John Broskie's Guide to Tube Circuit Analysis & Design

11 October 2018                                                            Post Number 442


RMAF 2018
Did my expectations come true? Was the 2018 RMAF filled with renewed vigor? Have audio companies experienced increased sales due to the booming economy? Were there more or fewer attendees? What about the exhibitors, more or fewer? And, most importantly, the sound, was it better or worse than years before? All good questions.

The quick answer is that I felt good leaving the 2018 RMAF, so much so that I expect this to be a comparatively short RMAF summery. Renewed vigor was amply on display. Although there seemed to be fewer exhibitors, the same amount of attendees seemed to roam the hallways.

I sought out answers from the audio company representatives. I asked and was told that while 2016 had been brutal, 2017 and 2018 were definitely looking up in terms of interest and sales. My optimistic balloon was popped when a sagacious fellow utter two words: Survivor Bias. Yes, indeed. You can only interview the survivors, not the deceased. He went on to say,

"Many audio companies are out of business and those still around might be seeing increased sales simply due to fewer competitors. And if the economy really is picking up steam, the survivors will make a killing."

As for my general impressions of the average sound quality, I would say it was better than previous years. Part of the credit must go to DACs just getting better and fewer server headaches on computer-based systems. The other part is that many products just got better, from headphones to loudspeakers. For example, the PSB M4U 8 headphones sound much better than their previous version. The Vanatoo active loudspeakers sound even better and they were amazing to start with; their Transparent One Encore model only cost $600 the pair (available now for $499) and are perfect for anyone who needs a pair of loudspeaker for his computer or bedroom. Insanely good bargain.

One of the advantages of being part of the press, besides the US constitutional protection and preference, is that The Press is allowed in two hours earlier than the general public. Well, at least that is what is posted at the RMAF website and that was the practice during the last seven RMAFs I had attended. Not this time. I was denied the promised easy movement and solo listening sessions, so I rushed to the top floor, so I can use the stairs to get to the lower levels with gravity assist. Walking into the rooms on the eleventh floor, my ears weren't disappointed.

I brought my USB memory stick, but it was only half useful, perhaps less than half useful. Recent Apple Mac laptops and tablets no longer hold USB ports and many fear USB-born viruses—and a few rooms sported only LP playback. Still, I did get to hear about a dozen listens to the same song sung by the Spanish singer, Bèas. Interesting results.

I fell in love her Album, Stolen Diamonds, two years ago and I have heard it many, many times on loudspeakers and headphones. The first third of the song, Moon Over Bourbon Street, she sings a capella, which is excellent for testing a systems clarity and cohesion. The track I use to evaluate speakers is Moonlight In Vermont, whose first third is purely instrumental.

Well, at this RMAF, only one system did the recording justice and that system cost $140,000, the DEQX/Kyron system, which included Kyron dipole loudspeaker, along with subwoofers and amplification that hold internal active crossovers.

The DEQX portion is a digital line-stage amplifier that is built in into the Kyron amplifier chassis, but that does not come close to describing what DEQX processor does. Here is quote from the DEQX website:

DEQX uses real measurements of each individual speaker to generate a precise correction filter that restores the amplitude and timing coherence of the acoustic signal from the speakers. DEQX correction works with regular speakers and multi-amped active speakers.

Only after the individual speakers are calibrated for true high fidelity does DEQX correct for the room's acoustic modes — resonances present in all listening rooms that muddy and confuse the bass region.

Single-ended and balanced analog I/O
5-input DAC
Digital outputs
Roon Ready network streamer
Full-featured remote
Speaker calibration
Room correction
Subwoofer integration
HD-Active linear phase crossover
Multiple listening profiles
Preference EQ ("forensic tone control")

At the other extreme, costing only $3600 the pair, was the RF-7 III. I was surprised by how well the new Klipsch flagship Reference-Series speaker filled the room with big sound. To be frank, I haven't liked any new Klipsch loudspeaker that I have heard in the last twenty or thirty years—and I am big Klipsch fan. In high school, I owned one Klipschorn corner loudspeaker. Well, the RF-7 III tickled my ears. It was not perfect by any means, but its departure from neutral was towards a gutsy, muscular—dare I say it—manly presentation, ideal for blues, bluegrass, country, rap, rock, and most jazz. It seems that, ironically enough for a company known for its horns, Klipsch has finally tamed their new high-frequency horns designs. So, if you have heard the RF-7 III's predecessor, the model II, you have to hear the RF-7 III. Best of all, this loudspeaker efficiency comes in at 100dB, which means mega tube friendly.

