21 May 2016

Book Review:

Samuel Johnson

Designing High-Fidelity Tube Preamps
by Merlin Blencowe

Review: Designing High-Fidelity Tube Preamps
Good news: Merlin Blencowe has written a new tube-related book. My copy arrived a few days ago and I have enjoyed reading it immensely. To be frank, however, that was not my expectation. Why not? His new book is a fat 431 pages long and I haven't forgotten the effort I had to summon to get through the 233 pages of his Designing Power Supplies for Tube Amplifiers, which wasn't a bad book in the least, but which was not at all quaffable. Indeed, that older book is chock full of useful and important information; and I still recommend it, although I believe it is now out of print. At the time I read Designing Power Supplies for Tube Amplifiers, the thought that crossed my mind was that a French or Italian or Spanish reader who did not read English would still get a lot out of the book due to its hundreds of schematics and many formulas. Should Merlin one day choose to give it a rewrite, the book could remain just as thick, but many portions would be compressed, while others expanded.

(By the way, as you might have guessed from my many posts, I am a voracious reader. When I am lucky enough to be staying at a B & B, the first thing I hunt down is the bookshelf. In addition, I read writers such as Sir Thomas Browne and Samuel Johnson and Seneca for pleasure. Believe me, if you can read Browne, you can read anybody.)

Second, as I looked over the table of contents for his new book, my impression was that it looked very much like a textbook. Not a good omen. Textbooks weigh upon the soul—heavily. No matter what the topic, all textbooks seemingly run into the identical dreary sameness. Books that were either written by or written for the living dead.

Third, reviewing books written by your friends (or acquaintances or enemies) is tough. William Hazlitt in his essay, On Reading Old Books, explicates the problem:

"I have more confidence in the dead than the living. Contemporary writers may generally be divided into two classes—one’s friends or one’s foes. Of the first we are compelled to think too well, and of the last we are disposed to think too ill, to receive much genuine pleasure from the perusal, or to judge fairly of the merits of either. One candidate for literary fame, who happens to be of our acquaintance, writes finely, and like a man of genius; but unfortunately has a foolish face, which spoils a delicate passage:—another inspires us with the highest respect for his personal talents and character, but does not quite come up to our expectations in print. All these contradictions and petty details interrupt the calm current of our reflections. If you want to know what any of the authors were who lived before our time, and are still objects of anxious inquiry, you have only to look into their works. But the dust and smoke and noise of modern literature have nothing in common with the pure, silent air of immortality."

Here is the list of chapter titles:

Chapter list:
  1: Essential Electronics
  2: Practical Components
  3: Fundamentals of Amplification
  4: The Small-Signal Pentode
  5: Noise, Hum and Microphony
  6: Advancements in Amplification
  7: The Cathode Follower
  8: Compound Amplifiers
  9: Controls
10: The Phono Stage
11: Power Supplies

The urge to quote Samuel Johnson again is great—well, at least it is from me. I will resist the urge and quote myself instead. The following was from my review of his Designing Power Supplies for Tube Amplifiers.

Today, however, it is not enough that a book be well written, illustrated, researched, and organized, as it must pass the new test, the web test: If I can find all the same material on the web for free, do I really need to buy this book?

The answer is yes, as you cannot find everything contained in Designing High-Fidelity Tube Preamps on the web. And even if you could, you wouldn't want to try, as all the effort and all the false starts and all the twists along the way would takes months, if not years. Moreover, I must mention that his new book is not only theoretical, as he provides gobs of practical, get your hands dirty, advice for actual construction. In short, for less than $40 Mr. Blencowe delivers a tube-audio treasure. There can be only one conclusion: buy this book. His web site lists three sources:

Lulu.com
Amazon.com
Amazon.co.uk

I assume that the Lulu link would provide Merlin with the most profit, so I would start there.

Since no book review would be complete without some nitpicking and grumbling, I will not disappoint you. But first, I would have liked to provide another quote from Johnson, but I could not find it. The gist of what the dear doctor said was that the writer of essays (blog posts also count) has a much easier time than the writer of a thick book does, as shorter piece of writing exempts the writer from encountering logical inconsistencies and plain bone-head mistakes that are implicit yet hidden in his short work, but which would be revealed in a longer work. Moreover, reviewing is much easier than creating. This reminds me of what I wrote back in blog number 116:

I once had lunch with an audio magazine editor. He told me that he gets several offers a day by eager audiophiles who want to write audio product reviews. He sneered, "As if it takes any talent to write an audio review; hell, give me four pieces of gear and four hours and I'll write four long product reviews. But if someone can write a decent article on how SACDs are mastered or the history of some great recordings or explain how Thai audiophiles are different from Chinese or Japanese audiophiles, then I am very interested."

