Well... Lynn has lots of wild ideas, but the thing is that they are very well thought out, and they are based on years of technical experience with Tektronix. I learned many years ago in academia, and as part of my main career in forest ecology research, to listen to really smart people with wild ideas. Many of them were simply ahead of the mainstream thinking. The mainstream often ends up there.....eventually.
Lynn suggests improvements and where practical, I try them. We finally ended up with a larger chassis that could accommodate all the things we wished to try. They are still under 19 inches so they fit any rack, since 19 inch is the traditional rack width. To my ear, it worked beautifully.... The final touch was the addition of old school gas VR tubes to further isolate the input tube supply from the drivers, and of course the move to KT66/6L6 or KT88 drivers. I think we are done finally.
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And I am very much looking forward to fellow enthusiasts hearing the Raven preamp and Blackbird power amp. I’ve been a voice in the wilderness for about twenty-five years ... neither a member of the SET fraternity (well, maybe on the edge of it) nor mainstream Audio Research/Jadis/Conrad-Johnson push-pull pentode big-watt amplifiers dominating the hifi shows. A handful of people built the Karna amps, but many abandoned the difficult project halfway through.
Don was one of the very few who persevered through two years of building prototypes that were far off the beaten path of mainstream tube gear. He’s had plenty of hands-on experience with the fiendishly difficult Citation II, the most complex amplifier of the Golden Age, and his own designs, the Valhalla (6L6) and Kootenai (KT88).
Of all the people I know in the industry, Don is the most qualified to honestly tell me what is unrealistic and pie-in-the-sky, and what is practical and a good solution. He’s been there and done that. Oh, and he has good taste, too, which isn’t that common in the industry.
You might think I’m being snarky about the "good taste" but I am perfectly serious. The industry has plenty of competent engineers, and whole hifi shows filled with high-powered marketers, but good taste? It’s not all that common, and I’ve been in the industry since 1973.
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@tinear123 No. Production starts in late November when I go to Salt Lake and teach the guys at Spatial the builds. I would expect a review by late spring and perhaps an audio show in the west somewhere next summer. That is kind of going to be the schedule I think. There will be a review pair that could potentially end up at a show in the east next year, but that would be Spatial Audio Lab's call. I really wish that folks could hear what I am listening to in my living room. It would be fun to have any of you over. There will be a complete setup in Salt Lake City area by January and I am sure audition arrangements could be made for anyone who wants to hear it along with a top end Spatial Audio Lab speaker. Of course this doesn't help any of you out East....
It is definitely the plan to have a review setup of both preamp and amps and have it reviewed by legitimate reviewers next year as early as possible. Perhaps sometime next year we can have them appear at a show in the east somewhere.
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Hi Lynn & Don... Will you be at Capital Audio Fest with the Blackbirds?
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A current source could replace the resistor, but in practice, the performance is very similar to a current source, so it’s rarely done even in modern amps.
Usually in a differential amplifier, the plate resistors are matched if both halves are driven. In the case circuit of the above description, only one side is driven. So if a CCS is not used, the plate resistors can't be matched; one side must have a slightly higher plate resistance to compensate for the mu (gain) of the tube of the un-driven side, so as to get equal outputs from each half.
A good CCS eliminates this problem (which is nice since in the real world you can't count on the mu of each section to be equal or matching that of the tube specs on paper). A good CCS is both inexpensive and reliable, allowing the tube to be removed from the circuit while active (hot plugged) without damage.
Further benefit can be had from placing a CCS in the output tube cathode circuit, if you control the output tube(s) bias using fixed grid bias. The cathodes are tied together and the CCS feeds them; thus improving the differential effect of the output section, which reduces distortion and makes it slightly easier to drive due to increased gain.
Of course, if you have the input tube be a differential amplifier too, it can accept a balanced or single-ended input and can have excellent performance if a CCS is used for this stage as well. But its not a good idea to direct couple both plates to the succeeding driver stage; its OK to do one but the other should be capacitively coupled so as to prevent DC offsets of the first stage of gain from causing distortion in the driver.
The original BAT VK-60 of the 1990s used a differential input direct coupled to a differential driver; to deal with the DC offsets a potentiometer in the cathode circuit of the input tube allowed the plate voltages to be equalized. I found this approach to be problematic (we had tried that back in the early 1980s; one obvious problem is that it requires the user to make this fairly critical adjustment...) and often causes more problems than it solves.
So in the quest to keep the number of coupling capacitors down but retain easy operation, we started using a differential cascode voltage amplifier. The advantage of this was that all the gain of the amplifier was in a single gain stage, consisting of three dual-section triode tubes, one for the input differential amplifier, one for the top of the cascode, being plate loads for the bottom tube sections, and finally a 2-stage Constant Current Source for the circuit, tied to a B- supply of equal potential to the B+ supply. The CCS prevented changes in the AC line voltage from affecting performance of the voltage amplifier from 107VAC to 126VAC the difference was only 17 parts per million. So you couldn't see any performance change on an oscilloscope over that range!
So that allowed for enough gain, low distortion (once the correct operating point was set up), and only one pair of matched coupling caps (of a small value, in our case only 0.1uf, further minimizing the sonic impact of the coupling caps). They drive a pair of cathode followers which are direct coupled to the output tubes. So the power tubes obtain their bias voltage from the driver; therefore the bias and DC Offset controls are in the grid circuit of the driver tube. This allows for instantaneous overload recovery and rock solid bias control of multiple high-capacitance triode grids, with low frequency response to 1 or 2Hz no problem at all.
If you use a coupling cap in the critical area of the grids of the output tubes, it must be large so as to get good bass response since the grid bias network must be of relatively low impedance to properly control the power tubes. This means that the driver tube has a difficult load to drive and the large coupling cap can cause blocking distortion and slow the overload recovery. While this really isn't much of a problem driving pentodes, using this topology to drive triodes is a bad idea IMO/IME.
I've been describing how our OTLs work but obviously this would work well with a 300b too. We've managed to get our OTLs to 0.5% THD which is pretty low distortion for a zero feedback circuit! SETs by contrast tend to be about 10% THD at clipping which might be only 7 Watts. Since the OTLs tend to be much higher power capacity, the tendency is, for any power level the SET might have, the OTL has distortion that might be 2 orders of magnitude lower or more at the same power level. This is why they tend to be so much more transparent than SETs. I've no reason to think this cannot be applied to a 300b circuit with similar results; SETs have the distortion they do out of the topology rather than the power tube that is used. So a pair of 300bs could be used to much greater advantage!
