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"Tubes are voltage amplifiers - very, very linear voltage amplifiers. But they are not very linear current amplifiers and what are we doing in the buffer? Current! So while we had a six channel box, we decided, let's do two channels of cathode follower and two channels of our buffer and two channels of hybrid. We found that the old tube system with our new buffer had the continuousness and the holistic sound. However, it had colorations with various cables and cable lengths and was unpredictable depending on the length of cable you are driving. You could never be sure how it would react. Under ideal conditions, it sounded spectacular. Add a long run of cable or power amp with different input impedance and the magic was gone and the character of the music completely changed. There was no real-world consistency. We found that the MOSFET actually is very much like a tube. However, there's no voltage amplification, just a current buffer with actually a negative gain of 0.9, so you lose a little bit of gain going through a buffer.

"Initially we switched between the hybrid cathode follower and our new tube buffer but the cathode follower was way too dark. You could easily tell that it was not very modern sounding and very 'tubey' instead. VTL's current buffer and the MOSFET buffer were the two left in the shootout. The MOSFET buffer sounded really neutral and did not seem to change no matter what cable you put o, and no matter how long a run. Even with high capacitance cables, the MOSFET did not seem to care. We were getting tons of voltage and current out and it became clear that this was the way to go. The challenge was to make the MOSFET sound like a tube. As I said, it has tube-like characteristics but there are problems. MOSFETs have very high gate capacitance so you have to make sure that the tube driving them can really couple into i, because tubes have very high output impedance. When you're trying to drive high output impedance into large gate capacitance, you're going to have a roll-off. The challenge was to make those two match and interface properly."

"The MOSFETs provide no voltage gain but are capable of massive current to drive loads as low as 600 ohms to the full 30V output, with an amazing 25-ohm output impedance rising only slightly at lower frequencies. Once we got past this point, we realized that this preamp was going to be far more neutral driving a variety of amplifiers (solid-state or tube) with virtually any type of cable. I did not want anything external to influence the performance of this preamplifier and that included AC. So we built multiple precision regulated power supplies for the B+, B-, filaments and the logic - everything is regulated. In the B+ case, it has to be precision regulated because you don't want any shift there. In fact the power supply has twelve separate regulated power supplies."

When Luke talks about overcoming sonic bottlenecks, potentiometers and volume controls are at the top of his list. He considers wiper-based potentiometers inherently flawed and noisy. Separate controls for volume, balance and offset add complexity and degrade the sound. Contact-based rotary switches with discrete resistors do not provide enough steps, leading to the perception that the volume is either slightly too loud or too low. VTL's solution is a combination of optical encoder, processor logic, 24 shielded reed relays and precision resistors. The beauty of the system is that the volume, balance and individual input offsets are all managed by a single unified system, simplifying the signal path so that no matter what the volume, balance or offset parameters, the signal flows through no more than one relay per leg and two resistors. Luke's position is that a sealed relay contact is far less detrimental to the audio signal than a potentiometer with its inherently noisy wiper.

Once you've optimized the volume and balance control system, now where do you place it in the circuit? There are three choices and each has a trade-offs. "We only had two stages, the gain stage and the buffer. The volume control can be in one of three places. You can put it before the gain stage, between the gain stage and the buffer or after the buffer. If you put it after the buffer or between the gain stage and the buffer, you need extra buffering on the control to prevent it from changing your output impedance or changing the matching between the gain and the buffer. Extra circuitry will affect the sound, therefore I believe there is only one place to put the volume control and that is before the gain stage. So really, you are attenuating the input signal. The problem is that the gain stage puts out full gain all the time so you hear the gain hiss with that. People did not want to hear any noise. Some were putting 12AU7s into the 7.5 to lower the overall gain. The 12AX7 has a huge amount of gain and with very low feedback, there's nowhere to get rid of that gain. In the old days, we would have slammed on the feedback throttle and lowered the gain that way. You would have gotten better error correction, more linearity, all of those specs things, but you don't get good sound quality that way."

However, placing the volume control at the input signal combined with the inherent high gain of the 12AX7 led to one of the few niggling criticisms of the original Series I. Since the input signal is being attenuated and amplification occurs further downstream, the amplifier section is essentially run "wide open" into the speakers at all times. If there is slight tube rush or the tube develops noise, you'll hear it from the tweeter regardless of the volume setting. In order to enjoy that silky black dead silence, the designers had their work cut out for them.

"We were kind of perplexed as to how to respond to our customers, some of whom had very high sensitivity power amps such as the Lamm. One of the Lamms has 32dB of gain. Customers wanted zero noise - absolute dead silence from the tweeter. So when we were doing the phono stage, we saw that the 12AU7s can handle a lot of current. As always, when you keep them in their linear range, they do not draw grid curren, and they have lower plate-in impedance so they can interface better into the MOSFET. That led us to the Series II version of the 7.5. And we said, how do we make this work in the 7.5 line stage? Just plugging a 12AU7 into a 12AX7 circuit is not ideal. Yes, it lowered the gain but it certainly did not sound as good as the 12AX7."

Don't get the idea that the original 7.5 had old-fashioned tube noise - not even close. Yet it also did not have the dead silence of the best solid state. The change to the 12AU7 circuit in the Series II pushes the noise level to silky blackness. With no input, it takes bringing my ear within 6 inches of the tweeter to hear anything; therefore you have absolute dead quiet when listening to music in the seat. Part of the reason is due to gain reduction. The original 7.5 Series I had a high gain of 26dB balanced and 20dB single-ended. The Series II lowers that to 20dB balanced and 14dB single-ended.

Third in the VTL hierarchy of sonic bottleneck design problems was feedback, one of the few terms that by itself can set off heated debates between and among both audio designers and listeners. DarTZeel uses no global feedback while others find it necessary and valuable, assuming moderation when used in a well designed circuit. One point of agreement is that it has some audible effect.

