Monthly Archives: January 2013

Introducing the the New Statement Series Amplifiers and Preamplifiers

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STATEiQfrontTHE STATEMENT 450 iQ MONOBLOCK POWER AMPLIFIER
The Statement iQ power amplifier is the first in a series of VAC components featuring the new VAC iQ Intelligent Continuous Automatic Bias System (patent pending). The VAC iQ System builds on the unprecedented performance of the original Statement Series and takes it to a whole new level with a set of critical enhancements.
product page
product specs and fact sheet (pdf)  

STATEphonetopTHE STATEMENT PHONO PREAMPLIFIER
The VAC Statement Phono Amplifier is designed for extreme applications. You can tell this the moment you examine the back panel and notice that there are two DC umbilical cords and large Amphenol aerospace connectors, a reminder of the massive dual mono power supply. Unusually, you see that there are inputs for four independent phone sources. Not only that, but each input may be of the conventional single-ended type or may be fully balanced.
product page
product specs and fact sheet (pdf)  

STATElinetopTHE STATEMENT LINE PREAMPLIFIER
The heart of the Statement Line Amplifier is a high current, low impedance, balanced Class A1 triode vacuum tube amplifier. Its inputs and outputs couple to the outside world through high precision wideband transformers. While inherently balanced in design, the transformers themselves accomplish conversion to single-ended operation in a sonically transparent way when required.
product page
product specs and fact sheet (pdf)  

Why Negative Feedback (and high damping factor/low output impedance) May Be a Cure That’s Worse Than the Disease

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By Kevin Hayes

To obtain a damping factor greater than (or output impedance less than) a modest value, negative feedback must be employed. Negative voltage feedback works by taking a portion of the amplifier’s output, and injecting it back into the amplifier’s input such that it reduces the amplifier’s effective gain. The feedback signal contains both the desired output and any error the amplifier has made; thus, the feedback tends to reduce the gain and the error in equal amounts. From the 1940′s to today, many designers strive to put as much feedback into their designs as possible.

Traditional theory gives feedback high marks, but this analysis changes when we consider that the “error” signal is fed back into an amplifier that is non-linear and makes mistakes (otherwise there is no need for any error correction). Due to this, feedback may lower the overall level of distortion products, but it also multiplies the order of the distortion products. For example, if an amplifier naturally produces second harmonic, feedback will create a second harmonic of that second harmonic, which is the fourth harmonic. If the basic amplifier has second and third harmonic distortion products, the fed-back amplifier will contain the second, fourth, sixth, and ninth harmonics. As is well established, the higher orders of distortion are most easily heard and more objectionable to the ear than lower orders, and odd orders more offensive than even orders. Thus it is possible to lower the level of total distortion products and yet make the distortion more audible and offensive to your ear.

The application of negative voltage feedback also reduces an amplifier’s measured output resistance, i.e., it raises the “damping factor”. Here again, the measurement fails to capture the essence of things. In the case of a feedback amplifier, better control of speaker motion is said to occur because the speaker’s excess motion creates a voltage (the back e.m.f.) which enters the feedback loop via the amp’s output terminals. The amplifier then acts in a manner opposite the error signal to correct for it. However, like many theories, this is an oversimplification and, in practice, the opposite result may be obtained. There are several reasons:

1) The motion of a speaker’s voice coil former may not match the speaker’s acoustical output due to cone break-up modes and room acoustics.

2) The motion of the coil former is being sensed by the voice coil. The coil is designed to be a good driver, but is a lousy sensor, primarily due to its high inductance, which will create phase anomalies in the back e.m.f.

3) The back e.m.f. may pass through a cross-over network, which will again alter phase and frequency relations.

4) A differing back e.m.f. from another driver may be summed in via the crossover, making a composite signal that does not match either individual driver.

5) The speaker leads may cause additional phase shifts.

By the time the error signal reaches the power amplifier, it is arguably an erroneous error signal. As the power amplifier attempts to correct for this signal, it may actually do the exact wrong thing with respect to the speaker’s acoustic output. Subjectively, I have noted that high feedback amplifiers tend to give the bass a “one note boom” on certain speakers, and tend to create an electronic glaze in the midrange, possibly attributable to this process.

Another interesting thing is that the damping factor number cited in spec sheets lacks a context. In an actual system, the damping imposed upon a speaker is dependent upon the resistance of the complete connection loop, which includes not only the source impedance of the amplifier, but also the speaker cable and the voice coil of the speaker itself (and, of course, crossover elements where a passive crossover is used). The old Audio Cyclopedia (2nd ed.) on page 1120 goes through an example of this (which the author terms ‘true damping factor’) for a simple, single 8 ohm loudspeaker driver. To summarize:

Amp’s published damping factor “True” damping factor
8 1.14
16 1.23
32 1.28
infinite 1.33

Interesting, isn’t it? No matter what one does, the true damping factor of the system never exceeds 1.33. There simply is no point in pursuing an astronomically high number for the spec sheet, and doing so will cause one to add complexities that tend to reduce the overall sound quality.

