THE REAL STORY ON SINGLE-ENDED AMPLIFIERS

It's hard to remember the last time any sort of "high-end" craze swept the entire industry the way the "single-ended" craze has over the last several years. It seems that almost weekly some other company has jumped on the bandwagon, offering their "unique" version of a single-ended amplifier. This article will attempt to offer a simple explanation of the technology, and some of the other often misunderstood factors involved with making single-ended amplifiers work for you!

Single-Ended: What is it?

Albert Einstein has been quoted as saying, "Everything should be made as simple as possible, but no simpler." Albert was obviously a single-ended man at heart, as the single-ended amplifier, in its purest form, is a monument to simplicity. Most of the common audio amplifiers that we have come to know and love over the years operate on a principal known as push-pull. In order to produce the output in a push-pull amplifier, the input signal must be altered. The input signal must pass through what is known as a phase-splitter stage, which in addition to passing the input waveform, also creates a waveform that is 180 degrees out of phase (that is, the exact opposite) of the input waveform. One half of the push-pull output stage is driven with the amplified input signal waveform, and the other half is driven with the waveform that is 180 degrees out of phase with the input. This creates a waveform which essentially swings positive with respect to ground through the load ("push"), and a waveform which swings negative with respect to ground through the load ("pull"), much as the AC waveform in common household current. In addition to complicating the circuitry, this scheme virtually always creates crossover or notch distortion at the zero point, where the positive and negative halves of the waveforms are attempting to match up.

Contrast this approach to the single-ended amplifier, where the input waveform is preserved without modification. The input waveform simply goes through a simple input stage directly driving the output stage, and is passed directly to the speakers without any unnecessary circuitry getting in the way. Think of the single-ended amplifier as an audio preamplifier, simply with enough power to directly drive loudspeakers! Crossover or notch distortion is zero, as there is no attempt to reconstruct a waveform made from two separate pieces.

Single-ended amplifiers exist today which use virtually every type of output device available, including bi-polar transistors, FET's, and triode, tetrode, and pentode tubes. Their basic principal of operation is the same, though the circuit implementations may be radically different.

So, if Single-Ended is so simple, how come single-ended tube amplifiers tend to be more expensive than push-pull?

There are several reasons for this, but the main common reason is the output transformer. Single-ended output transformers are very specialized, very complicated to make correctly, and virtually all of them must be wound by hand. There is a fair amount of "black magic" involved! As a result, anywhere from one-half to two-thirds of the cost of a single-ended amp is in the output transformer(s) alone.

By comparison, the building of output transformers for push-pull tube amps is much less exacting. Many of them are wound by machines on a production line, keeping the cost as low as possible.

So, if Single-Ended is so great, how come all amplifiers aren't made this way?

Good question! It's a question of "audio economics", but first, a little history.

Back in the golden days of audio and theater, long, long before the mega-powered home theater of today, tubes were king. They had to be, as there was no solid-state. Theater was considered "art" and so was audio design. More specifically, Western Electric was king. The theaters of long ago were filled with the sound of low-powered single-ended triode amplifiers, driving very efficient horn speakers. Yes, these simple set ups filled entire theaters with all the volume and dynamics required for involving performances!

When solid-state happened upon the scene, less expensive, higher-powered push-pull amplifiers became the rule of the day. Suddenly, much more power was available, and it was inexpensive. Suddenly, speaker designers no longer had to be worried about making speakers efficient and using simple crossovers. Everyone who had a wood shop and a soldering iron could throw some drivers and a crossover in a box and call it an audio loudspeaker. And, what's more, they could easily come up with an inexpensive, high-powered push-pull solid-state amp to drive it. Audio systems became driven by testosterone, rather than by music. The adoring public was there, demanding bigger speakers and more powerful amplifiers, and many flourishing companies were happy to oblige. Low-powered single-ended amplifiers and simple, efficient loudspeakers, basically vanished from the American scene. If it were not for our European and Far Eastern counterparts keeping the flame alive, we may have never seen the "Single-Ended Renaissance"!

