Lot of solid-state in there, probably not my first choice.
300b lovers
I have been an owner of Don Sachs gear since he began, and he modified all my HK Citation gear before he came out with his own creations. I bought a Willsenton 300b integrated amp and was smitten with the sound of it, inexpensive as it is. Don told me that he was designing a 300b amp with the legendary Lynn Olson and lo and behold, I got one of his early pair of pre-production mono-blocks recently, driving Spatial Audio M5 Triode Masters.
Now with a week on the amp, I am eager to say that these 300b amps are simply sensational, creating a sound that brings the musicians right into my listening room with a palpable presence. They create the most open vidid presentation to the music -- they are neither warm nor cool, just uncannily true to the source of the music. They replace his excellent Kootai KT88 which I was dubious about being bettered by anything, but these amps are just outstanding. Don is nearing production of a successor to his highly regard DS2 preamp, which also will have a unique circuitry to mate with his 300b monos via XLR connections. Don explained the sonic benefits of this design and it went over my head, but clearly these designs are well though out.. my ears confirm it.
I have been an audiophile for nearly 50 years having had a boatload of electronics during that time, but I personally have never heard such a realistic presentation to my music as I am hearing with these 300b monos in my system. 300b tubes lend themselves to realistic music reproduction as my Willsenton 300b integrated amps informed me, but Don's 300b amps are in a entirely different realm. Of course, 300b amps favor efficient speakers so carefully component matching is paramount.
Don is working out a business arrangement to have his electronics built by an American audio firm so they will soon be more widely available to the public. Don will be attending the Seattle Audio Show in June in the Spatial Audio room where the speakers will be driven by his 300b monos and his preamp, with digital conversion with the outstanding Lampizator Pacific tube DAC. I will be there to hear what I expect to be an outstanding sonic presentation.
To allay any questions about the cost of Don's 300b mono, I do not have an answer.
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Here is Eleckt schematics: http://4tubes.com/2-SCHEMATICS/BY-BRAND/ELEKIT/TU-8600/TU-8600R-Amplifier-300B-Schematic.jpg Global negative feedback, parallel 12au7 driver, 12ax7 input, semiconductor bias control and power supply stabilizers. chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/http://4tubes.com/2-SCHEMATICS/BY-BRAND/ELEKIT/TU-8600/TU-8600R-Amplifier-300B-leaflet.pdf |
I second what Don just said. Building a kit is far more rewarding than messing around with cables or component-swapping. You get to hear for yourself what XYZ capacitor sounds like, instead of reading random comments on the Internet. I can’t emphasize enough you just can’t trust what people say on the Internet ... but you can trust your own perceptions, first, last, and always. It’s super educational, and focuses your attention on tuning the power amp, which is where it should be. Likewise, if you build your own speakers, you get to tune your own crossover. This is where the big payoff is, and it trains your ear in what to listen for. Amp and speaker tuning set the sound of your system. Find out how to do that, and you are most of the way home. |
For someone looking for an entry point to 300b single-ended sound, the Elekit is a good way to go. I liked what I heard at a show. Equally important is how helpful and frank and honest Victor Kong (VK Music) sounded when one talked to him. He did not exaggerate what the gear can do and shared information on limitations, design compromises and the like. I have more confidence in gear when I know that the person behind it is trustworthy. |
@alexberger Since an SET is a constant current load insofar as the power supply is concerned, it won't make much difference if you use tube or solid state rectification. But you might want to consider what happens when the AC line voltage changes. The filament of a tube rectifier cools off when the AC line goes down- so the B+ voltage drops more than one might expect since the rectifier gets less efficient. . Plus you'll find that they each have a 'sound'. If you set up HEXFRED rectifiers properly they are about as neutral as it gets. FWIW we run a separate power transformer for the driver section in our OTLs. We did this so as to prevent any modulation in the output section power supply from affecting the driver. This reduces IMD. Our OTLs run class A2 and have dual output section power supplies so modulation of the supply could be a concern. In an SET a separate power transformer for the driver isn't going to have the same effect since the power supply for the B+ should be nice and quiet anyway. The separate transformer would be useful if you planned to direct couple to the power tube with a cathode follower though. |
Hi @atmasphere , I will use this rectifier for the driver and the input stages. So i don't need a very high current and power capability. |
@alexberger You might consider HEXFRED rectifiers. They are ultra fast, ultra soft recovery and so are less prone to 'diode noise' (which is actually an interaction between the capacity of the diode junction and the inductance of the power transformer), lower than even Schottky diodes. One tip - unlike silicon rectifiers, the maximum current rating of a HEXFRED cannot be exceeded even for a few milliseconds- there's no 'surge' rating. But they come in some pretty robust current ratings- the smaller ones are typically 8 Amps. There are 1200V versions too. DHTs like a 300b do not have a cathode so cathode stripping during warmup isn't a thing. |
Hi @donsachs , I have a question. I bought and received a power transformer for the driver tube's power supply. This transformer has a central tap for the rectifier. I also bought 4 schottky diodes. What type of rectifier will sound better: a central tapped full wave rectifier or a bridge rectifier?
