A U D I O... P A P E R


A SINGLE ENDED TUBE AMP FOR THE MASSES

by Steve Deckert
August 1996

A design log that was written during the development of the
Original Zen Triode Amplifier

 

August 1996

High Fidelity Engineering (Decware) has a clear focus that revolves around preserving the forgotten past of hi-fi in the 1960's before it took a major step backwards from the perspective of sound quality. By that I mean that since the invention of non-discrete IC based solid state receivers, over all sound quality has declined when compared to the older tube counterparts. Believing this gets harder and harder with each generation. Kids don't know what records are, and they think tubes are light bulbs. And quite frankly it is completely impossible to relate to the kind of sound quality I'm talking about unless you've been exposed to it. Kind of like a man trying to imagine what it must be like to be pregnant and give birth, something he'll never know... at least not in this lifetime. 

If you're the average person with a stereo - even a fancy new surround sound stereo then you enjoy the company of approximately 98% of all the people with stereos... and I'm sorry to say that presenting this amplifier in a way that you can grasp (because it goes against everything you know) may be no less complicated than a pregnant man.

The passion behind this idea for me comes from the desire to expose the common man owning the common stereo - to the magic of triodes, with a secondary objective of messing with the common audiophile who believes specifications are the 11th commandment. Someone should have done this long ago but because of the unorthodox thinking required to pull off such a feat I don't suppose other companies wanted to take the risk. For example if I tried to market a 6 watt amplifier with around 10% distortion who would take it seriously? 

An entry level "audiophile" amplifier that would be affordable to the masses has been my dream for many years now. To find a way to make it affordable and make it appeal to the average person is obviously the difficult part. The passion to do so comes from past personal frustrations of not being able to afford what I want and not being able to enjoy what I have because I've heard what I want! (been there haven't ya!) I can assure you most of us have a hard time justifying the crazy amount of money required to obtain a true high fidelity system.

The general perception of an audio system to the masses is pretty much anything you could find at a "digital ready" electronics supermarket. Well guess what... the masses all have ears as good as any audiophile and the ability to appreciate the same things that result from high end listening. Those who've never been exposed to three dimensional high fidelity can hardly be chastised for not appreciating the difference between the two, or for not being aware that there is a profound difference.

The science of sound and harmonic resonance is far and away the most complex and least understood science on earth in my opinion. I would say that the monumental joint effort of all the people and the equipment required to send up an Apollo mission would be simpler than building a perfect playback system. It takes significant effort in design and a significant quality in parts to achieve good sound. It should be pointed out that the HIGH END audio market is oppressed with the monumental and expensive task of improving the high fidelity playback system we commonly refer to as the "stereo" and that is why a respectable stereo system costs between 15 and $30,000.00 Gee, that's about the price of an automobile!

September 1996

My objective with this design is to get the absolute highest quality... harmonically in-tact sound in the hands of the wanna be audiophiles. My research and experience over the years has taught me that tube circuits are the way to obtain this objective. If I were designing this on paper with specifications in mind I would have chosen solid state circuits. I really prefer the natural sound of tubes and while I have heard some solid state amplifiers that I could enjoy, they start at about $7500.00. The market is SATURATED with solid state gear and as I said, I don't feel it is the format necessary to complete this goal. 

Up until this summer I have been building Class A1 push-pull tube amplifiers of various designs with the intention of marketing one as the entry level amplifier. The part I've been wrestling with (like all small manufactures) is keeping the cost down and yielding terrific sound quality. Success usually seems to go to those who have achieved the correct balance of compromise (understanding of course that everything in audio is a balance of compromise). 

The push-pull tube amp designs seem to be the most popular because they can be fairly inexpensive to build, have plenty of power, and sound quite good when compared to mid-fi solid state gear. I really thought the answer could be found in a push-pull design for those reasons. 

