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Vacuum tubes on the circuit board inside an Audio Research Reference Phono 3SE

Audio Research technology

The engineering behind Audio Research

Audio Research gives its technologies names because each one solves a specific problem. Here is what each does, where it came from, why it matters in a system, and what you may hear.

With an all-out design effort, tubes will winSince 1970

Vacuum tubes, KT150 and KT170

Audio Research has amplified with vacuum tubes since 1970 because its founder concluded that, designed without compromise, they sound better. Its output tubes have moved from the 6550 to the KT120, the KT150 in 2014 and, in the Reference 330M, the KT170, each chosen for more headroom and longer life. Read the deep diveClose

What it does

William Z. Johnson built his first commercial amplifier in 1949, tried the first transistors and found them terrible, and founded Audio Research in 1970 at a moment when the industry had decided tubes were finished; an engineer at his first trade show told him he had set audio back twenty years. His position was never dogmatic, and the company has built solid-state and hybrid products from 1976 on, but its Reference line has stayed with glass. Johnson used common, long-lived tube types, the 6550, 6L6 and 12AX7, rather than exotic ones, and that practice continues: the KT120 replaced the 6550 for more resolution and dynamics, the KT150 arrived in the GS150 and the Reference SE amplifiers of 2014 with about 3,000 hours of life, half again as long, and the Reference 330M of 2024 uses six KT170s per channel, with more plate-voltage margin and dissipation, to make 330 watts. Every current amplifier carries a tube-hour meter and individually fused output tubes.

Why it matters in a system

A tube amplifier's behavior at the edge of clipping, and the way it interacts with a loudspeaker's impedance, are what owners describe as its ease and dimensionality; Audio Research's engineering is aimed at keeping those qualities while removing the softness and noise that tubes were once excused for. Johnson's other rule was that if it measures badly it will not sound good.

What you may hear

Hi-Fi News, measuring over 400 watts from the Reference 330M: valve amps slow, lush and lazy? There goes another myth. Enjoy the Music on the I/70: it paints musical reality more accurately than anything its reviewer owned.

The working essence of every Reference preamplifierSince 1999

The 6H30 triode

Every Audio Research line stage and phono stage since the Reference 2 Mk II is built around the 6H30, a Russian nine-pin dual triode that can swing far more current than the 12AX7 and 6922 the company used for its first thirty years, run in pure class A with no feedback. Read the deep diveClose

What it does

A preamplifier tube has to drive the cable and the amplifier's input without strain, and the 6H30 does it with a reserve the older miniature triodes never had: Warren Gehl, who has evaluated every Audio Research design for about thirty years, describes it as having the most robust gain and drive capability of any nine-pin miniature tube, chosen after extended listening for current swing, dynamics and lower noise and distortion. The Reference 1 of 1996 used eight 6922s; the Reference 2 Mk II brought in six 6H30s, the Reference 3 of 2004 ran four in a zero-feedback class-A circuit, and the count has been a measure of ambition since: four in the Reference 5 and the new Reference 7, six in the Reference 6 and the Reference 20 of 2026, eight in the two-chassis Reference 10, three in the PH9 phono stage and two in the LS-2. The power supplies are regulated with a further 6H30 and a 6550.

Why it matters in a system

The line stage sets the noise floor and the dynamic reach of everything behind it, and a triode with current to spare, running in class A with no feedback, is why Audio Research preamplifiers are described in terms of depth and air rather than tube warmth. It is the single most consistent element of the company's sound since the late 1990s.

What you may hear

Michael Fremer on the six-6H30 Reference 20: a superbly balanced, exceptionally transparent, dynamically powerful, nuanced preamplifier that lets you see so far into the soundstage.

Dual differential, end to endSince 2004

Fully balanced circuitry

Audio Research's Reference preamplifiers and power amplifiers, and the new D-80, S-100 and S-200, are fully balanced: each channel is amplified as two mirror-image halves, so noise common to both cancels and the signal travels by XLR from the preamplifier to the output tubes. Read the deep diveClose

What it does

The Reference 3 of 2004 was the first Audio Research line stage built as a fully balanced, zero-feedback class-A circuit with a long-tailed-pair input, and the Reference 10 of 2012 described itself as fully balanced, all tube, pure class A, dual mono with zero feedback. The practice runs through the current range: the Reference 20 and 7 are balanced throughout, the D-80 of 2025 feeds a pair of KT150s per channel from a fully balanced differential path and offers XLR inputs only, the S-100 and S-200 are fully balanced solid-state amplifiers with dual toroidal transformers, and even the LS-3, the company's solid-state line stage, has a fully balanced differential JFET input stage. Asked whether the Reference 20 had transistors at its input, the company's answer was no: fully balanced, dual differential, all tubes.

