THE PUSH/PULL PARADOX: SAVING PUSH/PULL FROM ITSELF

SAVING PUSH/PULL FROM ITSELF

WHY DO MEN ARGUE ABOUT ART?

The first reason is that sex, sports and television can get very boring. The second reason is that men love to argue because apes love to argue. And the third reason is that men are vain…men who are artistic enjoy self glorifying arguments…like…push/pull versus single-ended circuits. If this vanity weren't forceful there would be nothing in museums.

Fashions change for a good reasons…boredom and expanded awareness. After almost twenty years of expansion of push/pull tube amplifiers they have finally reached a level of wretched excess and crudity that is completely antithetical to their inherent thermionic nature. Push/pull created a rebellion, its artistic antithesis, which is called single-ended circuits about eight years ago..in America. This return to ultra-simple circuits occurred because a tribe of men care about more about beauty than the size of their toys.

Each topology has its limitation, and even with the shortcomings of SETs they were preferable to the grossness of the dominant Williamson type pentode amplifiers.

The purpose of this the first article in a series is to accelerate the process of saving push/pull from itself and stimulate a new fashion in push/pull design

THE PUSH/PULL PARADOX

It is the most popular tube amplifier topology in America, and yet hardly anyone knows what it sounds like. Just because you have eaten a Mc Donald's Whopper you assume you know what a hamburger tastes like, and you don't…if the subtle beauty of the burger is your quest. The cry heard that explains why so many flocked to ultra-simple low powered single-ended circuits is "Something is wrong with the way push/pull pentodes sound…they lack the magic and finesse of SET DHTs". This statement is inaccurate because it should focus on the fact that the way American companies design push/pull amplifiers is out of fashion, tends to concentrate on maximum crude horsepower, and therefore suffers from a high degree of aural discombobularity. The reason we are going to see, very shortly, a new fashion of push/pull amplifiers, is because fashions change, and this is good news, because many of you need more than 20 watts of highly refined horsepower, and when done right, push/pull has its own sublime musical magic, that is unknown in America.

THE GOOD OLD DAYS

Twenty five years ago the Mc Intosh 275 and Marantz 9 defined the high power rung of the thermionic ladder. How could anyone need more than 90 watts?

When solid state designers realized that men would buy into the idea that more power was more macho the era of the mega-transistor amp began. In the mid 1970s the Phase Linear 700 was the shot that was heard around the world with its 350 watts per channel, and when you weren't using it for music, you could use it to cut glass, that is how bad it sounded. Japanese receivers were boasting 150 watts per channel, and the horsepower race was on, and soon speaker manufacturers were in the running. Speaker companies boasted that their speakers were between 2 and 1 ohm, which were perfect for solid state amplifiers, who could dump tons of crude horsepower. An amplifier that was putting out 300 watts into 8 ohms could boast 1,000 watts into 2 ohms in the hope of giving the ultimate audio erection. American men were getting horsepower and watts confused.

In the early 1980s Audio Research and Conrad Johnson recognized that there was a market for an analog of these big transistor amps and started to build high horsepower pentode push/pull amps. In a perverse way this made sense, because these amps definitely sounded better on these low impedance speakers than comparable transistor amps. If you could afford to buy these tube mega-muthas, you could afford to replace the tubes every six months, even if that was often at a cost $1,000 because this was a bad speaker/amplifier interface. I wish I had a dollar for every phone call and email I got that asked…"Why are my output tubes blowing up?". My answer is.."What do expect when you are using a 16 wheel truck on a Formula One race track?".

Because this is a sport of comparison…big push/pull pentode amps do sound better than big transistor amps, which is as obvious as saying a rose smells better than a turd, but really misses the point. These big push/pull amps compared to a "proper" push pull amp are musically mediocre for a number of reasons:

DC BALANCE: No one, not even Tube God, can get a large number of output tubes to be in both static and dynamic balance. Think of this as exactly the same challenge of keeping a chariot under control: race with two horses around a ziging and zaging course, and you know how hard it is to keep these horses in synch. Now you have a chariot with a dozen horses. You are now facing chaos. Here is a simple test: set your bias on all your output tubes in the normal way when your amp is idling. Make sure they are all are at exactly the stated bias point. Now put a sine wave signal into your amps so that is it operating at 50% of its rated voltage and measure the bias voltage again. I bet that all of the tubes have shifted their bias points. Just because your push/pull amp is in DC balance at its idling point doesn't mean it is in DC balance at its various power levels. This is one element of your push/pull aural discombobularity, and why less is more.

AC BALANCE (signal) is the world most difficult challenge. This is the challenge of designing an "ideal" phase splitter, which is impossible, because once you split a signal you can never, perfectly put it back together. There is an art to designing a phase splitter and let me tell you how to make it much easier: don't have your phase splitter drive the grids of a gaggle of tubes. Single ended fanatics, like me and others, spend a great deal of thermionic sweat on our driver stages and we only have to drive ONE grid to drive and if you look at the driver stages of the best single-ended amplifiers and compare them to the standard practices used in designing phase splitters in America, you would be shocked at how feeble they are.

