Product Review - Smart Home Theatre Systems 2X150VT Hybrid Tube/MOSFET Two-Channel Power Amplifier - April, 2001

Brian Florian

Introduction

"We would be pleased to send you a unit for review. But be prepared! We truly believe this is the finest amplifier on the market at any price."

Email from Norm Schneider of Smart Home Theater Systems

If you don't know Smart Home Theatre Systems, here is an introduction.  AKA Smart Devices Inc., they have been manufacturing audio equipment and related accessories for commercial cinemas since 1978.  And the reason you should get acquainted with this company is that they are gradually blowing the wind out of the old convention that cinema equipment is by some definition 'inferior' to high definition consumer designs.  They began their road into the consumer market with the CS 3X Jr, the first and still most cost effective add-on processor for bringing Surround EX to existing home systems.  At its core is the very same technology featured in their commercial cinema processor, the CS 3X.  More recently, they've begun to offer one of their subwoofers to the consumer, which is a gargantuan 180 pound monster with 18" driver (cast aluminum chassis of course) driven by a 600 watt digital amplifier.

Their newest product breaks new ground and promises to make a serious mark in the audiophile community.  In 1987 Smart produced the TA242 rack-mount amplifier and sold over 4000 to cinemas across North America during the hey-day of optical Dolby Stereo soundtracks.  With the greater power demands of Dolby Digital, the TA242 was discontinued in 1993, though many are still in use today.

Smart's engineers went back to that classic amp, keeping all that was good, replacing the old power supply with a much larger one, and retrofitting key circuits to turn it into a true audiophile piece of equipment, the 2X150VT.   I was trusted with one of just 10 pre-production units for review (full production began in late February).

Getting acquainted

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."

Each and every unit, including our pre-production sample, ships with its own individual Certificate of Performance (example shown at left) as measured by an Audio Precision 2-2232 analyzer at the final QA check, which plots power output and distortion.  Nice classy touch.

Truth be told, the 2X150VT is under-spec'ed.   Though the literature puts it at 2 x 150 Watts, allegedly all ten pre-production units measured over 180 Watts per side into 8 Ohms before clipping was recorded.  Nice to see honest specs for a change, though in this case it seems the manufacturer might be selling themselves short.  Further, as evidenced by the Certificate of Performance included with our sample, THD is not 0.9% into 8 Ohms 20 Hz to 20 kHz.  It's more like 0.15%.   Not too bad for a system with a vacuum tube in the signal path! Of course, specs for a high performance product are really just the minimum performance guaranteed. The actual performance usually is better.