WHAT? No entries in six years? WHAT HAPPENED?
No, I have not abandoned the development of the new vacuum tube project ideas announced in this section many years back. It is just that demand has led us in other areas and, as trite as it might sound, time really has FLOWN!
We do have the MPT-1 vacuum tube "MasterPiece" module developed already and it WILL be made available for do-it-yourselfers eventually. It is just that I do not have the time available to introduce it, and support it, as needed on the website as a product offeirng just yet.
You will also note that a lot of the design philosophy discussed below in regard to the proposed "DaviSingle 3-T" was subsequently diverted into the development of our new TB8-SE "DaviSolidSingle" solid state, single ended amplifier. That does not mean that the tube version of such an idea, as described below, is any less interesting.
We will have some exciting new "DaviSheen" vacuum tube custom projects on display on our site in the near future and we WILL have some more techincal ramblings documented in this section if, and when, time and schedule ever permits.
So, thank you for stumbling into this "backroom" section of the DaviSound website. Hopefully, there WILL be something more worthwhile posted here for you in the years ahead! Meanwhile, you may enjoy our original offering from many years back when time (in bottles and for bottles) was a lot less TIGHT!
"DaviSingle 3T" (Single Ended Triple Triode Triangle) powered monitor amplifer speaker system.
By Hayne Davis
We will address each parameter and, in each case, answer the age-old question ... "WHY?".
For example, WHY single ended? WHY low power? WHY vacuum tubes? WHY triode power tubes?
It seems I have developed quite a reputation on the message boards , and in other circles, as THE "anti-transformer" man.
I do find it so strange, when I scour the numerous "audiophools" sites across the net ... that so many of these "cases" can always seem to "hear" dozens of nuances in capacitors, different tubes, resistor types, WIRE (yep) and even, in some cases, the INSULATION on the wire (heaven help us!). Yet, these same "amazing ears" willingly wrap themselves around transformers without question ...
I have been listening to audio, critically and professionally, DAILY in some capacity, for forty years now (if you start counting when I was eighteen ...actually I was listening critically, AND professionally before that-just not on a daily basis).
I have listened to, analyzed, utilized and, at the least been exposed to, virtually EVERY kind and type of audio equipment there is. One of the first things I remember detecting along the way in my quest for "PURE" audio signal transfer, was the undeniable fact that transformers ALWAYS have a recognizable sound. Of course it varies and some sound different in different regards, each in its own way. Some sound "better" than others and some can even fool you ... for a little while.
The bottom line for me is ... transformers used to couple audio signals are always recognizable in some way to my ears! I did NOT start out prejudiced against transformers and I did NOT become educated in that way from reading someone elses opinions. I have tested myself on this perception many times in properly set up A/B situations and, while I may be fooled for a brief time by some of the very best designs, I ultimately can detect that "veiled" characteristic that my perceptive experience always exposes as an audio chain weak link - the audio transformer.
Now it may be argued that since speakers are themselves flawed inductors and a decided weak link (actually likened to AC electric motors!), anything you can distinguish on a speaker must REALLY be a "weak link"!
Ironically, and this "riles" many transformer enthusiasts when I tell them this, I don't even have to A/B a with/without chain to recognize that there is a transformer present! While I may not "bat the thousand" with the random perception that I do in A/B testing (which is always SO obvious to me), I can MOST always tell they are there from just listening a little while to a single transformed signal which, to me, will always ultimately reveal itself in "mud" (worst case) or subtle, low end "veil" (best case)! And, while I would, no doubt, notice it moreso on a monitor system I was familiar with, I have actually walked into a foreign (to me) control room on occasion and not only recognized "transformer sound" coming through the monitors during a mix, but actually pinpointed the sound signature of the equipment causing it, instantly, having not even yet seen their setup! Believe it or not, this is the honest truth and it has happened on more than one occasion!
