Audio Research VT-150 monoblock power amplifier

At the 1992 Winter Consumer Electronics Show in Las Vegas, Audio Research showed a line of reference products that represented the pinnacle of founder William Z. Johnson's life work as an amplifier designer (footnote 1). Although the all-tubed, fully balanced preamplifier and tubed monoblock power amplifiers were shown as works-in-progress, it was clear that these were products aimed at advancing the state of the amplifier art with no consideration for cost. Indeed, the reference preamplifier and monoblock power amplifiers were a return to Audio Research's roots, and a personal statement by Bill Johnson.

The reference products never made it into production—they would have been too expensive. But the design effort wasn't in vain; many aspects of their topology were adapted for the $4995 LS5 preamplifier and $12,000/pair VT-150 power amplifiers reviewed here. The VT-150 is the first Audio Research power amplifier in ten years to use vacuum-tube regulation—a design feature mandated in the cost-no-object reference products.

VT-150 monoblock power amplifier
One look at the VT-150 and you know this is an amplifier that means business. The large black chassis is fully 22" deep, and covered by a black ventilated cage. The VT-150 isn't the kind of amplifier to have gold-engraved front-panel legends; its look is decidedly utilitarian.

The front panel has two knobs and one small meter. The left-hand knob has positions for "operate," and one position for each of the four output tubes. The right-hand knob adjusts the bias, with the centrally mounted meter indicating the bias to the tube selected by the left-hand knob. Rather than ask the user to set each tube's bias individually, the VT-150 uses a servo circuit that automatically matches the bias of three of the output tubes to the one output tube that has user-adjustable bias.

Here's how it works: Move the right-hand knob from "operate" to "adjust V10." The meter becomes active, indicating the bias on output tube V10. After setting the bias with the right-hand knob, moving the left-hand knob to "check V11" shows the bias on output tube V11. This step just confirms that the servo circuit has set V11's bias identically to that of V10. This is repeated for the other two output tubes. I found it quick and easy to maintain the correct bias with the VT-150's front-panel controls and bias servo. In addition, the operate/adjust knob acts as a mute switch when changing preamps or interconnects.

The rear panel holds a captive AC power cord, power fuse, on/off switch, power-indicating LED (this lights up a dark room!), and seven very-high-quality machined binding posts. One binding post is the transformer center tap, the other three pairs are different balanced-impedance taps (8 ohms, 4 ohms, and 2 ohms). When using the grounded center tap, the positive terminals from the three pairs become 4 ohm, 2 ohm, and 1 ohm taps. If the loudspeaker has a fairly constant impedance with frequency, use the tap that matches the loudspeaker's impedance. If the loudspeaker's impedance swings wildly with frequency, the only way to select the right tap is by listening. The grounded center tap is provided for convenience if the VT-150 is connected to a device with a common ground—such as a loudspeaker switch box.

I used the 4 ohm tap throughout the auditioning, which worked the best with the Thiel CS3.6 loudspeakers. The 4 ohm tap provides the least voltage gain, but the highest output current capacity. Finally, the VT-150 has only a balanced input, with an input impedance of 200k ohms.

The VT-150's serious-looking exterior is mirrored by its internal construction. The six 6550 tubes (four output, two regulator), two large transformers (one power, one output), and huge block of filter capacitors are an impressive sight. Moreover, the VT-150 is beautifully laid-out, and executed with very-high-quality parts and construction. Everything in the VT-150 is beefy, from the thick, very sturdy chassis down to the clamps holding the filter capacitors in place. These are clearly "lifetime" amplifiers.

Looking at the circuit topology, the VT-150 is a fully balanced design; each phase of the input signal is amplified separately from the input jack to the output transformer. The input stage consists of a Sovtek 6922 (6DJ8) dual triode per phase. This feeds four 12BH7A tubes (GE brand) used as drivers. Each 12BH7A drives a 6550, the output tubes being arranged in push-pull pairs. The Russian 6550s are reportedly the best output tubes available in terms of sonics and long-term reliability.

