|John Broskie's Guide to Tube Circuit Analysis & Design|
19 January 2003
Last time out, I mentioned missing the LP’s 12 inches by 12 inch format, as it was just big enough to hold a piece of worthwhile artwork. I one reader wrote to confirm that we have indeed lost something that was worth preserving:
Just last week, I received a catalog from Crate and Barrel, one of my favorite stores, inside which, great looking 12.5 inch square record-cover frames are available for only $12.98. At this low price, why not buy three frames and use one to show off your favorite LP and one each for the cover and back liner notes (if you are like me, you probably own several copies of “Kind of Blue” or “Let It Be” or “Take Five,” so the sleeve wouldn't need to be split in half. Visit www.crateandbarrel.com for more details.
The international electronics magazine, Elektor, January 2003 issue holds an article I wrote about tube-based crossovers. Be sure to pick up a copy of this issue, as an extremely flexible tube crossover is detailed, allowing the use of 1st, 2nd, 3rd, and 4th order crossover slopes. The article includes a PCB layout and a link to free software. Visit http://www.elektor-electronics.co.uk/ for more details. (It wouldn’t hurt to encourage them to publish more tube construction articles by buying this issue and if you have never read an issue of Elektor, then you have missed one of the best sources for electronic projects and electronic theory in the world. Sadly, the USA lags far behind.)
Remember the fictional letter from the high-end tube amplifier manufacturer demanding both help and secrecy? Well, it prompted some interesting responses:
This isn’t the first time I have been accused of being anti-tweak, which I certainly do not believe I am, as I believe that tweaking by part replacement or tube rolling are fine pursuits, as long as that isn’t all you have to work with that is. If it is, then you have as much a chance of getting something worthwhile as a room full of chimps at typewriters has of producing a good sonnet. Well, actually the odds aren’t that bad, as most tweaking is imitative, aping no less, so it is more like Shakespeare sitting in a room filled with chimps, maybe one chimp could copy...
What follows is a reply I wrote to editor of Electronic Products (an excellent resource for the electronic enthusiast that I highly recommend by the way) after reading his editorial, wherein he denounced the pseudo-science mumbo jumbo of high-end audio uttered by those who, at heart, are anti-scientific. You know the stuff he decries: “Our new cables feature quark-aligned, time-space coherent, molecular-stabilized, non-diodic, macro-crystal, transimpedance-optimized, antimatter-free wire.” He concluded with:
Another issue of Electronic Products has come out and I am reminded that I haven't written in response to your last editorial. The topic was high-end audio: The sounds of science?
This is not going to be one of those letters that decries your lack of sensitivity for failing to hear subtle difference between five “9s” copper versus six “9s” copper. But then it is not going to be one of those letters that heartily slaps you on the back for trashing a group of spoiled snobs with more money than sense. No, this letter’s goal is much more difficult to achieve.
You see, I get both of those types of letters routinely, as I put a free webzine (www.tubecad.com) devoted to carefully explaining how tube-based circuits work, which requires precise terminology (remember when admittance and conductance were not synonymous) and a few long equations. Half of my critics see me as promoting a purely-objective undertaking that cannot be applied to an art, as exempt from scientific analysis as would be Paoblo Picasso painting a masterpiece.
In a recent issue, I tried to disentangle the popular understanding of how the SRPP (the totem-pole) circuit works from its actual functioning. The following is from the article:
The other half of my critics see me as denying progress and promoting a form of modern-day witchcraft. If it is newer, it must be better. Tubes are old, in fact, ancient, by modern standards; thus, worse than worthless. The logic is sound, but the premises are debatable.
(Many falsely believe that I hate solid-state amplifiers. In fact, I love them: I enjoy designing them, evaluating other designer’s solid-state circuits and topologies —Erno Borbely, John Curl, Nelson Pass, Douglas Self—even repairing them; I just do not like listening to them for more than a few minutes, whereas I can listen to a good tube amplifier for a few hours, before fatigue sets in. That marginal difference in sound is decisive for me.)