For those that might want to see more about how our OTLs work (and how this might be a topology for a 300b amplifier), there is a DIYaudio.com thread from several years ago that has a schematic and discussion. A lot of this would work very nicely with a 300b; for example the Circlotron output can be transformer coupled of course and have all the advantages (such as zero DC saturation of the output transformer) it offers.
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As a minor diversion, I should describe the "Golden Age" amplifiers I keep referring to. This aren’t just the amplifiers made in the Fifties and Sixties; it describes the majority of PP tube amps made since then, including today.
There were only a few basic Golden Age circuits, or topologies, as we like to call them. (Topologies omit circuit values, but are easily worked out once you know the tubes.) The first was the Williamson of 1948, but it had the drawback of marginal stability. Still, it dominated the US market until 1955 or so, when the much simpler Dynaco variant came in. (The Dynaco topology simply omits the driver stage of the Williamson and uses the phase splitter to drive the output tubes. More distortion but more stable.)
The Mullard became the prototype of many tube amps as the better-performing alternative to the Dynaco circuit, and is still widely used today. Let’s walk through it.
There’s a high-gain input tube, typically either a 12AX7 or a pentode like an EF86. This is direct-coupled to one half of a differential stage, with the other grid AC-coupled through a cap to ground. Because the grid of the diff stage is at 150 volts or so, the cathode is a little bit higher, maybe 155 volts. This requires a large value resistor that goes all the way to ground, so the diff stage is frequently called a "long-tailed pair". A current source could replace the resistor, but in practice, the performance is very similar to a current source, so it’s rarely done even in modern amps.
The diff pair are a pretty good phase splitter, and unlike the split-load inverter of the Dynaco circuit, audio-frequency balance is not too sensitive to load. It also has more drive capability than the split-load inverter, and unlike the split-load inverter, it has some gain, too. So a win all around.
And we’re not talking about a lot of parts here: 3 triode sections, and the output pair. A Dynaco is even simpler, with 2 triode sections, and the output pair. The only coupling caps with either circuit are between the grids of the output pair and the preceding circuit, so not really complex, and simple enough that a stereo chassis, running off a single B+ supply, is quite practical.
The point of the high gain (in the input section) is to give feedback something to work with. Feedback requires "excess gain" to work its magic; you need 20 dB of excess gain to get 20 dB of feedback, which will reduce overall distortion tenfold. In a pentode or ultralinear connected amplifier, the output impedance is way too high to use with most speakers. The feedback really comes in handy here: 20 dB of feedback reduces output impedance tenfold.
What limits applicability of feedback is loss of stability if too much is used (I’m not going to get into Nyquist Stability Criteria here, nor phase margin, settling time, etc.) In other words, if we slap in another gain stage and try for 40 dB of feedback, it will just oscillate. At full power. And take out a tweeter before damaging itself and letting the smoke out.
A more clever approach is wrapping local feedback around the most distorted stages, like the output section, and then add overall global feedback on top of that. This was done in the McIntosh, Citation II, and a few other amplifiers. This really gets the distortion numbers down, but clipping can get ugly, and settling time from transients can be an issue. Multiple feedback amplifiers can be quite sensitive to operating conditions. It’s more often seen in modern transistor amps as "two-pole compensation", and is not trivial to design.
Note: To puzzle out a schematic, by convention, signal flow is left to right, just like you’re reading this. To see what a tube is doing, look what the grid (the dotted line) is connected to. Often, there will be a coupling cap, typically 0.1uF. If it is much smaller than that, like 30 mmF or 30 pF, it is bypassing RF or has something to do with stability. Larger caps are cathode bypasses or power supply. The plates (the flat-topped dingus) is the output of the tube and typically heads to the right side of the schematic.
You usually have to stare at a phase splitter quite a while before the function becomes obvious. One side is quite simple, coming directly from the input tube, but the other side can be pretty weird. A diff stage can be puzzling, because the DC connection is a high-value resistor going to the other grid, and the AC connection just goes to ground through a 0.1uF cap. The "other half" is actually driven from its cathode, not the grid.
What gives away a split-load inverter, or "concertina" stage, are the equal cathode and plate resistors. This is a dead giveaway you are looking at an inverter, since no other tube stage uses equal resistors ... for one thing, it’s kind of useless for anything else, since gain is a bit less than unity.
I leave the "floating paraphase" as an exercise for the reader. I kind of like them, actually, because current drive for the power tubes is pretty good, although balance is only so-so.
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@atmasphere
perhaps this thread might have convinced you there is more than one way to reach audio Nirvana 😉 I’m sure the Blackbird is well worth hearing
Different pathways to audio nirvana is something I’ve acknowledged long ago. It’s an undeniable individual journey with numerous successful outcomes. What I have found to be most pleasing and satisfying for me certainly may not be the choice for another.
I don’t believe my comments above contradict this perspective. I was merely comparing two earnest efforts to build amplifiers that are vastly different in concept, design and implementation.
Charles
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@charles1dad My point was addressing a comment made earlier by Lynn about overshoot in amps employing feedback; simply that if you do it right its not a problem. The OP mentioned using Spatial Audio Triode Masters who were a dealer of ours and have used our OTLs and class D on their speakers. It didn’t seem that off topic, especially if we discuss the issues of signal coupling, operating points and the use or lack of use of feedback.
As I understand it, you are particularly enamored of SETs; perhaps this thread might have convinced you there is more than one way to reach audio Nirvana 😉 I’m sure the Blackbird is well worth hearing.
How does excessive transformer ringing can influence on sound?
Does it make it too sharp or bright?
@alexberger Ringing contains higher ordered harmonics which can be heard as brightness and harshness. You also get lower orders which contribute to richness. Both are colorations and will obscure low level detail.
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Hi @atmasphere ,
How does excessive transformer ringing can influence on sound?
Does it make it too sharp or bright?
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@atmasphere
our class D, which has less bandwidth owing to the output filter, nevertheless has a very similar 10KHz waveform, despite (well, actually because of) running 37dB of feedback;
This GaN class D balanced amplifier with copious utilization of NFB is literally at the opposite end of the audio design spectrum from the DHT (300b) balanced class A zero NFB Blackbird under discussion on this thread. Talk about traveling different roads toward the destination of Rome.
The in depth information presented here concerning the Black bird amplifier has me exceedingly curious to hopefully hear it one day. Its development is a fascinating story.