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Yes there are current production 45 tubes, but they are ridiculously expensive. As soon as Linlai decides to make one it will be much more reasonable and quite good I am sure. You can still get some of the TJ full music ones at about $350 a pair if you know where to look. I am sure they sound pretty good. There is no reason you cannot buy a quad of modern production for $700 or less if there was another source. Even so, if a 6V6 will sound pretty much as good for $50-100 per quad as a driver, then I would probably stay there. For a preamp tube... then maybe I would run a quad of 45 tubes, but I hesitate to design a preamp around tubes that are $700+ per set. |
@lynn_olson As best I can make out, Sophia and EML both make a type 45. Good news (though not exactly affordable) From the EML website; 45 years after the last American tubes of ’Type 45’ were made, EML was the first company, to take this tube back in production again, in the year 2001 |
I thoroughly understand your rationale with preference for current production tubes. The 6EM7 is inexpensive and lasts me 10-12 years with heavy usage. As mentioned before, I was just picking the brains of you and Lynn. I have zero doubt that the new 300b push-pull amplifier you two have developed sounds magnificent! Charles |
@lynn_olson As best I can make out, Sophia and EML both make a type 45. |
I am sure it is a great amp. Looks like my usual sources have 6EM7 for $15 a tube or so. If you love the amp, you probably already have bought a few for the drawer. If run conservatively I would expect them to last a long time, so a few spares will set you up for many years of listening. As Lynn said, I just cannot design an amp around extinct tubes. The exception are the various damper diodes and gas VR tubes. But there are tons of these out there for under $10 each and they last many years. When we ramp things up we will source a mass buy. It is always risky to design gear around extinct tubes... a delicate dance. You certainly don't use rare tubes. I think your 6EM7 is reasonably available so you should be good for years. |
Thanks @lynn_olson Here is a bit of information/listening impressions of the Roger Modjeski 6EM7 amplifier from a past Audio Circle thread. |
The tube Don, myself, and many other manufacturers would like to see go back in production would be a 45. NOS examples are astronomically expensive now, and surely production costs would be similar to, or less than, a 2A3. It’s basically a very simple tube, unlike a 6SN7 or a 6EM7, although "simple" is still plenty expensive relative to capacitor or transformer manufacturing. In the meantime, we’ll be using triode-connected 6V6’s, which operate in the same power range as a 45 (275 volts at 32 mA = 8.8 watts), have the same (triode-connected) Rp = 1800 ohms, and have excellent performance. Many choices with NOS and current production.
