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.
I just returned from Dallas, and I have to tell you that our room was one of the top two or three in the show. Danny Ritchie (GR Research) came by and told us our room was his pick for best sound of the show. Sam Whitt, the speaker designer at Spatial, just completely nailed that new Q3 speaker. It is superb.... Of course the preamp and amps driving it didn't hurt either:) |
If Spatial is at the Seattle show this year, yes, I will be there. I had a lot of personal stuff that precluded my attendance at Dallas, but it sounds like the SWAF was great experience and a good time for all. Don tells me there is a lot of serious tech in the new Q3 speaker, and of course Don and I have rolled in a lot of new tech into the preamp and power amp since the Seattle show. I expect I’ll be working with Joseph Troy Crowe on completing the notorious "Beyond the Ariel" speaker project started many years ago on DIYaudio. This will be a non-Spatial project, just between me and Troy, but it will be open-source. My work with Don and Spatial will continue ... in the short term, I need to write the product manuals. If there is demand for new electronic products, I will be part of that, but Spatial designs their speakers with their own design team in Salt Lake City. |
Alexberger, You are sure right about that. I have Don Sachs new preamp and his shoebox 300b monos and swapped out my Cube Audio Jazzon speakers for a pair of updated AR 2ax speakers and they sound extremely musical. A great front end yields benefits no matter what speakers you have. Rest assured, the Cubes will be back in my system shortly. |
I got into the amp game after auditioning both the Audio Note Ongaku and the Reichert Silver 300B on speakers of my own design. I’ve been designing speakers since 1975, so I have a pretty good idea what goes in them and why they sound the way they do. Crossover design errors, diffraction artifacts, enclosure resonances, driver breakup, etc. are things I usually hear in a few seconds when I listen to commercial speakers. So I know speakers pretty well. What surprised me was the Ongaku and Reichert revealed things about my speakers I did not know were there. And different things between the two: the Ongaku revealed "space between the notes" in a way I had never heard before, as well as extraordinary, almost uncanny spatial realism. The Reichert deepened the emotional response to the music in a way that was outside the audiophile experience and not anything to with coloration or tonal balance. It was just a feeling, even when I was listening objectively and trying to assess what I was hearing. It just snuck up on me with a powerful emotional response. It’s one thing listening to a commercial speaker that’s off-the-shelf with unknown design goals, semi-custom drivers, and unknown crossover design, but quite another when the speaker is something you’ve taken from initial conception to successive versions to final prototype. You’ve pretty much heard everything it can do, and have already built 13 different crossover variations and auditioned it on eight different amplifiers. You know that speaker inside and out. And yet my speakers had undiscovered sonic aspects that were NOT part of source impedance (damping factor) or audible amplifier coloration. Things I’d never heard before. What kind of "distortion" gives MORE resolution, or a deeper, more profound emotional response? That’s nuts. Nonlinearity is not our friend; it steals resolution and separates you from the music with a haze of coloration. So what’s going on here? That’s when I started researching the history of vacuum tubes and started writing for different audio magazines. I don’t believe in copying or "cloning" the designs of others; not only is it dishonest, it reflects a lack of understanding on why a product sounds the way it does. You have to understand the "why" before you can go any further, and that took about five years before I came up with the Amity in 1997. Amplifiers are not as neutral as we would like them to be, and an amplifier with measured distortion in the parts-per-million range is not necessarily neutral sounding. They all have a sound ... Class D, GanFET, bipolar transistor, pentode, triode, and direct-heated triode. |