Of course, Samuel Johnson put it best:

Criticism is a study by which men grow important and formidable at a very small expense. The power of invention has been conferred by Nature upon few, and the labour of learning those sciences, which may by mere labour be obtained, is too great to be willingly endured; but every man can exert such judgment as he has upon the works of others; and he whom Nature has made weak, and Idleness keeps ignorant, may yet support his vanity by the name of a Critic.

               Idler No. 60 Saturday, June 9th, 1759

So, having placed my critic hat firmly on my head, what didn't I like about Designing High-Fidelity Tube Preamps? Tube circuits—there weren't enough interesting topologies. He covers the the two generic, single-triode circuits, the cathode follower and grounded-cathode amplifier, but not the grounded-grid amplifier or the inverted amplifier (aka grid amplifier), wherein we use the grid as a "plate" and the plate as a "grid." In addition, he omits the anode-follower (plate follower) topology. He then moves on to compound circuits that use two triodes, such as the bootstrapped pair (where a grounded-cathode amplifier cascades into a cathode follower and a capacitor attaches from the follower's output to the input tubes plate circuit, which is fundamentally a modified SRPP), cascode, cathode-coupled, CCDA, SRPP, and White cathode follower. He even covers the Aikido cathode follower, but gets the design example slightly wrong.

First, the protection diode is not absolutely needed, as the bottom triode would have to be hot to provide a sustained current path into the top triode's cathode. Second, the top triode should get its own cathode resistor, for if the bottom triode's cathode resistor is too large in value, no power-supply-noise null will be possible, as the bottom triode's transconductance will have been too compromised. Third, capacitor C2 is too small in value and resistors R4 & R5 are too large in value, as the bottom triode does present a fair amount of Miller-effect capacitance. See blog number 188.

Assuming that the input signal is noise free, say a CD player or DAC or tuner (in other words, an external signal source), then the following arrangement works well.

Because both triode share the same cathode resistor value, the ratio between the power-supply-noise voltage divider resistors is simple: the top resistor should be (mu - 1) times bigger than the bottom resistor. In this example the SPICE model of the 6DJ8 used a mu of 31, so the 300k resistor is the right value. Running the above circuit under a SPICE simulation produced the following result.

If we replace the 1µF capacitor at the top of the voltage-divider string with a 10µF capacitor, we get this result.

Indeed, you might use a 100µF electrolytic capacitor, which would extend the PSRR enhancement down to 10Hz; an electrolytic capacitor, however, does run the risk of current leakage, which presents a DC offset voltage to the bottom triode's grid (this might actual prove an advantage). Practically, however, we get the most bang per microfarad by moving the null center only down to twice the wall-voltage frequency, so 120Hz in America and 100Hz in Europe.

But, John, your version presents an output impedance of 400 ohms, whereas his version only presents 100 ohms.

Sure. But just how low an output impedance do you really need? If you are willing to accept a higher idle current, the two cathode resistors can be lowered to 180 ohms. Remember that the cathode resistors perform three tasks: they set the idle current, they linearize the triodes, and the top resistor protects the cathode follower from capacitive loads. If you are willing to forgo the last two features, another approach would be to eliminate the top cathode resistor and bypass the bottom cathode resistor with a big capacitor. If the output impedance needs to be something close to zero, then solid-state gear is the better answer. And if the load is a brutally-low 300 ohms, the impedance of many fine German headphones, then a push-pull output stage is needed, such as the following.

The above is a complete tube headphone amplifier for 300-ohm loads. As the White cathode follower has gotten my full Aikido treatment, the above circuit not only delivers big symmetrical current swings into the 300-ohm load, the PSRR is exemplary. Note that the ratio between the power-supply-noise voltage divider resistors has changed. Why? The assumption that the White cathode follower stage would see a noise-free input signal is no longer true, as the grounded-cathode input stage, in this example but not in all examples, leaks 50% of the the power-supply noise at its output. Thus, we use the classic Aikido formula for the ratio:

     R2 = R1(mu - 2) / (mu + 2)

where R2 is the top resistor; R1, the bottom resistor. See blog number 255 for more details.

What if you do not want a complete headphone amplifier, but only a White cathode follower stage? We can apply the Aikido magic to the standalone White cathode follower, but we will have to pay the price of a higher B+ voltage, 236Vdc instead of 200Vdc, as we lose so much voltage across the 3.6k current-sense resistor.

