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.
My first experience of a 300B amplifier was back when I was writing reviews for Positive Feedback magazine back in 1993. It had grown from a 4-page mimeographed club magazine for the Oregon Triode Society to a fat 100-page periodical with cartoons, editorials, and a staff of reviewers.
I was one of them, after submitting a series of construction articles for the Ariel twin transmission line loudspeaker. I was listening to a whole string of amplifiers when David Robinson, the magazine editor, dropped off not just the Audio Note Ongaku (which cost three times the price of my car) and the Reichert Silver 300B’s. So now I’m in the reviewing business, too. Oh well, not one to look a gift horse in the mouth, away I go, with a month’s free use of entirely new amplifiers on my brand-new speakers.
Which was quite a revelation. When you design a speaker, and you’ve been doing it for decades, you get to know them pretty well. What they can and can’t do, and what the overall character of the speaker sounds like. Nothing new to me, and the Ariel was my eighth speaker design, after my time at Audionics during the Seventies.
One of the last stages of a near-commercial design is auditioning the speaker on many different amplifiers, keeping in mind the impact of output impedance (damping factor) on the bass alignment and the crossover. The Ariels are designed to have low sensitivity to output impedance: transmission line bass with no impedance peaks, low-Q 2nd-order crossovers, very flat drivers that don’t need equalization, and a tweeter that is running flat out, with no attenuation, since the paired midbass drivers have matching sensitivity. So all amplifiers see an equal playing field, with a minimum of power disappearing in resistors.
The shock with both DHT amplifiers was a radically different sound than Class A transistor, Class AB transistor (with high slew rate), and push-pull pentode. Much higher transparency and much more vivid tone color ... out of a speaker that I knew very well, and was my own brainchild. This is what led me to design my own DHT amplifier, but not following the path of either the 211-based Ongaku or the Reichert Silver 300B. But definitely using the 300B for sure; no 2A3 came close, and 211’s and 845’s (with plates running at 1 kV) are extremely difficult to use.
Don has brought me up to speed on modern power supply design, things I didn’t know about in 2003, when the Karna was designed. That takes the Karna to a new level, and reduced it from a ridiculous four chassis setup with high-voltage Amphenol connectors to a much more sensible pair of monoblock chassis. One pleasure of working with Don is he will chase down every possible variant, build it, audition it, and let me know how it measures and compares to all the rest. Fortunately, we have gone full circle and have arrived at a Karna Mark II with far superior transformers and power supplies.
Once and alway, no participant on this forum has more erudite and encouraging responses to all things audio related than you. Charles, thanks as always for your upbeat posts, alway a pleasure to read. Mark
If you have followed this thread, Don and Lynn have made significant enhancements to the performance of the pre-pro mono's I have, yet I just can't imagine how much better the SQ will be after Don gets them back and updates them to the final production version
I eagerly await your listening impressions of the updated mono blocks. You certainly have terrific speakers to judge what you hear.
Well, Hawkrising, your last sentence might be correct but I studied Economics, not electronics, so I rely on excellent designers and fabricators like Don and Lynn for my front end gear and, in exchange, I compensate them for their efforts.
Aspirationally, the ultimate audiophile could take a stab at recreating the something akin the stellar Lampizator DAC's, like the $20K Lampi DAC in the Spatial room in Seattle which was the source for the music they presented. Totally stunning sound, but likely far beyond the skills of most audiophiles. I am not sure even Don or Lynn want to tackle building a DAC.
Don/Lynn vs. Ralph seem to have somewhat different notions of how these tube amps might be best designed and fabricated, which reflects different design criteria. That's cool. Lots of designers have different design philosophies... probably all of them. I haven't heard Ralph's 300b amps which might at some point be an interesting comparison to my 300b amps. ;-)
Don sent me his pre-pro 300b amps a few months ago which I have been using to power my very efficient Cube Audio Jazzon speakers and the sound is simply ethereal, like nothing I have every heard in my 50 years of audiophilia... a remarkable improvement even in comparison to Don's KT88 amp, which is outstanding, a legacy amp for sure, and the 300b mono's are better in every way. The musical notes simply hang in the air, suspended in the room in front on my listening seat. Matching of speakers is required for the 300b monos, but I drove some 86 db efficient standmount speakers with alacrity... my ears gave out long before the 300b amps did, with not a hint of distortion so as Don says, the 27 wpc amps rather sound like a 100 wpc SS amp. That is what my ears tell me so the amps will drive a wide variety of speakers that many other 300b amps simply will not.
