FROM LISTENER MAGAZINE FALL/2000

REINVENTING PUSH/PULL


Be a Beacon
of Beaudacity

This summer I took my vacation at one of the most fashionable spots on the banks of the Ganges river and sat in the lotus position on a bed of hot coals, meditated and listened to the sound of one triode clapping. I enjoy these dramatic changes of context because it frees my meta-mind to wander where it may…through space, time and harmonic…through the infinitude of the beaudacious cosmos of audio gizmology. As you know the unexpurgated mind will go where it will, and I had two "contacts", while in a trance induced by ingesting red M&Ms…one message came from Marcus Testiculitis (68-10 BC), the famous Roman philosopher, and the other from, Sir Humpty Dumpty (1723-1743), the famous English Ovarian. But it was not until I kayaked back to the USA that was I able to integrate these two seemingly disparate messages. I had to go all the way to India to clarify my mind about push/pull tube amplifiers.

Marcus Testiculitis spoke to me with great eloquence and this is what he said…"The beauty that swings between our legs, is just like our taste for beauty and fashion, which swings back and forth". Sir Dumpty, under the yolk of great anguish spoke to me with these mystical words…."If only the King's men knew what they were doing, they could have put me back together again".

What was the meaning of these strange messages about tube amplifiers? For those who know, no explanation is necessary…because those in the know are the connoisseurs that have rebelled against America's pentode push/pull orthodoxy and taken refuge in direct heated triode single ended circuits…..but I have compassion for all audio pilgrims searching for musical ecstasy…so follow the yellow brick road….

As you know there is a "wall of power" for SET amplifiers of about 15 watts, because of the inherent design limitations of SET output transformers. What if you are part of the "real world" and want and need more that 15 watts of refinement? What do you use? Do you use a 211, 845, or 805 "big mutha" SET or…is it possible to design a P/P amplifier that approaches the refinements of SETs? The answer is YES, if you a rebel with a cause.

Almost ten years ago we started to swing away from pentode P/P and towards simple single-ended circuits because (1) we got bored with the sound of high feedback pentode push/pull amplifiers with their accelerating complexity and aural discombobularity, and (2) in every man there is a natural evolution of musical consciousness, with the intensification in desire for the subtle, ineffable, the magical…the refined in music…which is the domain of single-ended circuits for a host of good technical reason.

A VERY SHORT HISTORY OF AUDIO TESTOSTERONE GONE AMUCK

As you read this I remind you of the value of compassion, forgiveness, and acceptance, especially when it comes to the audio arts, because they most perfectly reflect the good, the bad, and the discombobularity of the American male's soul. Let get to the roots of the wrong turn:

I too was a solder soldier in the trenches of the amplifier battles of the late 1970s when transistor amps boasted 1,000 watts into One Ohm. In a country where Wretched Excess is the dominant male religion, what could be cooler than that…..especially because we saw the appearance of one of the dumbest things the audio industry every created…One Ohm Speakers. How did tube amplifier manufacturers respond to this dumber than dumb challenge?

Boys will be boys, and we must compete. So what's wrong with 250 or 500 watt push/pull pentode tube amplifiers? Some day I will write a encyclopedia which explains the problems of high powered amplifiers, but…basically…intelligent men have to make a choice between music power and music refinement…you can't have both when it comes to amplifiers. This is the universal meta-continuum of beaudacity. Where do gifted listeners, you, belong on the power/refinement continuum?

THE CHALLEGE OF SYMMETRY

Push/pull circuits are the most popular form of amplifier topology…for both solid state and tube amplifiers, and these circuits are based on a notion called symmetry: that a whole music signal can be split in half at the front of the amplifier's circuit and then re-assembled perfectly, at the back-end of the amplifier; at the speaker outputs. Oh joyous illusion!

Nothing is more difficult than achieving symmetry in audio circuits. How difficult is it? It is so difficult that no test exists to effectively demonstrate its quality, though in the real world, the lack of symmetry, is apparent to the gifted listener. In most cases, like Sir Dumpty, once the music signal is broken, it can never be put together again…perfectly. I have now explained why single-ended circuits, which do not use a phase splitter, have virused America. Once you have experienced SET wholosity, you can hear P/P relative aural discombobularity. If you think that this problem is just endemic to transformer coupled amplifiers, the Atma-Sphere M-60, which is an OTL amplifier, also suffers from a high degree of aural discombobularity.

And then we get into the debate about feedback, which is an essential feature of pentode push/pull circuits…..which is really not a technical debate, but a debate about aesthetics. Pentodes, because they are not linear, which is the price we pay for their power, require various forms of feedback, both globally (front the amp output to the amp input) and locally to their screens.

