VTL MB750 monoblock power amplifier Page 2

What is an MB-750?
The MB-750 shares its basic circuit configuration with all of VTL's amplifiers—a paralleled input stage driving a long-tailed phase-splitter driver stage, followed by the output stage. The basic circuit, designed by David Manley during his pre-VTL work in South African recording studios, is simple, tried, and true. Following the signal path, an MB-750 starts with a 12AT7 dual-triode input, either paralleled in single-ended mode or as a differential amplifier in balanced mode, with each half of the triode handling one leg of the signal. From there the signal is sent to the driver stage, a differential amplifier consisting of two 6350s, one "push" and one "pull," paralleled to get the current necessary to drive the 12 6550C output tubes.

In a slight break with the competition, VTL achieves power by running comparatively high B+ voltages and low bias current (30mA on the 750), which leads to long tube life but necessitates the use of tubes capable of handling the voltage. VTL uses Svetlana tubes exclusively, individually tested, batched, and branded. Another break with standard tube-amp practice is the lack of multiple output taps. Instead, a longstanding VTL practice is to optimize the amplifiers' output for a 5 ohm speaker load, using the entire secondary winding.

The MB-750 is one of VTL's "Signature" series of amplifiers, meaning that they use one of VTL's proprietary wide-bandwidth, low-loss output transformers, introduced across the model line in 1996. A clean-sheet-of-paper exercise, the Signature transformers incorporate an increased number of sections and are optimally matched to a particular model's output tubes' plate impedance for improved coupling and better current transfer. Other design parameters include the use of approximately 6dB of feedback, an output impedance of approximately 1.5 ohms, and an input impedance of either 130k ohms single-ended or 274k ohms balanced.

The top two VTL models, the MB-750 and MB-1250, are unique in their use of a dramatic, stacked two-chassis configuration. The power supplies—separate supplies for the input circuitry and output stages—are on the bottom, the tubes and output transformers on top, the two chassis connected by an umbilical cord. In the dark, it's easy to think of the amps as a miniature version of Fritz Lang's Metropolis, the workers toiling below to power the glowing city above. The stacked configuration also helps isolate the tubes and signal path from the magnetic fields and vibration created by the power-supply transformers.

The two chassis are linked by side rails/loops (these double as carrying handles) constructed from black powder-coated 1"-square-section steel tubing, and each has its own removable ¼"-thick black anodized aluminum faceplate. The upper chassis is covered by a screen to protect the tubes from inquisitive fingers or cold dog noses. A standard IEC AC cord plugs into the lower chassis' rear and is flanked by two power-supply fuses. The top chassis' rear features RCA and XLR input jacks and a switch to select balanced (footnote 2) or single-ended inputs, VTL's heavy-duty binding posts, the triode/tetrode selector toggle switch, and a B+ fuse to protect the circuitry in the event of a tube failure. The front panels are adorned only by an on/off rocker switch and operate/mute toggle on the bottom and top chassis, respectively, and LEDs on both to indicate operation. The amp is nicely finished and quite handsome, but with each unit being 20" wide, 10" deep, 20" tall, and weighing a hefty 130 lbs, the MB-750s put a premium on gym membership and a weight-lifting girlfriend.

Fresh tubes
The review process began when VTL's Luke Manley arrived at my door with the MB-750s in tow, fresh from a Canadian hi-fi show. Because I wanted to use both balanced and single-ended inputs, and the available review pair was configured for single-ended only, Luke spent the first evening installing the balanced input option. After final checkout and assembly we installed the 750s in the top half of the Audio Artistry system, with the Ichibans driving the woofers. After an hour or so of warmup Luke tweaked the bias on all four amps and, satisfied that all was well, headed back to California.

Special treatment? Sure, but all VTL amps, and particularly the larger models, are intended to be installed and given their final tweaking in the customer's system by a trained VTL dealer/installer. To simplify the process and reduce the possibility of complications, the amps are thoroughly tested, burned-in for 48 hours, and biased at the factory, then shipped with the tubes in place.

Once everything was assembled, installed, tweaked, burned-in, and warmed up (see the System and Setup sidebar), what did all that tube power do for me? For starters, one thing that I expect from a mega-power amplifier is spectacular reproduction of dynamic transients, and the MB-750s didn't disappoint, delivering everything from rim shots that snapped my head around to effortless, freewheeling orchestral crescendos.

What really set the MB-750s apart, though, wasn't their raw power, but how precisely they moved through dynamic swings. During the fireworks at the opening of A Night on Bald Mountain (Chesky RC30), for example, the leading edges of transients were stunningly explosive, and my sense throughout was that René Leibowitz had the Royal Philharmonic in an iron fist, precisely if mercilessly leading them to their absolute limits, and taking me along—exactly the sort of double-overtime type of emotional roller coaster that this piece can give you in a live performance.

Footnote 2: A switchable single-ended/balanced input stage used to be optional, but is now standard on the MB-750.
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