I once walked into a studio and told the engineer I did not like the fact that he was using a particular brand of recording console because of the "low end mud". He asked me how could I possibly know that they were using that board since it had just been installed that week, had not been advertised, and he and I had JUST that moment walked into the control room completely out of view of it (unfortunately, to my ears, not out of "earshot")! I told him the simple truth that I could recognize the sound of the particular input transformers that they used in their mic amps in that particular console. He found this remarkable and could hardly believe it ... yet, to me it was simple matter of fact. I refer to ths perception as "veiled" to keep it simple but I actually perceive it moreso as if the "sound is in a box" of sorts ... a confined, restricted characteristic that is recognizable mainly in the lows and low mids. As another example, I once did a session on a Tapco mixer, which employed mic input transformers, and the input signature is still very recognizable to me whenever I drag that tape out for a listen although the the rest of the chain, including the Tascam recorders with their 1%, or higher, distortion contribution of another sort, were all transformerless.
I find it hard to understand why there are those who would actually PREFER to cast this distinct "veil", no matter how thin it may be, over their audio signals for whatever reason! Of course, there is the very likely possibility that many of those "tube-o-philes" who think they prefer the "sound of tubes" may actually be conditioned to the "sound of transformers" and find it desirable! If so, they would likely not enjoy the sound, or lack thereof, of our new monitor amp which will employ power tubes WITHOUT the output transformer. However, as will be seen, although we are desigining a "transparent" amplifier employing vacuum tubes, it will still be possible to "push it" beyond accurate levels, into the edges of soft clipping and rather high second harmonic distortion , for those who enjoy this "fat warmth" produced by single ended Class-A power when overdriven! In fact, I urge all enthusiasts who have not experienced OTL to try this version and listen to the tubes do their thing WITHOUT the "mud slinging" hunk of iron across them!
I know of NO other audio component, aside from perhaps the transducers we must employ at the front and the end of our chains (microphones/speakers), which audibly affect the audio signals as blatantly as do transformers. This is why I find it so ludicrous when I read of "weekend designers" (and so-called "pro" designers as well) who have paid huge multi-bucks sums for the most expensive coupling capacitors and special resistors whereby they "imagine" they hear all sorts of sonic improvements, while all the while coupling their circuits with a transformer, sometimes several!
I know of other veteran, pro audio engineers and recording engineers who share my astonishment over this. Some of them say it would, naturally, work out conversely that the extremists would "hear" the things that are not there while in denial over the things that are. I still find it very, very odd that this phenomenon even exists amongst audio enthusiasts. To me, after all these years, there is no debate. It is just a matter of fact. Most of us, who have followed similar paths of experience involving years and years (and more years than any of us care to count) in control rooms and audio labs, agree that we can, in no way, "hear" a PROPERLY chosen and applied capacitor, resistor or piece of wire, but we all agree on the transformer issue.
I have often wondered in "wide-eyed amazement" ("wide EARED amazement"?), how some of these jokers can spend hours swapping tubes in a circuit, "hear" all sorts of differences, and then write long flowery discussions about the "character" of different tubes without ever taking into consideration that these things they "hear", if indeed they do, may well be related to the various circuit components affecting and interacting with a particular tube in that particular circuit, and not because of distinctions amongst the tubes themselves! This obsrvation would be ESPECIALLY applicable in the case of transformer coupled circuits!
Anyway, the point is, to this designer, transformers should be eliminated in all attempted HIGH FIDELITY audio circuits or, otherwise, they should be called LOW fidelity circuits right out of the chute. Because, anyone deliberately choosing to use a transformer has, as a known matter of unarguable fact, relegated himself, at the outset, to inherent electrical restrictions that definitely affect discernible audio quality.
Now, the ONLY possible argument justification for even considering using a transformer in high fidelity audio circuits is in a vacuum tube power amplifier. In that case, a transformer is unarguably the most EFFICIENT way to couple a tube to a loudspeaker if maximum power transfer is your definition of "efficiency". However, NOT using a transformer in this application is not as Inefficient as many people seem to think! Tubes CAN be coupled to speakers very efficiently if you take a little time to overcome some of the, apparent, ignorance surrounding this, traditionally, mostly overlooked, avenue.
Let me "back up" a bit and interject here that, of course, it goes without saying that the preclusive "justification" for employing transformers, or any other "coloring" component in one's own personal listening environment , is simply because of user preferance! If one prefers to listen to transformers, or any other coloration, then so be it. Go for it ... (in your own home, personal systems)! Just, please, in that case, acknowledge and admit that "special preferance" to yourself, as well as to others you might influence, and get out of denial in regard to the unmistakable sonic restrictions and "colorations" imposed by transformers.