Why it matters in a system

Balanced operation cancels the noise picked up between boxes and doubles the signal swing available to the output stage, and in an all-Audio Research system it means the music is never converted from single-ended to balanced and back. The LS-2's inverting single-ended path is the deliberate exception, for owners who prefer the simplest circuit.

What you may hear

A black background behind the music and a wide, deep soundstage, the qualities for which the Reference series has been reviewed for twenty years.

A tube amplifier that does not need fiddlingSince 2017

Auto-bias

Since the Reference 160M of 2018, Audio Research power amplifiers set the idle current of every output tube continuously, compensating for line voltage and tube aging, so the owner never touches a bias adjustment and can choose among several output tube types. Read the deep diveClose

What it does

A push-pull tube amplifier needs each output tube held at the right idle current, and for most of Audio Research's history that meant a technician with a meter after every tube change. The company resisted automatic biasing for decades because the circuits it tried degraded the sound; Warren Gehl and the late chief engineer Ward Fiebiger developed one that did not, first used on the Foundation VT80 of 2017 and then on the Reference 160M, the first Reference amplifier to carry it. The circuit works constantly, adjusting for wear, and accepts 6550, KT88, KT120 and KT150 tubes in the 160M MkII. It is now on every amplifier the company makes with tubes: the Reference 330M, 160M MkII, 160S, 80S and 80X, the D-80, and the I/70 and I/55 integrateds, which the I/50 before them did not have.

Why it matters in a system

Correct bias is the difference between an amplifier that sounds as it did on the day it was auditioned and one that drifts; it also protects the tubes. For an owner it means a Reference amplifier is as easy to live with as a solid-state one, with tube life of around 3,000 hours shown on a meter.

What you may hear

Hi-fi+ on the Reference 160M, the amplifier that introduced it to the Reference line: music of all kinds breathes through it; a rare jewel, one of the best ever.

Designed in Minnesota for one amplifierSince 1970

Proprietary transformers and parts

The output transformer couples a tube amplifier to the loudspeaker and sets its bandwidth, and Audio Research designs its own, along with many of the capacitors, resistors and wiring in its products. The Reference 160M's transformers reach from 0.5 Hz to 110 kHz; the I/70 shares its output transformers with the Reference 75SE. Read the deep diveClose

What it does

Johnson's first amplifier of 1949 was transformer-coupled with a separate, electronically regulated power supply, and the transformer has been treated as part of the circuit ever since; in 1983 he claimed a flat response from about 15 Hz to 40 kHz from Audio Research's power transformers alone. The current output transformers are designed in-house and wound to Audio Research's specification by American subcontractors; the pair in the Reference 160M are E-core designs that account for most of the amplifier's weight and pass 0.5 Hz to 110 kHz, and the 160M MkII added a new high-energy-capacity transformer. Many of the capacitors, resistors and signal wires are also made to the company's design, some for a single model: the Reference 6SE upgraded eighteen proprietary capacitors and fourteen wiring runs over the Reference 6, and the Reference 20 introduced next-generation film capacitors in its supply.

Why it matters in a system

A wideband output transformer is what lets a tube amplifier produce deep, controlled bass and open treble; a narrow one is where the old reputation for softness came from. Parts made to the company's own specification are also why its products from different decades sound related and why nearly every one built in the last forty-five years can still be restored.

What you may hear

Bass and extension that reviewers do not associate with tubes. Jonathan Valin gave the Reference 160M his highest recommendation; Hi-Fi News found the Reference 160S almost too good.