Charles
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I changed the load resistor to 120KOhm, and the overshoot decreased by amplitude and attenuation time. But still, there is a notable overshoot. I measured a frequency response and there is a hump +1.7dB at 35KHz. There is -3db at 19Hz and 47KHz.
Should I decrease the load resistor more to remove overshoot completely?
If yes, in which value range should be this resistor? For example, if I take a resistor less than 50K it can increase distortions.
@alexberger As you have noticed, if you are using a coupling (interstage) transformer, it will be needing proper loading to prevent ringing (distorting). You are nearly there with your technique so far; put a potentiometer across the output of the transformer, run a square wave through the active circuit prior (6SN7) and adjust the pot for minimum ringing (critical damping). IME its probably best if you leave a very slight amount of overshoot as opposed to rounding the square wave.
Its important that the driver to the transformer be active, since transformers transform impedance: Whatever impedance on the primary side, if it varies, will affect the critical damping value on the output side. Conversely, whatever is loading the output side will also load the input thru the ratio of the transformer. So you want to feed the squarewave to the active 6SN7 circuit so your loading value will be correct. Best to have the 6F6 running also for this very same reason, although the loading resistor will likely dominate that side of the transformer equation.
Once that is sorted out, you might find it interesting to measure the impedance at 1KHz on the primary side (you won’t need the 6SN7 in the circuit for that, but it would be a good idea to have the 6F6 in place and active) just to see what the load on the 6SN7 actually is. You may find that you have to adjust the operating point of the 6SN7 to obtain greater linearity (by adjusting the cathode resistor value if you have one); if you do that then you may have to readjust the loading value of the transformer since the source impedance of the 6SN7 varies a little with the operating point; in this way zeroing in on the optimal values.
Obviously you can stop at any point (call it ’good enough’), but in a zero feedback design I’ve found that the more you pay attention to refining things like this, the more it pays off in the end!
@lynn_olson I agree overshoot in a circuit using feedback is bad!
But if the feedback is applied properly you’ll get no overshoot at all. Our OTLs don’t exhibit any squarewave overshoot, being zero feedback and free of inductors in the signal path. The 10KHz waveforms are pretty good since they have wide bandwidth; our class D, which has less bandwidth owing to the output filter, nevertheless has a very similar 10KHz waveform, despite (well, actually because of) running 37dB of feedback; the difference being the residual sine waveform imposed by the switching frequency.
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@alexberger Please report back on the sonic diff between the A-107 and RC coupling. Completely different amps, but I much preferred all IT coupling. Curious to hear your impression
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Hi @donsachs ,
Yes Dave Geren at Cinemag do great job!
I use Hashimoto A-305 IT between 6f6 (driver) and 300B. The square wave was almost perfect at 10KHz, 20KHz, 30KHz. I use an 80K Ohm grid resistor on 300B. But I’m not sure this resistor is needed! The -3dB frequency response is from 6Hz to 95KHz. This IT is extraordinary on measurement and sound.
For 6sn7 I use Hashimoto A-107 that is not as perfect but has more primary inductance that is essential for higher input impedance 6sn7.
Hashimoto A-305 is designed specially for SET, but A-107 is universal IT that can be phase splitter or SET 1:1 or SET 1:2.
I just installed A-107 and it has zero break-in time. So I can’t comment how does it sound compared RC (V-Cap CuTF, AN Silver Tantalum) that were before. It need more break-in time.
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Now’s a good time to discus the difference between overshoot in feedback vs non-feedback amplifiers. Despite similar appearance on the scope, they are caused by completely different mechanisms.
* Overshoot in a feedback amplifier is quite malign, since it indicates the onset of oscillation, something that can destroy the amplifier and the speaker it is connected to. It is caused by the amplifier running out of phase margin, possibly the result of a reactive load, but also the result of design oversights in the feedback loop.
* Overshoot in a non-feedback amplifier is quite different. Now, it might be the result of high-transconductance tubes self-oscillating, but this can prevented by grid-stopper resistors and good layout practices. Normally, though, it is merely transformer overshoot, the result of phase shift at the edge of the passband, and has nothing to do with stability. That is what we are seeing here.
Something to be considered about ultrasonic behavior in the time domain: if there is no spectral content in the frequency range of the overshoot, it will never be stimulated in the first place. It never happens.
This is the difference between overshoot in a feedback amplifier and a non-feedback amplifier: in a feedback amplifier, it is a warning sign, like the LOW OIL light in a car. You ignore it at your peril. In a non-feedback amplifier, it has nothing to do with stability, since there is no feedback loop to induce oscillation. It is simply the behavior of passive parts, in this case, the input and interstage transformers.
As you can see, Cinemag has done a very nice job here. (Same photo as above, just tidied up a little.)
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I just cannot resist. This is a poor photo of the square wave on the Blackbird interstage transformer between the 6V6 and the 300b at 1 KHz and about 30% power (7 or 8 watts). There are NO grid resistors or grid stoppers or networks of any kind. The primary of the transformer is wired directly to the driver plates and the secondary is wired directed to the 300b grids. The KT88 looks as good or better.... Lynn and I were quite impressed. Dave Geren at Cinemag knows what he is doing..... This is an all tube amp with no feedback anywhere.
Yeah, it sounds like it looks.....
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@tinear123 Hi.. write Cloud Sessions at Spatial audio lab and he will know what is in the crossover. I am sure they can build something for you.
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I went through that with the Amity amplifier in the late Nineties. Nice square waves vs sonics. The amp sounded best with NO grid resistor, and the overshoot was a non-factor. Your amp may be different, of course. Keep in mind that music sources will never excite the overshoot, since it’s all in the far ultrasonic, and most studio mikes are all gone by 25 kHz.
The tube doesn't need that 510K resistor. The DC impedance of the IT is a few kohms at most, and that stabilizes the tube at low frequencies. If the driver is really unstable, maybe the 510K resistor helps, but that's usually the function of a grid stopper resistor, something quite different.
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I have a technical question for tube gurus @lynn_olson @donsachs @atmasphere
I put the intrastage transformer Hashimoto A-107 between the 6sn7 input and 6f6 driver tubes in my SET amplifier. The load resistor connected to the 6f6 grid was 520KOhm. I measured a square wave output and it had a big overshoot. I changed the load resistor to 120KOhm, and the overshoot decreased by amplitude and attenuation time. But still, there is a notable overshoot. I measured a frequency response and there is a hump +1.7dB at 35KHz. There is -3db at 19Hz and 47KHz.
Should I decrease the load resistor more to remove overshoot completely?