I read an article on the net recently written by an "old timer" tube enthusiast who was attempting to indoctrinate "newbies" on the proper use of vacuum tubes. His comment on OTL power amplifiers went something like this -which serves to point up my use of "ignorance" as the choice of terms to describe the state of OTL understanding amongst most tube unthusiasts -
Just about everything in that statement is wrong except the part about tubes themselves!
Actually, tubes are not good power amplifiers PERIOD! This is because they are, inherently, rather low current devices when compared to their bi-polar transistor counterparts. But, what many "old timers" do not seem to understand, is the fact that cathode follower configurations can function VERY well as power ampifiers, especially when two or more are paralleled with proper current summing, and driven by the proper source. If they were to take the time to THINK it out and actually EXPERIMENT in this regard, they would likely be astounded at just how well transformerless outputs can perform when properly designed and applied. And, if they have an ounce of honesty in their ear/brain connections, they would have to be more than astounded by the crystal clear sonic results of "ironing out" their designs!
Transformers contribute to so MANY impurities in an audio signal, and they have so MANY known degrading characteristics, it is hard to know where to begin. They have limitations in regard to frequency response, phase response, distortion, artifact contribution (ringing, hysterisis, core saturation) the list goes on and on and on! So...WHY BOTHER? The ONLY place they do ANY good at all is with impedance transformation and power transfer. On top of that, to be anything even slightly better than noticably atrocious, they usually have to be, by far, the most expensive componet of the amplifier that uses them! So, the question is, why have so many, for so long, been so willing to overlook so MANY flawed characteristics in favor of just one, rather minor, positive? It kind of reminds me of the flawed perception of the masses in regard to a certain presidential administration of the United States!
The singly argued chorus that keeps on refraining is that term, "efficiency". So, let's tackle that one (since when have the majority of Americans been considered a collectively "efficient" people by nature!). As we do so, always keep in mind our opening observation that when it comes to power amplification, vacuum tubes are "inefficient" instruments to begin with whether or not they are "transformed". So, utilizing a transformer does not make them "efficient" as power amplifiers, it just makes them slightly MORE efficient! And, as we shall see later, if high impedance speaker loads are employed, transfomers then leven loose their single power transfer virtue!
Such being the case, if one chooses a tube over a transistor for a power (high current) amplifier, one is ALREADY choosing inefficiency over a transistor. Not only that, one is choosing inefficency in many other practical aspects besides power transfer including consistent availability, component size, heat dissipation, and costs to name just four other considerations.
It thus becomes like the old joke whereby the blonde asks the gentleman "just what do you think I am?" and he answers ...
So, if we have already chosen an "inefficient" tool to do the job, why worry about a little more "inefficiency"? Or, maybe we should just consider a more realistic definition of what efficiency really is for an audio amplifier aside from power transfer.
Another good reason to avoid transformers (and transformer advocates) is having to hear continual references to that sickening nickname tossed around by these types ... "Tranny" ! After scouring the Internet and reading more than I could stand, I now tend to "run" from any page using that term as I do any site having the term "Labs" incorporated into their name!
When we use the term "efficiency" in relation to audio audio amplifiers it has always been, traditionally, referenced to output power. How "efficient" an amplifer may be normally refers to how much power it can deliver to a load under given circumstances of power supply voltage, current and signal.
But, suppose we looked at "efficiency" in altogether another way? Instead of always thinking in terms of power output, suppose we made our single desirable amplifier characteristic to be its ability to deliver the ultimate, highest quality reproduction of sound, period. It may, likely, then turn out that the most "efficient" way to acheive that goal is NOT the same as the most efficient way to to deliver power to the load. Not at all! In fact, it could very well turn out to be just the opposite!
Our design philosophy, then, for our new monitor reference amp will be based upon our redefining the meaning of "efficiency" in relation to amplifiers! Actually, to be more accurate and as will be ultimately seen, we are considering a system efficiency transfer whereby we transfer some of the "efficiency" burden from the amplifier to the load ... to the speakers themselves.