A local store builds and sells their own high end tube gear and their own horn-based speakers. For most of their demonstrations of the better horn systems they use their 350B amplifier. One day when the boss was away, two of the younger employees spent the afternoon swapping in speakers acquired in trade-ins to see how they sound. While all the speaker sounded quite different, they all sounded amazingly good to these two. They could not believe how good even Polk speakers sounded. It then dawned on them that the amplifier mattered a lot. One did not get that kind of "transformation" of the speaker with the likes of Conrad Johnson amps, for example. That is not to say that the CJ isn't good, it is just that the 350B amp had a particular character to its sound and that character played well with a wide range of speakers. I tend to find that tubes related to the 350B do sound nice with a wide range of gear. I tend to like 6L6 and KT 66 tubes (similar to 350B). I run a somewhat similar, albeit much less powerful tube, the 349 (in pushpull) and I like it a lot. |
@lynn_olson - Is the Raven Preamp AES48 standard balanced? They don't have a price listed for it that I can find. |
I'm very much hoping their is a build thread for the Speakers to be designed. This is an area I am progressing into, as I have recently on a few occasions heard Speakers that are a a bespoke design and are as a end sound, what I believe are a challenge to my ESL's. My personal preference for a Speaker does seem to have changed and a curiosity is now taking hold. Even the idea of modding owned Cabinet Speakers and attaching Internal Wiring to a Drivers Voice Coil as suggested by another Speaker Builder on here, has my curiosity and is intended to be experienced when advisories are offered. |
I believe the preamp is $5500. Search for spatial audio lab, find the site and look at the revelation series and preamp and you will find it. On the preamp, it is fully balanced and transformer coupled to output. Pin 1 is shield and grounded to chassis, pin 2 is XLR + phase and pin 3 is XLR - phase. It will drive any power amp with XLR connections, and will happily drive a 10K amp input impedance. |
I fully defer to Don, who is part of the loop of the Spatial team, who manufacture the preamp and power amp. I’m more like the system architect, guiding the project in a certain direction, and I’m not as aware of the manufacturing and production side of things. One benefit of the output transformer in the preamp is it does accurate phase spitting, even if there is a small gain mismatch in the 6SN7 tube. Other balanced designs tend to rely on feedback for output balancing ... the Raven is zero feedback, either global or local, so transformers do the balancing and unbalancing conversions. The input transformers are bypassed for the special case of XLR (balanced) sources. For example, many DACs have XLR outputs, since many converters inside the DAC have balanced outputs. On the other hand, tube phono preamps are often single-ended, so have RCA outputs. The Raven caters to both, with a source selector on the remote control. The Khozmo volume control is fully balanced, and so is the 6SN7 that follows it. |
OK thanks @donsachs and @lynn_olson I missed that price when I looked at it the first time online. A significant point you made Don is the fact that it will "happily drive a 10K amp impedance" I have a new Coda amp on order and the XLR input is exactly that! I sent some questions to Spacial Audio regarding deposits and time it will take to get one. I want to keep a tube preamp in the drive train. My old one was 25 years old! Thanks! |
@charles1dad Yes, it was a very tough decision but I cannot lift the damn thing anymore and re tubing it is nearly $2000. I want to simplify my retired life a bit. I just picked it up from VAC and it is fresh as a daisy for the new owner. New tubes with about 100 hours on them. |
Did anyone who went to the Texas audio show hear the 300B amps in the Classic Audio Loudspeaker room? They were powering a set of field coil speakers. I believe they were prototypes of Atmasphere 300b amps.
https://www.facebook.com/story.php/?story_fbid=861906829281196&id=100063856610499&_rdr |
@lynn_olson @donsachs I’ve been in touch with David Whitt about the pre amp, he said they are shipping the first build cycle in May of this year. He also stated that there is a 6 week build cycle. It seems you are still making changes to it at the moment, at least from what I gathered in this thread? Would it be wise to hold off a bit for a couple months until you determine the final product? I am not in a hurry, but searching for my end game preamp and want te best version of it. |
Hi. It is a very subtle change that I find inaudible, but on paper it is slightly better. The xlr inputs now go directly to the Attenuator and the rca go through a 1:1 transformer to convert to xlr. The previous version ran all inputs through the 1:1 transformer. Honestly, the Cinemag 1:1 transformer is so good that it has something like .25 degree phase shift at 20 Hz. You cannot see it on the oscilloscope and it is sonically transparent. So running the xlr through it was not an issue, but I figured out how to go direct in for the xlr. Again, I changed my preamp and I really cannot tell much difference on the xlr between input transformer coupled and direct. I doubt I could reliably tell in a blind test. The transformer is that good. So if you are using predominantly rca inputs to the preamp there is no change. If you are using the xlr inputs, then there is a change on paper, but you would be hard pressed to hear the difference. |