If you have followed this thread, Don and Lynn have made significant enhancements to the performance of the pre-pro mono's I have, yet I just can't imagine how much better the SQ will be after Don gets them back and updates them to the final production version. However, I have no doubt that I will have one of the finest 300b amps possible at any price so always with Don, it is "promises made and promises honored."
The final point I'd like to make is that Lynn envisioned this employment of 300b tubes in an earlier iteration of it and he and Don put their collective wisdom together to update the design with these new mono's. Just as with fleawatt 2A3 power tubes, the 300b power tubes have a sonic signature, to my way of thinking and to my aging ears, are unlike any other tubes in power amps and I have had many of them over the years. Once you have heard them, you can't unhear them... so long as they are matched properly with compatible speakers which in fact will include a lot of speakers that many other 300b amps will not drive properly . Cheers.
As I read the last few posts, it seems like objective approaches contain a subjective element (the proverbial yin-yang). Designers can perform the same experiment and draw different conclusions based on what they hear or don't hear. Perhaps the ultimate audiophile is the guy who designs his own gear and seasons to taste. :)
@atmasphere First off, this thread was started by a fellow who has a rough first prototype of a zero feedback push pull 300b amp, so of course we are discussing that. I agree that in that circuit you hear everything. I will tell you that even in circuits with feedback I will not use electrolytic caps in the signal path. I also do not use them in my power supplies, but only in the filament supplies. In my experience electrolytic caps make circuits sound grey and hazy compared to a very top quality film cap. You can wait forever for a lytic to run in, you can bypass it with small film caps, and whatever else. When I replace them with a very good film cap I hear the difference. To each their own. I also hear differences between resistor types in key spots in circuits, whether the circuit has feedback or not. Again, my experience, so I build amps with the parts types that I favor.
But there are choices that affect the sound. If you need a 1 watt 1K resistor you can use any type on a cathode, but different types have different sounds. The cathode bypass cap might need to be 100 uF. Different 100 uF caps sound very different. All will satisfy engineering standards, but parts and layout choices have profound effect on the final sound.
When you run zero feedback, the circuit has no ability to reject things like this. So everything makes a difference. However, for something like an electrolytic bypass, I think you'll find that as long as the part is good quality, the big differences you hear will be more about the part forming up over time: they will arrive at the same place sooner or later.
For reasons that are not clear, various brands of metallized polypropylene capacitors sound quite different from each other, and there is little correlation with DA and DF parameters. Based on measurements, they should all sound the same.
Sometimes you have to do your own measurements because the specs of the manufacturer don't always tell the whole story. If you use a precision differential amplifier to drive the caps in question, you can measure how they behave and differ from one another while in circuit.
Let’s talk technical about DHT filament power supplies. My friend John Atwood built a low-RF filament supply a few years back (100 kHz or so), but he made the discovery that the filament inductance of DHT tubes was all over the place, making it difficult to assess how much power was actually going into the filament. This is a big deal because DHT tubes require very tight control of filament power, preferably 5% of specification, or better. In practice, the RF supply had to be individually tuned for each tube ... and it didn’t sound any better.
An aspect of even very clean sinewave AC heating are "hum sidebands" ... not hum per se, but IM distortion harmonics that are displaced by 100/120 Hz on each side of the fundamental and about 60 dB down. It is clearly visible on a good spectrum analyzer, and is caused by small temperature fluctuations on the filament modulating emission, which in turn modulates the forward gain of the tube ... not much, but it is measurable. (The discovery of hum sidebands led to the experiments with RF heating.)
An aspect of DC heating is where the virtual center-tap appears. If the virtual center-tap is in the middle of the filament, the inherent balance of the filament can give a free bonus of 30 dB or more of additional noise rejection. Considering how difficult an additional 10 dB of noise rejection can be, 30 dB is not to be sneered at. And the whole DC supply has to float, relative to ground, while appearing symmetric from the viewpoint of the filament (mimicking an AC supply in that respect).
Although I don’t enjoy "tuning", it is an unpleasant necessity for speakers and power amps. For reasons that are not clear, various brands of metallized polypropylene capacitors sound quite different from each other, and there is little correlation with DA and DF parameters. Based on measurements, they should all sound the same.
On a system with moderately high resolution, subjective differences appear that can mimic crossover balance shifts and driver swaps. I found during development of the Ariel, back in 1993, that cap substitutions required 0.5 to 1 dB crossover adjustments to subjectively offset the colorations ... and this was with pink-noise test stimulus, not music.
You can really get into the swamp comparing silver vs copper wire. This should not be audible at all, and I have heard of no convincing argument why any differences are audible. You can go out on a limb and compare silver oxide vs copper oxide, and various weird sources of corrosion, but it’s all very speculative, and again, no useful measurements to be had.
On the other side of the objective/subjective fence, I have heard of well-known speaker designers who never audition their new speakers ... they do it all by numbers, then walk away. I frankly didn’t believe it when I first heard that about fifteen years ago, but other folks confirmed it, so I guess it happens. So it is possible to ignore "tuning" and let the product sound like whatever.
But in the speaker world, it is widely recognized that a "perfect" zero-coloration speaker is impossible at the current state of the art, so it comes down to choosing which set of parameters are most important. Speakers are still very imperfect, compared to any other audio component.
In principle, it should be possible to design and build a zero-coloration amplifier. I started my career in audio design in 1973, and haven’t heard a "perfect" amplifier yet. They all have a sound, and a little bit worse, topologies tend to have distinctive sounds. But that’s my personal experience, not necessarily the experience of others.
If a customer, or reviewer, is in the fortunate position of finding that all well-engineered amps sound alike, that’s great! You can sure save a bunch of money, skip over tubes entirely, and just buy the latest Shenzen-made confection for a few hundred dollars.
I can build an amp with entirely different parts and wire and have it be exactly the same from an engineering standpoint, but have a very different sonic presentation. I believe that is what Lynn is alluding to.
Exactly my take away from Lynn. Most of the builders of Japanese tube amplifiers are degreed electrical engiineers from their native universities. European educated electrical engineers with Nagra, Gryphon, CH Precision etc. No question that although their products are at opposite ends of the audio spectrum, they all adhere and practice engineering principles they were taught.
You can certainly follow sound engineering practices and yet have vastly different sounding audio products. I’m sure that Porsche and Ferrari are built adhering to engineering principles and yet drive/feel very differently. A product’s performance can be tailored to achieve a desired goal.
We all apply sound engineering practice. We overbuild all power supplies and over spec all parts. But there are choices that affect the sound. If you need a 1 watt 1K resistor you can use any type on a cathode, but different types have different sounds. The cathode bypass cap might need to be 100 uF. Different 100 uF caps sound very different. All will satisfy engineering standards, but parts and layout choices have profound effect on the final sound. We make these choices to achieve the sound we desire. There might be 2 or 3 different tube types that would be appropriate for a place in a design, but can have very different sounds. I can build an amp with entirely different parts and wire and have it be exactly the same from an engineering standpoint, but have a very different sonic presentation. I believe that is what Lynn is alluding to.
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.
FWIW Dept.: We've never had a house sound nor 'voiced' our circuits.
My comments thus far have simply been based in sound engineering practice. Engineering after all made audio products possible, has kept airplanes in the air and provided reliable power when you want light in your house.
It will always work to apply sound engineering practice to circuit design, plain and simple.
There are a lot of little tricks to DC filament supplies that make them sound better too. It isn't rocket science, but you can get a bit more performance and noise rejection by connecting them properly:)
I don’t think DC, IF well done, is any inferior to AC in DHT though. But there are many ways to get DC done. So it gets down to implementation at the end.
The commercially available 300b amplifiers seem to bear this out. By a wide margin they are DC heated rather than AC heated. AC is utilized by the DIY crowd more (Relatively speaking) than commercial manufacturers.
For DHTs such as 300B, AC at main frequency is hard to be hum free. But some have tried AC at a high frequency well above human hearing limit with decent result. Sadly this is mostly in DIY world, hence very few commercial offerings with high frequency AC DHT filament supply.
I don’t think DC, IF well done, is any inferior to AC in DHT though. But there are many ways to get DC done. So it gets down to implementation at the end.
@alexberger Hi. I just tried schottky diodes maybe 5 or 6 years ago and I liked the sound. They have a fast recovery time. They also have very low voltage drop so are good for small filament supplies, but the main reason is the fast recovery. I didn't hear any difference compared to the super high priced hexfreds or other types in the sort of supply I build. The automotive schottky diodes are not expensive and they are available in voltage and current ratings that far exceed requirements of the circuits I am building so there is plenty of headroom, and you can get to-220 versions that fit everywhere. So basically, they work well, sound really good, and are readily available.
Regulated high voltage supplies are also a very hot button issue. I know what I prefer from having built the same circuit with multiple power supply topologies and critically listened to each. The way I do it costs more money and certainly has to be worked out so that it is very reliable, but once you get to that point.... As I said, I value absolute blackness between notes and very fast transient response delivered by supplies that are grossly over rated for what they have to deliver. Other people value other things..... to each their own path.
As DIY I can adjust my amplifier to my own taste preference. But compared to experienced Hi-End equipment designers I have less time, money, experience and knowledge. My BCs in Electronic Engineering helps me a little bit. I do digital ASIC design for my living that is far away from analogue electronics.
I learn much when reading the rationale of opposing positions. AC versus DC filament heating is one of the classic topics when discussing tube amplifier circuits .
@charles1dad Well, let's just say I have experimented and know which I prefer:) The 300b amps have a dedicated 5V regulated DC supply for each 300b tube and a 6.3V DC regulated supply for the drivers and input tube. The preamp has a 6.3 V regulated DC supply for the pair of 6SN7 tubes. I value blackness between notes:)
The preamp and amps use regulated DC filament supplies in all positions
I know there are some DHT amplifier (Particularly with regard to SET) purists who insist that AC heating is sonically superior to DC heating.I have to say though that some of the very best DHT amplifiers I have heard utilize DC heating.
No doubt that either can work fine with excellent implementation.
I am pretty much the last guy on the thread to answer your question as "DC heaters" is one of a million design criteria that Don, Lynn, and Ralph understand very well and I think one of them will deign to respond to your question. I think the dead silence of Don/Lynn's amp is a function of a variety of factors... best I can put it.
Has anyone that is using Linlai E-6SN7 tubes in 300B amps found them to be noisy? Kind of like a ground hum but not very loud. The system was quiet with the previous RCA VT231 tubes. Sorry if I'm off topic a little. They were favorably mentioned here so I gave them a try. Thanks.
In my SET amplifier is IT coupling between driver and 300b and RC between an input and a driver stages.
I think I understand why the tone issue appeared. I played with driver tube B+ voltage and current. The cathode resistors 680 Ohms weren’t changed. The driver is 6f6 in triod mode. In begging, the idle current was 20mA. Then I increase it to 23mA and then to 26mA. So, in the middle setting 23mA, the sound character was faster and rough with flatter musical instruments tone. With 20 and 26mA, the amplifier sounds more like SET, the tone is more reach, and speed is more realistic and natural.
I should mention SE tuning is not the same as balanced-amp tuning. The dominant coloration with SE are the tubes themselves, and it requires artful selection to avoid heavy additive coloration. The fad for 2-stage SET amps makes this worse, since high-transconductance tubes are not designed for audio, and distortion can be all over the place using tubes designed for RF use. Selecting designed-for-audio tubes in a 3-stage amp makes things simpler and more manageable.
Unfortunately, common design practice in Golden Age push-pull pentode amps is not helpful in designing a balanced non-feedback amp. All of the 1950’s and 1960’s Golden Age amplifiers use feedback as a required part of the design, and the "balanced" part of the circuit (the output section) is typically running in Class AB. You have to reach back to the 1930’s to find useful non-feedback Class A designs.
Once pentodes and beam tetrodes took over, feedback came along with them, and that changed the overall emphasis of the contemporary designs. The search back then was for more power, more efficiency with B+ supplies of 500 volts or less, low measured distortion, and cost and weight reduction.
That search reached an end when high-powered transistor amps replaced tube amps around 1966~1968. Transistors dominated the broad consumer market with the exception of guitar amps, which kept the tube factories going. The decades-long Japanese fascination with triodes finally came West in the early Nineties, where it created a niche market in the high-end sector. (Helped along by Joe Robert’s "Sound Practices" magazine.)
It really helps that tubes are now so popular in the true high-end sector of the market. Transistors ruled the market in the Seventies and Eighties (with the exception of Japan), and the Nineties were an era of transition and growing acceptance of tube electronics. The home theater and 500-watt crowd are still all-transistor because they need the efficiency, and Class D will give them even more efficiency.
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.
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.
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.
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.
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.
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:
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.
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.
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.
What @pindacdescribed 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....
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.
@lynn_olsonAnother 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.
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.
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".
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.
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
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.
@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.
I have had the Linlai tennis ball 6sn7 tubes in my DAC for 9 months running 24/7 unless I'm gone for an extended period of time, no microphonics so far. I'm sure the old nos tennis ball tubes could become microphonic because the support structure is far different.
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.
@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!
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