I suppose I have always had an inclination in that direction as a designer since "efficiency" to me correlates to my basic nature which is to always try and do the most with the least and with the least amount of waste involved. That being the case, one would think that it would go against my grain if I were to go in opposite directions in terms of traditional amplifier efficiency and, normally, it would.
But, long ago I learned that, to a great extent, both accuracy and "listenability" (a loosely deifned term of mine to mean the ability of an amplifier to be listened to for hours on end in critical monitoring without creating listener fatigue) in monitor amplifiers was inversely related to high output power. In my estimation, given reasonably efficient speakers, more than 10 watts in a monitor amplifier is not only unnecesary, but undesirable as well.
We spend a LOT of time on our website within the TB-8 On-Line Manual belaboring the benefits of lower powered monitor amplifier design with efficient speakers. In so doing, we point out that the universally accepted "watt term" is the most useless yardstick by which to measure the volume and quality of sound coming from a speaker!
The ONLY electrical paramater that offers any true, fixed merit as a "yardstick" for measuring audio volume capabilities is SPL (Sound Pressure Level) measured at a proper, consistent distance from a loudspeaker. Assuming the amplifier has enough headroom so as not to clip the waveform at any useful level of operation thereby sending squarewaves to the speaker, then its output power rating is fairly useless. This is because regardless of load, at least, right about 2/3 of the amplifier output capability is utilized in delivering the initial first watt of "power"!
With even the WORST of today's inefficient speakers (down around 84 dB at 1 watt across the speaker and sound level measured at a distance of 1 meter), a SINGLE WATT of power can still deliver the same volume level, or SPL, of a loud automobile (or medium loud motorcycle) at the same distance! Consider, further, that with a typically modestly efficient speaker (91 dB 1W/1M), a symphony orchestra can be faithfully reproduced at the same volume level (SPL) of a live concert with only 1 watt of power across the speaker! It is a well known fact that the orchestra at full volume will generate about a 91 dB average SPL at the ear of a listener at average distance in the hall. Amazingly, very few people, some pros included, ever seem to fully grasp these relationships! Furthermore, most people never realize just how little power is actually being used to deliver a given sound level from their speakers at normal listening levels.
After the first watt is developed from an amplifier across a speaker load, the laws of physics and diminishing returns rapidly come into play. Just remember, it takes a doubling of power to acheive each 3 decibel increase in sound level! Conversely, a 101 dB efficient speaker will deliver just as much sound at only 1 watt, as a common 84 dB efficient speaker driven by 65 watts! This illustration alone should serve to show what we mean when we stated earlier that the watt is a useless "loudness" reference for sound systems and why speaker efficiency is every bit, if not much more, important as amplifier power for volume of sound produced.
So, all of these relationships considered, a low power amplifer that will deliver around 2 watts of true fidelity does not seem so strange a design consideration.
Then, if we discover that the most EFFICIENT way to obtain excellent audio quality is to actually eliminate some of the very things in our circuits that provide for higher power output in some cases, it now makes VERY good sense to purposely design a "low power" amplifier.
I suppose I must digress just a bit here to furnish the obligatory description of the difference in push/pull topology as opposed to single-ended for those who still may not know what this means or why it relates to "efficiency". To put it as concisely as possible, a "single-ended" amplifier utilizes just one amplifying device (or several in parallel) to amplify the complete cycyle of the incoming waveform; whereas, "push/pull", as the name implies, uses one device to "push" the upper excursion of the alternating signal wave while a another device is used to "pull" the lower excursion. In order for this to happen, the "SINGLE ENDED" (abbreviated SE) amplifier device must be biased to be "turned on" at all times whereas the "push/pull" devices only need to be "turned on" while their respective halves of the waveform are present. By nature of the current and voltage mathematical relationships involved, a push/pull stage using a pair of devices can typically deliver four times the power output of a single device of the same type operating SE.