In addition to being all-tubed and fully differential, the VT-150 uses tubed regulation on the B+ rail. All early Audio Research amplifiers used tubed regulation, but this was abandoned for solid-state regulation. Even Audio Research's highly regarded Classic series used solid-state devices as regulators. The VT-150's tubed regulation stage thus represents a return to the company's roots.

Specifically, the power supply uses a 12AX7 and a pair of 6550s in the 420V B+ regulation stage, with solid-state regulation for the other power-supply rails. Not only is the series pass element a tube (the 6550), but it's driven by a tube (the 12AX7).

The filter caps are housed behind a shield, connected with a large buss-plate, and covered with a clear plastic cover for protection. With an energy storage capacity of 420 joules, you don't want to accidentally touch the capacitors (footnote 2)

When I first put the VT-150s in my system, my initial reaction was that these amplifiers sounded a little closed-in, subdued in the treble, and lacked treble extension. I felt something was missing from the music.

It didn't take long to realize exactly what was missing: treble grain, etch, hardness, brittle textures, and that steely sound in the upper midrange and treble that we hear in reproduced music but not in live music. In fact, I was hearing—for the first time—reproduced music devoid of the metallic hardness I had thought was inherent in music playback.

My initial—albeit short-lived—reaction to the VT-150s demonstrated how we become inured to colorations in reproduced sound. The total lack of treble etch and grain was at first mistaken for a rolled-off treble, so endemic is the whitish, steely quality of the other amplifiers I've auditioned—even other tubed amplifiers.

Even compared to the superb VTL 225W Deluxe Monoblocks, the VT-150s were noticeably smoother and more liquid. Before I heard the VT-150s, the VTLs had been my reference for treble purity, midrange liquidity, and naturalness of timbre.

Footnote 1: See my interview elsewhere in this issue.

Footnote 2: A Joule is a unit of energy storage, and is a function of both the capacitance and the square of the voltage. Because tubed amplifiers have very high voltage rails, they have much greater energy storage in their reservoir capacitors than do solid-state amplifiers.

Audio Research Corporation
6655 Wedgwood Road N., Suite 115
Maple Grove, MN 55311
(763) 577-9700

Ortofan's picture

... 40dB below the fundamental, one has to wonder whether you're listening to the music or to the distortion?
Is it those relatively high levels of both harmonic and inter-modulation distortion that contribute to the allegedly "magic" sound quality?

Did RH ever compare the VT-150 with the Threshold S/550e?

Bogolu Haranath's picture

These ARC VT-150 mesurements are somewhat similar to recently reviewed ARC Ref.160M and ARC Ref.160S (reviewed in Feb. 2020 issue) :-) ........

Bogolu Haranath's picture

BTW .... A rose by any other name still sounds (smells) the same :-) .........

Bogolu Haranath's picture

Sound to die for (S2D4) :-) ........

JRT's picture

Not sure how that VT-150 SE differs from the VT-150, but it's interesting regardless.

More commonly the output transformer of an "ultralinear" configured amplifier (eg Dynaco MkIV) simply includes taps on the primary to connect to the screen grids, so those taps share the same B+ power supply voltage with the anodes, which requires suboptimal voltage on the anodes to prevent too much at the screen grids, and that compromise often results in lower than optimal voltage at the anode and higher than optimal voltage at the screen grids with shortened life and degraded performance. This amplifier (at least the SE edition of this amplifier) takes the better approach of providing a separate center tapped winding for the screen grids, allowing those to be operated at reduced power supply voltage than the anodes, which significantly reduces nonlinear distortion and lengthens usable life of the output tubes. Also there are four separate cathode feedback tapes, one for each output tube. For an unchanged level of global feedback, utilizing cathode feedback tapes further reduces nonlinear distortion and reduces output impedance by including the transformer in a local degenerative feedback loop with the output tubes' cathodes.

Lorton's picture

I had this amp, worked well on my watt puppies, wounder how it would sound today and if it will hold against current ARC.