Tube Power Amp Reviews

Doshi Audio first crossed my radar at the 2010 Rocky Mountain Audio Fest, where I wrote that the sound of the company's tubed monoblocks and preamp, connected to Wilson Audio Sasha loudspeakers via Transparent XL cables, "excelled in midrange strength as the system threw an exciting soundstage." After auditions of Doshi-Wilson pairings at many subsequent shows had convinced me that Doshi's products could truly sing, I contacted company owner and product designer Nishith "Nick" Doshi to inquire about reviewing one of his amps. Given that, in his home system, Doshi pairs his Monoblock V3.0 amplifiers ($35,995/pair) with Wilson Alexia 2 speakers, I presume that the presence of Alexia 2s in my own reference system helped convince him to send me a pair of Monoblocks for review.

Introduced at AXPONA 2016, the Doshi Audio Monoblock V3.0 has three gain stages, and its four KT-150 output tubes operate in pentode mode. It outputs 160W into 4 ohms, remaining in class-A to 120W before shifting to class-B. When Nick Doshi and his teenage daughter visited me in Port Washington to set up the Monoblocks, he declared that I would "never see" class-B.

Description
Nick Doshi came to consumer electronics through a stint as a recording engineer in New York City. After work on live radio broadcasts led him into a career in broadcast frequency and antenna systems, transmitters, and phono preamplifiers, he had an epiphany when he discovered that the very simple, modified 1950s RCA circuit in a friend's Counterpoint unit brought him "closer to the musical event.

"This led me to work with very simple circuits," he said during his visit. "I brought a lot of my experience with pro gear—which has to provide long, fault-free service—to Doshi Audio. Everything was designed for long service life, and to operate without too much heat stress. Power supplies were intentionally overspecified: our UK-sourced Sowter EI core output transformers and US-sourced custom-made Toroid toroidal power transformers were designed not to run close to their design maximums of over 400W continuous output. Even if a resistor dissipates 0.25W of power, we typically use a 2W resistor, so that thermal stress does not shorten lifespan, and it will sound the same over time.

"The KT150 output tubes in the V3.0 are designed for 60W dissipation, but typically run under 30W, at half their design maximums. We have people who have put 4000 hours on a pair of output tubes before they've needed to change them. People can also use KT88s or 6550s if they desire."

The Monoblock has just one pair of output taps. "They'll work fine with 8 ohm speakers," Doshi explained. "[But] people can custom-order any output impedance they want. The problem with multiple taps in amplifiers is, fi your amplifier has feedback around the outputs, then what tap do you take the feedback from? Also, when you have multiple taps, you can't optimize the coupling from the primary to the secondary in the transformer. I'd rather lose a couple of watts but retain output control and output impedance. Ultimately, multiple taps are pretty meaningless. There is no loudspeaker that has a flat 4 ohm or 8 ohm impedance, so why are we killing ourselves over something so ridiculous?"

Central to the Monoblock's casework is a ½"-thick Corian top plate, the front edge of which is supported by two vibration-absorbing columns. The amp's chassis, made from 14-gauge nonmagnetic stainless steel—it acts as a shield without setting up eddy currents—is partly suspended from the top plate via elastomer fittings, while its rearmost edge rests directly upon the user's equipment support, via two short, vibration absorbing feet. Inside the chassis, all of the tube sockets and other parts are fastened to a plate that's also made of stainless steel, which is itself pliantly fastened to the underside of the Corian top. By suspending the subchassis from both the top and, effectively, the main chassis, Doshi prevents acoustic vibrations from affecting the tubes and other components, which are hard-wired, point to point. The mains and output transformers are contained within a stainless steel cover, the inside surfaces of which are covered with the same damping paint used to promote low noise levels in submarines.

The sockets for the seven tubes are each machined from a solid piece of Teflon and have gold-plated, oxygen-free copper (OFC) contacts. "That socket costs me 15–20 times more than sockets other companies use," Doshi said. "I won't compromise on that, because five years from now, it will have the same clamping force on your tube pins and preserve sound quality. Even our 75W amplifier uses exactly the same parts quality.

"All Doshi components are built for music lovers, and targeted and voiced for somebody who tends to listen to music for long periods of time. Because we will never grow above 10 US dealers, we are able to use quality parts that larger companies can't touch because their overheads are so much higher, and build products to our standards for music lovers. My name is on a product. If it doesn't represent the best of me, then I'm not going to build it."

Features and Functions
Nick Doshi and I set the Monoblock V3.0s on Grand Prix Monaco amplifier stands, their acrylic shelves supported by the company's new Visco Void polyurethane dampers. The Monoblocks received signals from a dCS trio of Paganini transport, Network Bridge network player, and Vivaldi DAC. All interconnects and cables were Nordost Odin 2s. I listened mostly to files stored on USB sticks, as well as the occasional disc. The wiring for my dedicated AC power line is 8 gauge, and between that and my electronics I've inserted sufficient power conditioning—from Tweak Geek, Audience, and Nordost—to enable the bellowing of an elephant in heat to travel from Africa to the Bronx Zoo with maximal transparency.

