Any opinion on the 6EM7 as a driver tube for a 300b? I use a zero feedback 300b SET that has this configuration. I’m not an electrical engineer but can attest that it sounds splendid! 😊
Charles
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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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. |
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 |
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. |
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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Thanks @lynn_olson Here is a bit of information/listening impressions of the Roger Modjeski 6EM7 amplifier from a past Audio Circle thread. |
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. |
@lynn_olson As best I can make out, Sophia and EML both make a type 45. |
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. 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 |
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. |
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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@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 @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 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. |
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. |
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. |
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 |
It’s a safe guess it’s about the same ... in the 60 to 80 pF range. Most true 3-element triodes, triode-connected beam tetrodes, and triode-connected pentodes fall about there. So-called ultralinear mode is probably about half that. Pure pentode is a little higher than the socket capacitance, so about 6 to 8 pF. The 45 and triode-connected 6V6 are similar, with preferred operating points: 250 to 275 V on the plate, 28 to 32 mA cathode current, and dynamic plate impedance (Rp) around 1800 ohms. The big difference is gain and bias voltage ... the 45 has a gain of 3.8 and negative 45 to 50 volt bias, while the triode-connected 6V6 has a gain of 8 and negative 14 to 15 volt bias. But otherwise similar, including Miller capacitance. P.S. If anyone does bring back the 45 at a sane price, I’d love to see it in Arcturus blue glass. That would be very stylish, and a nod to its famous predecessor. Vinyl Savor (Thomas Meyer) discusses the 45
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I use the Linlai E-6SN7 pretty much exclusively. A very well balanced tube that rivals any of my NOS ones and I can buy all of them I want. The PSVane tennis ball shaped ones are also quite good, but I prefer the Linlai in my gear. Point is that there are some very good modern production 6SN7 tubes. |
+1 @donsachs ... I agree with the Linlai E-6SN7 tubes. I have a pair in my Aric Audio Super 6SN7 preamp, and it sounds "super" indeed. The Linlai are just so perfectly balanced in the preamp; nothing calls attention to itself; they are just as musical as can be, with a great soundstage, air, have a wonderful midrange with presence, and are speedy too. I have been a big fan of the ' 40s 6F8Gs w round plates, which also sound great, but they are hard to find, and when you do, they tend to be be microphonic, and some quickly turn noisy.... The Linai are remarkably quiet. I love them. The Treasure Globe 6SN7s from Grant Fidelity are also very nice, but I think I prefer the Linlai at this point. |
The treasure globes are most likely just hand selected psvane tennis ball tubes. They are indeed also a very good 6sn7 if they will physically fit in your application. The Shuguang WE6sn7 series was wonderful as well, but sadly all things Shuguang are out of production...... The Linlai WE300b copy is superb and what we will provide with these amps.... |
@1markr I have not heard the original WE300b, but I have heard many of the $400-500 per quad 300b tubes and then I heard the Gold Lion 300b, which was a definite step up. Then I got the Linlai WE300b copy and it is a far better tube than the Gold Lion to my ear. I am not looking for other 300b tubes now.... not when I can buy the Linlai WE300b copy for a rational price per quad. |
@1markr They look neat, but the globe is highly resonant. If the tube develops microphonics the globe exacerbates the problem. The regular Linlai tube does not have this problem. |
@donsachs based on your ears, and others I trust (e.g. Aric from Aric Audio) the Linlai’s will be a great choice for the $$. At 1/3rd of the cost of the real WEs, I don’t expect them to be WEs..… just looking for a reasonable 300b solution without breaking the bank! @atmasphere I guess I have been lucky with my pair of treasure globes. They haven’t gone micrphonic on me yet after hundreds and hundreds of hours… they are now relegated to “backup” duty. Funny how just a few years ago I could not stand China tubes, and now, here we are! They’ve come a long way! |
I have only had 3 or 4 pairs of the tennis ball psvane 6sn7 tubes come through here and I have yet to see a microphonic pair. I still prefer the sound of the Linlai E-6SN7 though. @1markr I think you will love the Linlai WE300b. They take maybe 50-100 hours to really come into their own, so be patient. Just put them in and forget about it and listen. In a week or two or three they will really open up. I am sure the super expensive original WE300b, or some of the pricier other 300b types may be slightly better, but these are so good I really don't care. There are far greater returns from changes to amplifier topology than switching 300b types after you hit a certain level of quality of 300b. Also, I am listening to them in my push pull 300b amp that is the original subject of this thread. I cannot say what they sound like in a SE amp or preamp or wherever else you may be trying them. They are very good in my Lampi Pacific DAC as well, but I prefer my 46 tubes with adapters to any 300b there. So application matters a lot. |