@donsachs How do you deal with the variable loads which the preamp will be driving? With no feedback, the transformer can ring or express the inter-winding capacitance if its not loaded correctly. |
The Raven is not very sensitive to loads. In practice, power amps range from 10K (typical solid-state) to as high as 470K for a handful of vintage tube amps. Most modern tube amps are 100K. Plus whatever cable capacitance is there, along with the Miller capacitance of the input section of the power amp. So 100 to 400 pF is typical. The range of loads is predictable and well known. What dominates the transformer performance is the source impedance, not the load. The source impedance from the preamp tube is much lower than the load, so it heavily dominates the transformer performance. Transformers don’t much care where the low impedance is, primary or secondary, so long as it is there. The hard transformers to design are the interstage transformers, since they are driving nothing more than a grid (without grid resistors). So the load is effectively nothing more than the Miller capacitance, which is fortunately a known quantity and can be designed around. Similarly, the source impedance is also known, since it is the plate impedance of the preceding tube. We use transformers that are purpose designed for the impedances and the power range they will be exposed to in-circuit. (Not off-the-shelf general-purpose transformers.) One nice thing is our transformer designer has access to modern magnetic simulation tools that were not available back in the Thirties and the Fifties. |
In terms of load abuse, plugging planar headphones (20 ohm to 32 ohms) into the Raven preamp will definitely load down the preamp and is not recommended. No harm will result, but the preamp will slide over into Class AB operation and distortion will rise to higher levels. Headphones with impedances of 300 ohms or higher (most dynamics) are fine. We may design a future project that can accommodate planar headphones, but these require a genuine 1 to 2 watt Class A power amplifier, which is outside the scope of a preamplifier intended to drive power amplifiers. In other words, a tiny but very high quality power amp with extremely quiet power supplies. Designing (good) amplifiers for planar headphones is not as simple as it appears at first glance. |
Of course, it is tempting to design a special-purpose headphone amp. I own HifiMan HE1000 Stealth (V3) planar headphones myself, and I use a modest all-in-one DAC/headphone amp to power them. But ... the headphone amp market is quite crowded, and is dominated by Chinese products selling from $200 to $1600. On the domestic front, Geshelli is cleaning up making AKM-based DACs and Sparkos discrete-opamp headphone amps at entry-level pricing. As far as I can tell, headphones, and headphone amps, is the most competitive sector of the audio market right now. So even if we spent several months to a year on a special headphone amp, I’m not sure we’d sell many. Something to think about, though. |
Transformers are aptly named as they transform impedance. They do not isolate impedance. If the source impedance is low that will change the correct loading on the output (as opposed to a high source impedance), which will be found to be exactly one value. Above that value the transformer will ring; below that value it will roll off highs. This phenomena is well known and is why Jensen Transformers specifies the loading to be used with their SUTs depending on what cartridge is used with them. If the load is too high impedance, the inter-winding capacitance will come into play as well, causing the FR to no longer be flat, and in extreme cases the transformer may fail to express its turns ratio. The proper termination will yield the widest bandwidth out of the transformer. This why in the old days when balanced lines were used, the termination standard was always 600 Ohms so the designer would have a pretty good idea of what to shoot for. A rheostat, placed across the output (between pins 2 and 3) can be used to load the transformer for optimal operation. Or the transformer can be designed for 600 Ohms with a termination switch using a 600 Ohm resistor built into the equipment (this is how Ampex solved this issue on their 351 tape electronics). That way any higher impedance load such as 10K or 100K is negligible and would not affect the transformer performance. The latter approach is why we did with our P-2 balanced line preamp 30 years ago.