The benefit of feedback is lower output impedance, a much lower total harmonic distortion, of one kind, and higher dampening of the bass…but there is a huge price to pay, and it is in those areas that audio test measurements can't measure, but we can obviously hear. For example: If you could install a variable feedback control of your pentode push/pull amplifier you would notice that the more feedback you use the more your amplifier starts sounding like a transistor amp…the glorious tonal clarity, the effervescent midrange, the ethereal open spaces where music flies…all disappear. The music sounds congested, confused, and two dimensional…very tight, anal and uninteresting.

Then we are faced with the output transformer wall: Designing a transformer is a very difficult balancing act, and the minute you must design a high powered model you must vastly increase inductance, and that means you are creating a nice big "smoother outer" of your musical signal.,,,.all the music detail disappears…all of the delicacy is gone. My first book, Understanding Tube Electronics, (1982), went into great detail on this subject.

Before I started New York Audio Labs I was a push/pull pentode amplifier junkie, and literally owned every tube amplifier ever made, and I reached the conclusion that all things being equal, smaller tube amplifiers were able to reveal the emotional soul of music much better than larger ones Pictures of my own custom- made low powered pentode amplifiers from this time are viewable on the Triode Guild web site, under TRIODE GUILD MUSEUM OF ART. Note the huge regulated power supplies…from 1977.

A PROZAC PAUSE: Nothing feels worse than a heavy duty dude criticizing our favorite objet d'armour. I am sure many of you love your pentode push/pull amplifiers, and are hurt that I am not saying kind words about them. Many of the new Listener readers are probably confused why I am criticizing these amps when they sound so much better than solid state amplifiers.

The good news is that our Japanese audiomaniac bros' who turned us on to DHT SETs are now, in their search for more power/refinement, revitalizing a PP amplifier circuit concept that existed long before Williamson type pentode PP became popular. This is a very good thing, because if you are up against the "SET power wall" you can now confidently climb over it. Here is a vision of the near future:

SIMPLER CIRCUITS: Look at the Sun Audio 300B push/pull schematic on the Triode Guild web site, and the schematic of 6BH4 amplifier. How sublimely simple. Compare that to some of America's favorite pentode amplifiers, with their complexity. LESS IS MORE because symmetry is much easier to achieve when circuits are simpler and parts quality is higher. PP amps with less output tubes are also easier to balance, and are much more reliable. Simple circuits can be hardwired and that is a big sonic advantage over printed circuit boards.

DIRECTLY HEATED TRIODE OUTPUT TUBES(DHTs): Why aren't more American amplifier designers using directly heated triode tubes, which have superior tone to pentodes and create a much more authentic aural matrix? These tubes are much more linear than pentodes, and because they only have three elements…NO FEEDBACK IS NEEDED. There are so many high quality affordable DHTs that ZOUNDS, you can create some very exotic sounds. You have over a dozen different brands of 300Bs, and each has a unique aroma. 2A3s in push/pull is from the 1930s and is returning because it is so juicy and just check out how many different brands you can now buy. Pentodes tend to sound more similar. But remember this limitation….we are on the hunt for refinement, which means accept the limitations of these tubes…meaning NO MORE THAN FOUR TUBES IN THE OUTPUT STAGE. With some of the new higher power DHTs that means you can get over 100 watts with just four tubes. If you need more power than this, your speaker head is screwed on wrong.

POWER SUPPLIES: An amplifier is a modulated power supply, and American orthodoxy isn't, but should be STIFFER…POWER SUPPLY QUALITY MATTERS. A separate/independent well filtered, or regulated, power supply for the front end and output stage is the path to ultimate refinement, clarity and bass response. Do not be impressed by "computer" grade electrolytics. Use tube rectifiers, even if it means using the higher current models or creating a full wave bridge because solid state rectifiers are tonal arsenic.

PHASE SPLITTER: A phase splitter divides the signal and if this isn't done with perfect symmetry nothing you can do can put Sir Dumpty back together again. Books are written on this subject because nothing is more difficult than designing a quality tube phase splitter, which is why transformer phase splitter are back in vogue. This is the oldest and newest way to grock the refinement we want. Why aren't they used more often in America? A tube phase splitter cost about $10 in parts…and transformer phase splitter can cost between $60 and $300. Five years ago there were only two companies making transformer phase splitters, now there must be at least ten. My experience with the Sun Audio 300B push/pull amplifiers and the Lundahl phase splitters permits me to rave about them. These little iron gizmos are the key to unlocking the beauty of push/pull amplifiers, if the rest of the amp is designed properly. Again, liberate your brain, and use the web to research this subject.

