The difference really seems to come down to distortion. Tubes make more harmonic distortion, but it tends to be of the lower orders (2nd, 3rd and 4th). These are considered musical by the human ear/brain system.
Transistors in general make less distortion, but what they make tends to be of the higher ordered harmonics (5th and up). These are considered amusical by the human ear/brain system.
The brain uses the higher ordered harmonics as loudness cues, so if they are distorted you will get two results: the sound pressure will seem louder (and IMO/IME a stereo should not sound loud even if it is) and it will sound brighter, due to the human ear's extreme sensitivity to these harmonics.
The ear/brain system translates all distortions into tonality. So it converts the distortions of tubes into things that audiophiles describe as 'warmth', 'lushness', etc.
Otherwise both amps might measure flat on the bench, but the ear has a tipping point and will often favor tonality generated by distortion over actual frequency response errors.
There are ways to design tube amps to not make so much of the lower ordered harmonics without creating more of the higher harmonics (fully differential and balanced is one way). There are also ways to design transistor amps to make less of the higher ordered harmonics, and ways to make them make more of the lower ordered harmonics (single-ended, such as some of the Nelson Pass 'First Watt' amplifiers).
Once you understand that its all about how the amps distort (and I don't mean clip or overload, rather at normal listening levels) then you are a long ways closer to understanding what the differences are between them.
One other important difference is how the two technologies overload. Tube amplifiers overload gently and resist making higher ordered harmonics until really pressed hard. Transistor amps make lots of higher ordered harmonics as soon as they are clipped- and so its usually a bad idea to overload them as the clipping is amusical and irritating.
This is why you can often get by with less power with a tube amp than with solid state. It is also why guitar players tend to prefer tube amplifiers, as they often overload the amplifier as part of their 'sound'. Some of the best-sounding transistor-based guitar amps (Sunn comes to mind) were built in such a way that they made a fair amount of lower ordered harmonics- which is why they have a certain following as they are 'richer'.
Since transistor power is considerably cheaper than tube power, to get around the clipping problem a solid state amp will have a lot more power. The clipping problem must be dealt with as music tends to have powerful transients which makes it difficult to reproduce without distortion. |
I find tube amps to typically sound brighter than ss amps. Your experience here is unusual! The majority does not share it. This might have something to do with the speaker you are using- it might have a much higher impedance at high frequencies than lower down. |
^^ Actually Mapman this is not entirely true.
Because tubes usually are more linear you can often build lower distortion circuit with them- and often without feedback. That is a lot harder to do with semiconductors!
Most power transistors incorporate a non-linear capacitive aspect into the junction of the device that is part of the price of building the junction in the device. The capacitive aspect is magnified by the amount of current put through the device, and in case some don't know it, current is a big deal with semiconductors. This capacitive aspect contributes to higher ordered harmonic distortion and is part of the semiconductor signature that is very hard to eliminate! Some semiconductors take advantage of this capacitance- for example most modern FM radios are tuned by the use of a varactor diode, which changes capacitance as the voltage applied to it changes.
Tubes have a similar capacitance but it is far less significant, lacking the non-linear aspect and mostly is an interaction between the input capacitance and the source driving the tube and is known as Miller Effect; IOW can affect bandwidth if not managed.
Further, distortion at clipping can play a much bigger role in the sound of many stereos than you suggest. Musical transients are where the action is- and if for whatever reason the amp is forced into clipping for an instant it can have audible effects. There are a number of threads on this forum asking about the difference between tube and transistor power as for some reason tube power seems to be more profound than transistor power.
The reason for this is how the two distort- when a transistor amp clips its instantaneously audible- when a tube amps clips it may not be audible until the amp is clipping fairly hard. Instead, it will make more higher ordered harmonics that interacts with the human ear/brain system in a way that will make the sound seem to be louder than it really is.
In a nutshell this makes the tube amp seem more powerful than a transistor amp of the same power as the outright clipping of the transistor amp seems to describe a limit that the tube amp does not seem to have. Its important to understand that this has a lot to do with how we perceive loudness and that the tubes play into that in a way that transistors don't.
The difference of course is revealed as soon as you put a sound pressure meter in the room- then it will be seen that both amps are playing at the same volume.
Anyway, the result of this is that the common wisdom is that if using a transistor amp you need a lot more power to avoid audible clipping.
But I agree, avoiding clipping altogether is preferable :) |
The problem here is that often the ear will give weight to distortion perceived as tonality over actual frequency response.
A classic example is the brightness of transistor amps. On the bench they might test with the same bandwidth as a tube amp but will sound brighter. Its due to trace amounts of higher ordered harmonic distortion, which the ear translates into tonality: brightness.
IOW it may be flat but it may not sound like it. Not that you can get flat frequency response out of a speaker anyway- just look at their response and you will see that getting flat frequency response is a fool's errand. |
^^ This.
Music is process in the limbic centers of the brain- unless some aspect of the sound is altered enough that the brain detects a problem- and then the music processing is transferred to the cerebral cortex.
