S-400 Series II Reference Stereo Amplifier


Since its introduction in 2004, VTL’s S-400 Stereo Reference amplifier has been valued by music lovers around the world as a high power, high performance tube amplifier–renowned for its ease of use and refined, natural depiction of music. The new Series II version of the S-400 uses the same circuitry as its bigger brother, the Siegfried Series II Reference monoblock amplifier, in an impressive single chassis design. The inclusion of VTL’s latest engineering advances brings the renowned sonic performance of the S-400 to even greater heights.

For the Series II, VTL, has completely re-worked the entire signal path. Upgrades include a fully balanced differential input stage driving a differential phase splitter and a lower impedance push-pull output stage with a dramatically improved, fully balanced and enhanced interleaved and coupled output transformer. Equipped with the same “SmartTube” technology as the Siegfried II, the S-400 deploys Auto Bias and Fault Sensing, precision-regulated power supply technology, and bi-directional RS-232 control, giving unparalleled flexibility and ease of use.

The S-400 Series II gains further sonic benefits with a shorter, faster and fully balanced negative feedback loop, with zero global negative feedback. The negative feedback loop completely eliminates ringing and requires no capacitor compensation to maintain critical phase integrity and information. The result is an amplifier that remains stable even under the most demanding loads.

Adjustable precision-regulated plate, screen and bias supplies hold the output tube operating point constant even under AC and main power supply fluctuations, and stabilizes the critical power supplies yielding tonal stability and sonic integrity, especially during complex, dynamic signal conditions.

Another new feature is a user adjustable Damping Factor feedback control that allows the user to adjust the amplifier’s output impedance by varying the amount of negative feedback. Impedance can now be precisely set to suit the listener’s taste, and to improve control of the loudspeaker loads to deliver best performance.

The 3 possible settings are:

  1. LOW – Lowest damping factor, good loudspeaker control, most natural sound.
  2. MED – Better loudspeaker control, with some impact on sound quality
  3. HI – Best loudspeaker control, with a little more impact on sound quality, but on speakers that need the control the sonic improvement is clear

Finally, the amplifier has been substantially re-voiced with premium Mundorf silver oil capacitors, for a sweeter, more extended top end and mid range tonality that sounds more relaxed, with better flow and integration. Input stage capacitors are also bypassed.

Even a brief listen to the S-400 Series II Stereo Reference amplifier reveals the successful implementation of the new technology. A lower noise floor creates a velvety black sonic background from which more of the nuances of music can emerge. The soundstage is taller and deeper, yielding better defined and more delineated instrumental images. The mid bass has more authority and control while the critical midrange exhibits a new effortlessness, better reflecting the natural colors and timbres of musical instruments.

Overall, the new amplifier has an uncanny ability to sound quick and agile, yet still possessing the ability to scale up effortlessly, with a sense of almost unlimited power. Listening to music through the S-400 II amplifier is an exciting and invigorating experience.

S-400 II Back Panel

Functionality features

  • Fully balanced differential circuit – single-ended input signal produces balanced signal at output
  • Re-designed fully balanced differential input and driver stages for increased signal swing, bandwidth and stability
  • Zero global Negative Feedback
  • Shorter, faster feedback loop for greater tonal control without phase shift.
  • Improved handling of loudspeaker loads for greater signal stability.
  • The amplifier is unconditionally stable, without the need for any capacitor compensation.
  • New current source for greater sonic integrity in single-ended mode
  • Lower impedance output stage for improved loudspeaker control
  • Dramatically increased interleaving and coupling in the balanced VTL proprietary Reference output transformer, for wider bandwidth and zero ringing
  • Variable user-adjustable DF feedback control to vary output impedance and damping factor for optimal matching to speaker load
  • Software microprocessor controlled
    • Tetrode/Triode Switchable
    • Auto Bias, Fault Sensing
    • Current in-rush limiting
    • Standby Mute function; with reduced tube current for increased tube life
    • Fully bi-directional RS-232 control
  • 300 watts per channel in tetrode; 150 watts per channel in triode
  • Uses twelve 6550 or KT-88 tubes
  • Adjustable precision-regulated plate and screen supplies for stable operating point; discrete regulated input and driver supply for greater signal resolution and tonality
  • Adjustable precision-regulated bias supply for greater signal resolution and noise rejection and improved isolation from mains supply variations; output operating point does not change with AC power fluctuations
  • Improved Autobias and fault sensing system to operate in fully balanced differential mode; amplifier is fully protected against output tube failure; no need for precision matching of output tubes
  • All premium Mundorf silver oil caps in signal path
  • Film bypass of power supply caps for greater HF resolution
  • Extruded aluminum front panel with luxurious, modern look – black and silver options available
  • Precision regulated B+ and screen supplies
  • Rigid construction and improved ventilation for lower operating temperature
  • Factory upgradeable from Series I


pdficon_smallS-400 Data Sheet

Technical Specifications

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
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)

Rocky Mountain International HiFi Press Awards 2018

RMAF 2018 International HiFi Press Award
The S-400 Series II Reference Stereo amplifier received the Rocky Mountain 2018 International HiFi Press Award for the best Stereo Amplifier.  This award is nominated by a group of journalists from 14 publications located in 11 countries including the US, Germany, UK, Hong Kong, Mexico, Poland, Australia, Greece, Sweden etc.

Diamond Dogs |The Audio Beat

But the S-400 II/Sasha 2/WATCH Dog system sets the performance bar extremely high and in doing so establishes not just a benchmark that will serve to keep more extravagant efforts honest, but one that will consistently deliver superb musical results in its own right, irrespective of what happens at higher prices or with bigger boxes.” “this is a system that does just one thing: it brings performers and their performances into your home — and it does it brilliantly. It delivers on the promise of the products it contains and it delivers — in the most straightforward, direct and uncomplicated way — on the promise of high-end audio itself.”

Read the full review here …

- Roy Gregory, “Diamond Dogs: Adding Subs to a VTL/Wilson System,” August 12, 2016, TheAudioBeat.com

S-400 Series II Reference Stereo Amplifier

“The power to grip, and the confidence to let go.”

“By allowing the user to match the amp’s damping factor more closely to the speaker’s low-frequency characteristics (and interaction with the room), VTL gives control over the critical balance between dynamic response, image stability and musical definition that is so key to both our personal tastes and a system that successfully meets our goals.”

” What we have here is an amp that is at once outstandingly confident, musically coherent and understated, an amp that exposes just how incomplete and/or exaggerated so much high-end hi-fi actually is. It is absolutely stable (electrically and sonically) and impressively solid, yet has that all-important ability to scale its presentation to match the recording. With a natural warmth, neutrality and wide bandwidth, it goes deep and high, delivering crushing bass as required as well as air and delicacy that are beautifully integrated into the musical whole, rather than obvious or attention-grabbing.”

“If James Joyce were alive today and wandered into my audio store, this is the amp I’d play him, its utterly unimpeded, deeply expressive delivery nothing short of a musical stream of consciousness: watts, big watts, small watts, as-many-as-you-need watts, whenever-you-want-them-watts, with the authority to deliver that final, definitive full stop.”

Read the full review here…

- Roy Gregory, “S-400 Series II Reference Stereo Amplifier”, The Audio Beat, May 6, 2016