Thanks to a group of "single-ended visionaries", led by people such as Dennis Had of Cary Audio, Gordon Rankin of Wavelength Audio, and Peter Qvortrup of Audio Note, the Single-Ended Renaissance did occur recently in the US⁽¹⁾. Not only did it occur, but also the interest in single-ended amplification has been on the increase among reviewers, manufacturers, audiophiles, and "home-brewers". What was once thought to be a "flash-in-the-pan" is here to stay!

Single-Ended, Low Power, and Speakers

The single-ended topology is inherently lower powered (and much less complex) than its push-pull counterpart. One of the reasons single-ended systems can sound so wonderful is due to this simplicity. Therefore, logic also dictates that less complex output stages may get us closer to musical nirvana - that is, single output tubes sound more "pure" than parallel tubes, and fewer solid state devices sound more pure than more. Also, simpler output tubes, such as triodes, sound more pure that tetrodes or pentodes. This tends to follow in the push-pull realm as well. Think back to when you may have had a chance to hear at least two similar push-pull amplifiers from the same manufacturer, one being higher powered than the other. If used with proper speakers which could easily be driven by either amplifier, you probably remember that the lower powered amp sounded more "pure" and "musical", that is, less was getting in the way of the music. You were hearing the magic of simplicity.

Following on the simplicity theme, we now come to the most important factor in making a single-ended amplifier work for you. Pay attention: You must use proper speakers! The single-ended amplifier/speaker interface is the most critical of all. Most of the bad press that has been leveled at single-ended amplifiers in the past (and indeed, most of it remains in the past, as a good many reviewers now understand the importance of using a "proper" speaker), such as "fat midrange", "wimpy bass", and "loss of high frequencies", can be attributed to using a speaker which was inappropriate for the amplifier.

So, what is a proper speaker? The proper speaker for a simple amplifier is, you guessed it, a simple speaker! Proper speakers for single-ended amplifiers are typically simple, two-way designs, with simple crossovers. There are basically two different philosophies of speaker design out there today. The simple philosophy pays very careful attention to the characteristics of the drivers and the design of the enclosure, such that the crossover can remain a simple first or second order filter, with the minimum number of components in the signal path. The contrasting philosophy is to use an array of drivers in a complicated system enclosure and fix it all in the crossover, meaning that the crossover will be complicated, and have a large number of components in the signal path. While there is nothing inherently wrong with this second philosophy, and a lot of highly touted and successful speakers were designed with this philosophy in mind, it just doesn't work with a low-powered or single-ended amplifier. Complicated crossovers eat power, leaving less power to produce sound, and if you only have 9 watts or so to start with, you are rapidly losing ground!

So, who makes proper speakers for single-ended? Without naming names in this article, let me simply say that we represent a number of such manufacturers, and they are typically companies based in the UK and other parts of Europe, as well as a number of smaller US manufacturers. Please call us for specific recommendations for your particular application!

More on Speakers: Impedance and Efficiency

Ideally, the simple speaker for a single-ended application will have a nominal impedance of 8 ohms, and will not have any excursions which drop significantly below 8 ohms over its frequency range (above 8 ohms is OK). This is because, in keeping with the philosophy of simplicity, most single-ended designs are also zero feedback, meaning that there is no active correction for load variations. Virtually all of the speakers which satisfy the simplicity requirement for single-ended also satisfy the impedance requirement, as the two go hand-in-hand.

What about efficiency? If you can find an 8-ohm speaker with high-efficiency and minimal impedance variation below 8 ohms, you have found the ideal candidate! They do exist; we sell several of them, but they are not common. So, if you have to choose one over the other, forego high-efficiency for reasonable efficiency and a "nice" impedance. That is because, in general, average to even loud listening is done at sound pressure levels of about 90 dB. Given this number, a speaker with even an 88 dB efficiency and a 9 watt amp will produce reasonable sound pressure in a small to average size room. If you try to minimize room interactions by getting your speakers out into the room and listening somewhat in the near field, this combination can work, even in a relatively large room!

Single-Ended: Is it for me?

It's very difficult to say. Like everything else in audio, it depends on your own personal tastes and perceptions.

