A direct-coupled, active-loaded, parallel-feed output SE 45 amplifier.
-Design by John Tucker, with contributions by Mike LaFevre, John Camille, and Kim Jenkins
“Actually the lineage of that amp goes back to a lunch at some crummy Chinese restaurant in Vegas in 1997, during CES. JT and I had been looking over some of the competition, and while waiting for our chow mein we put together a design on a napkin that we felt improved upon one or two of the more radical designs we had encountered that day. I think JT still has that napkin. “
-From a post By Doc Bottlehead on the Magnequest Forum, http://www.audioasylum.com/forums/magnequest/bbs.html
When musicians gather to discuss their experiences playing a particular work, the notes may be discussed with some anecdotal humor about notoriously difficult passages, but the real discourse is about tone.
Visit any internet site devoted to guitar, or piano or trombone. There are endless posts about what fingering or breathing technique best produces the tone, the sound of the music. Notice I didn’t write “classical” or “classically trained” musicians. I know from following around a rock group back in the Sixties that even guitar thrashers work more on the ‘sound’ they’re trying to achieve than getting all the notes right. After all, if both you and your audience are unable to read music, what’s the difference? Or perhaps we’re talking about Clapton, Townsend or Hendrix, whose fingers magically go where their Muse leads. Getting caught up in their technique, though, will cause you to miss the best part of the emotion that produces the music. It’s the tone that really tells the tale.
Now classical musicians have the notes in front of them, and early in their studies, they pass the goal of getting all the notes right. With that out of the way, they spend many years pursuing tone.
I would like to draw a comparison between ‘playing the notes, on the one hand, and ‘tone’, on the other with the idea of ‘detail’ and ‘tone’ in the re-creation of music by an electronic device. When you hear something new on a recording after an “upgrade”, you have left behind an inferior ‘player’ that couldn’t get all the notes right. When the pursuit of detail has finally been satisfied, it’s time to judge the tone of the device.
45 Out Of Nowhere
“Speakerman” called me up about two years ago and told me I had to go to Columbus and hear his new amplifier from a company called Yamamoto. I knew that Yamamoto had been shot down and killed somewhere over the Solomon Islands in 1943 so I was fairly curious. More so because the Speakerman’s house is a shrine to 70’s and 80’s audio equipment and it’s rare to find him with something new.
Upon arriving, I found him listening to John Lee Hooker’s Chill Out through Great Plains Audio Altec 704 co-axial’s, mounted in a pair of cast-off cabinets selected from his garage full of same; more for the fact that the drivers fit and the volume was about right than for any sonic attributes. I sat right down and immediately noticed two things: the sound of Hooker’s voice was absolutely magical and the amplifier was this little “plasticky”-looking thing powered by a pair of tubes resembling shrunken 2A3’s and a couple of mushrooms for drivers. Turned out the mushrooms were Western Electric 717-A’s and the power tubes were NOS RCA UX-45’s.
The magical sound I heard that day drove me to go after a 45 tube amp of my own. I couldn’t afford the Yamamoto so I bought the DRD 45 kit from Welborne Labs. It was disappointingly “dry” sounding to my ears which I attribute to the 6N1P driver tube. Others seem to love it but I quickly sold it and moved on.
Most of a year went by until I made the acquaintance of Steve Brown, who had moved to the area from Portland, Oregon. Steve’s one of those tube-heads who was fixing the neighbor’s color TV’s and building Heathkits when he was 14. The Pacific Northwest is a hotbed of DIY electronics activity and Steve seems to have built just about every tube circuit you hear about on the Web: amps, pre-amps, phono stages, everything. We’ve since become good friends and he has shared a great deal of his knowledge with me, patiently explaining how electronics work in a way even I can understand.
I came across the Simple 45 article and forwarded it to Steve for his opinion. Being a believer in ‘parafeed’ and constant current source drivers, he immediately liked what he saw. It was agreed that we would each build an amp as a joint project, which was especially fortunate for me as I could never have done it alone. Over a period of six months we discussed and began to acquire the various parts specified in the circuit.
We began with the electro-magnetic components: chokes and transformers. Steve already had on hand a pair of Magnequest EXO-03 chokes from another project and we both thought highly of the Magnequest line. I bought a pair for myself and we began to discuss output transformers. His favorite James Doddington transformers are all air-gapped and hence unsuitable for parafeed, likewise for the economical Hammonds which weren’t boutique enough for this project anyway, and so we went back to Magnequest. As much as I desired high quality, I was put off by the prices of the Magnequest iron one sees used in projects like the Bugle 45 amp that pops up all over the internet boards. I recalled seeing an ‘economy’ line of transformers from Magnequest called Robin Hood though and contacted Mike LeFevre, the man behind Magnequest, to see if there was a version that would be suitable.
