Smart Theatre Systems 2X150VT Hybrid Tube/MOSFET Two-Channel Amplifier

New in appearance; used briefly (less than 10 hours). Intended to put into a reference system. Sounds superb!

SMART Theatre Systems was a company out of Atlanta, GA that made commercial theater systems from the 1979 to the early 2000s. The so-called “Stainless Steel Stradivarius” is a two-channel power amplifier with 6922 full triode input tube housed in a stainless steel top chassis with a tube hiding behind a small transparent window.

Smart Home Theatre Systems 2X150VT Power Amplifier

Power Output: Greater than 150 Watts Per Channel into 8 Ohms, 240 Watts/Channel into 4 Ohms, 500 Watts into 8 Ohms Bridged

THD: 0.9% 20 Hz - 20 kHz, Both Channels Driven

IM Distortion:  Less than 0.01% at 150 Watts into 8 Ohms

MFR: 2 Hz - 160 kHz at 1 Watt - 3 dB, 6 Hz - 100 kHz at 150 Watts - 3 dB

Signal To Noise: Exceeds 100 dB referenced to full output into 8 Ohms over a frequency bandwidth of 22 Hz - 22 kHz

Input Impedance: 50 k Ohms

Input Sensitivity: 1.85 volts RMS for 150 Watts into 8 Ohms

Rise Time: 10 kHz, 60 volts p/p square wave, 10% to 90%: 2.5 microseconds

Slew Rate” 10 kHz, 60 volts p/p square wave: 30V per microsecond

Damping Factor: 300 @ 1 kHz into 8 Ohms, 60 @ 10 kHz into 8 Ohms

Size: 5.8" H x 15.8" W x 10.1" D

Weight: 23 Pounds (10.45 kg)

MSRP: $2,250 USA 

The 2X150VT is on the light side, as far as amplifiers go, but it is deceptively so.  Almost all of its 23 pounds are in the toroidal transformer and massive heat sinks which themselves are the sides of the unit.  The rest of the construction is mostly of light but strong stainless steel.  This two-tone black-silver appearance strikes me as remarkably original and very smart looking. It's sort of retro, with a bit of art-deco and a hint of contemporary.  It works for me.  On the front is a power switch to the right and on the left a row of LEDs for each of the two channels.  These last are calibrated to indicate power levels of 5, 50, and 150 watts being delivered into 8 Ohms.  Also on the front is a small window into the unit, candidly displaying the dual triode tube input stage which we'll talk about later. 

On the back we find an IEC socket for the power cord, permitting easy replacement with an exotic one should you desire.   The speaker connections are a set of very sturdy 5-way binding posts, to either side of which is an output fuse.  There is a switch to lift the ground, which as the manual suggests, provided the best results for me in the "lift" position.  There is also a switch to run the unit in bridged mode where it will deliver 500 watts (yes, that's 500!) into 8 Ohms.  The gold plated RCA inputs are floated in that they are grounded neither to the power supply or the chassis.  Finally, gain adjustment (volume control) knobs are provided for each input.  Though it is recommended they be left at "full", some systems may realize a better signal-to-noise ratio by lowering the gain in the amp and increasing the output signal of the preamp. On the whole, the build is unusually solid and has a robust feel: top and bottom stainless steel "pans" are fastened to the heavy heat sink sides (an optional rack-mount kit is available). 

Inside is a decidedly simple and logical layout, starting with the toroid power supply in the middle.  These are made completely in house by Smart to their own high standards, and in the final production run, they will be coated with lacquer to damp any mechanical humming (though our sample was remarkably silent without this treatment).  Behind the 900 watt (0.9 kVA) transformer are two massive 22,000 microfarad capacitors.  That's not an obscene reserve, but it is certainly above par.   There are six MOSFET output devices per channel and a Sovtek 6922 serves as the dual triode input stage (one triode for each channel, with both triodes being contained inside the same glass envelope).  The circuit operates with an output rail voltage of 65 VDC (from 93 Joules of energy storage) but a driver rail voltage of 75 VDC.  Oscar Neundorfer, Chief Engineer at Smart, comments on this:

"If the output stage and the drivers are run from the same rail, with this circuit topology, the drivers cannot drive the output devices hard enough to reach clipping. Therefore, the power output is limited a bit. Since we were trying to achieve a higher output power than we had previously obtained when using this amplifier module, we implemented higher driver voltage rails as compared to the output device rails. We could have used the same HIGHER voltages for both drivers and outputs, but this would have given us basically the same power but at a higher heat dissipation. Therefore, the different rails are used. The high current supplies are isolated from the driver supplies to a large extent. So the drivers are seeing cleaner and more stable power and are not unduly influenced by the demands on the output device rails.

Also, this design does not use current equalizing resistors in series with each output device as some other designs do. The output devices' drains and sources are directly in parallel. This results in lower output impedance and higher current capability by itself because there is no current limiting effect due to the resistors. The penalty for this is that the output devices have to be current matched so that one device does not attempt to hog all the current. Therefore we match every output device in these amps. This is a time consuming process, but the results are well worth it."

Reviewed by Brian Florian in Secrets of Home Theater and High Fidelity, April 2001: “This amplifier is a true gem!  While I am not in a position to categorically endorse his opinion, I can safely say that this is the best sounding power amplifier I have ever heard in and around the $2,000 mark.   There are generous amounts of power and current, while the sound is genuinely (you guessed it) pleasing.”

Reviewed by Brian Bloom in Audiophile Audition, February 2002: “If you long for the lack of irritation and hardness found in many typical solid-state hi-fi systems of today, then the Smart 2X150VT may be the answer. On the surface it may look like an amp from the past, but it is surely a different beast altogether. It delivered a pleasant, non-fatiguing sound without resorting to the rolled-off, dynamic restricted, power-limited sound present with many heat-generating all-tube designs, or, the sterile, analytical sound of many solid-state amplifiers. The Smart amp is small and compact, but attractive and powerful. If you have a system that is lacking in resolution due to a poor amplifier, or needs some warmth to get you through the harsh, cold sound of certain CD's, then the Smart Theatre Systems 2X150VT power amplifier should be on your list of amplifiers to audition.”