The supply is in series with the musicSince 1949

Regulated power supplies

Audio Research treats the power supply as part of the signal path, because, as Johnson put it, it is in series with the audio signal whether we like it or not. Its preamplifiers regulate their supplies electronically, often with a 6H30 and a 6550 doing the regulating, so the audio circuit sees a clean, constant voltage. Read the deep diveClose

What it does

Johnson's first amplifier had an electronically regulated supply in 1949, and the SP-10 of 1982 put the supply in its own chassis, a layout the Reference Anniversary and Reference 10 kept. In the current range the PH9 phono stage regulates its supplies with a 6550WE and a 6H30P to better than 0.01 percent line regulation; the Reference 20 of 2026 redesigned its supply with new film capacitors and reduced noise on the regulated rails by an order of magnitude; the Reference 7 uses solid-state regulation of the same platform; and the LS-3 is dual mono with separate power transformers and linear supplies. The power amplifiers store energy on the same principle: the Reference 160M holds 622 joules, and the Reference 250 of 2011 had fifty percent more capacitance than its predecessor.

Why it matters in a system

Every note an amplifier plays is drawn from its power supply, so noise and sag in the supply become noise and compression in the music. Regulation is the reason Audio Research preamplifiers have the black backgrounds and the dynamic reach that reviewers remark on, and why the company's two-chassis designs put the supply in a separate box.

What you may hear

Quiet backgrounds and dynamics that do not flatten at volume. Michael Fremer on the Reference 20: dynamically powerful, with a soundstage you can see far into.

A meter that floats in front of the tubesSince 2018

The GhostMeter

The Reference amplifiers' power meters are etched into the rear pane of a two-layer transparent faceplate and lit by hidden LEDs, so the needle and scale appear to float in front of the glowing output tubes. The lighting is adjustable, and can be switched off. Read the deep diveClose

What it does

The GhostMeter arrived with the Reference 160M of 2018, the first product of a new design language from Livio Cucuzza, then the McIntosh Group's chief design officer, who had also styled the Foundation series, the Reference 6 and the Reference Phono 3. Two acrylic panes sandwich a needle and two LEDs, with the dial etched into the rear pane; the result is a power meter with no bezel and nothing between the owner and the tubes. The Reference 160S has a pair of them floating in front of its eight KT150s, the 330M, 80S and 80X carry them, and the 160M MkII describes its meter as a window to the output tubes. The S-200's meters sit on a solid black background instead, and the LS-2 and LS-3 touch displays use the same typeface. The D-80 and the integrateds do without a glass front; the I/50, I/55 and I/70 use illuminated LexieTube displays.

Why it matters in a system

It is the detail that makes a Reference amplifier recognizable across a room, and it is the one part of the design an owner may want to adjust: Jonathan Valin found that lighting the scales darkened the sound slightly, so the brightness can be turned down or off.

What you may hear

A meter that reads the music and a chassis that shows what is making it.

Maple Grove, MinnesotaSince 1970

Hand-built, twice tested, auditioned

Every Audio Research product is assembled by hand in Maple Grove, Minnesota, built to order one at a time, bench-tested twice, burned in, and then listened to by the company's sonic evaluator before it ships. The output tubes are burned in for 48 hours and matched on the company's own fixtures. Read the deep diveClose

What it does

The boards are stuffed and soldered by hand; a computer-controlled soldering machine was sold because the boards it made did not sound as good. Each unit carries a production card signed by the technicians who built it, which the customer receives. Power tubes are burned in for 48 hours and matched for transconductance and plate current, and the yield is demanding: around 300 KT150s might be processed to find the 32 for a pair of Reference 750 SEs. Finished units are burned in for two days, tested on the bench twice, and then auditioned one by one in the company's listening room on Magnepan 20.7 loudspeakers by Warren Gehl, who has been chief sonic evaluator for about thirty years and approves every new design as well as every unit; if the listener does not sign off, the unit does not leave the factory. The company moved from Plymouth to a 20,000-square-foot facility in Maple Grove in August 2018, keeps a secured room of original parts going back to the early 1970s, and can restore nearly every product of the last forty-five years.

Why it matters in a system

A product that has been listened to before it is boxed is a product whose sound is known, and an amplifier that can be serviced for decades is one worth buying for decades. Johnson's rule was that the product should be serviceable or capable of being returned to a serviceable condition.

What you may hear

Consistency between the review sample and the one in your rack, and a reason that forty-year-old Audio Research preamplifiers are still in use.