With regard to damping, we believe the best approach is to design the amplifier to have as low an output resistance as possible in a static sense and use little or no feedback.

As we often say, in a battle between theory and the real world, the real world always wins. Or, as Daniel von Recklinghausen once said, “If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you’ve measured the wrong thing.”

The VAC Cinema Bypass Inputs

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cenemaOur preamplifiers and integrated amplifiers introduced since 1998 herald a new era in VAC design philosophy, and address the preamplifier’s changing role in today’s state-of-the-art home entertainment systems. The increasing number of digital audio formats (DVD, MD, DSS, etc) and the growing popularity of multichannel music and cinema software presents the audio designer with a new set of challenges: it is no longer sufficient to create a component that flatters traditional stereophonic program material. A cutting-edge preamplifier must also accommodate a wide variety of sources and perform flawlessly with both music and movies.

The new VAC components do, without any compromise to the purity and fidelity of traditional two-channel sources. They allow cost-no-object stereo systems to integrate seamlessly into a home theater context when needed.

Basically, the VAC Cinema Bypass input is a fixed gain input that allows your source component to control volume, and passes this signal through to the stereo power amplifiers without further processing. It is similar to a tape monitor input, except that it bypasses the volume control, thus eliminating confusion when switching between stereo sources and home theater sources. It allows a cost-no-object stereo system to integrate seamlessly into a home theater context when needed, without any degredation of two channel performance from traditional stereo sources.

The Cinema Bypass input may also be used with stereo components that have their own level control, such as d/a converters.

VAC and the Marantz Classics

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In 1995, VAC entered into an agreement with Marantz Japan to recreate the legendary Model 7 Preamplifier, Model 8B Stereo Amplifier, and Model 9 Monoblock. These models were made variously between 1958 and 1967, and have been sought after by collectors world-wide as the finest examples of the audio engineering art of their day.

vacmji

Unlike the McIntosh and other modern “reissues”, the Marantz Classics were exacting recreations of the original models. Labor intensive point-to-point wiring was retained – no printed circuit boards here. Extensive archeology by the VAC engineers uncovered original vendors and designs for virtually all key components. Except for a few changes mandated by international safety regulations, these models are as if straight from a 1960 showroom.

Given the nostalgic nature of these components, it was truly amazing to discover that they still can beat many modern audio products, as attested to in several reviews in the late 1990′s.

VAC’s Marantz Classic Model 9 was the best selling high end amplifier of any type in Japan in 1997.

All good things must come to an end, and so production of the limited edition Marantz Classics ended in 1998. They were followed by a short run of an all new VAC designed integrated amplifier called the Model 66. Also in development were the Model 77 and Model 99, thoroughly modernized versions of the Model 7 and Model 9; unfortunately, the difficulties in the Japanese economy caused a partial break up and reorganization of Marantz Japan, and these projects were lost in that confusing time.

It was a privilege to work with Marantz Japan and the many fine people there at that time, including Mr. Sato, Mr. Shimoguchi, and Mr. Sarashina.

VAC continues to supply many key components for the repair and restoration of these three classic Marantz models.

From time to time, VAC assists other manufacturers on audio projects. In one notable example, VAC resurrected the Teletronix LA-2A levelling amplifier (compressor) for Bill Putnam, Jr. at Universal Audio, and manufactured the first several hundred units for them.

When it comes to understanding vacuum tube audio and its history, we are second to none.

Tubes, Transistors, and Science

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headerTubesThe transistor has been around for over 65 years now, and solid-state technology must be considered mature. Further developments are likely to be evolutionary in nature, such as size reduction, rather than revolutionary. This is just the nature of innovation, a point of diminishing returns for a given technology. Radically new ideas for solid state amplifier circuits are unlikely.

The triode vacuum tube has been around for 90 years and the beam power tube for 61 years. So, vacuum tube technology and circuit design is also quite mature. Further major developments are unlikely. In fact, the improvement of modern vacuum tube amplifier performance over units from the 1950s is due primarily to changes in other components, such as capacitors and resistors, and attention to detail.

So, today when we compare a solid-state audio amplifier with one using vacuum tubes we are observing a showdown between two very mature technologies. All of the improvements in auxiliary parts are available in both types of amplifiers. And, lo and behold, the vacuum tube still produces superior sonic performance.

What accounts for the tube’s ability to survive and dominate the modern high end audio world? Many would say that it is because the tube produces a pleasant distortion. However this is just not the case. A well designed tube amplifier can produce vanishingly low levels of measured distortion (.01% and less is easily obtainable in preamplifiers) and extremely wide frequency response. The small amount of distortion produced in a tube circuit is mostly second harmonic, which is the type most easily disregarded by the ear.

