AUDIO RESEARCH SP-11 + SP-11PS

Second-hand. In good conditions. Shows minor signs of wear. Comes with manual and cables. Original packaging not included.

The Audio Research SP11 is a two-chassis unit featuring a hybrid circuit that combines six vacuum tubes with solid-state devices such as JFETs and MOSFETs. This technical synergy aims to merge the warmth and musicality of tube sound with the precision, quietness, and transparency of semiconductor circuits. The result is a unit that still impresses today with its ability to deliver extraordinarily clear, dynamic, and detailed sound reproduction, without compromising on naturalness or engagement.

The line stage uses sophisticated hybrid gain blocks, a result of the ingenuity of Audio Research founder William Johnson, employing tubes for voltage gain, JFETs for noise reduction, and MOSFETs as low-impedance output buffers. These stages work in conjunction with global feedback circuits and multi-stage voltage regulators, ensuring operational stability and tonal consistency. An additional JFET in the input stage, encapsulated within a feedback loop, ensures wide frequency response, low distortion, and consistent behavior even at high volume levels.

The front panel highlights the unit’s completeness and versatility: beyond the standard input selectors, it features two separate volume controls — Gain and Level — allowing fine adjustment of dynamics and optimal matching with the output levels of various sources. This setup, particularly useful during the transition period from vinyl to CD, helps avoid overload and ensures consistently controlled listening. Available inputs include phono, tuner, CD, video, and a spare, while the output can be selected as either inverting or non-inverting, depending on the listener’s phase preference.

The phono module, a flagship feature of the design, is compatible with both MM and MC cartridges, thanks to the selectable load impedance with five values directly accessible from the front panel. Internally, it features four active stages with a global feedback network for proper RIAA equalization, along with a selectable subsonic filter that attenuates unwanted resonances below 25 Hz while preserving the integrity of the musical low-frequency content.

Construction is meticulous, focused on maximum reliability, with a separate power supply and multiple regulation stages to ensure stable and clean voltage to each section of the circuit. Protective measures include the use of diodes and JFETs in unconventional roles, helping to prevent damage in the event of tube failure or powering the unit on without all tubes installed.

The rear panel of the main chassis features a clean layout of gold-plated RCA connectors — seven pairs for inputs and six for outputs, including those dedicated to tape recorders. The external power supply, which matches the preamp in size, also provides control of auxiliary outlets, improving convenience in complex home setups.

Sonically, the SP11 sits somewhere between the euphoric character of its all-tube predecessor, the SP-10, and the analytical neutrality of more recent solid-state stages. It lacks the lush warmth of early Audio Research models but offers a more transparent, precise, and deep sound, still maintaining the fluidity and musicality that tube enthusiasts appreciate. The phono stage is particularly praised for its ability to handle high-output MC cartridges and deliver around 76 dB of gain, sufficient for most analog needs without the need for an external step-up device.

The SP11 is, in every respect, a classic piece of hi-fi, a preamplifier that blends advanced engineering with timeless musicality. Its complex structure, refined sound, and exceptional flexibility still make it a sought-after choice for those who demand the very best.

Technical Specifications:

• Type: Hybrid solid-state/tube preamplifier with separate power supply
• Inputs: Phono (high-gain MM); Tuner; CD; Video (audio only); Tape 1; Tape 2
• Outputs: Tape 1; Tape 2; Main 1; Main 2; Invert; Direct
• Controls:

  • Gain; Level; Balance

  • Mode: Mono; Reverse; Stereo; Left; Right

  • Input selection: All except Tape

  • Cartridge load impedance

  • Tape Monitor; Copy switch; Copy mode (1-2; 2-1)

  • Manual Mute; Polarity (Invert/Normal); Bypass; Subsonic Filter

• Frequency Response:

  • High-level inputs: 1Hz–100kHz ±0.5dB; -3dB at 0.2Hz and 250kHz

• RIAA Accuracy: 20Hz–40kHz ±0.2dB
• Harmonic Distortion: 0.005% at 2V output from 5Hz to 30kHz; typically 0.001% midband
• Voltage Gain: 30dB line level; 76dB phono
• Noise:

  • -110dB at 2V output, minimum gain

  • Line input: high level, max gain, 106dB below 1V input

  • Phono input: 74dB below 1mV input

• Dimensions: Power supply and preamplifier: each 48.3 cm (19") W × 14 cm H × 39.4 cm D (including front handles and rear clearance)
• Weight: PS-11: 6.4 kg; PS-11PS: 6.8 kg