Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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long answer

Posted by Soundmind (A) on August 20, 2006 at 06:50:02

In Reply to: Why tubes? posted by Lynn on August 19, 2006 at 08:55:45:

First of all, nobody actually knows why some people prefer the sound of tube amplifiers, all anyone has is conjecture, most of it based on pretty flimsy evidence and faulty reasoning, the credentials of some of tubes best known proponents notwithstanding. The preference is hardly universal. I certainly don't and neither do a lot of other people. I don't know if anyone has ever taken s survey so the percentage of the market preferring them is unkown. One short possible answer is that many so called "high end" loudspeakers are voiced with the typical peculiarities of tube amplifiers taken into account. When they are used with solid state amplifiers, they sound relatively thin and shrill. Therefore the instinctive conclusion is to blame the amplifier. There are valid marketing reasons why speaker manufacturers would want to voice their speakers for tube amplifiers, not the least being that they know a fair proportion of the high end market which is supposedly more savvy will own them. And for those who own solid state amplifiers, well in typical casual comparisons by inexperienced listeners who do not know what real acoustical instruments and accurate reproduction of them sounds like, they actually prefer this kind of sound as long as there is enough deep bass from a subwoofer to make its inaccuracies not sound like it has no bass at all. The fact is that most of what we perceive is based on frequency response alone.

Fifty years ago, all amplifiers were tubes. Most speakers had little or no high end. JBL said prior to 1958, they never had to confront program material with information over 12 khz. Tweeters were shrill horns or small paper cones which had poor dispersion, low power handling capacity, and broke up easily if driven too hard...or were much too big. For example, Altec and Western Electric offered a popular 8" tweeter. With the arrival of acoustic suspension speakers, there was a need for more powerful amplifiers than most tube units could offer except at high cost and even with the arrival of dome tweeters, early models often had a high end rolloff. After the initial weak solid state amplifier designs of the early 1960s which had high harmonic distortion at low power output due to improper biasing and were easliy destroyed by accidental short circuits in the output by customers, manufacturers built much better units and they sounded brighter. The speakers of that era sounded much better used with them so that by the late 1960s the vacuum tube amplifier was just about extinct. With the arrival of inexpensive high quality tweeters in the 1970s it was possible to build much brighter loudspeaker systems. And they sold. This allowed new manufacturers to enter the market having tube units make a comeback and Conrad Johnson was among the first to re-enter long after Marantz, McIntosh, Dynaco, Scott, Fisher, Harmon Kardon, and the other major players had abandoned them. The tendency towards increasingly brighter sounding speakers left more critical listeners dissatisfied. Tube amplifiers with their typical high end rolloff was one answer audiophiles turned to. (Another was wires having very high shunt capacitance.) Traditionally, the frequency response of amplifiers is measured at one watt with a resistor for a load. That may have told consumers and engineers a lot about their differences in 1955 but by 1985 it left a lot to be desired as amplifiers which obviously sounded different driving real world speakers measured almost the same by these traditional methods. But by that time, few if any engineers were interested enough to develop entirely new measurement standards. They had moved on to other more interesting projects considering that the amplifier problem was largely solved and for people like me, it was.

As an amplifying device, vacuum tubes have many drawbacks for practical reasons such as heat, inefficiency, deterioration over their lives, necessity for high voltages, microphonicity, just to name a few. They are expensive to install as well, not merely because of their own cost but because of all of the other requirements associated with them. But on the whole, as electronic amplifiers they are approximately equivalent to transistors, both being able to be designed into circuits where nonlinear distortion is well below the threshold of human audibility and where frequency response of the audible range is flat with one major exception and that is for use as power amplifiers for driving loudspeakers where they have a serious drawback. The typical tube amplifier is connected as what's called a common cathode circuit with a plate output. It has a very high output impedence of the order of 6000 to 15000 ohms. To deliver power to drive speakers having an impedence of about 4 to 16 ohms an impedence matching transformer is used in over 99.99% of all tube amplifiers. This device is invariably responsible for most audible degradation, especially to frequency response. By contrast transistors use a common emitter collector output circuit or its VFET equivalent which has a source impedence of well under one ohm. This makes them an ideal choice for driving 4 to 16 ohm loudspeakers efficiently transferring lots of power with at most a single large DC blocking capacitor or no intermediary device at all. The result is less high end rolloff, flatter more extended frequency response in both the low bass and high treble, and much less non linear distortion. In fact, even with modern equpment, manufacturers can build solid state amplifier whose distortion is so low, it challenges the best of that equipment to find any at all. But connected to speakers voiced for tube amplifiers, a sound system's solid state amplifier can and often will make music sound bright, even shrill, and thin. Don't blame the amplifier, blame the speaker...and the unwillingness of users to use corrective tools such as equalizers to compensate for them. So this niche market for tube amplifiers as well as other niche markets for antiquated technology such as phonograph record playing equipment and wires to mitigate overly bright sound systems persists...for now.

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