VTL Siegfried Series II Reference monoblock power amplifier

Big tube amplifiers were once scary monsters reserved for those who didn't mind heavy maintenance, careful tweaking, and the occasional explosion. Blown tubes required replacing, preferably with pricey matched pairs, then biasing with a voltmeter. Optimal sonic performance required regular bias monitoring and adjusting, and because of current surges on startup, you had to choose between leaving the heat-producing monoliths on, or turning them on and off for each listening session, thus shortening the life of the tubes.

For all of those reasons, the received wisdom was that tube amps should have no more than two push-pull pairs of output tubes per channel. Nevertheless, in the mid-1990s, some designers soldiered on. Audio Research came out with its 16-output-tube 600 Reference, and VTL had its 24-tube MB-1250 Wotan both monoblocks.

Among the sonic rewards for all the time, trouble, expense—and, at best, moderate reliability—was enough harmonically rich power to drive inefficient speakers to large-scale sonic magnificence, even in big rooms. Aficionados of mega-powered tube amps felt that big, solid-state amps sounded dry and harmonically bleached, with shrunken soundstaging. For those who didn't want their music hard, bright, and edgy, and who could afford them, big, multi-tube monoblocks were the only way to go.

Microprocessors to the Rescue
VTL introduced the Siegfried Reference in 2003, and updated it to Series II status in 2012. A modern high-powered tubed monoblock, the Siegfried includes microprocessor technology in an attempt to eliminate the problems plaguing older multi-tube designs. The idea was to create a big, powerful, multi-tube amp that was reliable, user friendly, and didn't burn out tubes so quickly.

By carefully controlling the power supply regulator, the microprocessor delays and gradually ramps up the plate voltage applied to the output tubes. VTL claims that this feature is found in no other brand of tube amp. The microprocessor also oversees automatic biasing for each tube, as well as sensing of faults in the output tubes and monitoring AC power, power-supply overcurrent, and regulator heatsink temperature. The same system makes possible on-the-fly switching between triode and tetrode modes for the output tubes. In short, the Siegfried II is constantly checking that its performance is optimal. We should all be so conscientious.


In my ground-level, slab-floored listening room during a ridiculously cold winter, I can't think of any amplifier I'd rather have had installed than a pair of Siegfried IIs. I timed this review for the winter, hoping it would be a cold one, and got more than I'd wished for. I can state with confidence that the Siegfried II, which in tetrode mode outputs 650W into a 5 ohm load, or 330W in triode, can also serve as an effective space heater. True, a DeLonghi heater costs 1/1000 the VTL's price of $65,000/pair—but it doesn't sound nearly as good!

The Plant
The Siegfried Series II Reference is a massive edifice of extruded, anodized aluminum 2' tall by 2' deep by almost 1' wide and weighing 200 lbs. Loaded onto a dolly, it rolled easily into my listening room. I wouldn't want to have to carry a pair up or down a flight of stairs.

At the top of the faceplate is a small display panel. Below this are three LEDs, and below them a small plate. Remove the plate to reveal six buttons with their own LEDs; these allow you to change a few of the basic operating features, and to display such information as how many hours of operation the current tube set or the amp itself has logged, as well as access diagnostic functions.

To more closely monitor the Siegfried, you can display the bias setting of each tube, use the fuse tester, or check out some other operational parameters, but I suspect most users will leave the panel in place unless they encounter a problem—and in many months of use, I didn't. You can also access this information via a computer connected to the Siegfried's RS-232 port.

On the spacious rear panel, above the heatsink, are a pair of gold-plated binding posts, RCA and balanced XLR inputs, a 12V trigger socket, an IR input, the RS-232 port, and access to the fuse bay for the plate and screen voltage supplies. Below the heatsink are five more fuse holders, the main On/Off switch, and the IEC AC socket. The binding posts are easily accessible—important!

Circuit Improvements
The Siegfried Series II Reference's relatively simple all-tube, low-negative-feedback circuitry comprises three stages: input, driver, and output. Tubes are a single 12AT7, two 12BH7s, and a dozen paralleled 6550s or KT88s, all easily accessible just under the removable top grate.

In addition to producing prodigious power, the 12 output tubes provide a significant additional benefit: they sufficiently lower the output impedance to allow the use of a simpler than usual output transformer with a low step-down ratio. The secondary of this transformer doesn't have multiple impedance taps but is optimized for operation into 5 ohms. VTL claims that using the entire secondary for a single output results in low leakage inductance and superior high-frequency response: –1dB at 100kHz.


VTL's list of claimed improvements for the Series II Siegfried include: a fully balanced differential input stage driving a differential phase splitter, a lower-impedance push-pull output stage, a fully balanced feedback loop (the original was single-ended into balanced input, which VTL says had some problems with stability and a slight tendency to ring under heavy power), zero global feedback, zero capacitor compensation to maintain critical phase integrity, a new and more heavily interleaved and sectioned output transformer to extend the frequency response, with a smoother rolloff, and zero ringing. In addition, the original's 6350 tubes are replaced by 12BH7s, which VTL says are like "high-powered 12AU7s capable of driving the output stage well into clipping before overload."

