Sereno 300B

MONOBLOCK AMPLIFIER

Effortless.

The All New Analog Ethos Sereno 300B

Single-Ended Triode Monoblock Amplifier

10 Watts Per Channel

$4500 / pair

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The 300B is regarded as one of the best tubes for high fidelity audio, in particular in a single-ended triode (SET) configuration. Not only does it have outstanding sonic characteristics, but it has enough current handling capability to produce more output power than most tubes in single-ended designs. In the case of the Sereno, 10 watts per channel.

The Sereno is a monoblock amplifier pair--two identical amps, one for left and one for right. Each has its own independent power supply and entire amplifier circuit. This provides performance benefits in channel separation, and also accommodates larger transformers than a stereo amplifier might typically use for reasonable size and weight, allowing other performance advantages. Monoblock amps require a preamp to control volume or provide other features such as input selection.

The 300B is a demanding tube. Every 300B amplifier is, underneath, a story of how to swing enough voltage to drive the 300B tube with authority. Most amps will push a driver tube hard and then couple the signal through a capacitor to the 300B. This is not a bad solution, assuming it's done well and uses a quality capacitor. But where other amps place a capacitor, the Sereno places iron. A triode-strapped 6V6 tube drives the 300B through an interstage transformer. It's an easy load on the 6V6, and the tube's plate rests comfortably at the supply voltage, allowing it to operate in its best conditions. Eliminating the capacitor also avoids blocking distortion, a brief stall that can occur on heavy transients when driven to clipping.

In fact, there are no coupling capacitors in the Sereno at all. The front end uses a 6SN7 first gain stage that is direct coupled to the 6V6. The signal chain from input to output is as direct as possible. There is no global negative feedback used or needed.

Rounding things out for the tube selection is a GZ34 (5AR4) rectifier tube for the power supply. By splitting into mono block configuration, the power transformer and rectifier tube operate well below their rated limits. The directly-heated cathode filament of the 300B is rectified and regulated for silent, black background.

The amp includes an RCA input connector, with an optional upgrade available for XLR balanced input with a premium quality input transformer.

The Sereno is best suited for speakers 90dB/W/m or higher. The standard amp is wired for 8 ohms, but can be wired for 4 or 16 ohms according to your preference, and a multi-tap option is available for flexibility. The chassis is available in walnut or cherry, in a beautiful mid-century modern design with brushed aluminum top. Each amp is built to order, made by hand. All tubes are included. Most orders ship in about 3-4 weeks.

Designer's Note:

A single-ended triode produces a predominantly second-harmonic distortion profile — that's inherent to the topology. What separates designs is everything above the second. I spent most of my development time on bias points and loading, working to keep the third modest and the fourth and higher down near nothing. That clean decay isn't a flavor I dialed in; it's what a single-ended stage does when it's biased and loaded properly, and getting there took more iterations than simply pursuing a lower THD number.

At the one or two watts you'll mostly use with efficient speakers, this is a subtle thing. It should be. Vanishingly low distortion was solved decades ago, and solid-state does it best. That isn't what this amplifier is for. What I can tell you without qualification is this: when I sit down to do listening tests on the Sereno, they always run longer than I intended!