OUTPUT TRANSFORMER QUALITY: Everyone has been bellyaching about how hard it is to design a high quality single-ended output transformer. Forget about it…it is much harder designing a high quality push/pull because the "BALANCE" the winding is so critical to achieve symmetry. A OPT for DHTs is very different than one designed for pentodes because it doesn't need feedback windings for the screens. Spend time on the web researching all the name brand American, European and Japanese transformers. Give yourself an transformer education. Read Menno Van derVeen's book available at plitron.com. By taking the lower power DHT route you will get more quality for the buck. This is the heart and soul of your amplifier. Never be impressed by chrome plated or beautiful transformer covers..it is what is inside that matters.

GRID CHOKES/PLATE CHOKES: Like I said in my Lundahl Armada amplifier, by using these "magnetic" loading devices, as opposed to high value resistors something magical, but unexplainable happens to the quality of the aural matrix. This very old circuit concept from the 1920s has still not yet been incorporated into a modern push/pull amplifier…but that is what swinging pendulums are all about. That is the good news. What I am suggesting here is replacing a 25 cent part with a part that might cost $60…that is the bad news.

DRIVER TUBE: I can't go along with the orthodoxy of using a "pea shooter" 12AX7 or 6922 as a driver tube. The very least you want is a tube like a 6SN7..and more. As you know from my Lundahl Armada amplifier article I believe it takes a great amplifier to drive the grid of just one output tube, so when we are talking about two or four, we must have a very robust driver circuit. Why not use the AV20, or a 2A3 as a driver tube?

And of course paradox rules…because many of your are wondering why I don't suggest the above for a very refined lower powered pentode amplifier. And you are right, but that is a different article.

GLUING THIS TO 4th WAVE MODALITY

It is a beautiful winter night. The snow is falling all around your log cabin. A fire is blazing in your stone fire place. Your hunting dog is curled up by the fire. On the sheepskin covered couch, before the fire, is your favorite super model..naked, smoking a cigar, sipping a brandy and reading Listener Magazine…while you are swapping inputs between your single-ended and push/pull DHT amps…and your David Berning 300B ZOTL, the world's only SET that uses no output transformer. Why are you doing this Mr. Sensitive? The answer is obvious. The aural matrix…the harmonic, space, time continuum is different for different amplifier topologies, and each has its own unique form of beauty…and life is short…and it is our job to BE ONE WITH these vastly different, subtle forms of musical beauty.

Take your favorite music and experience it through a DHT SET and DHT P/P, of comparable sophistication, and you are experiencing your musical consciousness from a completely different point of view…both equally valid…both at the highest level of refinement.

A 20-50 watt DHT push/pull amplifier that embodies the design features I mentioned will be a thrilling experience….and something that you never heard before. I can't emphasize this too strongly….the DHT PP amp I described creates an aural matrix that is a completely unique and unknown in America. Yet, it will not be easy to fully appreciate this unique sound, because there are still so few speakers available in America that are properly designed for any type of tube amplifier.

Personally I prefer P/P on rock and roll because this digital music is inherently compressed, two dimensional, emphasizes its bass line, and is very wide. Having some experience in producing rock and roll music makes me not look for what is not there. Of course the sad news is that, generally speaking, orchestral music can never be recorded at a high state of refinement, so the best way to get the most juice from this music is with push/pull.

When we are listening to music where clearly the music is refined, or recorded in a smaller scale, and, especially voice, then SET is best for extracting maximum musical juice. Here we get a more three dimensional, deeper, more airy...but less dynamic aural matrix. The criticism that SET amplifiers have relatively poor bass and top end response relative to P/P is becoming relatively less true with the new SET OPT designs…but still exist.,,but who cares…. magic is always more powerful than brute power.

CLOSING THE GAP

Contrary to the marketing hyper, we are only at the very beginning of the tube audio revolution. In many ways the "tube gap" between America, Japan and Europe is widening, and I hope our new President will respond to this challenge. Happily we, the gifted listeners who read Listener, are now returning to our aesthetic roots where refinement and subtlety triumphs over wretched excess. If you need power, and most men do, buy yourself a Dodge Viper, and thrill in burning rubber.

What is the new American revolution? It is based on you knowing much more about audio circuits, and demanding more from designers and manufacturers. This is like a little boy growing up and realizing that his father is not God. This is the end of the notion that an audio designer, or an audio reviewer has better musical taste, or greater sensitivity than you. It is the acceptance of most fundamental musical truth…every musical/audio device is a musical instrument, has a unique personality, and therefore must be tunable, so it will be able to reflect the ever shifting tides of our musical taste. This is the rejection of the bogus, Low-IQ notions of musical accuracy and neutrality. It is surrendering to our compelling need for musical ecstasy, which is as unique, personal and eccentric as our signature. There is only one rightful authority in your musical domain…your musical soul. Let the force be with you.

Viva La Revolution

Dr. Gizmo: www.meta-gizmo.com

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