The Monoblock's rear panel includes both unbalanced (RCA) and balanced (XLR) inputs, the latter buffered by a high-quality op-amp. To their left are two toggle switches: the top toggle lifts the ground at the RCA input to prevent ground loops when the unbalanced input is in use, while the bottom toggle selects between the two inputs.

When Nick Doshi visited, he urged me to use unbalanced interconnects. By e-mail, he elaborated: "The XLR input is converted to single-ended before the tube input section. [While] the XLR input is extremely high quality, and has textbook specification for noise, common-mode rejection etc. . . . the tube section of the amplifier is a [an unbalanced] input design, and theoretically, the [unbalanced] input, which bypasses the balanced-to-[unbalanced] conversion section, should be the purest path.

"I have included the best balanced input section I could because there are some users that have made substantial investments in XLR cable, and may want to use them rather than changing everything out wholesale. The other reason is that a substantial number of amp owners are actually in our industry, and need their reference amplifier to be able to handle both [unbalanced] and balanced. Lastly, in cases of severe environmental interference, a balanced connection will offer better rejection of noise."

At the center of the front panel is a fair-sized, three-digit display, below it a row of four small blue indicator LEDs, and above it two more. When switched on, the screen can display the elapsed hours or bias in mV of each output tube. When the hours reach 999, the readout starts over again at "1."

The display and LEDs are surrounded by four small, clearly labeled buttons. The Power button, to be used after the amp has been turned on with its main power rocker on the rear panel, activates the main relay that switches on the Monoblock. When Power is pressed, the top right LED blinks as the Monoblock warms up, still in Mute mode. Simultaneously, the display shows the elapsed tube hours. After 30 seconds of warmup the display goes dark; a single blue dot at the bottom right of the display indicates that power is on.

Pressing Display awakens the screen. Pressing the Func button cycles the display between elapsed hours and the bias of each tube. The four LEDs below the display correspond to numbers engraved next to each output tube's socket on the Monoblock's top deck. Each press of Func takes you to the next tube, whose LED lights up as its bias is displayed onscreen.

Mute cuts off the Monoblock's inputs and outputs; the Mute toggle on the rear panel mutes only the speaker outputs.

Biasing affects tone and dynamics
The Doshi Monoblock V3.0's four KT150 tubes require manual biasing. Although Doshi doesn't yet offer automatic biasing, they will once Nick Doshi perfects a way to ensure that the tubes are used "conservatively."

Doshi says that his bias circuits are "quite complex, with two separate bias circuits for the push and pull sides of the circuitry. You basically have to check tubes every 300–500 hours." Should an output tube fail, a microprocessor control unit turns the amplifier off and activates the corresponding LED beneath the display to indicate which tube has failed.

Adjustable bias lets the user shift the operating point of the output stage in small increments, thereby affecting the amplifier's tonality and dynamic behavior. As Doshi explained by e-mail:

"In lowering the bias reading, you are lowering the standing current in the output stage, which slightly affects the output impedance of the output stage, which then interacts with the complex impedance load of the loudspeaker. Generalizing the effects thusly: 1) Lowering the bias current will 'cool' the sound, and provide a far more linear sound that may be preferable for a very warm-sounding front end. Lowering it too much will deaden the sound, with all the connotations to soundstage depth, width, and dynamic impact. The tubes are simply not awake enough. 2) Raising the bias moves the output tubes more into class-A. This will 'warm' the sound and also push the midband forward just a bit, so as to make the voices more palpable and 'tubey.'"

The output tubes' factory bias setting is 190mV. "The amplifier can safely run with the bias over 220/230, but it will shorten the life of the output tubes somewhat," Doshi said. "With the speakers I have owned, I have always found the sweet spot to be around 190."

To adjust bias, press Mute and then Func to cycle through the tubes' current bias readings. Insert a small screwdriver in a hole next to the chosen tube, and turn the screw within to left or right. Small adjustments can have a big effect on the bias setting, I found.

During his visit, Nick Doshi first set the bias of the Monoblocks' tubes to 190mV, listened, then lowered it to 180mV to better gauge the interaction of the monoblocks with my system. The next day, after listening to the sound of an Alison Krauss track stored on his portable HD and played through my reference dCS Network Bridge via an AudioQuest Diamond link, he reset all tube biases to 190mV.

"Her voice needed a touch more body and presence, to my ears," he clarified by e-mail. "I have the same Alexia 2 speakers, and that particular track is very familiar to me. I don't listen too much for tonal variations, since they are too system-specific. I listen for dynamic behavior and room overload. If the midband is open, smooth, and without bite, if the loudspeaker energizes the room at low and high levels without fatigue, and if the soundstage leaves the plane of the speakers forward and back, I know I'm on the right track."

Clearly, I needed to get a handle on the Monoblock's sound by playing with tube bias myself.

"Off We Go To Music Land"
Those are half of the words of the first and only song in John Thompson's Modern Course for the Piano: The First Grade Book that I learned to play when I was seven. The song's second and final line, "Play with ear and eye and hand," which I embellished with a two-note, quasi-symphonic finish that would have made the four-year-old Mozart wince, became my theme song as I set about biasing up and down and all over the town.