Audio Research writes: “The SP20 was our last SP model, a full-function preamplifier with both a tube line stage and a tube phono section in one chassis. It is highly sought after on the used market. The SP20 was the only Audio Research preamp ever made with a headphone input. The tubes in the line section powered the ‘phones using a buffer, sounding very good. There are a total of four 6H30 tubes, two per section, and the SP20’s display would show a red alert symbol when the tube hour meter reached 4,000 hours.
The SP20 was feature-laden, with input gain offset, programmable input labeling, and many others.
In the closeup photo you can see select JFETs in the phono section are encapsulated in silicone to maintain a stable thermal environment, and different gauges of Ohno copper wires used for the line and phono sections. The SP20 was also the first Audio Research product to use the new coated boards.”