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Good, although brief, discussion of the 6EM7 on Thomas Meyers’s Vinyl Savor page. It looks like a TV tube, not designed for audio, so NOS examples might be somewhat variable (the application in the deflection circuits of a TV would only have moderate requirements for linearity). No current production for obvious reasons ... vacuum tube TVs disappeared fifty-five years ago, along with the tubes that went into them. But there’s probably plenty of old stock. Analog vacuum-tube TV’s, particularly the inexpensive B&W models, had pretty bad picture geometry. Once integrated circuits took over in the early Seventies, picture geometry got a lot better, and all of the many different service adjustments went away. NTSC and PAL color TV is a lot easier when all the complex signal processing is inside a single chip, instead of several tubes with many adjustments. Collectors prize old 21" round-tube color TVs (1955 to 1967 vintage), but they are not easy to keep running (with many adjustments and 26 to 28 hot-running tubes) and CRT refurbishing services went out of business about ten years ago. Would I use a TV-only part in a new design? No, I would not. Zero chance of LinLai or JJ putting it back in production. |
Hi @alexberger Understood for certain. Given the extensive backgrounds of Don and Lynn I was just curious if they had any hands on experience with the 6EM7, that’s all. Charles |
Hi @charles1dad , I am not a tube amplifier design guru like Lynn, Ralph and Don. But here is some information about your amplifier. https://www.audioasylum.com/cgi/vt.mpl?f=set&m=75278 The 6EM7 is a very strong tube for the driver with a very low output impedance 750 Ohm. The interstage transformer is 2:1. So on the one hand, it requires double voltage swing from the driver tube but on the other hand the bandwidth of IT in high frequencies will be twice wide and driver current twice as big. As I understand, 6EM7 can give 300 volt peak to peak output, that is good enough. I don’t know how 6EM7 is compared to 6L6 in terms of linearity and sound quality and signature. |
What makes the 45 or 300B "hard to drive" is that it is good practice to have the majority of amplifier distortion in the final power device, not the driver. In other words, the driver should be cleaner than the 45 or 300B ... which are the lowest distortion tubes ever made. In a feedback amplifier, the source of the distortion doesn’t matter much ... the feedback sweeps it all away. Which is why the substitution of the higher-distortion 12AU7 for the lower-distortion 6SN7 in the mid-Fifties didn’t matter much, since feedback was in universal use by then, and it didn’t show on the low-resolution distortion analyzers of the day. To be honest, zero-feedback amp design is kind of a cult audiophile thing. For that matter, any kind of tube amp is a cult audiophile thing. If distortion numbers come first, THX or Class D are the answer, end of story. Don’t mess with tube amps, just buy the solution off the shelf. It’s an esthetic decision to build zero-feedback amps, whether bipolar transistor, MOSFET, or vacuum tube. I think it is good practice to design low-distortion zero-feedback driver sections, but I have seen (but not heard) all-transistor amps, with lots of feedback, used as 300B driver sections. Which begs the question, why use a 300B at all, if it’s just an expensive distortion generator? Returnng a little more seriously to the original question, there are a lot of SET amplifiers with marginal driver sections. I’d go out on a limb and say the majority of 300B amps on the market sound mostly like overstressed driver sections, not like a 300B. That’s why some of this discussion might sound like we are at cross-purposes. If state-of-the-art SINAD numbers are your goal, please look elsewhere. Forget all tube amps, whether pentode, triode, or hybrid amps. If you want a taste of "tube flavor", get a preamp with a 12AU7 in it. I own a charming little Xduoo TA-10R, which is a AKM 4493 DAC, a 12AU7 gain stage, and a simple two-transistor Class A output stage. 2 watts per channel, sounds great, and all for $320 from Apos Audio. Lots of power to drive planar headphones, and a fun alternative to the usual Topping or SMSL. Designers of zero-feedback 45 or 300B amps have different goals, which are also different from designing re-creations of Golden Age PP pentode amps. A lot of it comes down to esthetics and design philosophy. |
@atmasphere the ST70 power transformer spec was allegedly 2 x 6.3 V windings @5A each. Pair of el34 is 3 A, even a pair of 6sn7 is only 1.2A, so that should be only 4.2A. Should be safe |
@alexberger Mostly because of the voltage swing. RC coupled circuits are a bit inefficient if you also want current to deal with that input capacitance (which admittedly isn't all that much). That is why transformer coupling or better yet, direct coupling, does the job better.