Mike and I had several hour-long conversations in which he explained his ideas on transformer design, the merits of parallel-feed output, and what was done to keep the prices of the Robin Hood line low while retaining the essentials of high quality – things like channel frames for the windings and less exotic core materials. There are also fewer windings, which result in a bandwidth that goes down to 30Hz instead of the 20Hz achieved by the more expensive units. According to Mike, getting that extra 10Hz of extension requires the transformer to double in size. As desirable as that is, it allows the transformer to “loaf” at 40, 60, 80Hz, etc., the cost-quality trade-off yields a still very good sounding transformer priced significantly lower, just $99 each. I asked Mike what would happen to the price of these Robin Hoods if they were made from nickel instead of steel, and the answer was the price would get pushed up out of reach. Imagine my delight when they arrived and the laminations were “pin-striped” with some nickel lams at the $99 price. Mike turns out to be generous not only with his time.
I wouldn’t want anyone to get the idea that I truly understand the highly technical merits of parallel feed vs. conventional air-gapped output transformers; but I came across an article by Steve Bench on the ‘net that summarizes the difference:
“The “parafeed” (parallel feed) uses a large choke to provide the DC voltage to the anode (of the power tube, bold characters are mine). This choke must be large in value, and provides a relatively high impedance (a good thing) at audio frequencies. The output transformer is capacitively coupled to the anode (of the power tube)… The advantage of this kind of mechanism is the choke and the transformer can be individually optimized: the choke for saturation capability, and capacitance; whereas there is no DC in the transformer, so it can be made relatively smaller, providing both lower capacitance and leakage inductance with respect to its primary inductance. This is a long-winded way of saying that it is possible to achieve wider frequency response (especially in the bass which is why the Robin Hoods work so well).
There are four other advantages to this circuit. First, the capacitor inserted allows a low frequency “extension” … The second is that additional low frequency poles can be more easily controlled…The third is reduced “hum”. ..The fourth advantage of this circuit is since the transformer has no high voltage DC on it, the transformer can be replaced with an “autoformer” (a single tapped winding) allowing the output autoformer to be further optimized.“
In regard to the possible disadvantage of a large capacitor in the parafeed output, Mike Lefevre has this to say, ‘the power supply is also in series with the output and it has of necessity very large capacitors. An additional one that has the merit of reducing hum and blocking DC really isn’t all that bad a thing’.
For the rest of the components, based mainly on Steve Brown’s experience we chose a 47 microfarad Solen film cap instead of the electrolytic at the output of the power supply and Audio Note Tantalum resistors in the signal chain. I had a really good experience with building the Ella amplifier kit from Diyhifsupply so I took Brian Cherry’s recommendation of his Obbligato capacitors and we used an oil version for the parafeed cap. This is also where we sourced the tantalums and the Teflon tube sockets I insisted on to get as close as possible to the quality of parts in the Yamamoto amp that had started it all. All the wiring is soft-annealed pure silver, 24 gauge for everything but the ground buss, which is 14 gauge.
Steve’s West Coast experience came into play in regard to the Constant Current Source driver input. He custom-built boards with a much higher AC impedance than the original design for additional hum rejection and a far steadier current. These are both attributes of using CCS instead of merely a resistor and his Gary Pimm-influenced design rally delivers the goods.
Another revision of the circuit came about due to the solid-state full wave rectifier failing under load. Prospective builders of The Simple 45 should learn from our experience. The turn-on power surge goes over 500 volts DC! That took us from the solid-state diodes to tube rectification of the power supply using a 5AR4 rectifier tube. The beauty of a tube rectifier is that full power doesn’t reach the circuit until the filaments are fully warmed up which happens gradually. Solid state diodes are “instant on”. The cathode resistor is also way under-specified and it too failed. We replaced it with a 50 watt aluminum housed part. Such are the vicissitudes of DIY amp building and much is learned from the process.
It’s important to note that this is a “direct-coupled design” amplifier. There is no protective capacitor between the driver and power tubes. Opinion varies on a lot of aspects of amplifier design, but direct-coupling is fairly well acknowledged by all as better sounding. Greater care, however, must be taken in the circuit design to keep destructive current from reaching the output.
One of the few ideas I could contribute to the design was that I wanted to have a large chassis with a symmetrical layout. I had a picture in my mind of the aesthetic of the finished amp to match the 1930’s vintage of the UX-45 tube. Steve and I also spent a great deal of time moving parts around on a 12”x17” board to get the layout just right in electrical as well as aesthetic terms.