This is literally why some systems are emotionally involving while others are not. Obviously the goal would be to keep the processing in the limbic centers. |
Ralph, can a ss amp have such low distortion as to make that comparison not tenable? I suppose a better question would be, have you ever listened to a ss amp without the typical characteristics you ascribe to them? I have heard one solid state amp that was quite musical- it also cost $100K and made 100 watts total. It was also zero feedback. It was better than most tube amps I have heard. But it is the exception by far. I've yet to see a solid state amp with such low distortion as to not be bright- in fact it seems that the lower distortion units are more irritating to the human ear. That this occurs is nothing new- audiophiles have been commenting on this for decades. Norman Crowhurst wrote about this issue many years ago- back in the 1950s- we are not going over new ground here... I am not an advocate of 'distortion of tubes' either; I prefer as little distortion as possible, but I am pragmatic in the understanding that distortion is part of the tonality of any amplifier because that is how the human ear behaves and all amplifiers have distortion. |
Unsound, just because an amplifier cannot double its power as impedance is cut in half is not the same thing as saying that the amp cannot do flat bandwidth on a given loudspeaker. There is more to it than that! |
^ I got that and meant the same thing. "Typical speakers".
Just because an amplifier does not double power into half the impedance does not mean it cannot drive such a speaker perfectly well, without frequency response variation. Doubling power is not a requirement for that. |
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Actually global negative feedback cannot to that in an amp. What it can do is force the amp to put out *half* has much power into an impedance twice as high- and therefore satisfy the voltage drive model of 'typical speakers'. So add enough feedback to any tube amp and it can do that with ease and many do. when you do see a spec on an amplifier that does double power as resistance gets lower is that a sign of a better design? No- it is simply a sign that the amp has a very low output impedance and has the current available to double power. That is very different from a 'better design' which has a qualitative aspect; I prefer tubes so I think some good tube amps as often being a 'better design' than many transistor amps. Another way to put this is our ears hear sounds without regard to how tubes or transistors work. It just happens that tubes do a lot of things that the ear finds more pleasant/less irritating, for example tube amps often have less odd ordered harmonic distortion. That is why they sound smoother, as the ear translates odd ordered harmonics into brightness and harshness. So if an amplifier is designed to produces less of such distortion, in my book its a 'better design'; doubling power as the load impedance is cut in half has little to do with that. |
Atmasphere, In that most (all?) speakers draw power with varying impedances and those impedance changes will correspond with sensitivity changes (again we're talking about typical speakers here) unless the amp(s) can change power output appropriately there will be deviations from linear frequency response. Yes- and enough global negative feedback will allow nearly any amplifier to do that. Its not about being able to double power, its about the amp being able to act as a voltage source. Plenty of tube amps can do that. If we add about 20 db of feedback to our amps they will act like a voltage source too. Again, the amp does not have to double power as impedance is halved. What it does have to be able to do is cut power in half when impedance is doubled in order to behave as a voltage source. |
^It works the same in both directions, and global feedback is not neccesarily a prerequisite. If the output impedance of the amp is low enough then feedback is not required. An example is the Ayre amplifier. An example of a tube amp that acts as a voltage source is a Mac 30 from 55 years ago. |
^^ If the impedance of the speaker is too low the output power of the amplifier will be lower and distortion higher. The lower impedance taps are provided to deal with this. It might interested you to know that the voltage rules that we are talking about ( http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php) were initially laid out by Electro Voice and MacIntosh back in the days when tubes were king (late 1950s). |
^Them it would appear that the Mac 30 is not capable of adapting to the variable impedance swings of typical loudspeakers without deviating from linear frequency response? No, not at all, unless the amp is set up on the wrong tap. If the speaker is 4 ohms in the bass and 8 or 16 ohms in the highs, you put it on the 4 ohm tap and the bass will be correct as well as the highs. It will be just as flat as a transistor amp on the same speaker. |
^^ The nature of the dips should be examined. If at crossover points which is a common point of a dip, they will be of no consequence.
The amplifier power will be signal dependent of course, and the output into 8 ohms will be 1/2 of its 4 ohm power (30 watts) and 1/4 of that into 16 ohms if the signal is full bandwidth.
IOW, feedback causes an adaptive response in the amplifier. |
Actually there are a good number of tube amps that can do that. Any Mac for example. Ever hear of the Wolcott? Henry made a point of advertising that his amp could operate as a true voltage source.
All amps have higher distortion into lower impedances. As far as that goes, there is no argument for a 4 ohms speaker in the world of high end, not if sound quality is your goal. Why force all amps to have higher distortion??
You still need the impedance taps otherwise you may not be getting the most out of the amp. |
You didn't look deep enough. From the Wolcott site: http://www.wolcottaudio.com/WA_presence.htm: What if we told you that our very different tube amplifier had highly accurate autobiasing, a nearly infinite damping factor, acted as a pure voltage source into almost any real world speaker load, |
I don't see anything that supports your statement- perhaps you could point it out? |
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