@donsachs So 'only' 0.9 amps more... I think I've seen too many power transformer failures in my life, although that's mostly been due to bad filter caps. I treat old power transformers carefully- to avoid Bad Things happening. |
@atmasphere Hi.. you are only running 3 6sn7 tubes. Input tube and then a long tailed pair driver on each side using a single 6sn7.... |
@donsachs Unless the power transformer is replaced, I think its a bad idea to drop in a board using 6SN7s as much as I like that tube. 6SN7s have a 600mA filament; running 4 of them is 2.4 Amps! The transformer would burn up in due time. Yes, you can install a supplemental transformer...As much as I like the ST70 (it has decent output transformers) IMO when you get that far into it, I feel like starting from scratch with a chassis that is large enough to really do what you want is a better move- and leave the ST70s for what they are good at: inexpensive and competent tube power. |
@atmasphere Actually, I have rebuilt a few ST-70. I agree, the rectifier is inadequate, but so is the driver section, which is at clipping at or just before the output section clips. I have heard a couple of drop in boards and the octal version was by far the best. Much better amp with 6sn7 input tube and 6sn7 drivers. Still, the small case keeps you from installing a really nice C-L-C supply built with only film caps rather than lytics. When modded it is a cute little amp, but hardly worth the trouble compared to other designs in my book. I recall another vintage amp or two that used 6GH8 or similar for driver section, probably Scott, but too long ago. Yes, we re-wired the sockets to take more common variants of the triode/pentode combo tubes. They are all nice little amps, but not in the same universe as the topologies we are discussing in this thread. However, would make nice little systems though with good speakers. I recall a Fisher flagship el34 amp that was really rare that actually sounded quite good when restored as well. So long ago I forget all the model numbers! When I was doing all the vintage work I could probably have rattled off the model... |
I’ve said it before and I’ll say it again. This is such a great thread. Oh! And hi @eddie138 ! Nice of you to drop in! |
@lynn_olson They were quite stable and reliable. But the circuit is carefully designed around some serious issues with phase margin, which is why you really don’t want to try any serious mods other than updated coupling caps unless you do the math and think it thru.
@donsachs The real weakness of the ST-70 is the rectifier. Its not got enough current and so is the most likely tube to fail in the amp. If you replace it with solid state the B+ will be too high. It is possible to use dual rectifiers but you have to replace the power transformer to do so. Triode Electronics of Chicago makes a power transformer that drops into place. You can use the filter can location for the other rectifier; the power supply caps can be installed beneath the chassis. There are adapter sockets on ebay that allow you to run a 6GH8 in lieu of the 7199; that gets you a far more common tube that also runs the amp with a bit lower distortion. |
Playlist Here’s what I’ve been listening to for the past year. Probably zero percent overlap with Don Sachs, and only 10% overlap with the folks at Spatial. It’s a mix of dense heavily-produced rock, London Sound, and German techno. Joywave: Traveling At The Speed of Light (7:34 version) Joywave: Smokestacks Big Data: Unglued Big Data: Put Me to Work Disclosure: Help Me Lose My Mind (featuring London Grammar) Soul II Soul: Keep On Movin’ (demo quality) Elderbrook: Inner Light Elderbrook: I’ll Find My Way to You Ulrich Schnauss: Far Away Trains Passing By (whole album) Phosphorescent: Song for Zula Tears for Fears: Elemental Tears for Fears: Woman In Chains (demo quality) Preferred Listening Format: * Shadow Vector, Surround Master, or DTS Surround system * Open-back planar headphones * DHT triode system Probably half these songs would get me thrown out of most hifi show demo rooms ... certainly the first four, for sure. They’re all really emotionally intense, which is why I listen to music. Both "Keep On Movin’ " and "Woman In Chains" have an unworldly beauty unlike anything else, and Ulrich Schnauss’ "Between Us and Them" is quite a sonic excursion. I’ve been lucky enough to have several experiences that changed my life ... when I was ten years old and heard stereo for the first time, and heard a live performance of The Messiah the same year ... fifteen years later, hearing Shadow Vector quadraphonic for the first time ... twenty years later, hearing the Ongaku and Reichert DHT amplifiers on my new Ariel speakers ... and ten years after that, my own Karna amplifier. Profound, deeply moving, out-of-body experiences. That’s why I find audiophile discussions of "accuracy" to miss the point. I don’t listen to music to sip a single-malt whiskey, smoke a Cuban cigar, scratch my chin, mark up a twenty-point checklist, and write long reflective essays about this or that subjective aspect of the sound. That’s silly. What’s the point? Who cares what XYZ critic thinks? It either gets me flying or it doesn’t ... and if it does, how high?