This summer while playing with single ended tube amplifier designs for my own personal stereo, I stumbled into some results that forced me to take a real second look at exactly what are watts? You know you read these adds for those five watt triode amplifiers that start at around $5,000.00 and go way up from there and wonder why would anyone pay that much for such a thing if it only has a few watts? 

Lets just say some enlightenment from the audio spirits came upon me and it was enough to realize that at a normal listening level most of the musical content can be found in the first magical watt of power. I then realized that the priority in high fidelity reproduction should be focused on that first watt. I have been trying to truly understand this for some time now, and in particular been trying to define in my own research the reason why solid states watts seem less that tube watts to the ear, yet equipment measurements would indicate that they are capable of achieving the same amplitudes. 

Solid state stereo gear has a tendency to sound thin, and quickly run out of headroom (clip) when pushed. Tube amps are very different. If you compare a 40 watt tube amp with a 100 watt solid state amp or receiver, the tube amp will put more music in the room, and get louder every time. You will find that at nominal listening levels the loudness button is needed to get the solid state amp to sound full-bodied, yet the tube amplifier sounded warm and full with a dead flat signal. This is a great example of how "watts" are not "watts" and a prelude to a secret only the most advanced audio gurus will share / and that is that specs in audio gear mean nothing. 

Why is it that a 10 watt musical instrument amplifier such as a guitar amp will in real life (and on stage) get loud enough to split your brain in half, yet it seems to take mega bucks and major stereo gear with 100's of watts to reproduce the same sound in your living room, a room that remains a fraction of the size of a live performance? Perhaps it's because everyone is going about it wrong, confusing convenience with performance.

October 1996

Enough rattling on... Two weeks ago I completed prototyping the circuit for a single ended low power tube amplifier and have been listening to it ever since. The schematic is at the top of this page. At this stage I have already decided that this will be the chosen design for the project. The actual cost will decided by the cost of the output transformers. One of the main reasons single ended tube amps are so much higher in cost than their push pull counter parts is that the output transformers are completely different. In a single ended design the output transformer must be designed to handle the DC current at the bias point, so a special transformer must be used. It features an air gap that optimizes the coupling at low frequencies and the DC current that serves to lower the permeability of the core. Without the air gap, the iron will saturate under too little DC bias to accommodate the needs for a single ended triode. Too much gap will reduce primary inductance so that the lowest frequencies will not pass without attenuation.

All other things being equal, the output transformers (or IRON as I call it) has the final say in the resulting sound quality. When you're looking at different iron for a design, you find that the standard push-pull output transformers range in price between $75. and $400.00 ea. (per channel.) and you find the single ended output transformers range in price from $150. to 1200.00 ea. That BTW is why the single ended stuff is so expensive. 

I have a 50 watt (ea. channel) tube amplifier that I built up to a reasonably impractical extreme and I have been using that as my personal reference piece. It powers an efficient pair of speakers in my main listen room (over 90dB) and in all honesty (with feedback off) sounds better than any other push pull amplifier I have compared it to. In fact a version of it was my original idea for this project, and has been for years because I liked the sound so well..

My specific design goal has wavered a bit in the past months as I get ready to do this. If I am going to market a tube amp to people who have never had the joy of listening to one, and given the solid state, cranked up with the loudness on and tone controls engaged listening habits of those people, how should it sound? Do I go for power so it will stomp their past systems, or do I go for pleasure so it will reveal to them the inner levels of music. In other words, do I give them what they already have but just a lot better, or do I give them an opportunity to discover a magic in music that they are unaware of. It really gets into a psychology issue, one that I have pondered for almost 10 years now. From a business standpoint I would make more money with the prior.