@spazzghettie Looks like a nice amp. LC coupled and they use all DHT tubes. On the downside it is single ended so it won't have the bass punch nor the power of a push pull 300b. Also, I have moved on from LC coupling as I prefer custom interstage transformers, but that is my taste and it doesn't mean that Allnic amp is not a wonderful sounding amp. I also prefer solid state rectification and regulated supplies in power amps, but again, my taste. I am sure that is a very nice amp if your speakers can live with a 10 watt amp and you like the sound of tube rectification in power amps. It also has 6 dB of negative feedback. That isn't necessarily bad, but our amp project has zero feedback and you can tell. So again, they have made a nice compromise with good tubes, obviously good parts and a bit of feedback. I am sure it sounds really nice. |
Don I’m convinced of the merits of zero NFB tube circuit amplifiers implemented appropriately. Is this harder to pull off successfully? Does utilizing some degree of NFB confer a bit of a safety net? I inquire because its use is nearly universal with power amplifiers. Zero NFB amplifiers are a small niche. Charles |
NFB covers a lot of sins. You have to do everything right to have a stable circuit without it. It is actually considerably more complicated than that, but that is a good way to think about it. That allnic amp ad boasts a perfect square wave response. The feedback helps with that. It doesn't mean you cannot build a nice amp that uses NFB, but that "air" and sense of "realism" that you treasure is hindered by NFB. I can see what is inside their amp and have a pretty good idea what it sounds like. It is a dance. We have been working on this amp for quite some time and it will go into production late fall. We have tried many coupling methods and topologies and the fact that there is zero feedback lets you hear major differences between them. Again, there are many great amps in the world, most of which have some amount of NFB to make them really stable and limit distortion. That approach works, but has costs in terms of realism and spaciousness. That said, it doesn't mean those amps sound bad. Just different. I know the sound I value and I cannot get it with NFB, but others have different tastes or the need to drive speakers which are not at all tube friendly. So there are many paths.....and different sounds associated with them. There is no right way, just what we like. |
Negative feedback is the right choice for the vast majority of amps, particularly direct-coupled solid-state, where you can pile on the gain and use that "excess gain" to minimize distortion via feedback. To oversimplify, if you have 20 dB (a 10:1 voltage ratio) of excess gain, you can have 20 dB of feedback, which will reduce the distortion in direct proportion to the feedback ratio ... in this case, ten times. Pretty slick trick. In practice, as the excess gain goes up, and the feedback ratio increases, problems with stability creep in. Marginal problems with stability result in overshoots on square waves, and as it gets worse, brief periods of near-oscillation, and then full-power oscillation, which usually destroys the speaker. So you have to take account of the total phase shift on both ends of the spectrum, which includes the output transformer if it is included in the feedback loop. The phase shift of an output transformer typically limits tube amp feedback to no more than 20 dB, but this can be evaded by having multiple nested loops, as in the Citation amplifier deigned by Stu Hegeman in the early Sixties. But now we get into the (much) deeper waters of both slew-rate limiting and settling time, which are interrelated. That’s beyond the scope of this discussion, but they are limiting factors in any feedback amplifier. Multiple feedback designs can achieve impressively low distortion figures, but settling times can be much longer, since each nested feedback network has to leave saturation, return to controlled operation, and return to zero with its own time constant. These are not trivial design concerns, and made more complex by load dependence ... a reactive loudspeaker load will decrease the phase margin of the amplifier, and that in turn leads to longer settling times. As the phase margin erodes, settling times get longer and longer, until the amplifier breaks into self-oscillation. The other consequence of loss of phase margin is an increase in distortion, mostly at high frequencies, with the limit case of oscillation, which can be considered 100% distortion, with the output effectively decoupled from the input. For obvious reasons, great care is taken in the design phase to avoid oscillation, but there are amplifiers where stability is conditional on the load, with transient overshoots visible under some conditions of load and input stimulus. This was a serious problem with first and second-generation transistor amplifiers. (Which were designed with nothing more than slide rules and nomograms, so you can’t really blame the designers back then.) Nowadays, software modeling programs allow designers to avoid the stability problems of the early transistor amplifiers. If you want to jump down into the rabbit hole, read about "Nyquist Stability Criterion", followed by "Slew Rate Mechanisms" and "Settling Times in Feedback Circuits". For advanced practitioners, read about "Mixed Feedback Designs" and "Combining Feedback and Feedforward".