If yes, in which value range should be this resistor? For example, if I take a resistor less than 50K it can increase distortions.
http://www.tube-amps.net/HP_A107.htm
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@Donsachs I'm interested in the specifics of the X5 crossover upgrades please.
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I see Lynn has spilled the beans on the driver tube change. I have listened to the 6L6 (Russian 6P3S-E), the KT66 (Shuguang black treasure KT66), and the KT88 (My stash of a quad of the now unobtanium Shuguang WEKT88). Of course I have spent almost two years listening to the 6V6 as the driver before this. In my system the KT88 wins hands down. It has midrange and bass that hits you in the chest at moderate listening levels. That is in a good way. It is vibrant and rich and tonally correct. It has all the highs of the other tubes, but sounds less thin. If you put any of these tubes, including the original 6V6 in this amp and had never heard any other driver, you would still think it was the best or one of the best amps you had ever heard anywhere. But the KT88 is just superb. That doesn't mean that in someone else's system they wouldn't prefer a KT66. I also have EL34 to try, but haven't bothered because I think it the worst of the octal output tubes of that ilk and never understood why anyone liked them. Mine are Mullards too....
Let's just say that with the KT88 you FEEL the music in a way you do not with any of the other driver tubes. You sort of LIVE the performance. It is quite striking, and the larger chassis of the final design, coupled with the very conservatively designed power supply, allows for the amps to easliy handle the roughly 14 watt dissipation of the drivers vs. the 7 watts of the original 6V6. It still runs very cool for a class A 25 watt tube amp.
What I will say is the output section is basically a supercharged amplifier of the driver section sonics. You clearly hear the nature of the driver tubes. This is of course true in other amps, but I have never heard the effect like this. Usually a driver tube change is audible, but not night and day. Imagine all the things you love about your classic KT88 amp with far less distortion and that KT88 rich full sound on steroids. You are inside the music in a way that none of the other drivers do in my system, in my living room. You get that KT88 sound without all the nonsense of feedback, RC coupling, whatever. Instead you get an effortless, breathless, KT88 rich sound with lightning fast transient response. This is related to slew rate and I will let Lynn discuss that if he chooses. What matters is how it sounds:) So now you have an amp with all large plate tubes, no feedback, that is lightning fast and will produce 25 watts all day long with tons of current. The previous 6V6 version has easily driven 85 dB speakers. The KT88 version... :)
Just to be clear, this is my system so those of you wanting to get a feel for it can know what I am using:
Lampizator Pacific DAC with 46 DHT tubes run XLR. Dac is modded to eliminate a cap between DAC board and output stage resulting in a very slight pop when it changes resolution at the beginning of a track. I find this benign, and the clarity is increased. Just saying so you know this is better than a stock Pacific.
This drives the Raven preamp and Blackbird amps, cabled with Paul's best Anticable XLR v. 5 something..... Speakers are Spatial audio X5 with seriously updated crossovers. I never heard the X5 wake up like this....
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Last but not least, although four driver tubes are required, they don’t have to be matched quads. Matched pairs are fine. It doesn’t matter if the drivers in one amp are at 50 mA each and the drivers in the other amp are at 55 mA each. The difference can’t be measured and won’t be heard.
So if you have a stash of priceless MO-Valve KT66’s, which were manufactured in matched pair sets for the amps of the day, go ahead and use them! They will be operated very conservatively (far more so than most power amps).
There are a pair of pin jacks on the top panel to check if plate voltages match. Measure across the pin jacks with a basic DVM, set the DVM to measure DC, and if you see 3 volts or less, you’re good to go.
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Let’s do a little quick math to see how hard the driver is working, compared to an output tube. The gain of a 300B is 3.9, according to this Western Electric data sheet. The interstage transformer has a moderate gain of 1:1.2, so the net voltage gain of the output section is 4.68. In power terms, that’s a ratio of 21.9, or 13.4 dB.
The driver is working at 0.2137 times the voltage swing of the output stage, into an open load that only has 60 pF of capacitance, and only draws current when the 300B grids pass into the A2 region at 80 volts swing. The driver can push the 300B grids at least 20 volts positive, and surprisingly, the 300B remains linear in this region, with no visible transition when it goes from A1 (negative bias) to A2 (positive bias).
There are no charge storage effects, unlike conventional RC coupling, so the transition between A1 and A2 is seamless, and recovery is immediate on departure from the A2 region. In addition, if there are any nonlinear grid impedances from the 300B as the drive voltage goes up and down, it is swamped by the orders-of-magnitude lower plate impedance (700 ohms) of the driver section.
There are further subtle benefits of transformer coupling. The 300B grids do not go into the positive-grid region at the same time: they take turns. This means both driver tubes are available to drive whichever grid needs current, not just one. And the paired drivers aren’t just paralleled, but in a Class A balanced circuit, so most distortion is cancelled.
This is important with an output tube with distortion as low as the 300B; the driver should be as clean as possible, yet capable of peak-to-peak 200 volt swings. At 30 kHz (which is a 38 V/uSec slew rate).
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If we chose, we could have a switch that would alter the quiescent current for the driver section, so 6V6’s could be accommodated as well. All of these tubes have the same pinout, but the optimal current for the 6V6 is in the 25~30 mA range, while the 6L6 and KT88 are happiest in the 50 to 65 mA range. (By comparison, the optimal range for a 6SN7 is 8 to 10 mA per plate.)
If the switch is in the wrong position, the 6L6 and KT88 would be under-biased and have increased distortion, but no harm done. If it is the wrong position with 6V6’s installed, though, they would burn up in minutes, and if one shorted, would take out the interstage transformer as well. So a visit back to the factory, all from one user mistake. You don’t want to give the user the power to damage the amplifier, just from a single switch setting.
A limited range switch to trim between 6L6 and KT88 would not harm either (in the wrong position), so that’s an option. But optimizing current for the KT88 still gives plenty of current for the 6L6, and we’ve found these types aren’t all that sensitive to current settings, just so long as there is enough.
A general rule-of-thumb in vacuum tube design is setting the plate dissipation somewhere between 50% and 75% of max rating, with 65% to 70% usually considered a good balance between tube life and overall performance. Unfortunately, many KT88 amps take the tubes right through their ratings, so new types like KT120’s have been created for these amps.
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Good to see someone designing this way, I see too much premature tube death from a lot of high end audio designs.
