March 2026

Sneak Peek World Premiere Review!
Audio Research I/70 Fully Loaded Integrated Vacuum Tube Amplifier Reviewed
In-depth review of Audio Research's fully-loaded I/70 integrated stereo unit including the optional Hi-Res Audio DAC and MM/MC phono stage: performance, design, and reference-level listening notes.
Review By Ron Nagle

   I can trace my interest in high-end audio back to the 1970s, long before the internet, and the 1995 launch of Enjoy the Music.com. Before we discuss today's review of Audio Research's impressive I/70 vacuum tube integrated amplifier, let us step back to the very beginnings. My first serious Audio Research audio components, as back then, I recall vacuum tubes versus transistors controversy was still raging on. That was when some audio wags predicted the death of vacuum tube audio. I credit Audio Research and the late Harry Pearson as the two stalwarts that fended off the Japanese avalanche of cheaper transistor-powered audio components at the time.

And that was the time when the shifting public interest was to transistor audio. Audio Research Corporation (ARC) was founded by William Zane Johnson in 1970 and was originally located in Minneapolis, Minnesota. They are one of the longest U.S. companies of vacuum tube-powered high-end audio equipment for audiophiles and music lovers still in operation. The company is known as a pioneer for advancing the state-of-the-art of audio reproduction as we know it today. Audio Research gets much credit, in my opinion, for reintroducing the vacuum tube as a primary audio amplification device.

Back In The 1900s
Move that calendar page forward to the spring of 1994 and to my very first published review of the Audio Research SP9 preamplifier, which appeared in the long-gone Audiophile Voice magazine. As a matter of record, I reviewed the two upgraded versions, the AR SP9 MKII and the AR SP9 MKIII preamplifier. I subsequently purchased and still own that SP9 MKIII review sample. Over these many years, I have maintained a keen interest in the house sound of Audio Research's vacuum tube components.

Today In The 2000s
Audio Research amplifiers evolved from the very beginning with a classic, harmonically rich envelope of sound. Today, they can be made as articulate as any Gallium Nitride (GaN) transistor amplifier. But historically, they always managed to find a balanced sound between the two. Without further ado, let me answer question number one: how much? The standalone Audio Research I/70 vacuum tube integrated amplifier, as reviewed here,  is $9950. There are two great options available. For an additional $2000, you can include two additional input functions. They are a Hi-Res Audio DAC circuit card and a separate Phono Amplifier card (MM / MC phono stage amplifier), with adjustable gain and MC loading from 50 to 1000 Ohms, which will set you back a reasonable $1000 for each card. Or if you order both with the purchase of a new I/70 unit, then you can save $500 off the cost of the pair ($1500 for both). And last but not least, the Audio Research I/70 vacuum tube integrated amplifier can be purchased in either a silver or black finish.

Build Quality
Audio Research's review sample I/70 Integrated amplifier is so new that it is one of the very first made within the production run. Consequently, there was no owner's manual or list of specifications provided. (I made a phone call to AR, and I was told they were still not available.) Consequently, ruler in hand, I measured the amplifier; it is 16.5" wide and 13.5" inches front to back and 8.25" high. The mid section of the chassis is vented with a band of very attractive open grill work. The body of the amplifier is built like a tank with billet aluminum plates 5/16" thick. The plates are part of the bottom cover and the top of the chassis.

The transformer enclosure is vented and has a lovely commemorative badge marking 1970 as the year the company was founded. I believe that is also the rated Watts per channel (Wpc) power specification of the I/70. Notice the enclosure, it is constructed from the same 5/16" thick aluminum plate. With all the tubes in place, this very sturdy integrated amplifier weighs an impressive 55 pounds. The double-boxed Audio Research I/70 review sample had a separate box inside containing four KT120 vacuum tubes and three ECC88 driver tubes. The KT120 tube is a beam pentode-powered amplifier made by Tung-Sol featuring a plate dissipation rating of 60 Watts.

Audio Research's I/70 vacuum tube integrated amplifier has automatic bias and can self-adjust bias to match other pentode power tubes. It will accommodate a 6550 tube and KT88 tubes, also the KT90, and of course the supplied KT120. The smaller ECC88 is a gold pin double triode driver with separate cathodes made by Sovtek. It is similar to the 6DJ8 and 6922 family of dual triodes.

