Audio Research SP-11 (1985)
A two-box hybrid preamplifier marking the midpoint of Audio Research’s shift toward neutral, transparent sound.
Overview
The Audio Research SP-11 is a high-end preamplifier that appeared in 1985, representing a pivotal moment in the company’s design philosophy. It bridged the gap between the fully tube-driven SP-10 and the more solid-state-oriented SP-9 and SP-14. As a hybrid design, it combined vacuum tubes and transistors in a sophisticated topology aimed at achieving exceptional clarity and depth without the listener fatigue often associated with solid-state gear. At the time, it was considered one of the most complex preamplifiers ever released, offering precision and transparency that won over many new audiophiles—even as some traditionalists criticized its use of global feedback and high parts count.
Design
The SP-11 is a two-box preamplifier, with the main power supply housed separately. It uses triple-regulated power supplies and features a hybrid gain block in each amplifying stage, combining a 6DJ8 triode, a power MOSFET, and one or two JFETs. The gain block uses the triode as the load for a P-channel JFET in a cascode-like configuration, followed by a MOSFET source follower to lower output impedance. The valve provides all voltage gain, while the transistors operate in unity-gain mode. Feedback from the output to the valve grid via a voltage divider sets the stage gain. The line section adds an N-channel JFET before the gain block and encloses the entire circuit in a global feedback loop. The phono stage includes four active stages with its own global feedback. Options include variable gain and an inverted output.
Context
The SP-11 appeared in 1985 at the midpoint of Audio Research’s experimentation with hybrid topologies. It sat between the traditional tube-based SP-10 and the more solid-state SP-9 and SP-14. It exemplified a shift away from the "warm and cuddly" valve sound toward a more neutral, detailed presentation. The SP-15 later replaced the SP-11 at the top of the range, using MOSFETs instead of valves in the line stage and adding balanced outputs. The SP-9 and SP-14 were less ambitious single-box designs with similar hybrid circuitry—SP-9 using one valve per stage, SP-14 using just a single dual triode in the phono input.
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