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The self-bias cathode resistor lets you choose anything between 6L6, KT66, 6550, and KT88. So far, the KT88 is winning, but the KT66 is very good also. We're shipping them with a matched quad of KT88's, but the user can use any matched quad in that family ... including NOS unobtainium tubes if they choose, because the operating point allows a life in excess of 10,000 hours.
I was reflecting on this approach vs simply using them as output tubes. Thanks to the gain of the 300B's, they're only using 1/3 to 1/4 of their potential swing, and the 300B's completely isolate them from the speaker load. So the only "load" is the 60 pF Miller capacitance of the 300B grids, not a loudspeaker. This puts them in the most linear operating region since they are essentially unloaded triode-mode power tubes, very different than the heavily loaded Class AB ultralinear of classical Golden Age amplifiers.
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Hi @lynn_olson @donsachs
What is changed in sound with the driver replacement from 6v6 to kt88?
On the one hand kt88 is much more powerful and low output impedance tube. On the other hand 6v6 is more linear. Did you try kt66 that is more linear and musical vs kt88.
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@whitestix
Amazing to watch the arc of the development and completion of these stellar monobock amps. Just wonderful
Agreed!
I wish them the upmost success. It been marvelous following this exceptionally interesting and educational thread.
Charles
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The Kootenai is no slouch: a classic Mullard circuit (as used by Marantz and others) with 6SN7 input and drivers, PP KT88’s biased into near Class A, advanced B+ regulators, all on a compact stereo chassis. A clear step up from restored Marantz, McIntosh, and Citation II amplifiers, and the best Golden Age style amp I've heard.
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Don and I were talking on the phone today, and I realized that, of the four to five key people involved, we might have two centuries of professional design experience in audio. I think Don and I might have a century; I filed my initial Document of Disclosure for Shadow Vector in 1973, and Don’s been doing vacuum tube amps since he was sixteen.
And the development arc has been interesting: the PP Karna 300B of 2003 has merged with Don’s PP KT88 Kootenai amplifier.
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Amazing to watch the arc of the development and completion of these stellar monobock amps. Just wonderful.
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Don and I are moving towards production of the Blackbird 300B amplifier. We’re now using triode-connected KT88 drivers running at twice the current (56~64 mA per KT88) and one-third the plate impedance (Rp = 700 ohms) of the previous drivers. Not only that, owners will be free to use matched pairs of their favorite 6L6, KT66, KT77, 6550, or KT88’s, with no bias adjustments needed.
The tube lineup is: 6SN7 input, KT88 drivers, 300B outputs, with VR105 shunt regulators. As before, fully balanced Class A from input to output. Special-design Cinemag input and interstage transformers, with custom Monolith output and power transformers. 18"/457mm full-width monoblock chassis with two isolated audio-circuit and power-supply sections, a slow-start circuit, surge protection, and a 12V trigger option. (No AC power goes to the front panel, just DC trigger voltage for the slow-start circuit.)
Don and I anticipate Spatial Audio (of Salt Lake City) will start manufacturing in November 2023.
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@stephenr First off, thanks for you polite post on this thread. I bet your 12B4 preamp sounds great. I have tried removing the Raven preamp and going direct to the amps from the Lampi Pacific DAC via xlr. It sounds very good. It doesn’t sound nearly as good as with the Raven active preamp in the chain though. Over the years I have experimented with various passive preamps built with top notch volume controls and none were as satisfying as a really high end active tube preamp in my system to my ear.
As for the direct coupling, no I didn’t experiment with it in this project. I have heard it in other amps. As Lynn has described above, there were reasons for avoiding it in this particular project. I am sure you can make it work well in your amp though and I suspect it sounds very good.
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If you do direct-coupling correctly, it can be very reliable. I don’t think it wise to direct couple throughout the circuit though!
The advantage of a direct-coupled cathode follower driver tube are several. First, there is more current available to provide linearity if grid current is produced by the output tube(s) (which allows for class A2 or A3 operation since both of those classes produce grid current), and no worries about Miller Effect, so very wide bandwidth (+30MHz!) is very easy (we bandwidth limit our OTL amps in the voltage amplifier section). The stability of the circuit is very high- in our OTLs (where one driver tube is controlling many triode power tube grids), bias adjustment only needs checking once every 6 months or so; often no adjustment needed. Distortion is kept in check in a way not possible if a cathode follower with coupling cap were used. We went to this circuit because its more reliable and prior to its introduction to the marketplace, OTLs generally had a reputation for being unreliable. This approach was key to making OTLs as reliable as any other tube amplifier; one of the reasons we are still in business after more than 45 years.
How this might work in an SET is a different story- yes, you need a B- supply to really pull it off, but if you want to do it with transformers you’ll have to pay that price too, since decent interstage transformers are not cheap- in fact, several times more expensive than a B- supply.
The other advantages of using a direct coupled cathode follower driver are that the driver prevents output tube conduction until the driver has warmed up and stabilized. This is likely not a concern when DHTs are used, but it is if indirectly heated, since cathode stripping can occur if the B+ is applied while the power tube is warming up.
Another advantage, probably the big one, is that small value coupling caps can be used between the voltage amplifier and the driver tube. Since coupling caps have inductance (since they are wound) no matter what materials are used they will always introduce some coloration on this account. By minimizing their value while still allowing for very wide bandwidth, the coloration they might cause is minimized and the inductive influence octaves above the audio band.
If course you could use an interstage transformer for this latter function as well; the advantage being that’s the least expensive place one could be used and most likely to win the most performance from the transformer.
At any rate, by using this approach you wind up with a very simple circuit that offers excellent linearity. One idea I’ve had for a while for an SET is taking advantage of the B- supply and building a differential amplifier for the input voltage amplifier. This would allow a proper balanced input as well as reducing distortion in that gain stage due to harmonic cancellation.
Of course if you already have a fully differential/balanced amplifier design from input to output, the advantages multiply.
We’ve been building a balanced preamp with a direct-coupled output (for which we have several patents) that uses servo control to prevent DC offsets at its output. The inputs of the preamp are direct coupled as well, but coupling caps are used between stages. The DC servo has proven to be one of the most reliable aspects of the circuit, which has been in production since 1989. During that time we simply have not seen any servo failures! The output of the preamp uses a Circlotron, like our OTLs, so there are no excessive voltages produced as the preamp warms up.