The result is equal current swings from top and bottom triodes and excellent PSRR.

Returning to Designing High-Fidelity Tube Preamps, the second minor grumble I could make is that the list of tubes that Merlin highlights is a tad too short. I would like to see him cover some sleeping beauties, such as the 6SU7, which is a 6SL7 but with tightly matched triodes. Or the 5963 and 5965, which are late-development dual triodes which are largely immune to cathode-interface problems.

Nonetheless, my complaints are few and small. Merlin Blencowe has written a fine tube-focused book. At less than $40, it's a bargain, if not a steal. Buy it.

 

 

 

What is the Best and Easiest Method of Studying?
A great question; yet it is one that is so seldom asked. Note the oddity of the situation: we spend a huge chunk of our life in school, trying desperately to learn, but we are seldom taught how to learn. When I was in college, I devoted much of my time to learning how to learn. Because I knew that the time I saved would be mine to waste, I became fiercely good at it. For me, offense was much better than defense; active over passive. For example, before cracking open a textbook, I made a long (sometimes woefully short list) of all that I already knew about the topic. For example, say the topic was Liechtenstein, I would list the little I know: it's a country, a very small country; it's somewhere in Europe, probably next to Germany; they probably speak German there. I also listed key items that I expected to encounter, such as, What type of government was there, republic or monarchy? What is its history? Who are famous people who were born there? What are its borders? What does it produce? Then, as I read the book about Liechtenstein, I would place a check next to items in my list that I had found and write down the answers to the questions that I had asked myself.

If the topic was something conceptual, such as psychology, I also listed key items that I expected to encounter, such as, What is the definition of the topic or field of study? What is its history? Who are the greats within it? What is its strict purview? What other studies overlap with it?

In other words, I never read passively; instead, I read aggressively as a hungry wolf in search of prey. Let's say you just bought Blencowe's book (try saying that three times fast) and you are keen to become a power tube fancier. Or, say you wish to read any difficult book well. Here are some quick tips: make the list of key items that you expect to encounter. With Blencowe's book, list things like current, voltage, power, resistors, capacitors, inductors, transformers, diodes, triodes, pentodes, transconductance. The more you can write down, the more are likely to learn and retain.

No matter the textbook's topic, photocopy the its table of contents; then while eating breakfast or waiting in line at the bank, review the headings and observe the book's structure. When reading the book, start with the last section of the chapters first, which should be the chapter's summery. Next, skim and glance through the entire chapter, paying attention to all the subheadings and illustrations and graphs. When looking at a graph, be sure to trace the plots with your finger or pencil point, as the physical act will help burn the plot in your memory. Now, start reading. Once you are done, skim over all that you have just read and re-read the summary or last section.

Two or three hours later, quickly go over the chapter again. Before starting the next chapter, re-read the previous chapter's summary or last section.

Never highlight as you read, as you do not yet know what is important—i. e. what is just an example and what is a key concept. After you have read the chapter, you can go back and underline what is truly important. Whenever you get one of those "Oh I get it" moments, stop studying and go outside and walk amongst greenery for ten minutes or so before going back to studying. Why? Your brain needs time to consolidate what it has just apprehended. Before going to sleep, quickly review your notes or skim through the chapters that you read earlier.

One psychological problem I encountered was my dread when confronting a super-thick textbook. "How could anyone hope to live long enough to read such an oppressively fat book," I asked myself. My solution was radical: I cut the fat book into chapters, with a kitchen knife. Instead of a single fat book filled with 1,200 pages, I ended up with 30 thin chapters, each only 40 pages long. Yes, slicing a book up is a sort of desecration, but I took some comfort in the belief that the textbook had it coming. (I like to read in bed. Although I am a big brute of a man, I hate heavy books, preferring many thin books to one fat monster. Computer books are the worst offenders.)

The key point is that most students view their workload as hurdles that they must clear before moving on to the next one; and once cleared, to be forgotten as quickly as possible. In sharp contrast, I viewed my required studies as poorly-prepared, nearly-inedible, unseemly-looking food that I had to chew over and over again before I could digest it fully, much like a cow eating grass. Education has been defined as casting fake pearls before real swine. We are not in control of the first half of that definition, but we are in control of the second half.

 

 

Next Time
More balanced designs.

 

    

User Guides for GlassWare Software
Just click on any of the above images to download a PDF of the user guides. By the way, all the links for the PCB user guides shown at the right now work.

Since I am still getting e-mail asking how to buy these GlassWare software programs: