The Nuvistor and Bob Katz’s Audio Blender (via Inner Fidelity)

Bob Katz has been writing a series of articles over at InnerFidelity for several years and they’ve recently taken a turn down a more experimental path. His most recent article details a device that mixes a transparent solid state signal and a Nuvistor signal biased to provide a distortion spectrum with just a small percentage of second harmonic. Check out his write up here!

nuvistor

A Nuvistor is a small metal and ceramic tube released by RCA just as transistors began supplanting vacuum tube technology in most electronics. They are a true vacuum tube with familiar triode operation and characteristics and an indirectly heated cathode. The most common Nuvistor in consumer electronics was the 6CW4 (high Mu) though there are several triode flavors and even a couple of tetrodes.

nuvistor cutaway.png

Because they were originally intended for radio and TV usage, Nuvistors enjoy very good bandwidth, low noise, and high gain (high Mu variants).  The metal envelope is integrated with the basing and the tube plugs into what RCA dubbed the Twelvar base. You can probably guess how many pins that had. With the Nuvistor, RCA also introduced the RCA Dark Heater, a lower temperature filament that guaranteed higher stability and less AC leakage. Despite this innovation, most Nuvistor heaters require around 1W to light (e.g. 150mA @ 6.3V).

The 8056 used in Bob Katz’s project has the following characteristics:

8056 characteristics.png

And the following very respectable plate curves:

8056 plate curves.png

With a modest Mu, low plate resistance, and very low B+, it’s no wonder Bob decided to marry this interesting tube to a solid state partner for his Blender. The 8056 heater requires 6.3V at 135mA. At this voltage and heater requirement, it’s close to being practical for modern portable devices. In their heyday Nuvistors were used in battery-powered and efficiency-critical applications like the US Space Program and military radios and communications equipment.

Would I ever build something with Nuvistors? It’s tough to say.  I’ve been on a casual hunt for tubes that might be suitable in a portable battery-powered application. Other candidates are the Korg Nutube or the sub-mini 6088. Like all things in this hobby, there are trade-offs. The Nuvistor 8056 heaters are hungry relative to these other options, but the other characteristics are very attractive. In all likelihood, I’ll try them all eventually. This is why I DIY.

Ode to the ST-70

dynaco st70.png

The ST70 is a beautiful and historic amplifier (and surprisingly compact if you see one in person). It’s also the best selling power amp of all time (at least so says Wikipedia). All things Dynaco inspire much talk here around the water cooler at the WTF Amps institute of higher learning about vacuum tube stuff. Here are some loosely organized tidbits and thoughts on amplification!

Generalized Topological Design Trends in Discrete Amplification

Forget for a moment that some amps are made with tubes while others are made with transistors. Deep down in their vacuum or silicon hearts they are really both just simple three-pin devices used to accomplish the same thing (gain). Forget all the audio-speak we abuse in our efforts to approximate the many facets of circuit performance. Forget the preconceptions we file away in our minds under “T” for tube or “S” for solid state. We aren’t thinking about tubes or transistors, right? Good.

To SE or PP (tee-hee)

Beyond all other aspects, the amplifier topology choice that impacts a design the most -in performance, efficiency, and cost- is whether the amplifier will be single-ended or differential. The difference can be boiled down to whether the amplification devices handle the entire signal through to the output (single-ended) or “split” the signal phases and re-combine them at the output (differential). Differential amplifiers are sometimes also referred to as push-pull. There is no such thing as balanced amplification, but that’s another discussion.

Single-ended amplifiers tend to be more inefficient in both a power consumption and economic sense. Because they are Class A by necessity, they dissipate more heat per watt of amplification. Single-ended amplifiers need a squeaky clean power supply to achieve a respectable noise floor because they do not benefit from the same kind of ripple rejection as differential amplifiers. They tend to produce more distortion, but the distortion that they produce usually has an even-order-dominated harmonic spectrum. Studies say even-order distortion harmonics are less offensive to most listeners.

In contrast, differential amplifiers produce less distortion when designed well, but what they do produce is dominated by odd-order harmonics, which are less pleasing to most listeners. Differential amplifiers are capable of far more efficiency than single-ended amplifiers because both output phases do not need to be “on” all the time. By nature, differential amplifiers reject power supply noise because they only amplify the difference between the phases and any power noise appears equally in both.

/r/outoftheloop

The distinction between single-ended and differential is the most fundamental taxonomy that can be applied to amps. The next most important design choice with regards to the circuit and its behavior is whether the amplifier will be open-loop or closed-loop. A closed-loop amplifier injects a portion of the output back into the circuit in order to correct non-linearities created by the act of amplifying with non-imaginary devices. This requires extra gain from the amplifier to be spent on suppressing these distortions. An open-loop amplifier is able to get by with less overall gain and enjoys more polite clipping behavior at the expense of generally higher THD. We are very deliberately avoiding the term ‘negative feedback’ here, by the way.

If you’re following along, you see that less-efficient single-ended amplifiers with less-objectionable distortion spectrum might naturally gravitate towards open loop circuits. Furthermore, you can imagine that more efficient differential amplifiers, with power to spare but a less pleasing distortion spectrum, are logical candidates for closed loop circuits. Your powers of comprehension do not fail to impress, dear reader. In practice a blend of single-ended and differential, open loop and closed loop, choices are made at the stage/component level in order to balance the relevant strengths and weaknesses, but the broader structure of amplifiers is usually one or the other.

WTF were we talking about again?

I’m going to tell you a secret now. Please do not react too loudly or cause a commotion. Come closer… Single-ended, differential, open loop, and closed loop has nothing to do with whether an amp uses tubes or transistors. Yes, that’s quite something isn’t it? While it’s true that historically certain devices and topologies are strongly associated one to another, this is a question of device availability coinciding with design trends and market demands, not choices dictated purely by the devices used.

This brings us back to the topic of the Dynaco ST-70. This is a closed loop differential amplifier running in Class AB, much like the earlier Williamson or Leak tube amplifier designs. The overall topology is not much different from current Class AB transistor amplification because these solid state amps are simply a continuation of the same design trend (AB differential, closed loop). While today we associate tubes with single-ended open loop design and transistors with differential closed loop design – and all the baggage these topologies drag about – the reality is that performance has more to do with circuit choices than with the devices used.

The ST-70 was in some ways a pioneer. Though it was not the first of its kind, it was the standard bearer of the contemporary design values. Today we prize much of the ST70’s topological progeny in solid state Class AB (whether integrated on a chip or built with discrete components) but we also revere designers such as Nelson Pass who is charting his own course through both open loop single-ended transistor and Class A low feedback differential amplification. Amplifier design is not so much a timeline as it is a spectrum; the limits to what constitutes good amplification (subjective as that may be) are found not in the parts choices, but in the creativity of the designer.

TL;DR: Design, not device, makes the amp.

Here’s Dan Fraser’s write-up on the launch of the modern ST-70 series 3 (Dynaco was purchased by Radial Engineering in 2014)

Listening on someone else’s system

Like many audio enthusiasts, I have a general philosophy for audio that guides me when designing (or shopping for) new gear. In a nutshell, I value an objective and empirical approach to design, but this is tempered by the notion that music is art. At its core, art appreciation is a subjective, and often situational, experience. Objective design for subjective ends reads like a paradox; designers have egos too and so maybe conflict between engineering and ‘the feels’ is inescapable. If you’ve been on audio forums or blogs long enough, you know that objectivity and subjectivity do not usually mix in the audiophile hobby. I’ll steer clear of that morass, save to add one recently encountered perspective.

Last weekend I delivered a preamp (design write-up on the way) to its new owner, J. We spent a couple of hours listening to his system with and without the new piece. J’s system is different than mine and the music it makes sounds different, too (including recordings I know). We both had fun going through albums and cranking up the tunes. In a way, it was a little like seeing a favorite group perform live. You know the music but can appreciate fresh nuance all the same. That we got to do so together, on a social level, only added to the enjoyment.

Now what if we were all uncompromising in our objectivity? What if all systems and designers pursued the same goal and weighed compromises equally? Worse yet, what if compromises did not have to be made and all playback was “perfect?” While I know this is ostensibly what many of us seek in the audiophile hobby, where would it leave the hobby on a social and experiential level? I would visit J and hear the same songs in the same way that I always do.

The art in music is not a one-way street. The lenses and filters we use to experience and share art enrich both the art itself and culture as a whole. The process of internalization, expression, and rebirth keeps music relevant and vital. I’m off into abstraction, but there is a kernel of truth for audio here, too: how terrible the tyranny of ‘exactly as the artist intended’ could be if we took it too literally.

You are the artist of your listening, the world is your mixing console, seek out new stereos, and all that jazz.

 

Guest Post at Audio Primate: JDS Labs CMOYBB Review

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One part market research and two parts DIY hobby service: click here for another review of a small solid state headphone kit/board at Audio Primate. JDS Labs has done an excellent job with this kit. Everything is clearly labeled, the board is good quality, and the documentation is excellent.

If you want a place to start with DIY amps and line-level gear, look no further than the classic CMOY.

Letters to WTF: Why do “neutral” amps sound different?

One of the issues is that although we can measure a heck of a lot about an amplifier, we can’t express those measurements in ways that are easy to digest and interpret for the average reader. We end up with data that is easy to misinterpret or take out of context. Part of the blame lies with consumers, who want to compare and contrast specs as a replacement for firsthand experience, and part lies with marketing departments that are being deliberately selective or unintentionally incomplete with what they publish.

THD is one of the classic examples of this. The THD as a percent often appears in marketing specifications, but it is not nearly so often accompanied by the all-important context. To make practical sense of a THD specification, you need to know what kind of signal was applied during measurement (frequency, level, single sine wave, multiples for IMD, program material, etc) and at what level the output was measured (voltage or wattage). Even when that information is provided, judging % THD across devices doesn’t give you a good comparative idea of the sound unless you know the harmonic makeup of the distortion spectrum (very rarely provided by manufacturers). All 0.X% THD measurements are not created equal in terms of your listening perception.

Power output is another example of specs that tell less than they should if we want to make a judgement on numbers alone. Like THD, power output requires context in the form of % THD at the measured level, the load used to measure (reactive load, resistive load, etc), and the signal used to measure. This information is not often given and so apples-to-apples comparisons that would allow someone to say with confidence that “these are equally neutral amplifiers” really is not possible. To make matters worse, most of these measurements are interactive, making expressing the results in a practical way more difficult.

While we hope manufacturers would include this kind of information in specs as we shop, it just does not happen. A big reason for that is the average person just doesn’t care. The responsibility falls to the niche press that reviews products and is able to measure them with a consistent method. But there’s a diminishing return on the amount of work it takes to measure and share all the data needed to give a clear picture of how something sounds. Even if the measurement work is done, it takes a certain amount of technical knowledge and experience to interpret it, so we end up with catch-all terms like ‘warm’ or ‘neutral’ to paint the picture. This audiophile language is as much a shorthand for all the measurements that aren’t being done as it is flowery jargon.

TL;DR It takes a huge amount of work to measure and express the findings if you want to capture all there is to say about how something sounds. Few are willing to do it, so we have words like ‘neutral’ that are part subjectivity and part experienced evaluation by trained ears. Even then, it’s not a perfect substitute for hearing something yourself.

Another Muchedumbre lives

I finished another Muchedumbre build with some slight variations.  This has two outputs and two inputs (easily switchable back to the 1+3 arrangement).  The power supply CLC filter uses all motor run caps instead of a mix of motor run and electrolytic. Other than these small tweaks, it is built as designed.

The wood apron is a very nice piece of walnut with a lot of prominent grain motion and color variation and the panel is inset rather than sitting on separate interior spacer boards. This is going to live a very happy life in Madison, WI.

I love kits, too

Especially when they are high quality kits.  Here are the contents of a TubeCAD Aikido kit that just arrived. John Broskie’s boards are top notch, the parts are bagged and labelled logically, and the included manual is excellent.  I’ll be building this kit up in a unique way (see TubeCAD’s article on the SRCFPP) and will post a build and my impressions in the coming weeks.

In the meantime, if you aren’t subscribed to and reading The TubeCAD Journal, you should be. Also consider contributing to John Broskie’s Patreon: for less than the cost of a Netflix subscription, you’ll support excellent vacuum tube DIY content and resources for everyone in the hobby.

USB interfaces for DIY audio measurements

USB interfaces

Looking for a better way to measure my line-level and amplifier projects, I decided to investigate some USB prosumer interfaces. Rather than options like the QA401 with its required software suite, the Keithley 2015 with uninspiring THD specs, or HP 8903 with a footprint and compatibility penalty, I wanted something small, flexible, and with performance good enough for tube audio. A USB audio interface will require voltage dividers for many measurements (whereas the lab equipment usually allows a higher Vrms input), but recording interfaces are inexpensive and flexible with software. Also, I don’t have the play money for an AP or dScope rig.

RMAA interfaces screenshot

These were all measured back to back on the same laptop with latest drivers and the same unbalanced cables. The same -1db level was used for all interfaces to get a relative distortion/noise baseline. RMAA doesn’t necessarily give an absolute and repeatable spec, but it is good enough for relative comparisons. All interfaces were measured several times; the displayed specs capture the “average” performance (calculated by eye).

  • The MBox 3rd Gen is an obvious winner in just about every regard. It was also a much more expensive interface when it was new.  MBoxes are no longer produced, but used interfaces aren’t difficult to find. This would be a decent basis for THD measurements of amplifiers (my intended use).
  • The M-Track 2×2 did rather well (as much as I hate to admit it) but doesn’t have two identical channels for this kind of thing (one TRS and one combo jack with mic pre). Wouldn’t recommend it for measurements for that reason.
  • The old Fast Track Duo (Avid branded bu made by M-Audio) blew chunks. Can’t rule out that my unit has some kind of issue.
  • The AudioBox USB looks good but there is a cross-talk issue. Possibly grounding with the unbalanced cables. The knobs are also too fiddly for fine adjustment in my opinion.
  • I really wanted the iConnectivity to perform the best here. In my opinion it’s the nicest piece of hardware. Unfortunately, the relative measurements don’t make it the best choice. It can be run from a 9V supply rather than the USB bus though and I may try that to see if there’s any improvement. No supply handy for this test.
  • The AudioBox 44VSL does pretty well (this is what I had been using for measurements). It also requires a 12V external supply and is a larger 4 mic pre interface, making it a little less convenient for a bench-top test setup. The 22VSL is smaller and may measure just as well (don’t have one to play with).

Here’s after some fine tuning the MBox levels in REW (sampling rate set to 96khz):

mbox-96k.png

This is close to the -110db THD Avid spec’d. All in all, I think I can live with the MBox for a while for my testing. Although all the caveats of RMAA and testing conditions/methodology apply, performance is on par with some specs I’ve seen on the cheaper audio analyzers and definitely a cut above the other USB interfaces here.

Link to RMAA software

Link to REW software