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The HK Citation gear was the most razor flat sounding, and the Fisher receivers were actually quite good. The Scott gear was also quite good, and my favorite small integrateds were the Sherwoods. Their little EL84 based integrated was quite good if rebuilt. None of these are as good as the really good modern tube amps, but only the really good gear. The transformers in the much of the old gear were really good, but they didn't have the quality signal path parts we have today. Also, they had all sorts of features that robbed signal quality, like miles of wire in the signal path to support things like stereo/mono and phase reverse switches, and of course tone control circuitry. Very long signal paths in many of the old integrated amps and preamps. The Dynaco ST 70 had good transformers, but the driver section was barely adequate and if you replace it with one of the more modern boards, especially the all octal tube ones, the amp is greatly improved. The small chassis limited the quality of the power supply you could fit in there though. Still, if modded, the ST70 is a great little amp for the money. |
They all had a different "house sound". Marantz was crisper and more resolved, a more hi-fi sound, and Fisher sounded like a really good console. Not sure if the H.H. Scotts had a consistent sound or not ... they were mostly known for their FM tuners. Dynaco was always the "value for money" brand, like a VW Beetle. |
I rebuilt a number of classic mac tube amps when I was restoring vintage gear. They all sound very nice, but the mac house sound is a bit soft to my ear. Sort of on the warm side. Pleasant sounding, but not the absolute last word in detail. The preamps sounded like that too. Very good gear if you like that sound and very reliable, but warm.... CJ gear was also a warm sound. |
Interesting circuit ... thanks for posting it, Ralph. It’s kind of weird the actual gain/driver tube is a powerful 12BH7, while the cathode follower is limited to 1mA or so from the 12AX7, which is about the worst possible choice for a driver. Very strange. Maybe an intentional current-limit for the power tubes? Also was not aware the global feedback network has its own winding, making four secondaries on the output transformer. Most everybody else samples the 16-ohm tap, which captures the entire secondary that powers the speaker. I kind of wonder if McIntosh did this to make the circuit hard to copy. From this distance, I wonder about circuit stability. I see at least two feedback networks, one nested inside the other, and the outer loop not actually sensing the voltage on the speaker terminals, but a separate winding (which will never be exactly the same, especially at high frequencies). The circuit has a massive amount of forward gain (12AX7 -> 12AU7 -> 12BH7), so the feedback networks are definitely active. |
Yes. R21 (120K; refer to schematic) sets the bias point of the 12AX7 (which to me seems a terrible choice for this application- a 12AU7 or 12AT7 would have suited better), which in turn sets the bias point of the power tubes. The operating point it based on the idea that as the tubes weaken, at some point you just replace them, rather than readjusting the bias as the tubes age. The output power is not a function of the bias- its affected by the condition of the tubes! That is why the operating point was chosen to be class AB2 so the actual operational point is not critical and the power tubes will run cool with long life, high power and low distortion (due to the various feedback means). A good driver tube will present the output tubes with a very consistent bias voltage over time- that circuit is quite stable and has some ability to handle some grid current in the output section. The bias rectifier and its power supply play a role in this. The original rectifier was a selenium device which has a larger voltage drop across it so if you renovate the amp and replace the rectifier with a new device (recommended- those old seleniums were terrible) this is a minor thing to pay attention to if you want to set the same operating point, although as I said its not critical.