VTL's variable Damping Factor (DF) feedback control, first used in the MB-450 Series III Signature monoblock, has been incorporated into the Siegfried Series II. With this control, the user can change the amp's output impedance by varying the amount of negative feedback applied; the resulting sound is claimed to be superior to that produced by separate transformer impedance taps. There are four Damping Factor settings: Low, Medium, High, and Maximum.

The power supply, located in the bottom half of the chassis features rubber-potted power transformers to reduce vibration, and premium Mundorf silver-foil capacitors bypassed with smaller caps.

In short, the Series II is a lot more than new makeup slapped on a 10-years-older face.

Setup and System
VTL's CEO, Luke Manley, flew in from California to set up the Siegfried IIs in my room, though more to supply brawn than brain. Installing and using the amps was straightforward; moving and placing these monsters requires at least two people.

Choosing the gear to hook up to the Siegfrieds was another story. With any high-performance product, proper setup and careful selection of associated equipment are critical for maximizing performance. Although my darTZeel NHB-18NS preamplifier is single-ended, with a transformer-coupled balanced output, we both felt the Siegfrieds sounded better driven balanced, via TARA Labs Zero Gold interconnects. WireWorld's Platinum 7 balanced interconnect, which worked so well in my system with Constellation Audio's Performance Centaur monoblocks ($54,000/pair, didn't sound as smooth, extended, or precise in transient attacks as the TARAs (footnote 1).

I didn't think I had any premium 20A power cords, so we used some mismatched 1990s-era ones I had in a box in the garage, plugged into the wall and then into the Shunyata Research Hydra Triton/Typhon power-distribution system, where they stayed: the backgrounds were obviously "blacker," the images far more solid, and the sense of space more coherent.

After about a week of listening, during which it became clear that something wasn't jelling, I called Shunyata's Grant Samuelson and asked if he could send me a pair of 20A Z-Triton Anaconda power cords. He suggested I try the new Z-Tron Alpha HC Analog ($1295), which is considerably less expensive than the Anaconda ($2995) and specifically designed for the high current draw of power amplifiers. The Z-Tron Alphas use a new, non-ferrite filtration system claimed to be highly effective in reducing noise by not producing such audible effects such as "dulling" or dynamic compression.

Waiting for the Z-Trons to arrive, I found, in a closet blocked by cartons, a pair of TARA Labs The Cobalt AC 20A cords. Those produced a considerable improvement in terms of the sonic coherence I felt had been lacking, and though some say that a single-brand cable loom is best, once Shunyata's Z-Tron Alpha HC Analog cords had arrived, spent 24 hours on the Audiodharma Cable Cooker 3.5, and been installed, the system finally snapped into the desired sonic focus and tonal and textural coherence.


I'd been certain the Siegfried IIs were capable of better sound than I'd been hearing because I'd listened to a pair of them in a friend's system driving Wilson Audio Specialties Alexandria XLFs—the same speakers I have at home. There, wired with Transparent Audio Opus cables, the Siegfrieds produced the rich, creamy center I hadn't been getting.

With the cabling finally settled, the system returned to the coherence and freedom from loose ends I'd enjoyed with the darTZeel NHB-458 monoblocks—which, at $144,500/pair, cost more than twice as much as the Siegfried IIs. Finally, I could sit down, listen to music, and not hear the system—although, of course, the VTLs didn't sound identical to the darTZeels.

Big Picture
About this time, Neil Young's Live at the Cellar Door (LP, Reprise 535854-1) arrived: Young in 1970, singing in an intimate setting, accompanying himself on acoustic guitar and Steinway piano, and recorded on analog tape by the late, great Henry Lewy. Lewy cared about the performer and the room—he closely miked Young and his instruments, but manages not to lose the 200-seat club, which is tucked in subtly behind Young, more sensed than heard. When the applause comes after the first tune, you realize that Lewy carefully miked the audience as well. Of course, even the greatest two-channel live recordings are spatially inverted, the sound of the audience seeming to come from behind the performer(s); this one sounded thrillingly three-dimensional and eerily transparent through the Wilson XLFs driven by the Siegfried IIs, and devoid of recording process detritus.

When I played Live at the Cellar Door at the appropriate (moderate) sound-pressure level, I seemed to be seated at the edge of the stage, with Young seated directly in front of me, and no microphones between us. When he strummed the guitar, it got appropriately loud, with ideal transient attack, the guitar's body perfectly reproduced behind the strings. Attack, sustain, and decay were rendered believably and with good balance, to produce both detail and emotional intent. Lewy's recording of the Steinway shows some masterful miking—harmonics, textural delicacy, and soundboard decay were all eerily accurate.

Footnote 1: If you're a cable skeptic, take this test.—Michael Fremer
VTL Amplifiers Inc.
4774 Murrieta Street, Suite 10
Chino, CA 91710
(909) 627-5944