The obvious advantage is a lack of coloration at the output of the preamp (most tube preamps use a large coupling cap, from which, as pointed out earlier, colorations are unavoidable) and very wide bandwidth; to the latter point the direct coupling allows for reproduction of extremely low bass that most tube preamps won’t even acknowledge. Since the preamp uses no feedback, controlling phase shift can only be done by having bandwidth down to 1/10th the lowest frequency to be reproduced, so its good to 2Hz. If phase shift is present, the ear perceives it as a lack of impact, even though the preamp might otherwise be perfectly flat at 20Hz. By direct coupling that problem and the often tubby bass that can occur are both avoided.
So while direct coupling (with or without a servo) might seem tricky, it offers great advantages and can be extremely reliable if properly designed.
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The thing about the transformer coupling is that once done correctly, it just works. It will not fail. It protects against a tube that may develop a grid short. Rare in smaller signal tubes like the 6SN7, but I have seen it. So for a commercial amp I just want something that works and the owner never has to worry about. If he or she gets a hum or odd noise from the system, a quick tube change will most likely solve the issue. Direct coupling has the chance for a tube failure to propagate to the next stage. Very rare, and if it was just my own amp that I could repair, then I probably wouldn't worry about it. But if you put 100 amps out into the world....sooner or later a rogue tube will appear. Lynn's point about servo circuits is also valid. It adds complexity and again, a possible failure point. If my own amp in my living room, then fine. I can fix anything. But if I put 100 amps into the world I don't want to see one fail. My business philosophy would be to immediately take care of the problem for the customer, but I would rather just avoid the issue entirely.
Every component and coupling method has a sound. I found the well designed transformer to have less of a sound than any other method, plus it is totally reliable. I find it more transparent than any other method as well, with subtle detail more audible. It has less coloration than anything else. I think transformers get a bad name because there are a lot a mediocre ones out in the world and that is what most people have heard. If you go all in on good ones they are quite spectacular. So that is the way we went.
As to Lynn's comments about capacitors, yes, I have heard a very large subset of the best caps available, bypassed and unbypassed, etc... You can happily live with many of the best ones, but the transformer coupling kills them all to my ear in this circuit. Once you hear the really good IT you realize that none of the caps can produce the tonality of the transformer. The instruments all sound just a little wrong with the caps. The transformer does the "piano is in the room" thing a LOT better in this circuit built this way. You may have a different experience in a different circuit built with a different power supply. RC coupling is certainly easy. LC coupling a little more difficult because you need a good anode choke and you have to physically find room for one. Direct coupling has advantages, but again, there are failure modes and colorations. The hallmark of this circuit is the absolute transparency and I just found the transformer sounded best, once a really good one was wound, and it will be trouble-free.
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@lynn_olson
It’s a dumb truism in audio, but simpler usually does sound better. Not that I’m a fan of 2-stage amplifiers or full-range drivers ... there’s such a thing as too simple. Every designer has to find the balance point between simplicity and complexity.
No doubt a painstaking balancing act to conquer.
Charles
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The problem of DC coupling vacuum tubes in a balanced circuit is maintaining DC balance ... during warm-up, in steady-state operation over hours, and as the pair age over the life of the amplifier. A small DC imbalance error in the first stage becomes very large in the second stage, resulting in a massive current imbalance in the second stage.
This can be servoed out by a housekeeping circuit, using a bit of analog logic, but if that ten-cent opamp fails, it takes out the entire amplifier. I have seen that happen while I was sitting in the listening room of the editor of the magazine I write for, Positive Feedback. A cheapo servo circuit in the preamp took out the entire power amplifier and the bass driver. $50,000 worth of damage in a few seconds. I don’t care how it sounds, that’s just bad design.
DC coupling without a servo basically doesn’t work. Small drifts become big ones over time, and the circuit will have to be manually re-balanced by the user whenever tubes are replaced, which will happen many times over the life of the amplifier.
I think the horror of transformers has been taken much too far. There’s a reason they have been used so widely in studios for the last eighty years. They are problem solvers. Output transformers take the pint-size currents of output tubes and multiply them 28 times, or more. Input transformers reject common-mode noise and RFI, presenting a clean, quiet signal to the input grids. Interstage transformers sends the power of both driver plates, summed together, to whichever grid needs it the most (grids take turns going into Class A2).
Looking at the driver section, I don’t see the appeal of a cathode follower drive circuit. Adding an additional stage is not exactly direct coupling, and it requires another regulated power supply with oddball voltages for both plus and minus. It’s not a simplification, it’s considerable added complexity, and for what gain? There’s no improvement in slew rate, which is controlled by the current available to drive the Miller capacitance of the power tube grid. The Blackbird has 32 mA of current from each side of the driver, far more than the usual 8 mA of many other amplifiers. The driver can even enter Class AB for a half-second or so, so 32 mA is not the upper limit for grid drive.
The least necessary transformer is between the input tube and the driver tube. The driver grid is relatively easy to drive, and no clipping is seen in that part of the amplifier. The biggest annoyance is the slowly drifting DC imbalance of the input tubes.
In an AC coupled circuit, it doesn’t matter ... it’s only a few volts out of 150 or more. In a DC circuit, though, it controls the bias of the driver tubes, which is a big deal. You really don’t want one tube at be at 50% power while the other is at 90% power, and you don’t to burden the user with meter and knob twiddling on a regular basis. Most of all, you never want to give the user the power to destroy their own amplifier with a thoughtless knob twist.
The 6SN7 DC balance will drift ... not by much, but by a few volts. I do not want it controlling the 6V6 bias point, and most of all, I do not want a solid-state servo circuit to control the 6V6 bias point. That circuit will fail sooner or later.
One option that was considered was a center-tapped inductor for the 6SN7 plates, with direct coupling to the 6V6 grids. But the performance of the inductor, against expectation, was actually worse than the dedicated transformer, and the transformer completely eliminates DC imbalance at the 6V6 grids. Like all transformers, DC is not getting through.
Don and I tried all the more complex options that would give supposedly better operation. They were worse. Removing current-source coloration is non-trivial and difficult in a very transparent amplifier. Bipolar transistors and MOSFETs are audible, even as plate loads.
All of the coupling caps were colored, some much worse than others, but they were all colored sounding. (Once you hear capacitor coloration, you cannot unhear it. Just ask Don.) That was a source of great disappointment. The high-value load inductors had their own set of issues, mostly excess stray capacitance that could not be removed.
I did not expect the transformer to win, honestly. Don and I tried everything else, and the more complex options were always a step downward. In a zero-feedback circuit, you hear every single part. It’s a dumb truism in audio, but simpler usually does sound better. Not that I’m a fan of 2-stage amplifiers or full-range drivers ... there’s such a thing as too simple. Every designer has to find the balance point between simplicity and complexity.