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One of the nice things about working with Cinemag is their custom design service. This gets us away from the many limitations of standard off-the-shelf transformers from other vendors. We specify source impedance, the range of loads (including capacitance), DC current flow, and expected DC imbalance. They show us what their computer model tells them, we test the physical prototype, see how it works on the bench and in-circuit, and go back and forth a few times until everyone is happy. Off the shelf, it's take it or leave it. This is what made both the Raven and Blackbird possible. I didn't think a transformer using the 6SN7 in balanced mode could be realized because of the quite high impedances involved. To my surprise, Cinemag showed me otherwise ... very clean square waves at 10 kHz with minimal overshoot. This is not an off-the-shelf part ... it's been designed for us. When we say the Raven and Blackbird use custom parts we aren't joking. Without them, they aren't practical. We use standard tube types because we want our customers to enjoy them for a long time, and we expect the 6SN7, KT88/6550, and 300B to be around a long time. Under the chassis, though, we use custom parts for critical functions. |
“Ralph, I see you introduced a PP 300B amplifier at one of the recent hifi shows. How do you think it compares to your Class D amplifier, in subjective terms? Since you designed both, you're the best qualified to assess the differences.” There are a few of us curious audio geeks interested in learning more. John Wolf of Classic Audio Loudspeakers used them with his field coil speakers and he said that the bass was incredible. Perhaps time to ditch the Novacrons he uses a lot for the new 300. |
That is the first thing you notice when you hear our 300b PP. The bass is amazing. It is not over emphasized at all, but if there is bass energy on the recording you hear it all. Most importantly, it is the quality of the bass, not the quantity. When I built the prototype I had never heard anything like it. The resonance in the body of instruments, the attack and decay of bass notes. Then we spent over two years improving the amps by eliminating cap coupling, improving power supply designs, working with Dave Geren through several iterations of custom interstage transformers, moving to Monolith output transformers, etc... Now the bass is otherworldly. So I can imagine a well designed PP 300b is going to have those qualities with a good field coil speaker and make people take notice. |
The prototypes at the show sound very similar to our class D amps; the big difference is the bass- the prototypes roll off at a higher frequency, and owing to no feedback and the OPT design don't throttle back their power when dealing with higher impedances, such as seen in the CALs at the show, which are bass reflex. So there was a bit of extra energy in the double impedance peak of the speaker. Its fun, but not correct. Essentially that OPT loads the tubes at their power peak rather than a bit higher where the amp would have had a better chance at being more like a Voltage source. If the next iteration of the OPTs had been available, the bass would have been a lot more like the OTLs but ultimately lacking the extension that up until now has been something that OTLs do better than any other kind of tube amplifier. The class D has the same relaxed mids and highs but is a little more transparent- its easier to hear what's going on in the rear of the sound stage if you have a 2-mic recording. I think we can do better with the 300bs by using some of the tech we use in our OTLs. So the next PP version of the amp will be quite a bit different- with LF bandwidth similar to our OTLs with no sacrifice in high frequency bandwidth.
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Wow, is that the first Atma-Sphere with an output transformer? Sounds like the 300B amp is well on track, and I imagine the next version will have current sources in the appropriate positions. I’m curious what your experience with a high-power current source in the output stage turns out to be. It didn’t work for us, but it did work for Allan Wright back when he visited me in the late Nineties. |
No, nor the first to be shown in public or sold. We make a small 5Watt/channel integrated amplifier that is suitable for desktop, bedroom, headphones or a main system if your speakers have the efficiency. That one uses EL95s which are a little brother to the 6AQ5, running class A1 ultralinear. The tube is very easy to drive so the voltage amplifier is a differential amplifier using a 12AT7 with a constant current source. The amp can sit on a single sheet of notebook paper with room left over but its built with good parts throughout and an overbuilt power supply. The chassis has its corners welded and ground so it can be polished and chromed. We've been having fun with a variety of finishes and color schemes; blue with black (the transformers be the contrast finish), blue with chrome, black with chrome, chrome with black, grey with red and so on. They are entirely handwired point to point. It easily keeps up with SETs of the same power; it has a greater amount of usable power owing to greater linearity. It also has wider bandwidth both on the bottom end and the top end (goes out to 100KHz). This amp is one of the projects we did which aptly demonstrate why SETs are impractical and obsolete. That's not to say we're not also having fun with SETs. We have a 300b SET project as well. It uses some techniques we use in out OTLs to minimize distortion driving the power tube.
Constant Current Sources are the key to getting differential amplifiers to perform. A PP output section is often wired as a differential amplifier so a CCS can work really well if output section is class A.