The Fiddly Bits
These are the user controls along the top front edge. There is a luxurious 1.5" diameter control knob labeled, Input / Setup. Just to the right of that is a black button with the word, Power, written next to it. Move to the right, and there is a headphone jack, and next to that is another 1.5" diameter black knob used to control Volume / Mute. We know Enjoy the Music.com fans feel that, perhaps, most importantly, there are two cylindrical housed LED Lexie-Tubes. These are modern versions of the old Nixie tubes, formerly used as a numeric display.

These are the communications center of the I/70. Unlike the older version, they can display both numbers and letters within the same glass envelope. With that ability, they can graphically display the source/input you select with letters. For example, (X) (L) indicates a balanced input. Change the volume setting and the Lexie-tubes display that level with a numbers readout.

Ins And Outs
These are the connections available on the rear chassis, including the speaker terminals. Looking at the rear panel from left to right, there are five pairs of RCA jacks. They are installed vertically, the top is the left channel, and the bottom is red/right channel. The first is labeled Pre Out; next to that is a chassis' ground lug. Next is the Phono amplifier card with two sets of separate inputs for moving magnet (MM) and moving coil (MC) cartridges. Next, a pair of RCA jacks labeled SE2 and Direct.

Moving further right, two balanced XLR input sockets. Next, a power cord socket and just below that a button-style fuse holder. There are two sets of three loudspeaker wire terminals labeled 0, 4 Ohms, and 8 Ohms. And the last feature is the optional Digital card. This has an antenna for Bluetooth, USB, and S/PDIF RCA coax input.

The Hollow State
Safely packed in a separate box are the four KT120 tubes and three gold pin ECC88 dual triodes. The four KT120 tubes and the smaller twin triodes are positioned in the box just the way they are to be positioned in the amplifier. They are numbered, V1 to V4, and the three smaller ECC88 tubes are numbered V5 to V7. Donning a pair of white cotton gloves, I installed all the tubes with no problem. It is the smaller nine-pin ECC88 that can be a problem. Check and see if all nine tube pins of the ECC88 are perfectly straight before attempting to install it.

Working
All glassed up, we need to see how it survived shipping. The easy way to do that is to connect it to my home system's Parasound preamplifier. Then connect that to the I/70 balanced input. This is not a quality ear test, just a quick way to see how it works. Press the black power on button, and the Lexie-Tube's fluorescent display starts to count down from fifty to zero. Without the instruction book, I spent a brief time turning the input/setup control just to see what the corresponding Lexie-Tube readout would look like. When it displayed (X) (L), that was the balanced connection from my systems preamplifier. Using my Arcam SACD player, I was able to listen to an hour and a half of Crosby, Stills, and Nash.

The very early immersive soundscape image was shallow and lacked definition. After 30 minutes, the (new) tubes, electronics, and output transformers started to take shape. Gradually, the stop and start edges of digital music phrases appeared, and the stage grew spaces around the players. Note: The top edge of the amplifier grew a bit hot to the touch, so I advise placing the Audio Research Corporation's I/70 integrated amplifier on an open shelf with nothing above it.

Reconfiguration
The logical way to find the true voice and fully review Audio Research's I/70 is to remove my existing home setup from the mix. Eliminate my preamplifier and the maze of wires connected to it. With the I/70 as a standalone component, I can connect source components to the appropriate designated inputs for all the line-level Phono and Dac cards. Along with that, I believe I have the perfect testing device in my Arcam CDS50 SACD/CD Network Player. It has balanced, unbalanced outputs and a USB, S/PDIF coax digital output, TosLink optical cable, and a mini arial for a Bluetooth network connection. Next, load the Arcam player with a super-quality Analogue Productions DSD / CD, like Nils Lofgren, Acoustic Live, and you have my home reference system. So this is one unvaried direct source feeding digital and analog inputs to the I/70.

The Sound
Play the track "Some Must Dream" from the Nils Lofgren Acoustic Live album directly to Audio Research's I/70 balanced input. Wow! This is the improvement I hoped for. Now I can see deeper into the venue and with far less grit, and subsequently, elements on the immersive soundscape became far more dimensional. Listening to "Some Must Dream" at one point, I heard a (swooping) sound deep in the background. I thought that I was hearing violins in the background, but it quickly resolved into a (Moog) synth imitating the sound of strings. I was able to remotely switch from the XLR balanced input to the unbalanced RCA input from the Arcam as it played the "Some Must Dream" track.

Switching inputs using the remote control makes A/B comparisons very easy. The differences between the two inputs are almost identical. But during one brief passage, I thought I could hear a bit more openness at the higher frequencies extension when listening with the balanced XLR connection.