Not married to vacuum tubesSince 2025

The solid-state line: LS-3, S-100 and S-200

Audio Research has built solid-state products since 1976, and in 2025 a newly formed solid-state engineering team produced the LS-3 line stage and the S-100 and S-200 power amplifiers: fully balanced, dual-mono, bipolar rather than MOSFET, in the same machined chassis as their tube siblings. Read the deep diveClose

What it does

Johnson said the company was not married to vacuum tubes, and its history includes the hybrid SP-11 of 1985, with a triode and a FET in every stage, and the hybrid Reference 1 of 1996. The 2025 products are the first all-solid-state line in years. The LS-3 has a fully balanced differential JFET input, a bipolar voltage amplifier and a class-A output, dual mono with separate power transformers, 20 dB of gain and a touch screen, in the all-aluminum chassis it shares with the LS-2, and takes the same DAC and phono cards. The S-100 and S-200 are fully balanced class A/B amplifiers with bipolar outputs chosen for purity, transparency and dynamics, 100 and 200 watts into 8 ohms and double into 4, with dual toroidal transformers, in a machined aluminum chassis with no visible fasteners. The Reference 330M and Reference 7 use solid-state regulation and driver stages in otherwise tube designs.

Why it matters in a system

They give an owner the Audio Research balance and build without tubes to replace, for a second system, a theater, or a loudspeaker that wants damping factor; and the LS-3 with an S-200 is the company's most affordable balanced pre-power pair.

What you may hear

The Absolute Sound included the S-100 and LS-3 in its series on the best amplifiers under ten thousand dollars.

A Reference amplifier in one boxSince 2021

The I/70 and I/55 integrateds

The I/70 and I/55 put an Audio Research tube preamplifier and power amplifier in one chassis with optional DAC and phono cards, a headphone output driven by the output tubes, auto-bias and the company's LexieTube displays. The I/70 uses two pairs of KT120 for 70 watts and shares its output transformers with the Reference 75SE. Read the deep diveClose

What it does

The I/50 of 2021, in six Cerakote colors applied in-house, was the product meant to bring a new generation to the brand. The I/70 of 2025 is its larger successor: two matched pairs of KT120 and three 6922s, auto-bias that also accepts 6550, KT88 and KT90, 4 and 8 ohm taps, balanced and three single-ended inputs, a 75-step volume control, preamplifier outputs, a headphone output driven by the KT120s, and slots for an AKM-based DAC card with USB, optical, coaxial and Bluetooth and a moving-magnet and moving-coil phono card with adjustable loading. Its machined aluminum, mesh and badging follow the Reference 330M. The I/55 keeps the I/50's look and its two pairs of 6550WE for 50 watts, and adds the auto-bias and the active tube preamplifier section of the I/70, so the owner no longer needs a technician after a tube change.

Why it matters in a system

An integrated with the Reference amplifiers' transformers and tubes is the simplest way into the Audio Research sound, and the cards mean a complete system is one box plus a turntable or a streamer. The I/70 is the one we recommend for loudspeakers of ordinary sensitivity; the I/55 for the smaller room.

What you may hear

Enjoy the Music on the I/70: it paints musical reality more accurately than anything its reviewer owned. Audio Research's own line is that the I/70 sounds like a big amp.

Three 6H30s and a regulated supplySince 2017

Phono stages

Audio Research's phono stages apply the Reference preamplifier recipe to the smallest signal in the system. The PH9 uses three 6H30 triodes in a short, clear path, with 58 dB of gain, five cartridge loadings switchable from the remote, RIAA accurate to 0.2 dB and a supply regulated by a 6550WE and a 6H30P. Read the deep diveClose

What it does

Johnson used moving-coil cartridges exclusively for a decade before 1983, and the SP-3's phono stage was what J. Gordon Holt praised in 1973 for modifying the signal less than any other preamplifier available. The PH9 of 2017, from the Foundation series, is the current single-chassis design: a trio of 6H30 dual triodes, 58 dB of gain suitable for most moving-coil cartridges, loadings of 47k, 1,000, 500, 200 and 100 ohms chosen from the remote so they can be compared from the chair, and an electronically regulated supply with line regulation better than 0.01 percent. Above it sit the Reference Phono 3SE, upgraded in 2020 with the parts and wiring of the Reference 6SE, and the two-chassis Reference Phono 10. The I/70 and I/55 take a moving-magnet and moving-coil phono card with adjustable loading.

Why it matters in a system

The phono stage adds the most gain of anything in the system, so its noise and its power supply matter more than anywhere else; a tube phono stage with a regulated supply is where Audio Research's approach pays off most audibly. We match loadings to the cartridge on our bench before a PH9 goes home.

What you may hear

A quiet, open presentation of records with the dynamics of the Reference line. The PH9 has been listed in Stereophile's Recommended Components.

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