OUTPUT TRANSFORMERS: I have my "Father of the Modern OTL" hat on and I can tell you that the problems endemic to all OPTS, become geometric when they get past 100 watts; they become big effective filters and choke off important musical information. Because these transformers are in the negative feedback loop, their distortion is being fed back into the feedback signal, making the problem of feedback worse. Then there is the issue of how you couple pentodes to the OPT, and this is such an interesting discussion, I once again recommend Menno Vanderveen's book available at www.plitron.com. Then we get down to the nitty gritty, my major complaint here, is the mediocre quality of transformers used in these expensive push/pull amps.

PENTODES: I grew up on pentodes. Yet for the past two decades I have kept away from pentodes because I still remember how the great "pentodes of yesterday" sounded and for the last ten years I have not liked what I heard from the new spread. When you compare the tonal quality, and quality of the aural matrix of pentodes compared directly heated triodes, directly heated triodes win for a bunch of reason that can be measured and can't be measured. My recent experience with the new JJ. Tesla pentodes gives me great hope. They, in many ways, sound like the good old days when pentodes were King. KR is introducing a KT 88 and that is also very good news.

POWER SUPPLY DESIGN: This is where marketing hype really gets down. This is a subject avoided like the plague. Amp manufacturers concentrate their sales pitch on power and describing the topology..ultralinear, etc, but completely ignore discussing their power supply, and for good reason…it is not properly designed, and I have written about this subject over and over again. Before you buy any tube amplifier ask the designer what kind of power supply regulation his circuit uses. You will be amazed at the benefits of just one choke, and of course active regulation is even better.

DIRECTLY HEATED TRIODE PUSH/PULL: In separate articles on this site about the Sun Audio amps you will hear my praise for what is completely missing in America: refined DHT push/pull. Many of you have asked me about the VAC 300B push/pull amplifiers and my response, based on listening to some of their models, is close, but no cigar. These don't come close to the quality of the Sun Audio Amplifiers. Just because an amplifier uses directly heated triodes doesn't mean that it is automatically better than a pentode amplifier. Intrinsically a directly heated triode amplifier that uses either 300Bs or 2A3s will be lower powered because you can't use lots of tubes because of the high input capacitance of these tubes…and you don't want to. Then there are the amplifiers that use 211s, 845s, and 805s, the big Class A RF DHTs. It is very difficult to do this right, and creating a high quality 10K OPT is difficult and expensive…but when done right there is real magic here.

WHY BOTHER?

You may not have a choice. If you need more than 20 watts and YOU probably represent 95% of music maniacs who love their "made for transistor" speakers you need a push/pull amplifier.

On the other hand when push/pull is done right it is a very ecstatic form of musical beauty that has its own unique aural matrix that is completely different than SET. It is wider, more uniform in its wave front top to bottom, and it is definitely more bombastic bass. What you give up is the 3D holographic wholosity of single-ended circuits and their harmonic purity.

AND NOW THE GOOD NEWS

LYNN OLSEN

THE GEORGE WASHING OF THE PUSH/PULL REVOLUTION

Every revolution needs its George Washington and Lynn is it. Most of you don't know who he is, yet among the savy he is considered a heavy duty savant. In the coming years he will be identified as the starting point, the leader of the new American push/pull revolution. In the same way, in 1992, they all laughed at single-ended circuits, we are about to see the beginning of a push/pull revolution, and you wont recognize it because it looks like nothing you have ever seen before in push/pull amplifiers, and Lynn has a beautiful vision of it to inspire you.

Now that the pendulum has properly swung towards single-ended circuits, , it is time to re-explore push/pull circuits from a completely new point of view. and Lynn is the first American to do it.

Lynn and I started to discuss this "push/pull abyss" about two years ago, when I raved to him about my experience with the Sun Audio 300B amps with transformer phase splitters. He sent me a schematic of his own very impressive design, which was a radical update of a very old, pre-Williamson concept..and looked way cool. Two years later his design has reached maturity.

Because Lynn is a Zen Buddhist thermionic techno shaman, aesthetic principals precedes and guides engineering. Judging by knowing them both, Lynn is the American closest to the spirit of Nobu Shishido which is probably explained by the fact Lynn lived in Japan for many years. I can think of no higher compliment. Now go to his web site and experience the place where the revolution is beginning. Because of the fertility of Lynn's mind I have asked him to create a compilation of his writing and articles, let's call it The Encyclopedia Olsenica, which you can download from his web site. Consider it "The Push/Pull Declaration of Independence", and it is an historic document. (address of web site). www.aloha-audio.com

CLOSE OF PART I

America is the land of powerful push/pull tube amps out of necessity because this is the land of the dysfunctional "transistor" speaker, but the fashion revolution, which is your ascending sensitivity to music is climbing up the thermionic ladder. A new generation of push/pull amps will be appears because YOU know more and demand more…you are The Declaration of Independence from Push/Pull aural discombobularity. Fortunately Lynn Olsen is here to guide the march. Spend time investigation this subject. Check out a bunch of Japanese web sites…and stay tuned to the next articles in this series.

e-mail Dr. Harvey "Gizmo" Rosenberg: drgizmo@meta-gizmo.com