A few years ago, a many-degreed, highly-gifted and intelligent senior analog-electrical engineer sat on my couch, belittling the premise that I would waste my time designing and building a tube amplifier, a single-ended one at that! But as the music filled the room, he shut up. Seven minutes later, the track nowhere near ending, I killed the music and I was about to offer a rebuttal, when he plaintively cried out in an anguished tone as if I had just accidentally thrown his only picture of his mother into a fire, “Nooo, don't! Why did you turn the music off? That was the first time in decades that I actually enjoyed listening to music.” How very sad, when you consider that he might never had the luck to break the decades-long fast. Once he recovered from his emotional outburst, he challenged me to a contest: he would use solid-state devices and I would use tubes and the goal was to see who could make a better Op-Amp. “Sure,” I replied, “as long as one of the design objectives is the ability to swing 300 volts peak-to-peak at the output.”
Fifteen years ago when an audiophile friend and I went to breakfast in Davenport , California, a small sleepy coastal town 5 miles north of Santa Cruz. We were eating our omelets when a young women sat 20 feet behind my friend and began to play a large harp. After a few minutes of live music passed, I asked my friend what he thought of the sound system. He became animated.
“So that’s it. That’s why you wanted to come to this restaurant: let me guess these new -age hippies are too cheap to buy a new PA system and they are running some old Dynaco gear. And they probably are running the original tubes,” he said triumphantly to me.
He then cocked his ear and said, “Yes, definitely tube gear, notice the flabby bottom end and the rolled off highs.”
“What about the mids,” I asked.
“Sure it sounds smooth, but too smooth, artificially smooth, no bite...just too romantic don’t you think? Real music has more of an edge to it,” he replied.
“You’re right, it sounds just like tube gear to me,” was my answer as we stood to leave and my friend was horrified to see real music being performed.
Neither the senior engineer nor the audiophile was converted to tubes, but they both thereafter would change the subject when solid-state or tube stereos were mentioned. I understand their position perfectly, as we all have our limits on what we are willing give up in order to embrace its opposite. I once came upon an odd sonic effect, where no sonic effect could result. I called up a friend and had him come over to verify what I was hearing (I did not tell him what I was hearing). He heard it too and asked what I was going to do. I told him nothing; I was going to pretend that it had never happened, as I could think of no good reason how the sonic effect obtained.
If I knew less about physics and electronics, I could readily embrace the technique for creating the sonic effect. (I once worked with this no-nonsense fellow who told me one about the grueling month he had spent dealing with a haunting of his family that was only resolved by giving a hundred dollar donation to a church whose minister preformed an exorcism for him. After he finished, I asked if he believed any of the steaming BS he had just told me. He replied that he didn’t. This confused me and I asked why he had paid the money and gone through the exorcism, if he didn’t believe in it. His answer was, “I don't believe in microwaves either, but if the microwave oven breaks, I am going to pay to have it fixed.”)Charming as these stories no doubt are, how does this justify $2,000 interconnect cables and magic rocks? First of all, I believe that anyone who sells $2,000 interconnects should not be allowed to die a natural death, failing that, horsewhipped with the cords. But at the same time, I have no quarrel with the audiophiles who tell me that those same cables transformed his system or, at the other extreme of expense, that four sheets of toilet paper in between his loudspeaker and their stands made his system sound ten times better.
In fact, I believe he is understating the difference. For the difference between recognizing and not recognizing is infinity. As Vanna White turns just one more letter, which is after all maybe only 2% of the available letters, the contestant recognizes the quote: “These are the times that try men’s souls...” Prior to turning that letter, there was no recognition; afterwards, total recognition. The result was not linearly related to effect. Marginal differences do not necessarily have only marginal results.
In Jude Wanniski’s great book, The Way the World Works, he develops his theory of marginality: marginal differences can, in human affairs, can have hugely disproportional effects. The camel's back is fine until the extra piece of straw, which is only a 0.00000001% increase in weight, breaks its back. The fastest runner in the world is famous and receives fat promotional deals; the second fastest runner, the one who is 99.993% as fast, is largely ignored and is offered no Nike endorsement jobs. The model who works for your local department store makes $200 per photo shoot, not the $200,000 that Cindy Crawford makes; yet she is only marginally less attractive than Cindy Crawford. If she were marginally more attractive, she would make $2,000,000 per photo shoot and we would worship her as a freshly born deity.