"We all know now that feedback changes the sound and my dad always believed it, although I think he did not realize just how much it affects the sound. He is actually quite surprised now when I tell him the results of our engineering and listening tests. In the olden days we did not do as much listening to the product as we do now, particularly with a live reference because today, we have two listeners, myself and Bea. When I told my father that Bea can hear the effects of 2db of feedback his answer was, ...bull something! There is no way. You can not hear that small amount of feedback.

"The process is one of balance. A perfectly designed and measured circuit can sound boring. If you don't have emotion, it's back to design. We always look at the number one culprit - how much feedback do you have and where is it? You struggle between the measurements, the top end response versus the emotion in the music."

In the end Luke believes in the value of a small amount of global feedback. "It's like a little spice in cooking. The 7.5 actually has less than 2dB of overall negative feedback in one global feedback loop. It's a very, very linear design that does not need much error correction."

Well, there you have it. Staying with the food analogy, the VTL recipe for sonic nirvana is: mix a simple differential tube circuit topology with low negative feedback, high current solid state output, ultra clean power, eliminate potentiometers, add a pinch of feedback, house the monster in an isolated chassis and build it like the proverbial brick sheeba house. Great concept but does it perform?

Setup is pretty straightforward. The master rear-mounted power switch generally remains on continuously and the unit in standby indicated by two dashes in the front display. Since the control system is microprocessor based with its dozens of initial programming options, you will need the well-written, spiral-bound manual to guide you through some of the setup functions and the myriad of customizable options. Enabling trigger output, suppressing the countdown timer, setting inputs for unity gain for home theater use, disabling inputs all require some combination of multiple button pushing. However, in regular use the unit is quite intuitive and once set up, you will probably not need to touch the manual ever again. The 7.5 stores your settings in non-volatile memory so they're retained even with power off. One option allows a settings lock lest you inadvertently muck up your programming.

Pressing the power button on either the control unit or the remote starts a 90-second countdown, gently warming up the filament voltage before applying the high plate voltage. This is said to extend tube life to possibly 4,000 or 5,000 hours. When the countdown reaches zero, the power light stops blinking and you're ready to roll 'n' rock.

Each button on the control unit sports an LED above it. In general the LED can turn green, blue, red or off, each color signifying a status of the settings. Once you get the LED color code down, it's not too hard. A quick glance and you know exactly what the 7.5 is thinking and doing. Pretty neat actually.

Rotating the volume control produces a very slight clicking noise through the speakers but it's normal and integral to the design. If your source component has a problem with DC at its output, the clicking becomes louder and it's an indicator to get the DC offset solved.

Even with its twelve massively filtered, precision regulated power supplies, the 7.5 is sensitive to external power cables and power conditioners. Luke recommends to always plugging the 7.5 into the wall outlet first, using that as a baseline and experimenting from there. Some AC conditioners will sound better, others will sound worse as manifested by reduced dynamics, a smaller soundstage and a more two-dimensional sound quality. I started with the wall outlet as recommended but ended up with a Furman balanced power conditioner feeding a Walker Velocitor. I tried all the various permutations and combinations and even threw my old PS Audio P150 Power Plant into the mix (yuck). The PS Audio was eliminated right off the bat - thin, cool and constricted. The Furman/Velocitor combination produced the most robust and dimensional sound with the lowest noise floor.

The 7.5 was connected at various times to the VTL S400 and darTZeel NHB-108 amps through 20 foot runs of balanced Transparent interconnect.

Remote controls are what I call negative impact devices. Good ones go basically unnoticed, disappear into one's listening environment, are taken for granted and thus ignored. Bad ones constantly irritate and provide a low-level annoyance to an otherwise quality component. Thankfully the 7.5 remote is excellent; weighty, full functioned yet easy to use, offering input selection, volume, balance, power, phase, mute and fade. I particularly like the fade function, which reduces the volume to level 20 on the display. If the volume is already below 20, it has no effect. By exclusively using vinyl for critical listening, this allows me low-level output while queuing up the next track - a small but nice touch.

Listening to music in the dark as I do, it's frustrating to fumble around for the volume control buttons or the mute when the needle is bouncing around in the run-out groove. By placing the fade button at the bottom left, it's easy to find even in the dark. Likewise the volume up/down buttons are located at bottom right for easy volume trim in the dark; certainly better than volume controls buried in the center of buttons you never use. The IR signal from remote has a very wide angle. Just point it in the general direction of the control unit and it works every time. The darTZeel remote on the other hand had an irritatingly narrow beam. At times I felt I was using a laser pointer. It almost required a head-on angle to communicate, which at midnight, after a few glasses of wine, was not always that easy.

Again, it's the small things that add up to either likeability or annoyance. Sometimes I wonder if the engineers and designers actually use a product before it is sent to market. The usability factor of the 7.5 remote is exceptional and the design team deserves kudos for their attention to detail. Its design exceeds both the Levinson and darTZeel. Additionally, a rear IR receiver allows the 7.5 to be controlled by either an IR extender or any one of the major home automation units with control codes uploadable to Creston, AMX or Pronto.

From I to II
Where do you take a commercially successful product still in its prime, lauded likewise by users and reviewers? Maybe just as importantly, why change anything? And if you are going in a new direction, how are you going get there? Well, in this case the 'where' is a fairly significant sonic upgrade, not one of those subtle tweaks noticeable only by the most golden of ears. The 'why' comes from customer and dealer input mixed with an internal VTL passion to be the best. And the 'how' is yet another 'Connection', in this instance the trickle down (really up) technology from the new VTL Statement Phono Preamp. Simply put, Luke believes that change is mandatory.