When I mentioned the 100dB per watt at 1 meter SPL figure to friends, I received raised eyebrows. The argument went like this: the tweeter in isolation might put out 100dB, but the loudspeaker as a totality can't. I searched for as many reviews for the loudspeaker that I could find online and all stated the 100dB figure. Of course, they might have all just blindly quoted the Klipsch spec sheet, but I would expect at least one to voiced some doubt about the 100dB SPL, if the speakers didn't thunder. (You can find PA and musical-instrument speakers that put 100dB, they usually hold more woofer area than two 10-inch drivers.) If you are in doubt, just bring your flea-powered amplifier to the audio showroom and give it a listen.


Omnidirectional Loudspeakers
I did get to hear two 360-degree radiating loudspeakers. The first was the MBL 101 E MKII, which I have heard called the "Huge Garlic Clove Speaker." If nothing else these loudspeaker are amazing for the eyes to behold, looking very 1930's futuristic.

The ears are also happy with them in a room, as they produce a natural-sounding stereo image. The second omnidirectional loudspeakers I heard was made by another German company, German Physiks, the Carbon MK IV—or at least I believe that was the model; I cannot be sure, as the flyer didn't reveal the model's name. The speaker uses an improved Walsh driver with built-in subwoofer.

I didn't like this model as much as I had like the smaller one that I mentioned so favorably in post 310 , but I can easily imagine it appealing to a different sort of listener, one who doesn't fret over extracting the last quantum of detail, but wishes to immerse himself in a lovely, expansive sonic environment, much he would enjoy dipping into a warm bath at the end of a hard day. Where most high-end loudspeaker tend to yell or spit at you, these omnidirectional (and good dipole) loudspeakers pour out music.


Aegir: the First Continuity™ Amplifier
Schiit Audio, the little Californian company that kicks sand in the face of bigger bullies daily, has come out with a constant-transconductance amplifier. This is a big deal. All class-A output stages, either single-ended or push-pull, run in constant-transconductance; all class-AB push-pull amplifiers do not. There are sound reasons behind audiophiles preferring single-ended amplifiers and class-A push-pull amplifiers. The Aegir amplifier puts out 10W of class-A power into 8-ohm loads and 20W of class-AB power in stereo or 80W in bridged mono.

Once again, this is a big deal. What is not a big deal is the cost, as the stereo Aegir amplifier only costs $799, which many would deem the appropriate cost of a budget headphone amplifier. It is easy to make a good $7,990 power amplifier; easier still, a $79,900 amplifier; but it takes genius to make a great $799 power amplifier. Genius and Schiit Audio are intentionally isomorphic.



If we are lucky enough to escape the outlawing of linear amplifiers, most if not all high-end amplifiers will sport constant-transconductance output stages in next decade or two. Why? For the same reason that we prefer single-ended and push-pull class-A amplifiers: constant-transconductance beats gm-doubling.

See posts 331 and 332 for more information on constant-gm output stages.



Where is the Best Sound of the Show Report?
I heard four truly great-sounding systems at the show, but as I know all the people behind these systems, I just don't feel comfortable naming the the systems.



Kick-Butt Cathode-Coupled Line Stage
After seeing and hearing many millions of dollars worth of high-end audio gear at the 2018 RMAF, I feel the need to return to the topic of Kick-Butt systems. Consider the line-stage amplifier. We do not need a lot of gain, which is revealed by the fact that many run passive line stages that offer no gain whatsoever.

Shown above is a simple +12dB line stage amplifier that uses an OpAmp. The volume potentiometer attenuates the input signal as needed. Okay, simple enough. Now, consider that we often do not need a signal gain at all, only a buffed output driver to drive the power amplifier and interconnect capacitance. Well, what if we break the line-stage into to separate functions: gain stage and output buffer.

Now, we get signal gain when we need sign gain. In addition, we use a high-quality stepped attenuator in place of the volume potentiometer. Even better would be to use two stepped attenuators, one in fine -1dB decrements and one in coarse -3dB decrements.

Actually, if you have ever used a stepped attenuator with -3dB decrements, you know that this amount granularity actually is fairly fine, not coarse. The logic here is that the fine stepped attenuator would also double as balance control, as each channel would gets its own fine stepped attenuator and both channels would share a stereo coarse stepped attenuator.

As you can see from the schematic above, a volume potentiometer could never be used in this application, as we need to effectively set up two series stepped attenuators per channel. In other words, we would just rotate the coarse volume-control knob and the switching internal to this stepped attenuator would select gain or no gain for us. Goldpoint makes a 47-position stepped attenuator that would have to be be hotrodded to work here, but I as I have know Goldpoint's owner for over 30 years, not much arm twisting would be needed.

Okay, if you are wondering where the tubes are, there here.