Why, then, would it not be better to always use push/pull? Well, again, if POWER and "efficiency" to that degree is your main concern as it usually seems to be in mainstream American hi-fi, then it WOULD be better to use push/pull. That is, in fact, how the typical modern, mainstream, commercial amplifier is invariably constructed. But, for those of us who migrate toward the idea that "one device is more accurate than two" when it comes to amplifying a sine wave or similar alternating signal, then the only way for that to happen is with a SE, Class A single device(s). Now, I know, it may be argued that, technically speaking, a properly "tuned" push/pull stage is electrically the "same thing" as a "single device" which does the job "more efficiently". That argument, of course, again only applies if you accept that traditonal limited definition of "efficiency" in audio circuits to mean only power transfer. The "key", here though, is that obscure, little qualifiying phrase ... "PROPERLY tuned".
Even if we discount the minimalist ideal that one is better than two if one will do ... we have to consider even the best push/pull stages to be more complex, if only just slightly so, thereby more difficult to keep "tuned" ... meaning properly "time aligned" and in precise "balance". Actually, it can be shown that wherever two symmetries are involved in audio electronics, there is no such thing as "exact balance". There are, also, just as many push/pull devotees who will shout that this makes no difference! And, they could be right. But, for those of us who have done the homework, and who have paid the dues, and who have, as a result of all this effort, determined for ourselves that there IS a marginal distinction that is readily discernable in blind testing which favors single ended Class-A over push/pull, even Class A push/pull operation, then there is no turning back. We will not be convinced to do so no matter how loud the shouts!
And, now, once we have made this determination we are left to speculate as to just WHY we can discern this distinction that preferances the SE mode. Since it is possible, to a limited degree, to operate push/pull in true Class A (in fact all Class A/B push-pull circuits operate full class A at low signal levels), it must not be the "class" of operation that determines this distinction so much as the imbalances involved with push/pull. I actually, personally, tend to suspect that it may be partially contributed to "time constants" relating to the minuscule alignment differences involved in the phase splitting process necessary to drive the two push/pull devices. And, it may also, just possibly, relate to the, as yet undetectable to tradtional measuring instruments, electrical artifacts and by-products of the on/off switching process itself! If so, this would be the most important distinction discovery to date!
I suppose, then, I should further explain to any non-technical reader who still does not know, a bit about the difference in Class A as opposed to Class B or Class A/B modes of operation. I mentioned earlier that a device is "turned on" in order to amplify. In order to "turn on" a device to conduct alternating currents, we "bias" it with a DC supply voltage and we, alternately, bias its control element in another "direction" so as to partially turn it on or to turn it off entirely as needed. To be considered full "Class A", our device must "idle" at all times so that it is always turned on and will thereby amplify the complete cycle of the incoming waveform without any interuption whatsoever.
For Class B operation, the push-pull devices need only be switched on for the period that their half of the cycle is present. While pure Class B is four times as efficient as Class A due to operating half the time at both half the voltage and half the current demand of Class A operation, there will invariably be a brief, small portion of the waveform cycle that is lost due to the "crossover switching" process. This leads to a particularly ugly (when viewed on a scope) and nasty (when heard on a speaker) form of distortion called "crossover" distortion. In order to combat this, the Class A/B mode was devised whereby each device is turned on just a bit longer than the time required for it to amplify its half of the wave, thereby overlapping conductance with the second device when it is turned on for the alternating cycle of the wave. While not as efficient as Class B operation, this mode of operation virtually eliminates, for all MEASURABLE purposes, crossover distortion. Crossover distortion can be further cleaned up by the proper application of negative voltage feedback.
Yet, still, my soon to be described tests have revealed to me that there is a subtle, but audible, distinction between SIngle Ended and push/pull!
It almost just seems so much "common sense" that one device could manage replicating a full wave cycle more faithfully than attempting to make two separate devices, no matter how similar, replicate a full wave from two distinct, and/or overlapping (merging), half waves. And, no matter how "theoretically sound" the electrical principles might be ... I now know that SOMETHING is audibly discernible between these operating modes so your guess as to what that SOMETHING might be may be as good as mine and ours as good as any!
Before you conclude here that those subtIeties and "textures" I am detecting and preferring in this mode at low to medium listening levels is just the old degree of second harmonic distortion inherent in the single ended Class A scheme as opposed to push/pull (which cancels second harmonics but generates dominate odd order distortion), consider that I can still detect that "subtle sweetness" when properly applied negative feedback has effectively eliminated ALL measurable THD down to below .008% at these levels!
