD and it really can only muster 70 watts at 3% distortion. Some of you may remember a rant I went on long ago on this forum about specifications not being measurements and there were some naysayers who didn't get what I was saying at the time. The naysayers were under the impression that specifications in the absence of measurements were just fine and surely they really were the same. Here is a classic example of why this isn't so.
This question was answered in the 1960s by General Electric (regarding how we can like an amplifier that seems to have poor distortion figures). The human ear/brain system regards the 2nd, 3rd and 4th harmonics as musical. OTOH, it uses the odd ordered harmonics, the 5th, 7th and 9th, as loudness cues- that is, to sort out how loud the sound is. Its important to understand that the ear is more sensitive to the presence of the 5th, 7th and 9th harmonics than it is to human vocal frequencies! It also helps to know that our ears are tuned to birdsong frequencies.
Armed with this knowledge we then take a look at the spectra of the amplifier in question and compare that to typical solid state. What you will see is that while the ARC has much more distortion overall, most of it is the 'musical' type. OTOH the transistor amp has more of the odd orders and so sounds brighter even though on the bench they measure the same from 100Hz to 10KHz.
I would not take ARC too much to task on the power measurements. There are two problems, at least one that may have escaped JA/Stereophile. The one that JA may have missed is line voltage- if measuring power, one should make sure that the line voltage is at spec when the amp is also at full power. You often need a variac to accomplish that. Depending on the amp, a drop of a couple of volts of AC power can rob the amp of a considerable amount of power, as much as 30%! Second, its quite normal to loose efficiency and bandwidth on the 4 ohm tap. Further, loading the 4 ohm tap with a lower impedance will predictably get vastly reduced power levels. IOW the 8 ohm measuremenjt is the one to be taken seriously.
BTW this opens the whole topic of impedance. Bluntly, in high end audio there is no good reason/excuse to build a 4 ohm speaker. If sound *quality* is your goal, the amplifier dollar investment is best served by an 8 or 16 ohm speaker, regardless of the technology of the amplifier. You can see this in the distortion specs of all amplifiers. Now if sound *pressure* is your goal, there is a slight advantage to 4 ohms. Further, if you are considering a tube amp, the measurements above should tell you that a 4 ohm load is obviously not serving that amplifier very well.
I'm not discussing how SS amps that measure somewhat close sound so different. This is more about a tube amp that is sold as a 75 watt amp at 0.6 THD and it doesn't come close to meeting those specifications and why in spite of that, it's still a damn good sounding tube amp.
This all has to do with distortion, not just that of the tube amp but that of the transistor amp as well. IOW, they both have colorations.
I imagine the ear's inability to distinguish gross distortions of the original signal fuels their belief in the non-audibility of differences between amps. Personally, I think I could easily distinguish a good SS amp operating under lots of headroom from a "75 watt" tube amp struggling to produce 40 watts. I tend to think those who can't hear tube warmth just haven't heard it enough, and don't quite know what to listen for.
Tim
The tube warmth does not apply to all tube amps- just those that have single-ended circuits in their topology. For example our amps are fully differential, and don't have any even ordered harmonics at all. Sunn, famous for guitar amps, made a series of solid state guitar amps in the 1970s that were known for their warmth. It came from extensive use of single-ended circuits in the preamps and power amps of those products. OTOH the brightness that is signature of many low-distortion transistor amps is simply coming from trace amounts of odd ordered harmonics, something that is typical of 99% of all transistors. IOW both tube and transistors have colorations if one is not careful about the topology.
I have no idea how SS "breaks up the sound more". What I do know is SETs exhibit primarily 2HD, which is generally perceived as less annoying and in fact may be somewhat pleasing, compared to most SS amps that suppress the 2HD and exhibit primarily 3HD. And that tubes have much higher output impedance and so vary much more with (speaker) load. And that the instrinsic distortion series for a single tube is factorial, which does in fact mean it is lower than a bipolar junction transistor's exponential series. In the real world I doubt it matters, especially given a little feedback. I still think the bottom line is that tubes tend to be more colored (but do not have to be by any means) and people like that color. Including me, while recognizing it is not as low in distortion, but my system is all SS at the moment.
BTW, as a preamp, tubes have the advantage of having more voltage headroom than SS for most practical circuits. Tubes are more-or-less "voltage-mode" devices while transistors (some types) are "current-mode" devices.
Feedback is a huge issue in amplifier design. If you examine the circtuit with respect to Chaos Theory, what you find is that the application of feedback to any amplifier creates a Chaotic system. If fact the formulae for feedback in an amplifier are identical to that of some classic Chaotic systems! So the amp likely appears to have a stable region, but then also has unstable (or less obvious, Chaotic response) regions as well. The stable window is the use of repetative signals like sine waves, often used for analysis. The problem is that audiophiles don't usually listen to steady state tones. They listen to waveforms that are constantly changing. Under such conditions, negative feedback acts as a destabilizing factor, often causing greater distortion than one would ever realize. With the advent of higher power computers, we likely have tools that can measure this stuff, but the industry lacks the ambition.
It is much easier to build a tube amp without loop feedback than it is with transistors. This is because tubes are indeed more linear. Now in case its not clear, the distortion that negative feedback adds to an amplifier is that of the loudness cues, the 5th, 7th and 9th harmonics. In essense, it will make any amplifier brighter, while it may not affect the actual frequency response on the bench.
Sorry about sentax/spelling errors. This site somehow disables my spellchecker.