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By way of comparison, neither the Raven nor the Blackbird use any form of feedback, either local (around the tube) or global (around the entire amplifier). The incoming audio signal only flows forward, with no secondary paths around the circuit. Further isolation is imposed by isolated B+ supplies for input+driver and output sections, so there is no secondary path for B+ power supply intermodulation (clipping in the output section has no effect on the preceding circuits). This means distortion is entirely the result of device linearity in the specified circuit. The gains are scaled so each preceding stage has 3 to 6 dB of headroom compared to the following stage, so in practice clipping only happens in the 300B power section. The B+ regulator for the output section has a peak output of 200 watts, so the only limiting factor is the peak current capability of the output tubes. The performance of the 300B pair sets the performance of the entire amplifier. By contrast, in a feedback amplifier (of any kind, solid-state or tube), clipping and/or slewing creates large error transients at the feedback summing node. This can saturate the input stage, which means the entire amplifier is now clipping, and can lengthen the recovery time from clipping. |
@lynn_olson Another topic of discussion that has been undertaken by myself recently. Note: As stated previously, I am no EE, I discuss notions with EE's and EE minded people. The recent discussions taken part in, has been about the signal travelling through the audio system with as little of a interruption or diversion as can be put in place, with the value of such a approach, being that a direct signal is able to be produced as sound with no loss of content, however miniscule, resulting from an earlier manipulation of the Signal Path. It was strongly suggested, a later arriving signal (it is going to reach the speaker) can be a info that when finally processed to a sonic, can be perceived as having a effect on the content of the produced sound, such as a smearing/masking of a particular dynamic, detail or frequency extension. A demo' was also carried out to assist with my helping understand this at a very basic level, and I would settle / will be settling for the measures shown to help reduce the effects of what is claimed to be a delayed signal. In the case of the delayed signal, it was made known both circuit design, topology and component selection can all be a contributor. In the case of the demo' it was quite obvious that at certain places within the replay, there was a Vocal that was more comprehensible and certain notes were sensed as being a rendition that had attained a step further to being a more honest presentation. When I read your post, it does look likely to myself, through your circuit design and time spent voicing the sonic, that many of the items that were 'above my head' to address at the time of my discussion are addressed in your 300b Amp' Design. The description certainly prompts the idea, that it would be a real pleasure to be able to be in a room with this 300b design one day. |
@donsachs @lynn_olson , Your detailed (And well written) explanations are greatly appreciated. It heightens one’s admiration for the decision making, knowledge, skill and simply hard work required to design and build very high quality excellent sounding amplifiers. This thread could legitimately be separately filed/classified as a teaching course. No doubt that many following this thread have learned a lot and expanded their knowledge base. Charles |
What @pindac described is exactly what I heard when I first built the rather primitive initial "silicon assisted" stereo version of this circuit with a CCS on the plate of the driver tubes and a single regulated supply for each channel. I could hear things like subtle inflections in vocals, or the resonance in the low notes of a piano in a way that I had not encountered in all my years of building and restoring amplifiers. I had heard the stirrings of such things in single ended triode amps, but not with the drive and authority that this circuit presents. So then we spent 18 months or so experimenting with every permutation and combination and ended up with mono block amps with dual independent regulated supplies and all custom interstage coupling, and some old school VR tubes as well. I have not heard anything like it.... and the final version walks all over the one presented in Seattle. It sounds like it does for the reasons described above.... |
Lastly, I have said this before in this thread. It is about your design goals. We could make a low powered amp that uses 45 tubes in push pull and it would produce maybe 5-10 watts and stay in class A. I am sure it would sound incredible. If you had 94+ dB speakers it would probably be plenty of power. But 45 tubes cost a LOT, and many folks have 88-90 dB speakers and maybe a larger room. So our amp is 25 watts, well really about 27 watts, and has easily driven 88 dB speakers to screaming levels. Would the 45 amp sound better on more efficient speakers? I don't know. It would certainly sound a bit different, and possibly better. We tried to make an amp that had a much wider appeal and used modern production tubes that didn't cost a fortune. So it depends on your design criteria. The Citation II amp will drive darn near any rational speaker. It has three nested feedback loops and is very stable. It is one of the few tube amps you could let idle away for an hour on a bench with no speaker load attached and it would not oscillate. That said, it doesn't have the clarity and just spaciousness of the zero feedback DHT circuit. It makes wonderful music and is non-fatiguing to listen to, but it doesn't have "the piano is in the room" sound of the 300b project. I know this because I rebuilt about 80 of them, and lived with one for a few years. I could rebuild one in my sleep:) Ultimately, it depends on what you want the amp to do. I want an amp that will drive a pretty large number of speakers and have the clarity of a flea watt DHT, with the drive and authority of a push pull amp. So that is what this project is about. Trying to get a sweet spot that will make a lot of people happy and not cost a fortune to re-tube, and to run the tubes at really sane operating points so they last a long time, while pushing the state of the art sonically. There is no single design that can make everyone happy and drive all speakers and retain all the sonic characteristics we desire. There are always compromises. Tubes vs SS, SE vs PP, etc... Within SS and tube worlds there are many topologies, devices, and tube types. So there are many choices. For me to say our way is the best is ridiculous. So our comments in this thread just point out why we have made the design choices we have. |