For those who feel that the transistor represents better objective science, consider this: Both the tube and the transistor have parameters known as stray capacitance. That is, just by physically existing there is unwanted capacitive coupling between various elements of the devices (ex: plate to grid, collector to base). These can not be avoided. In essence there are several small capacitors contained in each tube or transistor.

In the vacuum tube the dielectric for the stray capacitances is nothing, a vacuum. This is the finest dielectric known, having far and away the lowest losses and least dielectric absorption (the way in which capacitors color the sound by reradiating stored energy).

In the transistor the dielectric is silicon, germanium, etc. In other words, using each transistor is essentially as bad as sprinkling a few ceramic capacitors in the circuit. Given a choice, no audiophile would allow even polyester caps in the audio signal path, let alone ceramics. Add to this the fact that transistor design typically uses 200% to 500% more active devices than tube circuits do and it becomes readily apparent why transistor amplifiers display strange subjective characteristics, particularly at mid and high frequencies.

There are many other technical ways in which the tube is scientifically superior to the transistor. They are sonically superior as well.

Isn’t it time you listened to tubes?

The VAC iQ Intelligent Continuous Automatic Bias System

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iQcloseup24The VAC iQ System of intelligent continuous automatic bias (patent pending) is the result of 18 years of research and development by the engineers at VAC and represents the first time in history that each tube in the output stage of a vacuum tube amplifier is held at the optimal bias point (quiescent current) at all times, regardless of how loudly or softly the music is playing.

The result is sound that is always the best it can be, reduced distortion and noise, and elimination of tube failure drama, all without any effort by the user.

The VAC iQ Intelligent Continuous Automatic Bias System is, to put it mildly, a very big deal. It is found in the new VAC Statement 450 iQ Monoblock.

Background
When one “sets the bias” of an amplifier, one is adjusting the quiescent current of the output tube; audiophiles generally call this the idle current, while engineers note it as “Iq” (I standing for current, q for quiescent). It is the Iq that sets the operating mode (Class A, AB, or B) of a given output stage. A typical Iq for a KT88 tube would be 60 milliamperes, and listing tests have shown that variations of as little as 1 milliampere are audible.

In prior state-of-the-art, output tube Iq is set by one of two approaches, known by as  “fixed bias” and “cathode bias” (also known as “self bias” and sometimes incorrectly called “automatic bias” or “auto bias”). In this former approach, a negative voltage (relative to the cathode voltage), which may be adjustable in spite of the name “fixed”, is applied to the control grid of the tube; the more negative the voltage, the lower the Iq. In cathode bias, the grid is held at DC ground potential and the cathode of the tube returns to ground through a large resistance, such that the current flowing through the tube causes the cathode to rise to a positive voltage relative to the grid, such that the grid again is negative with respect to the cathode. Cathode bias has something of a self-centering action but is far too weak to establish and maintain precise Iq.

If Iq is set too low, sound quality suffers. If Iq is set too high, the chance of “run away” tube failure is increased. Between these two points, variations of 2% are distinctly audible.

Operation
If tubes were perfectly stable devices, one could set the bias once and never think of it again. In practice, the Iq of a tube varies with warm-up, with power line voltage, with the temperature of the output transformer, with age, with the volume of music being played through it, and randomly. Some of the variations are minor, and some happen over long periods of time, but one way or another, Iq should be watched and adjusted, both as a matter of sound quality and reliability.

The Difficulty
At first, this would seem like an easy problem to solve: just have an automatic circuit keep measuring the DC and adjust it all the time. The problem is that no conventional technique allows for this because of a characteristic of the tube called the “rectification effect”. Even though a vacuum tube is much more linear than a transistor, it is not perfectly linear, and that small nonlinearity causes part of the audio signal to appear as a DC component when you try to measure Iq while music is playing. This is the rectification effect, and it is inherent in all existing amplifying devices (tube and transistor) to some degree and is quite extreme whenever a tube reaches cut-off, as happens in all Class AB and Class B amplifiers as well as when an amplifier reaches it maximum power output (i.e., when it “clips”).

Many approaches were explored at VAC. One of the most appealing at first blush was to measure the DC plus audio signal and then subtract the measured audio signal from it, on the theory the (DC + AC) – AC = DC. As it turns out, this does not work due to the fact that musical waveforms are asymmetrical, and the location of the waveform’s true zero crossing, which is lost when the DC is removed, changes the measured RMS value of the energy present.

 Other Approaches / Prior Art
Some other approaches to automatic bias have been tried, and there are some amplifiers that do not have bias adjustments and thus seems automatic. The most prominent of these are now briefly summarized.