This is what happened... to completely solidify the decision. My new little single ended amplifier will run in either Pentode or Triode mode. In Pentode it benches 5.7 watts, and in Triode it does 1.8 watts RMS pure class A per channel. Because of the front end, and additional gain stage in the design it is possible to get louder than you could ever believe is possible with 5.7 watts. Anyway, ever since I switched the little guy over to triode mode, I have not had any desire to switch it back. It is the most natural real sound I have ever heard in this house. And the eerie thing is that it achieves a nominal listening level high enough to be exactly the same listening level I have been accustomed to. that's 1.8 watts Vs. 50 watts which is the same as a hundred watts or more in a solid state receiver. The enormous improvement in quality has made this new little amp my full time personal listening amp. My good ol' favorite just got bumped. As for the psychology issue, I will be going for the magic. 

In my observations over the years the reason people turn the volume up to the levels they do, is to gain the effect of physically feeling the music. This effect is the motivating reason for turning it on, and this effect is the ONLY effect that the quality of equipment has to offer so it's no wonder. Once you have spent an evening with premium gear like this little triode amp, you find that the physical effect you were accustomed to happens sooner and at lower volumes because the even order harmonics are in-tact and free of odd order harmonics found in solid state circuits. Then the big one hits you, another more profound EFFECT happens in addition to the physical effect - emotional effect. So you have one that strokes your body, and one that caresses your inner self adding a new found joy to the experience of listening to your stereo.

November 1996

Having my strongest talent in speaker design I have thought long and hard about the statement: "Your speakers have the most effect on how your stereo will sound - replacing your speakers first gets the most improvement." Being a speaker designer the temptation to accept that has always been strong, however I have been slowly and consistently disproving that to myself year after year. 

It is in my opinion the amplifier that makes the most difference in the sound of your stereo, and then your other electronics. I can safely say that your speakers are probably the least contributors to the sound rather than the most if your listening to good tube gear. It is not however usually NOT true when listening to mid-fi solid state gear. If you gave me a choice of a good tube amp and a pair of Bose speakers or a mid fi solid state amp and a $24000.00 pair of Wilson Watts, I would choose the Bose and the tube amp because it would sound better. I know blanket statements like this are hard to digest, but I believe it to be true from my own experience. 

There are things in this design that some engineers would whine about, perhaps lots of things, but I did what I did... to achieve the desired sound. Audio is so much more complicated than a square wave response on a scope. I feel a significant part of the complexity can be found in the topology of the circuit itself. Understanding that all matter in the universe resonates, including conductors such as wire, adds an exiting new level of depth to it. I should imagine a circuit would not be unlike a piano with all of its notes. Combining the right ones (based on their fundamental tones (resonance) in a way where the harmonics of those resonance's are complimentary determines how pleasing the outcome is. Since on the bench there is no real way to measure molecular resonance or see its effect on the flow of electrons it is impossible to know exactly what is going on. If we could become electrons and jump in for the ride, how smooth would it be? Would it be violent? How many times would you hit you head? And when we both popped out the other end of the circuit at the approximate same time would one of us be battered up and the other one no worse for the wear? And if so why? These are the things that intriguing me. Now that you have an idea where I'm coming from, I will spend the remainder of this log going over the design itself with the intent of helping anyone who cares understand the reasons why I did the various things I did with this design. 

 


 

 A SINGLE ENDED TUBE AMP FOR THE MASSES

Part II - The Design

 

November 1996 

If your not an amplifier builder this may become laborious reading after awhile, but initially you should understand that the audiophile's fascination with sound stage and imaging (the motivating factor for all tweaks) is well supported in this design. I have found that the determining factors that map your systems sound stage and control imaging are mostly if not completely governed by impedance matching. Finding the exact right setting on your pre-amp volume control for best sound could be an example. When you break-up a pre-amp and amplifier into their respective gain stages and begin to play around with the input and output impedance of each you start to see why no two combinations of amplifier/pre amp will ever sound the same. This is the reason for the large concern about finding "good matches" in equipment and why some good gear sounds bad. To all "miserable" audiophiles that have found out the hard way that money alone can't solve the problem, I would say in fact that this is the problem.  