I should note my description of feedback circuits is a grossly oversimplified, non-mathematical overview of a complex subject. For the curious, read about how op-amps are stabilized, and the concepts of loop gain, excess gain, dominant-pole compensation, and phase margin. Once you get a reasonably firm grasp of how it works, then read about slewing distortion and settling time. I tend to use settling time as a figure-of-merit when looking at op-amps, or more complex discrete circuits. It all comes together at the summing node, which is simply an analog comparator between input and output. In an op-amp, which has extremely high forward gain, the high gain of the op-amp forces the differences between the two nodes to zero. This is fine until the op-amps clips or slews, which creates very large error voltages at the comparator input. The large error voltage can force the comparator itself into nonlinearity, and feedback theory relies on a distortionless comparator. In addition, if the comparator is saturated, or if the power supply sags or is discharged, then recovery time can be quite long (tens or hundreds of milliseconds), much longer than the original clipping or slewing event. During this settling time, amplifier distortion can be quite high, since feedback is only partially effective. This will not appear in FFT harmonic distortion or multitone IM distortion measurements, which are taken over several seconds and then averaged. This is the gap in existing measurement techniques. Harmonic and IM distortion are averaged over several seconds, and do not sense events happening in microseconds or milliseconds. High-speed scope measurements are insensitive to distortion unless it is very high, such as 10% or more, where it becomes visible. Transient distortions, in the microsecond to millisecond range, are not seen. The key principle of non-feedback amplifiers is they are insensitive to transient upsets or interactions with the load. Steady-state distortion is higher, but there are no issues with phase margin or settling time. |
That depends on how the feedback is implemented!! If the feedback is sent to a non-linear point in the input of the amplifier which is used as a feedback node (such as the cathode of an input tube) then you can expect it to be problematic, as Crowhurst pointed out 60 years ago, and Baxandall 'rediscovered' 15 years later. In other areas of electronic design, feedback is known as 'control theory' and is very well understood. But in audio, it seems to get misapplied (and so gets a bad name) on a regular basis, then everyone points at feedback being the problem when its really just design flaws.
The settling time referred to above is a process of many amplifiers with feedback, but not so much opamps (unless overloading, which is easily avoided). In a nutshell, the reason you run into the problem described above is that part of the amplifier circuit is not in the feedback loop. So it can behave as described and as pointed out, lots of test equipment ignores this phenomena, although it can be measured if you have advanced gear. There is more at this link: https://linearaudio.net/sites/linearaudio.net/files/volume1bp.pdf If you don't want to read the whole thing, start at page 11, where the math is a bit lighter- but stay with it till the end of the article- its all relevant to this conversation.
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Following Lynn and Don recommendations, I added a separate filament transformer for 300B. It made the sound clearer. Then I added a separate transformer and rectifier for driver and input tubes. I use a Hexfred bridge (Ralph's recommendation) and C-L-C-R-C filters. With 30H chokes for each channel. Input tubes B+ is connected to the capacitor after the choke with R-C. I’m going to connect it to the driver B+ capacitor later when I move from RC to IT coupling between input and driver tubes. The 300B output tubes are fed by an old transformer with the 5u4g Linlai rectifier with CLC and each channel has separate 15H choke and B+ capacitor. This upgrade doesn’t break in yet. Two weeks and around 30 hour is not enough (despite all capacitors being previously used). But what I can hear is that the bass control, speed and rhythm accuracy are significantly, radically better than they were before. I can’t hear improvement in the midrange yet. I also can hear the tone of instruments loose a little bit "tube magic". I understand I need more break in to make a more accurate conclusion. |
Areas for improvement: The 5U4G rectifier is not ideal. I’d use HEXFREDs, high-voltage Schottky rectifiers (Don’s choice), or damper diodes for the 300B plate supply. Any of the three will have more dynamics and more vivid tone colors. The improvement should be immediately audible, two weeks will not be needed, you should hear it right away. RC coupling will sound more dull and compressed compared to dynamic loads, LC coupling, or IT coupling. What RC has going for it is resistor coloration is less potentially noticeable than the other three methods, which each demand very careful component selection. The final capacitor coloration of the filter sections will be audible, although less so than the cathode bypass caps, which are extremely sensitive to cap coloration. I wouldn't try mixing and matching Ralph's approach with ours. Ralph has his way of doing things, and his own unique taste in sonics, and we have ours. Most designers in this biz have a distinct "house sound" that they aim for, which results from design approaches and parts selection. |