Audio Research

Combining the best attributes of both tube and solid state sound has long been one of the Holy Grails of high end audio. Those of you who have been audio aficionados for quite a while may remember several, not completely successful, efforts to do this in the 1980s and 1990s. Now, two of audio's most prestigious companies are breaking new ground in this area. One of them, Audio Research, has introduced the HD220 tube/solid state power amplifier at $8995, a first for this company. (For a discussion of the other, new hybrid offerings, please see the Conrad Johnson section of this newsletter.) HD stands for Hybrid Drive because that's how this amplifier works. As in the all-tube REF 110, a JFET, low-noise input feeds a 6922 dual triode gain stage. This, in turn, feeds a 6H30 dual triode cathode follower which drives the outputs through ARC's proprietary coupling capacitors, first used in the REF3 preamp. This vacuum tube input/driver arrangement yields superb nuances, highly musical performance, and life like character at the front end of the amplifier. The output stages are comprised of twelve Thermal Trak bipolar output devices per channel, producing 220 watts per channel. These new devices have the unique ability to adjust their internal bias automatically in response to operating temperature, thereby assuring extremely consistent output performance no matter how hard the amplifier is pushed. Low-noise, fan cooling of the output stages keeps the output devices within their operating range. Power supply regulation is all solid-state to assure stability and consistency under high current loads, and power supply energy storage is an amazing 200,000 microfarads capacitance. These factors yield explosive dynamics and rock-solid bass impact and control.

Whether tube or solid-state, power amplifier designs from Audio Research have always emphasized the key criteria of musicality, lifelike dynamics, and realistic sound staging. Whichever technology was used was intended to serve these priorities in the music reproduction chain. Audio Research believes that the HD220 represents the highest level of performance heretofore obtained in any solid state or hybrid design. Its musicality is superb, with a degree of transparency and grainless purity that is nothing short of eerie, thanks to extremely low distortion and a low noise floor. Dynamics and bass control are thunderous and controlled, thanks to the Thermal Trak output stage. In short, the HD220 gives you the finesse and nuances for which tube amps are famous with the bass, drive, and speaker control at which solid-state amps excel.

The discontinuation of the 150.2 and 300.2 solid state models earlier this year created the need for a new, solid state power amplifier from Audio Research. The development of the HD220 discussed above positioned Audio Research to take advantage of that technology to create the SD135, a 135 watts/channel, all solid state version selling for $4995. Like the HD220, the new SD135 ("SD" for "Silicon Drive") uses Thermal Trak self-biasing output devices in a Class A-B design. Unlike the larger hybrid, however, the SD135 features an all-solid-state input stage using multiple JFET devices in each channel directly driving the output stage. This obviates the need for inter-stage coupling capacitors. By not requiring high-voltage regulated supplies and those expensive inter-stage capacitors used in the HD220, Audio Research was able to achieve a much lower suggested price. The result is an amplifier with amazingly fast response time, explosive dynamics, and plenty of current delivery for a wide range of speaker types. Bass is deep, controlled, and punchy. The noise floor of the SD135 is extremely low, rendering small dynamics and nuances with vivid clarity. Yet the SD135 is never harsh, dry, or etched in its presentation. Paired with any top quality preamp, you will hear the soundstage you expect in a highly refined and purely musical presentation.