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Yes, it’s difficult to get people to understand that Class A push-pull is quite a different animal than the much more common Class AB push-pull. I understand the reasons for Class AB ... it’s much more efficient, but Class A working is not the same, since device switching and the saturation region are avoided. It’s also kind of funny seeing the solid-state fraternity tap-dance around various sliding-bias schemes and the marketing attempts to call them "Class A". Well no, they’re not, since the clever sliding-bias system is always just a little behind the musical content. And genuine (thermal) Class A transistor amps require stupendously large heat-sinks or fan cooling. I remember all the marketing about "cool-running" transistor amps in the late Sixties. Yes, they ran cool if the Class A region was 1 watt or less, and the rest of the operating envelope was Class B. The A/B switching region was especially offensive with the quasi-complementary amps of the first and second generation. The third generation, in the late Seventies, finally had access to decent quality complementary devices, although dog-slow by modern standards. Class A working is actually easier with vacuum tubes, since you don’t need a monster heat sink. Solid-state reliability drops pretty fast above 80 deg C, so heat sinks and efficient thermal design are mandatory. By contrast, vacuum tubes operate quite happily at very high temperatures (short of red-plating). If you can accept lower power, it’s easily within reach by altering bias points and the primary impedance of the output transformer. I agree with you about the merits of single-ended vs Class A push-pull (or balanced). The only indisputable advantage of SE operation is avoiding the zero-crossing region in the output transformer, but this comes at a massive cost in core size and the requirement for a large air gap, which in turn erodes bandwidth. Skillful output transformer design can work around the problems of the zero-crossing region ... this is largely a solved problem. |
I have done every permutation and combination of ways to build this 300 PP circuit. I originally started with high quality CCS and then went to LC coupling, which sounded more natural and musical to my ear, and then finally went to full transformer coupling. As was stated above, this was impossible with off the shelf iron from even a very well know major European manufacturer. Those ITs rang like a bell at 15K and up. It took several iterations of custom design work in collaboration with Dave Geren at Cinemag to get it right. Dave's easily make it down below 20 Hz and out past 30 KHz with no oscillation and no loading network needed. We all have our taste, and all versions of the amp were wonderful, but there is magic in the final version that simply wasn't there with CCS or LC coupling. A lack of solid state coloration and wonderful tonality and presentation of instruments to my ear. The piano sounds like a piano, not a nice rendition of one. We are in production after two years of prototyping. Sounds like @atmasphere is early on their journey with a PP 300b. Certainly far enough along to demonstrate a prototype at a show, much as we did in Seattle last summer. It is undoubtedly a different topology than ours and I wish them well. I am sure they will get their version to where they want it to be as we have with ours. They will sound different of course due to the preferences of the designers. |
We started prototyping with DHTs seriously about 20 years ago. Back in the 1990s we built a 300b-based OTL which we showed at CES. The trick with the CCS is doing a good one. A lot of the circuits that you see on the web leaver performance on the table. We've had CCS circuits in our amps for the last 35 years. 'A lack of solid state coloration' is matter of avoiding the distortion signature that is endemic in so many solid state amps. This has to do with proper design of the feedback loop and a whole lotta loop gain in an amplifier design. The 'solid state' sound is just bad feedback application and is part of why feedback has garnered a bad rep in high end audio. Feedback has been known to give tube amps a 'solid state' sound too; this is because the feedback signal has been distorted prior to mixing with the audio signal its supposed to correct. Its like having an incorrect map to guide you through town. Norman Crowhurst wrote about this problem nearly 70 years ago but did not suggest a solution, despite it being rather simple. Peter Baxandall 'rediscovered' the same problem in solid state amps about 20 years later but he suggested more feedback, which doesn't solve the problem. Its only been in the last few years where we've seen self-oscillating class D amps where this problem has been solved pretty consistently. You can do it in class A/AB solid state amps too if you're willing to work out the 3rd or 4th order feedback loops that need to be involved. Most designers just use a resistor which won't cut it...