This is a cathode-coupled amplifier with my signature Aikido mojo thrown in. The problem of dissimilar cathode-to-grid DC voltages is solved by feeding the triode on the right a 5.85Vdc bias voltage. The problem of poor PSRR is solved by injecting a small portion of the power-supply noise into the right triode's gird. A constant-current source loads the coupled cathodes. An alternative approach would be to terminate the constant-current source into ground.

Why the two possibilities? If we terminate into the -12V power-supply rail, we could use an OpAmp to make a high-performance constant-current source. If we use ground as the termination, we could use an IC constant-current source, such as the LM334. Now, let's move on to the unity-gain buffer.

Once again, a 6SN7 is used, if only because they look cool. of course, many other dual triodes could be used, such as the 6CG7, 6DJ8, 6H30, 6N6, 12AU7, 12BH7, 5687, ECC99... The cascoded cathode follower offers a low input capacitance and an excellent PSRR figure, as the top triode shields the bottom triode's plate from the power-supply noise. Since the bottom triode sees a nearly constant cathode-to-plate voltage, its distortion is low. With a 6SN7, the output impedance is less than 500 ohms.

If you are wondering why a coupling capacitor was used when the buffer's output already sits at ground potential, the answer is that it may not sit at 0V, especially at startup, when the output voltage will be close to -12Vdc. If we wish to eliminate the coupling capacitor, which is what Kick-Butt design is all about, we must get clever.

The constant-current source has been replaced by a compliant-constant-current source (CCCS), which establishes a DC-free output and matches the triode's idle current. In other words, the CCCS does NOT work to maintain a fixed idle current; instead, it works to match the preexisting idle current and offers a staggeringly high output impedance. The two 200k resistors save the day, or rather the first minute, when the triode are still cold at start-up and not conducting, as they complete a current path for the CCCS to connect to the B+ voltage and, thus, still maintain a 0V output voltage.

Okay, let's put all this together.

Click on schematic to see enlargement

What we have is a truly kick-butt line-stage amplifier. We get signal gain only when we need it. We have a means for setting the balance between channels and setting a precise output level. In addition, only one coupling capacitor—and that coupling capacitor is only in the signal path when gain is needed. Wrap this circuit in an expensive enclosure and slap a $20,000 price-tag on it and everyone one is happy.




Book Review: The Clockwork Universe
This is not a tube book; it's not even a book about audio; it is, however, a good read, which is why I am reviewing and recommending it. I picked the book at a friends of the library book sale. Why? Although I did not know the author, Edward Dolnick, I had heard of his prior book, The Forger's Spell, which had been recommended to me. And although I have read at lot of books on the topic of the birth of modern science in the 17th century, this one seemed to offer many more and different perspectives on the event. For example, Dolnick fills in many interesting details on Gottfried Wilhelm Leibniz's life and personality. Well, 320 pages later, I was pleased that I had bought the book.

As I read the book, I kept waiting for Dolnick to fall off the historical tightrope, saying such ahistorical things as the Catholic Church tormented Galileo for depriving the Earth's golden, exalted position in the universe, when something close the exact opposite was true: the further you went from the heavens, the worse things got, with sulfurous Hell at the Earth's center. (Nor was Galileo a particularly nice guy, being much closer to a royal PIA.) Dolnick never fell. In addition, he has a degree in mathematics, which allowed him delve deeper into the mathematician mind. That's right mind, not math. He brilliantly explains, from a mathematician's perspective, why the Leibniz-Newton battle over who was the creator of calculus was so important to each mathematician.

In short, this is not the first book one should read on the topic of Newton and the 17th century, but it should be the second. I got the feeling that Dolnick wanted to fill in all the equally important stuff left out of the textbooks, such as what a hellhole life was in 1660 in England, what a universal genius Leibniz was (as Wikipedia points out, "Leibniz may have been the first computer scientist and information theorist"), what a horrific screwing Leibniz receiving from the Royal Society (secretly Newton), and how what a God-obsessed century was the 17th.

(I just figured out how to sneak some audio into this review. )

At the RMAF, as a group of us talked away, I mentioned that the unsung positive audio adjective was quaffablity, that rare attribute of great audio systems that makes us want to keep on listening. Well, this book was quaffable in the extreme. The chapters are short; the prose is lively; the anecdotes are new and interesting. Read the book and do yourself a favor.




Music Recommendation: Claudia Lovato (Beàs)
I mentioned the lovely the Spanish singer, Bèas, in my RMAF report. I have done some sleuthing and I discovered that her full name is Claudia Bèas Lovato and that she appears on at least one other album, Flying Attitude.

Mirabile dictu, i.e. wonderful to relate, Tidal Music Service offers it. Be sure to search by the album title, not her name. If you want to listen to her Stolen Diamonds album, search for its title, not her name.





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So many have asked that I had to do it.


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