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I’m no expert on the MC30, but it is very unusual. The cathode feedback (from a special tertiary winding in the output transformer) results in very low gain for the 6L6 power tubes, so the driver has to swing 100 volts, putting extreme demands on linearity. And I think it operates in nearly pure Class B, with a very small Class A region. This requires substantial feedback (which it has) to linearize the output section. The Class B operation requires very close coupling between the tertiary and primary windings, otherwise the circuit will have tube cutoff glitches with every zero crossing. So the whole thing is very much a package ... multiple feedback loops, a unique output transformer, a wide voltage swing from the driver, and cathode feedback for the power tubes. Almost the polar opposite of the Brook 2A3 amplifier, which relied on the linearity of the 2A3 power tubes instead of massive feedback. The high-power (30 watts!) Brook amplifier used sliding bias to keep the output section in quasi-Class A. It should be mentioned there was no awareness of slewing distortion at this time, because signal sources had very limited HF bandwidth (12 kHz) and limited peak energy. Phono cartridges were very primitive and could barely track LP’s at 5 grams. When the first writings about slewing distortion appeared in the late Seventies (25 years later), things were very different: moving-coil cartridges with exotic styli were flat out to 50 kHz, and cutterheads could put down tremendous levels on the disc. Tweeters were much better as well. |
Hi @atmasphere , McIntosh MC30 has the similar cathode follower driver 12ax7 to drive 6L6 directly. The output tube works in fixed bias in this schematics. Isn't it? Can you explain how the output tube bias is self adjusted? |
Don is being modest. The last year, going through the present, really made Don pursue every obscure byway of amp design, building and listening as he went, every step of the way. Don started with an obscure version that I called the Symmetric Reichert, which was literally a Reichert 300B done twice, with a phase-splitter transformer at the input. All RC-coupled. He built that and called me out of the blue, about a year ago. Don then tried separate B+ supplies for the input+driver and output section, and an interstage transformer between the driver and 300B’s. A few months later, Don used a triode-connected 6V6 instead of the hard-to-find 45 driver. Thom Mackris and Don independently tried this at just about the same time, pretty much on the same day. Don (but not Thom) then used active current-source loads for the input 6SN7, instead of resistor loads. That was the Stereo version Don built and shared with the Spatial team and the first customers. Next, replacing the active current source loads with custom Cinemag inductors designed for the purpose, and using the shoebox-format monoblocks that became the show amps. My Colorado neighbor, Thom Mackris of Galibier Design, has been following along in a parallel project, with a SET architecture, but with passive CLC B+ supplies and damper diodes for rectification. That’s where all of us were a month ago ... Don Sacks, the team at Spatial, and Thom Mackris. The latest from Don is an IT between the 6SN7 and the 6V6, replacing six other parts with a much simpler approach ... provided the IT was up to the task, which it is. The IT has turned out to be superbly designed, exceeding expectation, and also making our lives simpler. Don and I have gone full circle, and re-invented the Karna (after trying every alternative), with far more advanced power supplies that were not available in 2003. Don really has tried every topology, one after another, and carefully measured and auditioned each one. RC coupling, active loads, LC coupling, and now, IT coupling. By lucky coincidence, Thom has been walking a parallel path with his SE topology. All four groups ... Don, Thom, Spatial, and myself, have been exploring this zero-feedback approach for several years now. If other folks want to build transistor Class A, Class AB, or Class D, more power to them. Those designs have an entirely different set of challenges that have nothing to do with triode amplifiers. In triode amplifiers, the devices themselves are exceptionally linear, and the appropriate circuits take advantage of that. |
@alexberger And just like that you put your finger on why a direct coupled driver can be so effective. No bandwidth issues (able to go to DC) and plenty of drive for a hungry, highly capacitive grid. If the driver is pulled from the amp while its on, the output tube goes into cutoff. Similarly, the power tube does not conduct until the driver tube warms up. You might think the negative voltage power supply to be too expensive to include in the design, but its a lot easier to get right than the design of a good interstage transformer. If you think a type 45 sounds nice, try them in push-pull! |