During the first part of this year 2005, I had the opportunity to do a lot of extensive prototyping, measurements, as well as critical listening tests, as a result of several new vacuum tube designs having been commissioned by DaviSound clients. During these sessions I not only auditioned different designs and different components but I also A/B compared many traditional designs as critically as I could to see if there actually were, to my ears, any validity to some of the traditional claims about this or that topology "sounding better" in spite of similar measurements etc. I "lived with" each of these test circuits for some time ... long enough to become well aquainted.
Now I have already mentioned that speakers are a decided weak link. Keep in mind that I am a lifelong, experienced and much practiced, listener. As I have always pointed out to any young appentice who might have been seeking such advice ... in regard to critical monitoring, it is not so much the type of monitor speaker you use, so long it is of reasonably high quality (full response as flat as possible in the nearfield and with as little inherent distortion as is possible), but the important thing is that you are most FAMILIAR with this all important tool. You have to KNOW your speakers! I also think it is important to habitually monitor at the same level settings for critical evaluations.
During my recent critical evaluations on my known, reference monitors, I surprised myself to learn that there were more audibly distinguishable differences in circuit topologies than I had ever imagined there might be! For example, the higher powered designs, which were invariably some variation of a "push/pull" output stage variety, the need for which will be further discussed later, naturally delivered their best at their highest "kick in the gut" operating levels. However for moderate listening levels, the kinds of levels where this audio engineer had always preferred to do critical evaluation and mixdowns, the higher power amplifiers seemed to "loose something" especially when A/B compared, at critically matched source levels, on the same high quality studio monitor speakers used as a reference.
Now, taking into consideration my earlier "peeves" regarding "audiophools" who fail to consider all parameters, I tried to eliminate as many variables as I could in my own tests and tried to determine if I was only falling victim to some of the "experimenter's ear" syndromes I have written about elsewhere on numerous occasions over the years. I tried numerous high quality monitor speaker systems, and "lived with these" as I had the amplifier circuits, each variety, for long periods of time to be fully aquainted with each newly assembled test system.
Scoping down the assortment of tests and methods without much elaboration at this juncture, in order to get to the point, suffice it to say that through it all I did come away with the decided opinion, contrary to my previous design choices of the past, that there just really might be something to this SINGLE ENDED amplifier superiority argument!
When a modest power amplifier with limited internal "frill components", such as our solid state, TB-8 studio monitor amplifier was employed as the push/pull unit under test compared with a prototype single-ended Class-A design, the audible difference was almost undetectable. But under repetitive, criticaI scrutiny, I still managed to pick out "something" in blind testing (not much of my testing was properly, to form, the acceptable "Double Blind" variety; but, I believe I have devised a reasonably active "single-blind" method for my own purposes as a lifetime critical listener, so as not to delude myself!) most of the time that seemed to marginally favor the SINGLE-ENDED designs. This was especially the case for the tests conducted after extended listening periods (the same repetitive music source cycled for four to six hours as a close range backdrop to normal work activities). I came to the conclusion that SINGLE-ENDED invariably proved to be more "listenable" for long periods, providing less "listener fatigue" than the push/pull varieties!
Now, in all fairness to our ORIGINAL (non-bridge version) TB-8, which I still regard as the most faithful, modest power, (Class A/B push/pull) monitor amp I have ever used, the "listener fatigue" was honestly NOT an issue in that case. But, remember, along with other "minimalist approach" sonic considerations, the TB-8 design utiliizes only ONE PAIR of push/pull devices per channel and whatever irregularities are involved with push/pull operation would, logically, be minimized as opposed to the additive properties of those other designs where more stages are cascaded for higher power outputs as is the case with all the higher powered amplifiers of this mode.
Keep in mind, too, while you consider all of this that, although designers have supposedly eliminated ALL noticable crossover distortion by operating Class A/B and/or applying negative feedback, crossover distortion is always most noticable to the ear at low volume levels. High power Class B amplifiers operating at high volume can mask crossover distortion almost entirely.
