The Audio Research I/55 is a 50-watt-per-channel all-tube integrated amplifier that brings several of the design improvements introduced with the larger I/70 into the compact chassis of the I/50. It uses two matched pairs of 6550WE output tubes and three 6922 tubes in the input and driver stages, with an active tube preamplifier section providing substantially more gain and headroom than the passive preamp used in the I/50.
 
The result is an integrated amplifier with the familiar Audio Research tube presentation, while adding practical features that make the I/55 considerably more versatile. Auto-bias continuously maintains the output tubes at their proper operating point, eliminating the need for manual bias adjustment as the tubes age.
 
50 Watts of All-Tube Power
The I/55 produces 50 watts per channel continuously from 20Hz to 20kHz. Its push-pull tube output stage uses two matched pairs of 6550WE tubes, with 4-ohm and 8-ohm speaker taps for matching a wide range of loudspeakers.
The active preamplifier section gives the I/55 a wider gain range than the I/50. Audio Research specifically designed this stage to provide additional headroom with recordings mastered at lower levels, while the independent preamp and amplifier sections also provide stereo preamp outputs for connecting powered subwoofers or an external power amplifier.
 
Auto-Bias
One of the most important additions to the I/55 is automatic bias adjustment. The circuit continuously keeps the 6550 output tubes operating within their intended range throughout their service life. This simplifies tube ownership while maintaining consistent performance as the tubes age.
 
Digital and Analog Connectivity
The I/55 can be equipped with an optional DAC module using an AKM Velvet Sound DAC. The module accepts high-resolution digital audio through USB, optical, coaxial and Bluetooth connections, with incoming signals upsampled before conversion.
 
For vinyl playback, the optional phono module supports both moving-magnet and moving-coil cartridges. MM and MC gain can be selected from the remote control, while MC loading is adjustable from 50 to 1,000 ohms.
 
The standard analog connectivity includes three single-ended RCA inputs and one balanced XLR input.
 
Headphone Output
The I/55 includes a dedicated headphone output driven directly from the 6550 output tubes. This allows the same tube output stage used for the loudspeakers to power high-quality headphones.
 
The front panel uses two LexieTubes to display input selection and volume, while the compact 16.5-inch-wide chassis makes the I/55 easy to integrate into a wide range of systems. Like other Audio Research tube electronics, the I/55 is built and tested in Minnesota.
 
Key Features
• 50 watts per channel continuous, 20Hz–20kHz
• All-tube integrated amplifier
• Two matched pairs of 6550WE output tubes
• Three 6922 tubes: one input and two driver tubes
• Active tube preamplifier section
• Automatic output-tube bias adjustment
• 4-ohm and 8-ohm speaker taps
• Three single-ended RCA inputs
• One balanced XLR input
• Stereo preamplifier outputs
• Dedicated headphone output driven by the 6550 output tubes
• Optional AKM Velvet Sound DAC module
• Optional MM/MC phono module
• Adjustable MC loading from 50–1,000 ohms
• Dual LexieTube displays
• Remote control
• Hand-built in Minnesota