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VTL S-400 Series II Reference Stereo Power Amplifier

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  • Flagship Stereo Power Amplifier!
  • Ideal Partnered with TL7.5 Series III Preamp
  • 6 x 6550 Tubes per Channel


The S-400 is the flagship VTL stereo power amplifier and sits directly below the mighty Siegfried. With 300 watts per channel in Tetrode mode (150 in Triode) the S-400 is a seriously powerful and capable amplifier.
For the latest Series II revision VTL has 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.

As with the S-200 above another great 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 settings are:

  1. Low – Lowest damping factor, good loudspeaker control, most natural sound.
  2. Medium – Better loudspeaker control, with some impact on sound quality
  3. High – 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.

The immediate thing you hear when you listen to the S-400 is the noise floor, or the lack of one. You get an inky black background which really lets you hear everything that is going on in the music.

You get a huge soundstage which is tall, wide and deep which paints a superb picture of what is going on in the mix. The high power of the S-400 delivers real dynamics and most importantly control and the whole presentation is just effortlessly musical.

Listening to music through the S-400 II amplifier is an exciting and invigorating experience!

VTL Tetrode-Triode Switching Capability

The VTL range of power amplifiers offer a unique feature that makes them even more flexible: On-the-spot switching between tetrode and triode modes of operation that is easy to reach and user switchable.

At lower volume levels, and with efficient speakers, the sound of the triode mode offers a level of delicacy, liquidity, and sweetness to vocals, chamber and jazz instrumental quartets and solo instruments with 3-D imaging in the mid-range that will get you even closer to the real event.

Tetrode mode offers the detail, soundstage width and depth, and weight to the music that on large-scale orchestral or big band will transport you to the concert hall with all the visceral power and detail rendition of your favorite music.

The 3 settings are:

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

The S-200 bears a similar sonic signature to its bigger brothers, the MB-450 Series III Signature monoblocks. The mid bass authority and control is excellent while the midrange and top end reflect natural timbres and instrument colours effortlessly. The amplifier’s agility and speed, its very low noise floor, and its ability to drive a wide range of speakers are distinct features that few other tube amplifiers in this price category can offer. Pair it with VTL’s TL5.5 Series II preamplifier for one of the most compelling and seductive sounds available anywhere close to the price, put it with a 6.5 and the performance steps up considerably.

Standard valves are 6550 but KT88 can be chosen as a cost option.


  • 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

Additional information

Power Output

Tetrode: 300W, Triode: 150W

Valves Used

12 x 6550 or KT-88, 2 x 12AT7, 2 x 12BH7


113 Kg


W x D x H 290 x 610 x 610 mm

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