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I’ve been following … no … have been massively influenced by Lynn’s Karna amplifier and the reasons behind the topology. My first effort is on Lynn’s site as the PP813 Grand Aurora so I’ve been at this for about 20 years.
It has now morphed into all DHT with finals of GM70 so not part of the discussion in a 300B thread.
These types of amplifier are not easy to get right. Even though the signal path is ridiculously simple, it is still challenging and the supporting power supplies are anything but easy. Finally after all this time, I’ve succeeded in getting to the sound I was after.
Probably as I’m not in the position to commission a super IT and maybe as my current one is not so super, I’ve ended up DC coupling the 1st to the 2nd stage in a similar way to Kevin Carter’s method; choke loaded 1st stage.
Don mentions in the thread DC coupling was not an option for a commercial product, but I’m left wondering, was this method evaluated in the many permutations of coupling configurations for the front end?
I’ve also got a line stage, which again, is heavily influenced by Lynn’s designs; the Raven but using 12B4; initially used as a headphone amplifier. The system is better for it being in the chain than out despite having no gain.
Many thanks to Lynn for providing oodles of thought-provoking posts over the years to fuel the ideas leading to the success I’ve had.
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This has been an interesting thread. I didn't start it, nor did I expect it to have this long a life. It is very clear that there are many ways to build amps and preamps. I just want to restate my goal for this project. I am semi-retired. I wanted to build the best sounding preamp and amp combination that I could. A setup that I can happily listen to the rest of my days and that will drive a reasonable subset of the speakers out there. Within reason, I didn't care at all what it cost. Once done it will be produced by a partner and priced according to what it actually costs to build plus enough to cover their labor and margin to stay in business. It will be sold directly to customers though with no distributor markup. But it costs what it costs to build because the main goal of the project was to produce the best sounding preamp and amp that was possible without have some four chassis amp or gold plated case and $50,000 starting point. The thread has shown why we made the design choices we did and where it led us based on a combined (gasp) probably 70 years of experience with amplifier design and builds between Lynn and I. He is the historian as well, and I have rebuilt literally hundreds upon hundreds of pieces of classic tube gear, so I guess that makes me sort of a hands on historian.
At any rate, I thank everyone for their civility and free discussion of audio ideas that has made the thread enjoyable. I hope to meet a few more of you next year at the Pacific Audio Fest when I will be there with actually production versions of these for you to hear. Trust me, they are FAR better than the prototypes shown this past June.
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Yes, these amps sound very good after 10-15 minutes and are very consistent. They sound the same every day. I have the preamp and amps plugged into a Puritan supply, which does clean up grunge in the AC line. Basically, you turn them on and they sound quite good as soon as they can play music in 30 secs or so, but in 15 minutes or so the tubes are all warm and then they are pretty much on song until you turn them off. They may sound a wee bit better after 30+ minutes, but really 15 gets you almost all of it. Plus they run quite cool. You can leave your hand on the transformers or anywhere on the top panel even after the amps have been on for an hour. No, it is not a class D amp that barely warms, but they run quite cool for a class A tube amp.
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One of the maddening things about the original Karna amplifier was sonic variability. Some days, it would be a glimpse of Heaven, and other days, nothing special. It was always extremely transparent ... that’s the nature of the circuit ... but the tuning came and went. Most of the time it was quite good, but every now and then, it was extraordinary.
By contrast, SET amps are usually consistent, any time of day, due to massive 2nd harmonic masking all the high-order harmonics. Tuning a SET is straightforward ... which parts complement the dominant 2nd harmonic most gracefully.
One of my experiments was bringing out the filament circuits on a separate power socket. I bought the Audio-GD AC regenerator, and powered the filaments with that. Sure enough, the variability went away, and I discovered quite small variations in filament power had a big effect on the sound, overwhelming any other tuning decision, including tube swaps. But the Audio-GD liked to run its fans, despite the easy load.
So I was open to Don’s approach of regulating everything, using the proprietary regulators he’d been using so successfully on his 6L6 and KT88 amps. Sure, why not? I’d already been splitting the high voltage B+ supplies between input+driver and output section (to prevent crossmodulation between sections), and using high-quality regulators for a balanced drive for the filaments and heaters made a lot of sense.
(I should mention regulating filamentary tubes is not trivial, and the usual 3-pin low-voltage regulators introduce unacceptable colorations. Filament and cathode nodes are extremely sensitive to coloration, and balanced discrete circuits are required.)
Sure enough, variability gone, vanished, like the morning mist. No change in sonics depending on the time of day. No annoying and objectionable regulator coloration, which is the bane of high-end audio ... that obnoxious grainy transistor sound, coming out of an otherwise good tube amplifier. Not a trace of that, thankfully. Reliable, too, which goes with good regulator design.
This alone justified re-naming the amplifier. I suggested Blackbird (because Red-Winged Blackbirds are a common sight in Colorado), and to my surprise, the name appeared in the Pacific Audio Festival show guide. So Blackbird it is.
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The Blackbird amps are running the 300b tubes at about 400V B+ (plate - cathode voltage), and just a wee bit under 80 mA. About 31.5 watts at idle. They sound wonderful and should last a very long time. The tubes each have their own independent regulated filament supply and are right at 5 volts. I have measured a lot of them and it is always 4.99 - 5.01 volts. All the supplies are regulated so the tubes stay at that operating point. It seems to be a sweet spot for the 300b tubes, although I suspect that dialing the operating point up a bit for the Euro super tubes would be an option. The supplies are run very conservatively and could easily handle the extra current.
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....I try Real Hard to avoid 300 ib lovers.....
Oh.....
;)
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Don thanks for sharing what you are hearing with the OPT and tubes. I’ll have to hear these somehow someway.
I share my tube experiences because I know how hard it is to get all of these expensive things in one session so they can be A/B/C/D compared. Some I owned, some were loaned. Goes without saying that everyone will find their own preference. But meaningful differences exist. I roll till I find the right ones then stop. Some never do.
I was mistaken. Thought you did the PAC 2 upgrade already. From what I hear (from Fred) it is money well spent. If this was an option when I had my Pacific, I would have gone to 2 and stopped. End game. The PAC is DHT, the Horizon is not. Some consider the 46 as the best option. I never had any to try. I can say the EML 45 globe mesh are pretty special in a Thomas Mayer amp and the Pacific. But rare unobtainium now. But my everyday Pacific tubes were KR RK PX25. I ran these in my amps too, which is kind of funny.