Finally, in concluding this area of consideration, again, let's simply suffice it to say that I now believe, as a result of many months of daily testing, that there IS INDEED justification to the claim that for the ULTIMATE in electrical audio reproduction and amplification, we must consider the seamless, single-ended Class-A topology. However, we must also recognize that this topology is not necessarily a total panacea unto itself as will be brought out further on. And, of course, it is entirely impractical for very high powered requirements such as stage sound reinforcement and the like.
I should also mention that push/pull operation always has another inherent strike against it when utilizing vacuum tubes as the output devices since there are no complementary tubes available as there are transistors (the tube has no PNP complementary equivalent). This, of course, magnifies the problems of "balance" and phase error eluded to earlier.
I can almost hear the fanatical tube devotees who might be reading up to this point proclaim ... "AHA! He is leading to the conclusion that single-ended Class-A TUBE power is, audibly, the ultimate!". Well, while I almost hate to shatter that notion, I can NOT make that distinction entirely. I did, in fact, build some single transistor power stages operating Class-A , with the same, comparable support circuit considerations employed, that were virtually indistinguishable from the power triode operating at the same, low signal level, which would be in the 20 to 40 mW range. There were times when I thought I heard a "sweeter something" with the tube circuit when I knew it was in use. But, I failed to convince myself when I poured over my blind test results which was invariably about half and half ... the random expectation.
Now, for you transformer/anti-feedback devotees, yes there would be a drastically audible distinction between tubes in that case as opposed to OTL transistor single-ended Class-A. Remember I stressed "comparable support circuit considerations" employed. This means I employed the proper amount of corrective negative voltage feedback and maintained proper circuit component relationships within the respective, compared circuits.
So then ... WHY TUBES? Well, purely and simply, for the unique aesthetic values involved which, to me, are as important as ANY consideration. We have, at the outset, chosen to use a traditionally "inefficient" power (current) amplifier topology to drive our speaker system so we may as well use the traditional, ORIGINAL amplifier, the Class-A Triode! Whatsmore, in so doing, you can enjoy the "light therapy" offered by the glow of the tubes if you buy into that sort of thing. And I, for one, most definitely DO buy into it! I enjoy the idea of the "old bottles" and their warm heater glow, amplifying my audio signals about as well as do any of the tube "fanatics". I had already decided that I wanted to use tubes ... IF they could be proven to be as "clean" or as "accurate" as (or possibly better than) transistors.
Furthermore, most vacuum tubes just might be arguably superior than the best transistors for this uniquely demanding application simply because they tend to have slightly broader, useable "linear regions" inherently than do their solid state counterparts. So, although we are talking small margins here in that regard, that just might be one technical consideration for justifying their application. This, along with their more obviously well-known "soft knee" overload qualities, which are also highly desirable in a Class-A , limited power amplifier, tend to weigh heavily in their favor for this application!
Because, while I emphasized, in the earlier paragraph, that down in the full linear range of operation there was no distinction in sound, once you start to turn the volume up into the borderline and early overload regions, admittedly the "warmer", old "Triode effect" of soft clipping, fat second harmonics, gave the expected "tubey" sound that devotees of that genre know and love! So, why not have this "option" available as opposed ot the harsher, harder clipping onset of a similar class transistor stage?
Along with the myriad of myths still surrounding vacuum tube audio on into the new millennium, is the notion amongst some sophomoric design enthusiasts that "cathode followers never 'sound good' ". This myth was probably conceived and perpetuated by the same types who think that transformers DO "sound good"! Cathode followers actually come as close as you can come, with a simple tube stage , of producing power amplification with no (neutral) "sound" which, again, may be why most of the "audiophools" say they "never sound good". A well designed cathode follower stage, with no voltage gain itself, is faithful to the voltage amplification stage which precedes it and contibutes little else to "sound" aside from power gain!
Actually, the cathode follower getting a bad rap by the semi-informed detractors probably has a lot to do with the fact that it, by nature of configuration, employs 100% negative feedback (albeit so-called "local" , stage-confined, feedback).These same detractors would, without exception no doubt, abhor the practice of utilizing corrective feedback in any instance. They may mistakenly choose to call themselves "purists" in so doing but, in fact, they are just the opposite! Feedback is the most useful sensory property that I can think of! Were it not for "feedback" sent through our own human electrical circuits from our five senses, where would we be?