In all honesty, I'll admit I was a huge skeptic. The only reason I even tried listening to single-ended was because a wonderful distributor (Leonard Norwitz, formerly with Audio Note) sent me a single-ended amp and a pair of compatible speakers (both already broken-in) "just to try". I was reluctant to even hook them up - I had already convinced myself it couldn't sound good. How could this flea-powered amp produce any real dynamics? But, I was a little bored one night, so I unboxed them and hooked them up. I make a habit of not really listening closely to anything until it has broken-in (which these were already), or until it has warmed up a bit from turn-on. So, I hooked them up, turned everything on, and put in a CD. It all seemed to work, but I didn't pay much attention to the sound until it had warmed up for about an hour. Then I came back, cued up a favorite CD, and sat down in earnest. Less than thirty seconds into the first cut, my lower jaw was on my chest! I was hearing magic! Although I had listened to this same cut so many times on so many different systems, it was like I was hearing it for the first time. There was a sense of "rightness" there that I had never heard before. The sense that a layer of grunge was gone, that I had never even realized had been present before. This was truly something special. I went crazy, trying favorite cut after favorite cut, and reveling in every note! I'm sure every audiophile has a similar experience to talk about in a much different situation, but I can't remember when anything else has hit me like my first "single-ended experience"! And from that moment on, I have been hooked.

Not everyone who heard that particular system reacted in the same way I did (a good many did, however), and not everyone will relate to a good single-ended system in the same way. Again, just like cars or even personal relationships, it's a matter of personal tastes and perceptions. If you want to be receptive to single-ended systems, you must keep an open mind. After listening to higher-powered "slam-bang" push-pull systems for years, we begin to think that that is the way reproduced music is supposed to sound. We tend to be unreceptive to things that may present our favorite music in a way that sounds "different". Single-ended systems do sound "different", and one should not dismiss them because they may reproduce our music in a way that sounds "unfamiliar". It is our duty as audiophiles to carefully evaluate these subtle differences over a period of time, to see if those differences might actually create a more satisfying musical experience. For me, the proper tonality, sense of timing, intimacy, and involvement in listening to a good single-ended system, far outweighs the "slam-bang" I may be partially sacrificing.

So, is it for you? If you have a pair of complicated, hard-to-drive, and/or inefficient speakers that you dearly love and cannot bear to part with, don't bother. Single-ended amplifiers that could drive your speakers either don't exist, or would most certainly break the bank to buy. If you are a male who feels that his audio system is an extension of his testosterone, don't bother. Maximum "slam-bang" is your thing. If you want to reproduce the takeoff of an F-16 fighter at full volume in your living room, don't bother. That simply takes too many drivers and too much power. If you enjoy being a member of the "system of the month" club, don't bother. There still aren't that many choices for single-ended combinations out there to change systems every month. And, if you find that single-ended combination that works for you, you will keep it, requiring you to resign from the "system of the month" club.

If, on the other hand, you are an open-minded audiophile who is receptive to potentially starting over with new speakers (this may or may not be required) as well as a new amplifier, you may be a candidate. In addition, if you are looking for something beyond all the "me too" products, and believe that a system should ultimately be a musical experience, you are getting closer. Ultimately, the single-ended system candidate is a bit of a maverick, who is tired of searching for musical nirvana with the traditional approach. He/she wants a system that is driven by music, one that will keep them coming back to the listening room, and one that will remain satisfying for years to come. If that person is you, please give me a call! I can recommend some solutions that will get you there for much less money than you may have imagined.

Of course, if you favor the more traditional approach, still give us a call! We offer many unique solutions that will work for you as well.

Thank you for your time and attention!

Harold Cooper

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(1) Some of the "less conspicuous" single-ended visionaries included the original gang from the "Fi" shop in NYC, Don Garber, J.C. Morrison, Komuro, and Herb Reichert (formerly with Audio Note NYC). Also, several other writers and editors have been major contributors, such as Joe Roberts (Sound Practices and Fi), Eric Barbour (Glass Audio and Vacuum Tube Valley), Lynn Olson (Positive Feedback) and the self-proclaimed "Gizmologist", Harvey Rosenberg (Positive Feedback). The list can go on and on, but these guys were all into single-ended long before it was "cool".