Lastly, I will say that internet forums are both a blessing and a curse. There are people who tout one amp topology over another, or whether they prefer SS or tubes, or interstage transformer coupling over LC or RC, etc... When you read these things consider the source. There are lots of people who have strong opinions about what sort of coupling to use between tube amp stages, or even whether they should be direct coupled. We all have our biases. You need to figure out whether that poster has actually tried custom transformers for example or whether the opinion is based on some off the shelf midrange transformer. Or the person who states that coupling cap quality doesn't matter much, or they love this or that cap. The question is what have they heard before, what circuit is it going in, etc.... I have had several cherished notions overturned by this project, simply because I experimented thoroughly with different approaches and let my ear by the final arbiter. I started out with the silicon assisted tube amp approach and ended up with all IT coupling, like the original Karna amp. So my point is that you have to keep an open mind. I am definitely a tube guy and solid state makes me unhappy when I listen for long periods of time, but that said, if a pair of Ralph's latest and greatest Class D amps appeared in my living room I would give them a serious audition and chance to shine. I hope to hear them at a show some day. As I said way up above many pages ago, there are many paths to audio nirvana, and we all may have our own. |
Again, basically what Lynn said. At each stage of the project whether in the preamp or amps I discussed the exact parameters and even shared the relevant part of the circuit with Dave Geren at Cinemag, who is a very experienced master winder of small transformers. He models everything and makes a prototype and then I build with it and listen. Then I give my impression and measurements of the frequency response to him and we do round two where he tweaks the design and that is that. Dave is also a maestro of core materials and interleaving. The result is that all coupling caps are now gone from the input to the preamp to the output of the amplifiers. Certainly you can accomplish this with direct coupling in places. Every approach has strengths and potential pitfalls. One advantage of transformer coupling is the complete isolation of every stage and the banishment of hum, and also far greater immunity to ambient RF. Also, if a tube goes south, or a customer does something odd like put the wrong tube in the wrong place (yes I have repaired things where people did this), or pulls a hot running tube, any potential damage is limited to a very small portion of the amp. Direct coupling can lead to a daisy chain of failures, cascading through your amp. These are considerations for commercial gear, where numerous units will go to many environments over which you have no control. I can build whatever I want for my living room because I know how it works, and I can fix it. But if a commercial product I want reliability. Transformers are very reliable. I will say that I have had the luxury of semi-retirement to spend a over a year with these preamp and amp circuits. That has allowed me to try pretty much every permutation and combination of power supply and coupling topologies. We have finally settled on everything and the final tweaking is about done. When you are trying to make a living selling gear you generally get something that works and sounds quite good, and is reliable and then you make them. When you are semi-retired you can go down every rabbit hole until you find the exact sound you seek. Good enough isn't good enough..... So my comments above are based on this approach. I cannot tell you whether you will like all transformer coupling vs. RC or LC using your favourite capacitors. I much prefer IT coupling, but I had everything custom wound by a very experienced winder, and we did a prototype and final version in each case. I can do this because I have time and am willing to devote some money to the project. For your one off amp you are forced to buy off the shelf transformers, but there are some very good winders out there and if you communicate what Lynn discussed with them, you may well get a very good solution. Just don't expect to pick a transformer off some web site and have it work perfectly. You will have to communicate with the winder. I wish you all success. If you persist, then you can probably get a great transformer for your circuit. I will also say that if you heard the gear at the Pacific Audio Fest in June, you heard prototypes and the set in my living room is considerably better, both preamp and amps. Now they are about done. The cases will be redesigned by the folks at Spatial Audio and the amps are getting physically larger so we can fit one more mod under the hood, which I expect to bring the sound up yet another notch. Then they will be commercially available, probably late Q4 or early Q1. There are already 4 or 5 folks on the wait list at Spatial and I would expect them to have gear very late this year or early next. So if you are on that list, your patience will be rewarded with gear that is considerably better than what was shown in Seattle. |
