Audio Research D 70 (1972–1979)

The Audio Research D70 was the company's first solid-state power amplifier. Until then the brand's reputation rested on tube designs. William Z. Johnson designed it, and it was introduced in 1972. It stayed in production for seven years, through 1979, and originally sold for $1,200.

It was rated at 70 watts per channel into a nominal 8-ohm load, with frequency response spec'd at 20Hz to 20kHz and 0.5% distortion. The design was all-discrete, class AB, with a "dual-differential" circuit, a solid-state output stage with transformer-coupled drive, and a toroidal power transformer. The chassis combined wood and metal, and the output impedance was 4 ohms. The SP-3A preamplifier was its companion piece. Audio (June 1984, p.75) described it as the smallest of three power amplifiers in the line at the time, and as representing the company's then-current thinking on tube amplification - though the D70 itself was not a tube design.

The D70 was well-regarded in its day, praised for clean sound and solid build quality. Audio (June 1984, p.84) noted that the reviewer personally preferred the sound of the Marantz 9s to this particular D70, but acknowledged hearing good reports from listeners whose judgment he respected. Some contemporary reviewers found its sound a bit "dry" compared to tube amplifiers. Forum owners echo that. One calls it musical but hot-running and bias-sensitive, and another finds the bass thin and the treble harsh relative to tube amps.

The D70 is considered a collector's item now, with prices rising, particularly for clean units. A current US Audio Mart listing asks $1,200. Known service issues: capacitors are prone to failure and should be replaced routinely, some units have a noisy power transformer, bias adjustment is critical, and output transistors are hard to source.

Power output: 70 W/ch RMS into 8 ohms
Frequency response: 20Hz–20kHz
Distortion: 0.5%
Output impedance: 4 ohms
Original price: $1,200
Production: 1972–1979