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Back when I was playing around with 300B’s, I found that each brand had its "sweet spot". The authentic 300B’s, and the exact reproductions, seemed happiest between 65 and 75 mA. The European super tubes, between 72 and 85 mA. The monsters with giant 65-watt plates are probably happy between 80 and 95 mA. Power supplies need to deliver at least twice the quiescent current, preferably more.
If the current is a little low, they are rich-sounding, but also murky and dull, and if too high, super detailed but also wiry and hard-sounding. Very much like setting VTA in a phono cartridge ... there is a correct setting, and you know it when you hear it.
There is also a tiny range of allowable filament voltage, from 4.85 to 5.05 volts. This can change the entire character of the tube, from very dull but rich sounding to wiry and hard. For longest life, it should be exactly 5 volts, and leave the subjective tuning to setting the quiescent current.
The B+ voltage only had a minor effect compared to the other two parameters ... well, three, counting tube swapping. The other indirect-heated tubes aren’t as temperamental ... put them in the usual range, and they sound fine.
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@tuckia08 Thanks for your comments. I have heard a number of 300b now, but not the new WE 300b and I have not heard the European expensive tubes. When I tried the Linlai WE300b copy I stopped looking. I can get them for about $750 a quad from my supplier and so far the 4 or 5 quads have all been perfect with no issues. As for sound.. well they stomped all over all the $400-500 per quad tubes from China, and the Gold Lion reissue, which is a nice tube, very well made, but just cannot match the clarity of the Linlai WE300b copy in this push pull Blackbird.
The Monolith OPT is based on their standard summit series core. I don't want nano or amorphous. The amps have incredible micro detail and the tonality and timbre of instruments and vocals are spot on to my ear. They are rich and full sounding. I am happy with the summit cores. I was playing records last night because I am burning in a phono stage for a customer and shipping it today. I had not used phono with this final version of the amps and I have to say that I heard subtle details in the familiar records that I had not heard before, even with earlier prototypes of these amps. The summit series transformers do it for me:)
I too will be interested in user comments. We hope to be making them in November!
Yes, the Lampi Pacific is outstanding. Mine is modded a bit and I am considering sending it in for their upgrade. They will put the Horizon digital engine in there for about $4K, and do a few other things. Spendy. Once the 300b project is off the ground maybe I will gamble the $4K:) But the Pacific is just stunning as it is. I run a quad of NOS 46 tubes with adapters, so they are essentially a 45. The clarity is astounding. I wish we had this quad at the Pacific Audio Fest in that Pacific that Fred, the NA distributor for Lampi loaned us. Anyway, enjoy your Horizon. An end game source no doubt! Once you get a DAC that good... you cannot go back.
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They now dominate high-end audio, which was not true thirty years ago. I remember going to some CES shows with hardly any tube amps at all, never mind DHT’s, and now they are everywhere.
While it is correct that there are more tube amplifier producers (many of whom show at audio shows) than there was in 1958, its incorrect to say that tubes dominate high end audio (most of these producers are very low volume). The larger players who produce solid state products are still the bigger sellers.
I should have been more clear when I mentioned direct coupling as a method. Specifically, I meant using a cathode follower which is direct coupled to the power tube so that the bias of the power tube is obtained from the driver tube. For this one would need a B- supply, but if the best reproduction is a goal then that cost isn’t important. IMO using a driver tube in this manner does not result in higher ordered harmonic generation, at least insofar as our OTLs seem to measure out. In them the higher orders fall off at a faster rate than any SET I’ve seen. They sound smoother too. One thing we sorted out from doing this over the years is that the plate needs to be heavily bypassed to prevent IMD and harmonic generation. In our amps we found we had to use bypass capacity values an order of magnitude higher than one would typically expect; after trying lesser amounts we found the seemingly excess amount made a difference to the distortion. IMO this aspect of cathode followers is poorly understood.
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@tuckia08
EML 300b XLS - Pretty good. Not as nuanced as the best. Depth and width OK, but not the best. Good low end drive. Much better than everything listed above.
Takatsuki - Excellent tube. Best bass drive. Good Detail. Depth and inner resolution good, but not the best
In my experience I’ve found that the preference for a particular upper echelon 300b is very dependent upon the specific amplifier in use. Some of these top quality tubes are simply better fits in certain amplifier/systems than our others. It’s very difficult to draw an all encompassing generalization.
Charles
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First, loving the conversations here - delving into the correlated technical stuff. Everyone should have a Tektronix RTA for this hobby, lol. But the last time I used one they were over $50k. That's more than a good pair of interconnects🙄.
It's also intriguing reading that these amps will run Monolith output iron. Kind of curious which series they are based on, but maybe that's asking too much.
Tubes
Over the years I've used quite a few different tubes from NOS to current production - Shuguang, Psvane, JJ, Sophia, Linlai, KR (Kron), EML, Takatsuki, WE. Some brands aren't worth mentioning. In the past I preferred KR in amplifiers as they are wideband, dynamic, and have solid bass. But depending on the system, can come across as a little unrefined/aggressive. I liked them in what I had. (PX4, PX25, 242, and 5u4g)
But with 300b in what I've owned:
Art Audio - They make heat and conduct electricity. About the nicest thing I can say about them
Shuguang 98b - Meh. Pretty darn average across the board. Puts me to sleep.
Shuguang BT - Pretty good in a VAC Renaissance P/P amp. But other places are bright, lacking midbass weight, and little bass
Linlai 300b E - A good step up, decent bloom and detail, a little light in the midbass and down low
EML 300b XLS - Pretty good. Not as nuanced as the best. Depth and width OK, but not the best. Good low end drive. Much better than everything listed above.
Takatsuki - Excellent tube. Best bass drive. Good Detail. Depth and inner resolution good, but not the best.
WE 300b new production (2022) - Excellent detail, inner resolution, nuance. Holographic. Quiet. Appropriate bass weight and definition. Not as punchy as the Tak or the EML. That will depend on your system. 5 year warranty and projected long life. Love this tube in my SET amp. Well worth the money.
So I find a huge performance delta in the list. Spending the money will be rewarded, probably more in SET than P/P, but that's speculative.
Oh, regarding comments about DAC's made earlier, I concur with Don who has a Lampi Pacific 2 with DHT output, my Lampi Horizon dac is the most significant piece in my system. But there are no unimportant pieces.
Very curious to hear user comments once the production Blackbird and Raven hit the wild.
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