1)   Cathode bias. As previously noted, cathode bias provides some inherent correction, steering the Iq toward a desired point, but without the strength of action and precision needed. There is a second problem caused by the bypass capacitor which must by used around the cathode resistor, which is that it is charged up by any present rectification effect. The result is that when the amplifier enters Class AB operation or is clipped, the capacitor charges to a higher voltage; the bias voltage rises and the Iq is reduced, driving the amplifier toward Class B operation, and in some cases, beyond, with very high resulting distortion. Under some circumstances, the amplifier will produce high distortion for several seconds after it exits clipping.

2)   Some designers have placed a constant current source in place of a cathode bias’s cathode resistor. Unfortunately, cathode current must vary for amplification to take place, and in the end, the same basic problem with rectification effect occurs.

3)   Some designers run their output stage very close to Class B, with Iq in the 10-20 milliampere range. Part of the theory here is that even if the tube drifts, it is unlikely to drift so far as to cause a catastrophic tube failure. Of course, operation isn’t really optimized, and Class B operation has some significant sonic penalties. In some amplifiers, massive amounts of negative feedback are used to lower measured distortion; in others, things like DC restorers and other older techniques are applied in attempt to reduce the distortion somewhat, to small effect.

4)   A few manufacturers use an active circuit, usually a “set and hold” logic controller, to set bias during the amplifier’s turn-on cycle, and then hold that setting thereafter. The problem, as anyone who has adjusted a tube amplifier knows, is that a tube’s idle current changes a lot within the first half hour of operation. Thus, an amplifier that employs this type of “automatic self-regulation” is guaranteed never to be operating optimally. Due to this fact, additional protection circuits are added to detect tubes that run away due to the lack of continuous automatic bias optimization.

5)   As a side note, in the world of solid-state amplifiers, one often encounters adaptive sliding bias schemes. In such cases, the designer is not attempting to hold an optimal, steady Iq, but rather is intentionally varying it, trying to prevent cutoff from occurring for more than a fraction of a second at a time, effectively keeping the amplifier as close to a Class B Iq as possible while still being able to claim some sort of Class A operation. This is altogether different than the intention or operation of the VAC circuit. Interestingly, one current tube amplifier manufacturer uses a similar circuit that varies the Iq, which they term Adaptive AutoBias. This does not hold a continuously steady Iq.

 The VAC iQ Intelligent Continuous Automatic Bias System
After sixteen years of research, VAC reached the conclusion that is was not mathematically possible to achieve an automatic bias circuit that was theoretically precise. It was at that point that a new insight arose, giving rise to a new heuristic approach. Two additional years of research, modeling, experimentation, and testing proved that VAC’s heuristic approach results in an underlying Iq in each tube that is always within 1% of the set target Iq under all conditions, from silent passages to the most explosive and sustained musical peaks, and in the process, ensures that the individual tubes as well as the overall output stage are always delivering optimal performance.

The VAC iQ Continuous Automatic Bias System (patent pending) is a true breakthrough in power amplifier design. It is not inexpensive, but, once heard, it is indispensable.

What Else Does It Do?
In addition to guaranteeing unparalleled fidelity, the VAC iQ System greatly enhances the reliability of the amplifier in several ways.

First, “run away” tube failures are eliminated. Run away tubes arise from one of two mechanisms. The first is a “gassy” tube, the Iq of which tends to rise on its own until it goes into positive thermal feedback; the VAC iQ System prevents that cycle from ever starting. Interestingly, through observation of the operation of the VAC iQ circuit, it has been revealed that such tubes actually tend to be relatively “cold” tubes, which users ordinarily “crank up” thorough bias adjustment.

The second run away mode occurs when a tube is set to run at a relatively high iQ and then randomly drifts a bit higher or becomes physically hotter through sustained loud musical passages. The VAC iQ circuit detects any move to a higher iQ and automatically corrects for it, and once again, the run away cycle is never allowed to start.

As part of its operation, the VAC iQ System can tell when a tube is becoming weak and notifies the user that it should be replaced when convenient, while still making the best of its remaining life.

Lastly, it is always possible for a mechanical break to occur within a tube, resulting in an internal short circuit. No adjustment of bias can fix this, so if this occurs, the VAC iQ System automatically shuts off the high voltage power supply within a fraction of a second, long before a conventional fuse could blow, and illuminates a red LED to indicate which tube has failed.

 Summary
The VAC iQ Intelligent Continuous Automatic Bias System advantages:

- Always the best sound

-No user adjustment

- Supreme reliability

-Longer tube life

-Indication of weak tubes

-Automatic protection from and indication of failed tubes

 Final Note
The principles of the VAC circuit are applicable to solid-state devices as well as vacuum tubes. Development and licensing of these applications will be considered.

printable version (pdf)

VAC’s own Kevin Hayes will be discussing the VAC iQ System next week at 2013 International CES in Las Vegas January 8-10 in suite 30-125 and suite 30-127 at the Venetian Towers.