Feedback allows us to taste and season our food, steer our vehicles, launch our spacecraft, guide our missles, perform open heart surgery and ... remove artifitacts generated within an amplifier which ride on the output signal that were not present on the incoming signal!
Ironically, it is probably another property (fault) of their transformers themselves that likely contriubuted heavily to the "anti-feedback" train of thought in the beginning. Amongst that long list of "nasties" that describe transformer properties, comes "phase lag" and poor bandwidth integrity which would often cause multiple phase shifts of some form within the feedback loop. This kind of "feedback" would, indeed, as they like to say, "cause more problems than it solves". That and the fact that many "would be designers" often tried to "close the loop" with more feedback than was available from the open loop gain of a given stage. For a specific example, two cascaded triode stages employing so-called "global" feedback which are comprised of the "cultists" prefered 6SN7 would NOT be a wise attempt; whereas, a similar attempt using the less popular 6SL7 may work very well because of the inherently much higher Mu, thus available open loop gain, of the 6SL7!
So, you see, it is not the FEEDBACK ITSELF causing the poblem ... it is the attempt at misapplication that is the problem in those cases! Naturally, "sophomores" can not sort all of this out and make the proper distinction so they just label feedback as a bad thing period. This kind of thinking is akin to hitting a pothole on the road to Grandma's house and then, as a result, subsequently condemming and the entire route to Grandma's or, in some extreme cases, even forbidding going to Grandma's at all any more because there may be another "pothole" lurking on another road somewhere!
Unfortunately, this misplaced "logic" does perpetuate heavily amongst the "audiophools" but, worse, when they preach it from their website "pedestals", it has the potential to wrongfully influence a whole next generation of curious newcomers who would readilly accept the faulty premise without question and add it to their "bill of materials" for more shaky foundations.
Cathode followers behave very similar as their emitter follower counterparts in the transistor world, the latter of which make up 99% of the industrial and commercial power amplifier workhorses operating in the world today. Though there can be more complex variations, a cathode follower stage, in its most basic form, simply connects the plate directly to the supply voltage and uses a cathode bias resistor to ground in conjunction a high impedance grid resistor across the input to ground. The output signal is taken across the cathode bias resistor. This arrangement forces the output voltage at the cathode to "follow" the input voltage at the grid ... thus the name "cathode follower". It does this with near NIL distortion contribution and no phase inversion!
It also forms a very desirable audio transfer characteristic for any amplifier in the process! This characteristic is a high (non-loading) input impedance and a very low output drive impedance!
While a cathode follower has no voltage gain (actually it will always be slightly negative voltage gain of just less than 1 - but it is close enough to consider it as "unity" for all practical purposes), it DOES retain and offer the full POWER GAIN of the tube stage. This fact is seldom recognized by the anti-OTL types it seems! Since it "transforms" a high impedance to a low impedance with power gain in the process, it is in itself, an "active transformer" comprised of a vacuum tube instead of a hunk of coil wound metal!
We mentioned early on that tubes were poor current amplifiers compared to transistors. Well, while this is true to a great extent, it is somewhat countered, in my view, by the fact of their inherent nature to "swing" extremely high voltage potentials! In practical OTL application, this means that while you might not have voltage gain available from your output tube by not working it into a transformer, you can employ a higher voltage swing in your DRIVER stage working into the follower's high impedance grid. Then, the follower provides considerable power gain in "transforming" this to a lower impedance. If you parallel many current-summed, follower stages together acting as one, you can achieve very high power levels of distortion-free "tube swing" in this manner which is just how the higher power OTLs work. Of course, in that case, especially choosing single ended over push-pull, you quickly get into extreme "room space heaters" along the way with tremendous heat dissipation as a by-product.
However, as eluded to earlier, there is a way to level the power transfer "playing field" between transformer coupled power stages and those without transformers. We will discuss this attractive alternative in our next installment as we head toward the conclusion of our OTL design philosophy presentation!
THIS SECTION TO BE CONTINUED!