This ability to tweak is what I feel it will take to win the hearts of the serious audiophile and what it will take to be compatible with the expectations of the solid state conditioned masses. This is not because it has millions of bells and whistles (and it doesn't) but because the operator will be able to "discover" the best sound by changing the configurations of the amplifier. It makes sure that whoever buys it is sheltered from the alternative which is taking your chances with odds of getting just the right impedance match between all of your components.  
 

 

The schematic above is what I have prototyped and am currently listening to. The final version will be very similar to this. As you can see the amplifier has 3 gain stages, or optionally two, depending on where you select to have the input go to. In the two stage (direct) mode, the single passes through one resistor and one capacitor the tubes and the output transformer. I'm sure you'll find merit in this "less is more" approach when you listen to it. The additional stage making this optionally an integrated amplifier for all practical purposes gives additional impedance balancing and more gain so that you can listen to it at louder volumes.  

The tubes I am using are a 12AU7 and 12AX7 for the first two stages into a pair of 6BQ5's (EL84's) for the finals. I chose the single EL84 because it is a scaled down version of the popular EL34 but in my opinion has a better sound. The single EL84 uses less current which is making this amplifier a cost effective reality to build. I think it has better sound than larger tubes because the plates are small and tightly packaged around the screen grid meaning there is less distance for the flow of electrons between the two. The other tubes are perfect for this design because they are the most popular of their kind and the easiest to find. This means the guy that buys it can play musical tubes by trying several different brands without spending any real money. 

The biggest feature will be the switch for changing the amplifier operation from Pentode to Triode. If you need more output, and like you music to sound a little analytical with better specs you can run it in pentode. If you want to here a slightly warmer more pure musical sound with less power switch it to Triode. Obviously there are several combinations you can get from combining the features of this amplifier each with it's own sonic signature. You surely will be able to find one that you love.  

The outputs on this amplifier will run most any impedance of loudspeaker, 6 to 8 ohms being ideal (at least with the current iron I plan to use). The outputs can bridged by strapping the positives of both channels together if you happen to have two of these and wanted to run them as mono amplifiers, one for each channel. 

 

 


 

A SINGLE ENDED TUBE AMP FOR THE MASSES

Part III - The Chassis

 

 
"If you're going to market this thing so that it would appeal to the masses, it would be mistake to think the potential buyers are going to part the living room with it like the Red Sea... Nope, that only works when you live alone! 
 

Well I have pondered the chassis design for some time now. Not only from the perspective of cost and appearance, but from the resulting topology of all the parts. Things like heat and shielding become large considerations not unlike where the thing is intended to be placed in the room. Do you make it like a conventional tube amp and plop it down on the rug, or try to cram it into a more standard chassis like a 19" rack mount unit. 

A unique opportunity with this little amp lies in the simple fact that it is so small. The 6BQ5 output tubes are not much larger than a normal pre-amp tube, and have nowhere near the heat of a larger output tube. Because of this and the size of the transformers I could build this into a nice rack mount chassis the size of an average pre-amp (3.5" high).  

Okay, how to give the owner of this amplifier the satisfaction of hypnotically staring into the golden glow of the tubes... you know that's a must - too many people talk about it. I happen to be one of them. In fact I'm not sure sometimes if it's a vice or a gift, but I have a real hang-up with how things look. What followed was the all too familiar, slightly out of phase, molting period of several days when I walk around in circles a lot. Two days ago while I was meditating to music, (I was really enjoying some "FRESH AIRE" on American Gramophone being played on this amp) it hit me like a Zen Lightning Bolt in the forehead! An image of exactly what it should look like, how to lay out the entire parts topology, and flashes of it sitting in many different listening rooms blew into my head. I love it when that happens!  

From that image I did this conceptual drawing of it exactly as it appeared. The picture can be seen above. The ventilation will be through the top and bottom center of the chassis. The tube topology will be horizontal opposing channels with the rectifier centered between them in the back of a shielded "room" inside the chassis. I'm not sure how cost effective this chassis would be, but I will defiantly be building some.  
 

