S-400 Series II

Since its debut in 2004, VTL’s S-400 stereo reference amplifier has won the favor of music enthusiasts worldwide thanks to its high-power, high-performance tube design, ease of use, and refined, natural musicality. The newly released S-400 Series II inherits the circuit design of its high-end sibling—the Siegfried Series II Reference monoblock amplifier—and features an impressive, unified chassis. The incorporation of VTL’s latest engineering innovations elevates the S-400’s exceptional sound quality to a new level.
For this second-generation product, VTL has completely redesigned the entire signal path. Upgrades include a fully balanced differential input stage driving a differential phase splitter, a lower-impedance push-pull output stage, and a significantly improved, fully balanced, enhanced interleaved-coupled output transformer. The S-400 incorporates the same “SmartTube” technology found in the Siegfried II—featuring automatic bias and fault detection, precision regulated power supply technology, and bidirectional RS-232 control—to deliver unmatched flexibility and ease of use.

S-400 Series II employs a shorter, faster, and fully balanced negative feedback loop to achieve zero global negative feedback, thereby further enhancing sound quality. This feedback loop completely eliminates ringing, preserving critical phase integrity and signal information without the need for capacitive compensation. Consequently, the amplifier remains stable even under the most demanding loads.

Adjustable, precision-regulated plate and bias power supplies maintain constant operating points for the output tubes—even amidst AC line and mains fluctuations—and stabilize critical power rails, thereby ensuring tonal stability and sonic integrity, particularly under complex, dynamic signal conditions.

Another new feature is user-adjustable damping factor feedback control, which allows the user to regulate the amplifier’s output impedance by varying the amount of negative feedback. Impedance can now be precisely set to suit listener preferences and provide better control over the speaker load, thereby achieving optimal performance.

Three possible settings are:

The lowest damping factor setting offers superior speaker control and the most natural sound.
Medium – Offers good speaker control, though it has some impact on sound quality.
High – Provides optimal speaker control; while the impact on sound quality is slightly more pronounced, the sonic improvement is clearly evident for speakers that require such control.
Finally, the amplifier has undergone extensive retuning and incorporates high-quality Mundorf silver-in-oil capacitors, resulting in sweeter, more extended high and mid-range frequencies; the sound is more relaxed, natural, fluid, and cohesive. The input stage capacitors have also been bypassed.

Even a brief listening session with the S-400 Series II stereo reference amplifier reveals the successful application of this new technology. The lower noise floor creates a soundstage of velvety purity, allowing more of the music’s nuances to emerge. The soundstage gains greater height and depth, while instrument imaging becomes sharper and more distinct. The mid-bass is more powerful and tightly controlled, and the crucial midrange exhibits an unprecedented sense of ease, faithfully reproducing the natural timbre and tonal quality of the instruments.

In short, this new amplifier boasts stunning dynamic response; it delivers sound that is both agile and lively while effortlessly handling high volumes, as if possessing limitless power output. Listening to music through the S-400 II amplifier is an exciting and invigorating experience.
Fully balanced differential circuit—converts a single-ended input signal into a balanced output signal.
The redesigned fully balanced differential input and driver stages enhance signal swing, bandwidth, and stability.
Zero Global Negative Feedback
A shorter, faster feedback loop enables superior tonal control without phase shift.
Improved speaker load handling and enhanced signal stability.
This amplifier is absolutely stable and requires no capacitive compensation.
In single-ended mode, the new current source achieves superior audio integrity.
The lower-impedance output stage improves speaker control.
VTL’s proprietary balanced reference output transformers significantly enhance interleaving and coupling, thereby achieving wider bandwidth and zero ringing.
User-adjustable DF feedback control allows for adjustments to output impedance and damping factor to achieve optimal matching with speaker load.
Software microprocessor control
Switchable tetrode/triode mode
Automatic offset, fault detection
Inrush current limiting
Standby/Mute function; reduces vacuum tube current to extend tube life
Full bidirectional RS-232 control
300 watts per channel in tetrode mode; 150 watts per channel in triode mode
Utilizes twelve 6550 or KT-88 vacuum tubes
Adjustable, precision-regulated anode and plate power supplies ensure stable operating points; independent regulated power supplies for input and driver stages deliver superior signal resolution and tonal quality.
This adjustable, precision-regulated bias power supply enhances signal resolution and noise rejection while improving isolation from mains voltage fluctuations; the output operating point remains stable regardless of AC power variations.
An improved automatic bias and fault detection system operates in fully balanced differential mode; the amplifier features comprehensive protection against output tube failure, and precise matching of output tubes is not required.
All capacitors in the signal path are premium Mundorf silver-in-oil types.
Film bypass capacitors are used across the power supply capacitors to achieve superior high-frequency resolution.
Featuring an extruded aluminum alloy front panel, it boasts a luxurious and modern look—available in black and silver.
Precisely regulated B+ and screen supplies
Sturdy structure, improved ventilation, and lower operating temperatures.
Upgradable from Series I factory units.

Vacuum Tube Complement 12 x 6550 or KT-88, 2 x 12AT7, 2 x 12BH7
Output power Tetrode = 300W, Triode = 150W
20Hz – 20kHz <3.5% THD
Into 5Ω
Input sensitivity Variable between 1 – 2V, depending upon DF setting
Input impedance 48kΩ
Load setting 5Ω
Optimum load range 4Ω – 8Ω
S/N Ratio -90dB, 120Hz
Power consumption Idle = 560W, Full power = 1600W
B+ fuse ratings 100/120V = 20A Ceramic Slo Blo, 220/240V = 10A Ceramic Slo Blo
Logic fuse ratings 100/120V = 1A Ceramic Slo Blo, 220/240V = 1A Ceramic Slo Blo
Input fuse ratings 100/120V = 2A Ceramic Slo Blo, 220/240V = 1A Ceramic Slo Blo
Filament fuse ratings 100/120V = 2A Ceramic Slo Blo, 220/240V = 1A Ceramic Slo Blo
Secondary fuse ratings Plate 2.5A Fast acting 600V, Screen 0.75A Fast acting 600V
Dimensions W x D x H 11.5 x 24 x 24 inches (29 x 61 x 61 cm)
Weight Net: 250lbs (113Kg), 350lbs (159Kg)