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Hi Don, I just want to say that I appreciate your decorum and professionalism. No doubt that the two 300b push pull amplifies will sound differently precisely for the reasons you mentioned. I’ve greatly enjoyed the many insightful and educational contributions from you and Lynn Olsen in this terrific thread. I wish you and Lynn the best. Also Atma-Sphere with their endeavor as well. Charles |
Once and always, Charles sets the bar for thoughtful and positive responses to topics on this forum, which in 10 months regarding this thread, have always been very upbeat and positive. When we are treated to a master class in tube design philosophy, freely given, participants clearly appreciate the generous sharing of information by the masters: Don, Lynn and Ralph. It has been a real education for me. |
Very well put.👍 This thread you initiated has miraculously avoided the all too prevalent trolling and disruptive deterioration. Bravo!!! Charles |
Ralph brings up a very good point about feedback: the underlying theory assumes a distortionless summing point. (The summing point is the comparator input between signal input and the sampled output.) Any distortion introduced at this point of the circuit will be amplified without correction, and there is a real possibility of introducing new, higher-order terms that are not present in the forward path of the physical amplifier. Norman Crowhurst mentions this in passing in his Audio magazine articles in the late Fifties. Don and I go to some trouble to avoid even local feedback in the Raven and Blackbird ... all cathodes are bypassed in every stage, we do not use the Ultralinear connection in the triode-connected KT88, nor global feedback. Any distortion that is present is the result of first-order effects in the tubes themselves, not anything else. It also mandates selecting tubes that have a minimum of high-order distortion terms in the intended operating range. This overlaps with the philosophy of some SET amplifier designers, but we prefer to use complementary pairs to cancel most of the distortion without recourse to feedback (local or global). The transformers, if correctly balanced, cancel out most of the distortion before passing the signal to the following stage, thus, each grid-pair is fed a cleaner signal. This comes in particularly useful for the 300B grids, which can be a difficult nonlinear load for the preceding stage. Unlike RC coupling, when one grid demands current, both phases of the driver pair are available to deliver current to the 300B grid that needs it. This enables seamless and jump-free transition to A2 (positive grid drive) when the signal demands it. |
In essence, the signal flow is: Complementary Pair of 6SN7 sections -> Signal is re-summed in the transformer -> Complementary Pair of triode-connected KT88 drivers -> Signal is re-summed in the transformer -> Complementary Pair of 300B outputs -> Signal is re-summed in the transformer -> Loudspeaker Output. To minimize interactions between output section and the rest of the amplifier, there are separate and isolated high-voltage regulators for input + driver section and the output section. The only signal flow, either from the audio path or the power supplies, is from input to output. There are no secondary paths, at least to the 130 dB limit of filter isolation offered by the power supplies. A few SET amplifiers have a roughly similar signal flow, but only offer a few dB of RC filter stage-to-stage isolation between sections, so there is a signal backflow pathway between sections. Isolated power supplies resolves that problem. In a SET amplifier, 2nd-harmonic distortion builds up from stage to stage, but there can also be unpredictable level-dependent cancellation effects since signal polarity is inverted with each stage. |
If all these transformers disgust you, or you want to use local or global feedback, you can re-visit the Acrosound schematic from the late Fifties. Three stages, all differential, each RC-coupled to the next. It can be easily updated with modern current sources on the cathodes, giving precise differential action and almost no even-order distortion. The Acrosound circuit also offers some interesting options with cross-feed feedback between sections. If you are bold, you can DC-couple some of the stages, but you will need good servo circuits to maintain DC balance as the tubes warm up and then gradually age. The servo circuits also need to be failure-proof, since a runaway servo circuit can destroy the amplifier. |
Acrosound UL-II schematic. Note the input 12AX7 is direct-coupled to the 12AU7 driver stage, so DC drift from internal 12AX7 mismatch could be a problem. Technical analysis: The input signal goes to the top half of the 12AX7. The bottom half is connected to the feedback signal, which comes from a dedicated winding on the output transformer. The feedback summation occurs across the paired cathodes of the 12AX7. The plates of the 12AX7 are direct-connected to the grids of the 12AU7 driver. There is a HF phase-adjust circuit from the 100 pF cap in series with the 47K resistor. This improves the stability of the feedback system. The 12AU7 driver has a 5K 5W current-balancing pot in the cathode circuit, most likely to correct for any DC imbalance of the 12AX7. I suspect the 300p marking of the bypass caps on the 12AU7 is in error. This should be 2 to 10uF, whichever gives the best square-wave response. Similarly, there is a DC balance pot in the Bias 1 grid circuit of the EL34 output tubes. The 4.7 ohm series resistor in the cathode of each EL34 is there to meter current, which is adjusted by the 5K pot. Although not drawn, there would be test points for each cathode to ensure the correct bias point. Although not marked as such, this is a fixed-bias output section set up for Class AB (high power) operation. Correct bias setting for each EL34 is essential for long tube life.
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