 

 


 

 A SINGLE ENDED TUBE AMP FOR THE MASSES

Part IV - Personal Log

 

Feb. 1997   

I hoped it would be ready by now, but you know how that is! Currently I am waiting for the output & power transformers I requested that all transformers be hand wound by one particular man, (one of the few older fellows still alive doing it) and am still waiting. I believe it will be worth the wait.   

Last night (this is a classic example) a gentleman stopped by with his ADCOM amplifier to hear what it would sound like matched with a tube pre-amp. He had the most expensive and largest amplifier they make. It looked very nice. We listened to it for a half hour or so and he was impressed with the idea of a tube pre-amp. Between you and me, it sounded fair with the a few shortcomings of being a little dry and a little thin sounding unless it was really cranked. I don't remember how much power it is, 500 watts per side or something.   

As we finished, he asked what that pre-amp on the floor was like. He was pointing to the SE-6BQ5a. I told him that was an amplifier, currently running pure class A triode with a 1.8 watt output per channel. I went on to explain the whole thing to him. Well of course he wanted to hear it so I hooked it up where his Adcom had just been. The bass was full & rich, and everything sounded better.   

It was having no problems maintaining the 90 dB playback level we had been listening to and in his own words, besides saying he couldn't believe it 42 times, he said "At this playback level my Adcom's got NOTHING on this!" He was right.   

I am very excited about this project! With the perfectly flat hand wound output transformers, silver wire, and 100% polypropylene power supply there are very few amplifiers that will sound as good. The only ones that would stand a chance are other single ended triode amps costing as much as a new car.   

 

March 1997  

While waiting for my transformers, I have had some time to re-analyze my thinking on all aspects of this design. The only thing that is certain is the circuit itself. Parts and layout are still open to review. Certainly for the purists and seasoned audiophiles I am on track with my 100% poly caps in the power supply and signal path. Make no compromise on parts quality, including wire and solder and lets see how good- good can get.   

Perhaps if I'm going to make references to "Zen" in the design of this amp I should pay some attention to it. An interesting thing has happened this past month while waiting for the transformers. I was commissioned by a studio in New York to build a tube equalizer that would warm up the recordings. It would be the last thing in the signal path just before the master deck. They wanted that enchanting "tube sound" in their final product, the recordings.   

I took on the project with the approach that I would give them some basic tone shaping in the bass and treble area nothing else. I figured if they need to EQ it more than that there is something wrong elsewhere in the signal path. This being the case, the front end of the SE6BQ5 circuit would work perfectly. So would the basic chassis. This would give me an opportunity to explore some variations of the project, and let me really analyze the sound of the front end as a separate pre amp. This also gives me an opportunity to try some different twists on the power supply.   

Thinking about marketing and other non Zen thoughts I did a really stupid thing. I built the full blown power supply that goes in the Zen amp for this pre-amp project. I also made some changes to the solid state rectification using diodes and very large filter and B+ caps. Here is a picture of that power supply. A little overkill ha?   

My thought was build a solid state rectified duplicate of the tube rectified power supply I would be using in the Zen amp and see which I liked better. The problem was that I had a B+ of 435 volts and only needed around 270v. Not wanting to introduce solid state regulation (the natural solution) I tried to wrestle it down and work with it. The result was a poor signal to noise ratio that was audible.   

I solved this problem by coming back down to earth and just building the proper supply to do the job. I chose two 300 ma transformers, one to run each channel. I ran a full wave bridge off each one through a one ohm resistor, followed by three sections of filter caps. This gave me what I needed to accomplish a true dual mono unit. Why not right? I did the same thing with the heaters, using one large 100,000uf 20v cap on each side. Now I had a dual mono supply with dual mono DC filament supply and no solid state regulation. This is an obvious improvement over the first circuit.   