I apologize for making this sound confusing, but I wanted to give the readers of this forum a taste of what Don has been through. There are many possible ways of getting a zero-feedback amplifier wrong, and from the standpoint of mainstream audio engineering, all zero-feedback amps are wrong ... not just in practice, but in principle. An all-transformer coupled amplifier is especially wrong. The accepted path is DC coupling throughout, using transistors, with lots of excess gain for plenty of feedback. Modern feedforward techniques (Bruno Putzey, THX, et al) can get distortion into the parts-per-million range, so why look elsewhere? Unless your goals are subjective, and you have a weird hypothesis about linearizing each stage, as much as possible, without using feedback. That’s why Don took a gamble on the Karna topology, a circuit out of the late Twenties and mid-Thirties. |
This is entirely up to the transformer designer. They need to know the Zout, or Rp, of the tube driving the primary, and the load on the secondary, which will either be a pure capacitance in the 60~80 pF range, or paralleled with a load resistor, typically 100K or so. The method of extending HF bandwidth is interleaved windings, and this falls into the realm of modern computer modeling. Back in the old days, this was cut-and-try, now, it can be modeled. Interleaved windings extend bandwidth, but the interleave pattern has to be carefully chosen so there is no HF ringing into the intended circuit. (This is why they need to know the Zout of the preceding stage and the load of the following stage.) They will want to know your expected bandwidth, power handling within that bandwidth (particularly below 40 Hz), and how much square wave overshoot you will accept. And the DC parameters ... if SE, how much quiescent current does the tube run at, if the circuit is balanced, how much DC imbalance do you expect from the pair of tubes. This affects core gapping, which in turn dictates core size and transformer size. A small air gap linearizes the transformer, but can also double the required core size, which in turn affects HF bandwidth. The DC parameters are critical for the entire transformer design. As you can see, this isn’t a matter of selecting an off-the-shelf part, but consulting with the transformer designer and telling them what you need (and what they can do). They *might* have an off-the-shelf part, or they might not. If not, what is the minimum order, and how long will that take? I haven’t mentioned sonics yet. Aside from meeting minimum technical specs (which you and the transformer designer both agree on), there’s the matter of subjective sonics, and how it fits with the sound you are aiming for. This might sound trivial, but if the amplifier designer has no subjective sonic goal, you will not get there. "Perfection" is not a goal, it’s a marketing term, like "Perfect Sound Forever" for CD’s back in the Eighties. Are you familiar with the subjective difference between RC coupling, LC coupling, active current-source loads with capacitor coupling, and interstage transformer coupling? (With this amplifier design, Don built and auditioned each one.) This is very useful to know as the amplifier is tuned subjectively. Similarly, the driver stage design has a major effect on amplifier sonics, aside from inter-stage coupling. The driver section affects slew rate, HF distortion, and subjective colorations in the mid and upper frequency range. It’s useful to know the sound of a 6DJ8, 12AU7, 6SN7, and a power-tube (45, 6V6, KT88) driver ... they sound quite different from each other, and can dominate the sound of the entire amplifier. |
See page 21 for a construction article on a push-pull, zero-feedback 300A/300B amplifier. This is about the same time as Columbia introduced the LP microgroove record, so it’s very early days for high fidelity. What surprises me about this article is that 300A’s and 300B’s were even for sale to the public; they must have been pulling them out of prewar theater equipment At the time of printing, the only records you could actually buy were 78’s, and FM radio was very new. Note the primitive state of tonearms and phono cartridges ... also, 78’s had no standard equalization, so preamps had to cover several curves, including "acoustical" for pre-electronic records. The reign of the monophonic LP record was surprisingly short; ten years later, almost to the month, stereo LP’s were announced by all the major labels, and stereo cartridges and stereo preamps were also on the market. |