The whole of high-end audio lives in the realm of marginality: the $30,000 vacuum-tube amplifier's output is only marginally different from the average $200 Chinese receiver's output; as seen on the scope, no difference can be discerned. But, like the extra letter revealed by Vanna, small audible differences can have large effects.
But are those differences audible? No scientific proof has been offered that shows that they are. However, proving a negative is much more difficult than proving a positive. If someone shows us a pink crow, then we have proof of its existence; but how do you prove that no pink crows exist, without collecting every single crow in the world and checking for pink? Second, when we test for audibility we are not testing the sound, but the listener. If we were testing the sound, we would need only a microphone and no listener. Now what happens to people under test? Sometimes the act of testing influences the results. Imagine that you are being tested to see if you can tell the difference between your wife’s kissing and another women’s kissing. No problem. You intimately know your wife’s lips, their texture and temperature, their pliancy, her smell and feel against you, and her technique from years of avid kissing. The differences are not subtle.
Yet, I promise you, if you were blindfolded, placed on stage with a huge, loud audience, and Regis making jokes at your expense, you could not tell whether your wife or Regis was kissing you. This example is extreme, but the principle is the same. I know from experience how the act of being tested can alter the results.
A friend had stuffed his ears with cotton and sat listening to music. He wanted to prove that he the sonic corruption resulting from having audio signal flow through mercury contacts was so obvious that he could hear the difference with his ears stuffed with cotton. He was right he could hear the difference, as could I. But then we introduced an AB switch box that allowed random selection for double-blinded testing. Even removing the cotton did not help him. For the first half of the test, I was puzzled by his suddenly poor performance, as I could readily hear the difference, but then I could also see the switching in action. But when I operated the switching box in the random mode, I too had a hell of a time trying to discern the differences. If you believe the answer is obvious—power of suggestion—I am sure you are wrong.
While the power of suggestion is no doubt great, it isn’t always universal nor decisive. For example, I managed to attend the University of California at Berkeley, live twenty years in Santa Cruz and never become a liberal. One possible source of trouble is the stress of being evaluated. (I am always sadden by my eye’s poor performance in the optometrist's office and I find myself trying to guess the “right” answer, rather than just report what I see.) Another possible source of trouble is revealed in a recent study that showed we listen to music with a dominant ear. If we know the piece well, we listen with one ear primarily, as can be seen in our unconscious turning of that ear towards the loudspeaker. On the other hand, if the piece is unknown, we listen primarily with the other ear. Apparently, we listen for different aspects of the sound, rhythm vs. melody, tonality vs. dynamics, depending on whether we know the piece or not. My guess is that when we are placed in the hot seat, we switch from known to unknown and we switch ears and with that switch, we lose the faculty we need to discern those subtle differences. Crazy idea?
Try this test: take two diodes (1N4001) and solder each of them in parallel, but opposite in orientation, with a 10-ohm resistor. Now place a soldered pair in series with the leads to each loudspeaker. Now listen. If the no difference can be heard, or if it can be heard too readily, increase or decrease the number of diodes or the value of resistors. Once a very small difference is achieved that you can easily hear, have someone blind-test your ability to discern the difference.
So is scientifically rigorous testing worthless in evaluating the audiophiles claims? No, of course, not. But such testing must be careful to test apples and only apples. But then even if the tests were perfect and showed no positive results, why shouldn’t someone be allowed to remain adrift in the sea of subjectivity, unlike the Arts, their stereo systems are not government funded. And surely, the whole point to high-end audio is the sound of music, which, like drinking wine, is a truly subjective enjoyment.
Tube CAD does the hard math for you.
This program covers 13 types of tube circuits, each one divided into four variations: 52 circuits in all. Tube CAD calculates the noteworthy results, such as gain, phase, output impedance, low frequency cutoff, PSRR, bias voltage, plate and load resistor heat dissipations. Which tube gives the most gain? Tube CAD's scenario comparison feature shows which tube wins.
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