The next interesting development was the way I immediately dumped the very best poly caps I had (combinations of Multicap and Rel caps) for everything in the signal path. I let the unit burn in for a week and was surprised to hear it sounding as good and actually a little better in some ways than my pre amp. This is my own interpretation of the "Last Pass" for the popular Dynaco tube pre amp.   

The result was that the pre amp was "too good". In other words it was very very very fast, extremely revealing. On my high end system it sounds wonderful. Substitute a normal CD player, or mid-fi solid state power amp into the equation and the sound (for me anyway) was unlistenable. One of the secrets I know to be true in all musical sounding systems is balance. All pieces of electronic gear in the system must be of equal speed and quality regardless of the value.   

Since I wanted to accomplish a "tube sound" in a solid state signal path of a recording studio, this wasn't going to work. I needed something that warms, soothes, which translates into subtle filtering. Something that wasn't so fast and so transparent. To do this I removed all the high end caps and installed normal mylar caps very similar in sound to original caps used in the 60's. I also changed the resistors back to 5% metal films. Part of me thought I was really crazy at this point.   

With great anxiety I took the reworked unit back into the listening room and plugged it in. Get this... I was actually shocked when music came out, and then had to quickly remind myself that of course music came out. Just shows how brainwashed we get by listening to everybody else. I expected the sound to be so pathetic that it would justify my original implementation of the good caps. And it was... for the first excruciating 30 minutes... while the caps seated. But then guess what?   

The stupid thing sounded warmer, richer, and the highs were nice and sparkly and most important, the sound stage was still there! Hmmm. Okay, we can fix that, bring in the Sony power amp... and that 200.00 CD player over there. Now we have a mid fi system on high end speakers, a cripple from the start. Well, that was the best I had ever heard that gear sound. Gee, it's working, this thing is making flat dry nasty stereo gear jump back into the 3D high end sound stage. The difference is rather amazing.   

This means that the secret to a musical Zen amp for the masses is not going to be found in the high end caps. (A rather hard pill for me to swallow btw.) That means I am going to have to build it exactly like I just did this pre amp and that means I will have to make some "Silver Edition" units with the originally intended parts for those audiophiles who have the gear to back it. Consider the conceptual drawing I did of the chassis to be what the Silver or Special Edition model will look like if and or when I decide to sell some.   

Meanwhile, staying focused on the original intent of this design I have elected to go with a more straight forward look. And because of the success I just had with the studio's pre amp, I am going to offer two products. The Zen amp as intended, and a pre-amp. This way those of you who have the larger amplifiers/less efficient speaker thing happening can benefit from the corrective qualities of this pre amp. In other words we can help more people create a musical sounding system. See how things work out?   

Here is a picture of that hand wired pre amp.   
 

 



August 1997 

The iron samples have finally come in and I am very pleased with them.  Now all that remains is to build the little guy and see how it compares to the prototype. 

I've made all the decisions needed to build the first pre-production unit and have completed it.  I kept the layout symmetrical so the amp looks good. 

June 2001

Well, what a rush! This is the first time I've gone back to this page since August of 97. The Zen was optimized for Triode use only after the first 25 units were sold. Shortly there after the tubes were upgraded to SV83's and 6N1P's and it has remained basically unchanged since then. We have hand built over 600 of these amplifiers. In fact we have developed an entire line of Zen Triode Products along the way. Many Thanks to everyone who is supporting us by enjoying these magic little amps!

 

Steve Deckert

 

 

 

NOTE: In January of 2008 Serial #1000 of the little gray amplifier (Model SE84C) was built and shipped. The chassis was retired but the amplifier lives on in a slightly larger and significantly heavier black steel chassis with a front mounted gain control and 2 sets of inputs.  The new model is called the SE84C+.  A history detailing the revisions from the first zen to the model pictured above can be found in the articles section of the web site.  At the time this article was written I never dreamed we would have hand built and shipped 1000 of these, not to mention a few thousand more amps that we designed based on this concept.  Just goes to show I was on the right track, and it remains our top selling amplifier today.

 

 

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