In this picture of the first, solid-state rectified build, you see that none of the electro-magnetic components are oriented in the same direction. The large plate chokes (foreground) and output transformers (background) attached to the sides are at different angles to each other and to the power supply chokes in the center. Each of these emits strong electro-magnetic fields. The orientations you see here keep these fields from interfering with each other. We also wired the amp in three dimensions to maintain more ditance between the power supply wiring and that of the actual signal. If you follow the wiring closely you’ll see that the all the wires from the power transformer hug the chassis plate and those for the signal ‘go vertical’, up and away with the two output cables in the center way up in the air. This keeps AC power hum from polluting the signal and showing up at the speakers.
I put together a plywood chassis to test this design and the amp was first played utilizing it. It was at this point that things began to blow up.
The 1/8” thick top plate was sourced from Online-metals in aluminum, which I drilled for the numerous component holes. Keeping with the aesthetic and having an aversion to legions of round screw heads protruding, I used pan-head socket cap screws and countersunk all the attachment holes so the screws are flush with the top plate. That was tedious! I then took the plate to a local powder coating shop that applied the black powder coat for a very reasonable $25. The panel meters were merely a cool-looking touch that didn’t make it to the final design (they display output current).
I’ve always liked the look of Cherry wood and black trim so that was my original idea for the chassis. However, while I was applying various dyes and finishes to test pieces without very good results, I happened across a nicely figured Padauk board. I tried a wash coat of the amber shellac I had intended for the Cherry and was amazed at the result. The grain and color seemed so rich and alive! I hope you’ll all agree.
Listening To The 45
So now we had a finished project. How does it sound? Well, on first firing it up in Steve’s basement listening room, I looked at him, and he looked at me, and we were, as Steve later described, ‘grinning like idiots’! The bass was so articulate, the mid-range so rich and the air and extension in the treble was everything we had hoped for and more.
I couldn’t wait to play Chill Out to see if there was some of that Yamamoto Magic. Oh yeah! The unparalleled linearity of the 45 tube (1930’s National Union) together with the bandwidth of Mike Lefevre’s Robin Hood transformers and a touch of romance from Brian Cherry’s Obbligato oil caps combined to bring a life to all the music we played like I have seldom heard. In use now for about six months, this amplifier has demonstrated repeatedly, with diverse source components and speakers, that there is no detail in the music that escapes notice. It gets all the notes right and allows the listener to be drawn into the performance by the tone of the instruments and voices being portrayed.
I’ve been listening to a lot of Mark Knopfler lately, the Dire Straits Brothers In Arms, re-issued in both SACD and 180 gram vinyl, and All The Roadrunning, with his sweetheart (and mine), Emmy Lou Harris, also on 180 gram vinyl. Besides the participation of Sting on Money For Nothing and the well-known thunder on the Brothers In Arms cuts, I particularly enjoy the emergence of the percussion and wooden flute from a totally silent background on Ride Across The River. The way the Simple 45 portrays this passage is eerie and almost chilling. The husky quality of Knopflers voice on the soft passages there in his lower register is also noteworthy (pun intended) and allows a comparison with same on All The Roadrunning. With the Simple 45, you can clearly hear the passage of time between the two works. That huskiness is far more youthful sounding in the former and a fully mature voice in the latter.
It also doesn’t hurt that I have perhaps the best speakers on the planet, The XLH Audio 1812’s for the 45 amp to drive. The number one reason the UX-45 tube isn’t more widely used is that it outputs a mere 1.8 watts. The fact that less than two watts drive these 7 foot tall, 350 Lb. speakers to room-filling levels indicates two things: first, amplifier power is vastly misunderstood, and two, the 1812’s unique crossover and blend of transducers presents an easy load for a tube amplifier to drive.
In the months since then this amplifier has served to bring out differences in sources, cables and tweaks previously unheard or merely hinted at with other amplifiers, all the while remaining musical and fun to listen to. It’s not just ‘proud Papa Syndrome’ either. Everyone who has heard the Simple 45 has immediately been taken with the sound. As other amplifiers have come and gone in the review process, which include some great names and designs, it’s always a pleasure to come back to my reference amp and have no sense of loss. It has all the tone musicians strive for.
Doc Bottlehead and John Tucker may have had second-rate Chow Mein, but the design they wrote out on the proverbial cocktail napkin is truly haute cuisine! Of course, my pal Steve’s significant modifications have improved it to the point that it really needs another name. It’s not quite that simple anymore.
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