Let's Try The Hi-Res Audio DAC Card
Again, using Nils Lofgren's Acoustic Live, the track "Some Must Dream" as DSD source, the sound difference between the TosLink optical input and S/PDIF digital coax was, try as I might, aurally non-existent. Or possibly the difference a strong cup of coffee might make. Audio Research's I/70 vacuum tube integrated amplifier also has Bluetooth digital input. Alas, my subscription to Musicast is to a low-resolution free version, so I will only say that it works. At this junction, using the remote control, I was able to switch between seven separate inputs to Audio Research's I/70.

The Vinyl Path
Finally, we get to the thing I am most curious about. And that is the sound of vinyl LP discs, since the I/70 has an optional MM/MC phono stage.  For continuity, I was able to employ the Acoustic Live performance from both a DSD / CD and from a super quality two-disc vinyl album [Analogue Productions APP 090 ], both made from the same master tape. My favorite song on this record is "Some Must Dream". Now, as you listen, you can hear the subtly present live sounds of the surrounding venue. That space is, in fact, a complex mix of many small whispers of sound.

For me, and what may interest those reading Enjoy the Music.com, is a large portion of this quality is described by the word "continuousness," which is a term coined by the late Harry Pearson. It is used in part to describe an organic live presence that exists as you listen to a complex chord that slowly decays into silence. I would like to remind you of a problem inherent in Red Book lossy CD digital recordings. As the intensity / volume of a sound fades, it is treated as a least significant bit and dropped. The major difference between this vinyl recording and the same DSD / CD version is that the entire body of sound is retained as natural. It contains a complete range of frequencies, both odd and even.

With an analog recording, those overtones decay into silence naturally, just as they do in life. I wired an RCA connection from my Denon DL-301/2 cartridge to the Audio Research I/70 adjustable moving coil (MC) input. On every track, Nils Lofgren's presence appeared deeper in the center mix, surrounded by a halo of space, texture, and tonal depth. I have tried a hundred or more little tweaks spread out over decades. A hundred micro steps closer to a tangible reality, all of them hinting at what might be possible. I believe there exists a natural synergy between a vinyl recording and a vacuum tube amplifier that has never been adequately described. I believe even now, with all the advances made by digital technology, there is still something missing. I believe stop and start waveforms just do not let instrumental overtones have enough time to decay naturally.

Conclusion
I can't think of a single thing to improve, or that I can add to my home audio stereo system, to make it sound better. With the Audio Research I/70 integrated amplifier here for review at Enjoy the Music.com, music on vinyl LP created aural imaging as close to three-dimensional immersive reality as I have experienced within my home.  The I/70 vacuum tube amplifier paints musical reality more accurately than anything I presently own.

Why do I write about audio? It is because during rare occasions, I just might experience music with a deeper understanding and with greater emotional involvement. For a brief time, that's the gift Audio Research Corporation's I/70 vacuum tube stereo integrated amplifier has given me. In my 40-year long pursuit to bring the joys of music into my home, this is the best of times! Wish I could afford the I/70 to keep it here forever.

Audio Search's I/70 stereo unit is built like a tank and should last for decades. For some lucky soul, this vacuum tube unit with Hi-Res DAC and high-end audio MM/MC phono stage option will be a time machine that takes you back to memories of both today and long ago. Sit down, get comfortable, dim the lights, open your ears, and… enjoy the music.

Thank you, Audio Research.

Specifications
Type: Type: Vacuum tube stereo integrated amplifier with optional DAC and phono card.

Power Output: 70 Watts continuous 

Frequency Response: 7Hz to 30kHz (-3dB @ 1 Watt)

Tube Compliment: Four KT120 and three 6922 (one input and two drivers)

Input Sensitivity: 1.25V RMS for rated output

Input Impedance :100 kOhms balanced XLR, 48 kOhms unbalanced RCA

Volume Adjustment: 75-step volume control

Headphone Amplifier: Output is driven by the KT120 output tubes

Phono Stage: MC and MM gain settings, controlled via the I/70 remote.
      MC loading is also adjustable, from 50 to 1000 Ohms.

DAC: AKM Velvet Sound DAC chip for USB, TosLink optical, S/PDIF coax, and Bluetooth

Loudspeaker Output Taps: 4 and 8 Ohm

Dimensions: 16.5" x 8.25" x 13.5" (WxHxD)

Weight: 55 lbs.

Price: $9950, add $1000 for phono or the DAC card ($1500 for both)

Company Information
Audio Research
6655 Wedgwood Road North
Suite 115
Maple Grove, MN 55311

Voice: (763) 577-9700
Website: AudioResearch.com