Audio Research SP-11 MKII Vintage Hybrid Preamp- Fully Serviced

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General:
This legendary preamp requires no introduction among vintage audiophiles.

It represented the state of the art in tube preamps, and was also the flagship model from Audio Research circa 1985. It gives you everything you need in a classic Audio Research design that stands the test of time.

Many manufacturers have tried to surpass this performance and musicality in the past 40 years with little improvement.

Best of all it contains a very capable phono stage, eliminating the need for an additional box.

This particular SP-11 MKII is in fantastic condition. It was also serviced in house at SkyFi and fitted with strong tubes for ultimate reliability. Included will be its original owners manual/paperwork, its speciality cord for linking the two chassis, and its original box for safe shipping.

SkyFi Cosmetic Notes:
Light wear is preset on the covers of both pieces and on the handles of the power supply chassis. Overall excellent.

SkyFi Technical Notes:
This unit began its life as an original SP-11. We have the original QC cards from its production as an SP-11 as well as the QC card after it was factory upgraded to a MKII. It also went back to Audio Research for some light repairs in 1987 according to the service receipts. At some point after the factory upgrade, the unit was further modified. The mods involved bypassing the gain control and adding some additional power supply filtering to the phono stage. We identified and reversed these modifications and restored the unit to the MKII schematic with the exception of the balance control which may not be original. We also replaced a bunch of drifting and worn out parts in power supply chassis.

This unit is fitted with (3) RAM certified 6DJ8 in the phono stage and (3) USA marked 6DJ8 in the line stage.

Brand Background:
Audio Research continues its dedication to unparalleled musical expression and crafting components of the highest quality and lasting value for its customers. Our long-term dedication to service is peerless in this industry (Audio Research, with few exceptions, can repair anything ever produced during its entire history). We continue to seek out new and exciting ways to make music more real and more enjoyable. More than just a name or a marketing tool, Audio Research has come to represent exceptional performance and long-term value. Experience Audio Research – experience High Definition®.

Ownership:
Single Owner

Connections:
RCA Inputs, Four AC Power inputs,

General Sound:
Big beautiful musical sound with little fatigue.

Cosmetic Condition:
8/10 = Very Good. Excellent front faceplate, one minor flaw on chassis side or top. See our detailed rating description here.

Working Condition:
Working perfectly and tested in our lab and listening room.

Included:
Unit, Box, Manual

Packing:
Original Manufacturers Packing

Specs:
- Tube complement:
- (6) 6DJ8/ECC88 tubes
- Provides 29dB gain for high-level inputs
- Provides up to 75dB gain for phono inputs
- Rated output:
- 2V RMS
- Phono input impedance options:
- 47K ohms
- 100 ohms
- 30 ohms
- 10 ohms
- 3 ohms
- High-level input impedance:
- 20K ohms
- Output impedance:
- 250 ohms (main/direct outputs)
- 500 ohms (tape outputs)
- Bandwidth (high-level signals):
- 0.2Hz to 100kHz
- Harmonic distortion:
- Less than 0.005%

Dimensions:
19 inches (48 cm) Wide, 5 ¼ inches (13.4 cm) High, and 10 ¼ inches (26 cm) Deep (each)

Weight:
14 lbs.

Approximate Age:
1987

Link to Manual:
Click Here

Recommended Cables:
Kimber Kable - RCA Interconnects - Better

Kimber Kable - RCA Interconnects - Best

Kimber Kable - Phono Interconnects

Kimber Kable - BALANCED XLR Connectors - Better

Kimber Kable - BALANCED XLR Connectors - Best

Kimber Kable - Power Cords - Better

Kimber Kable - Power Cords - Best

Testing Process:
We start with a visual inspection of all internal components to make sure that there are no signs of heat stress or damage. Capacitors are checked for telltale signs of predictive failure including bulging, shrunken wrappers, or physical leakage. We also inspect the PCBs for discoloration from resistors or transistors that may have been running hot. On vintage units we often spot check select capacitors for value and ESR.

When we first power on a preamplifier we connect its RCA output to a Sencore PA81 Power Analyzer which simulates real world loading conditions and gives us an oscilloscope interface.

The first order of business is checking that the volume control works smoothly throughout its entire range with acceptable channel balance. This is accomplished by feeding a 1 KHz sine wave into one of the preamp’s line level inputs while monitoring the preamp’s output on an oscilloscope.

We then switch to a 1 KHz square wave to test the tone controls, loudness function, and filters where applicable. During this step we are watching for equal alteration of the test signal by both channels. This also helps us identify dirty controls that will need treatment.

Once the basic line stage functions are verified, we test each input individually. This is especially important for devices that use relays to select their sources. If the preamp is equipped with a phono stage we test that as well. We use an inverse RIAA filter which allows us to feed a reference test signal into the phono input with the proper RIAA equalization and level. A square wave or sine sweep is used to verify that the device’s phono stage is faithfully reproducing the RIAA curve. If the preamp under test has balanced inputs and/or outputs these are tested as well.

We finish up our bench testing with a listening test with our bench amplifier and reference speakers. During this test we check for hum or hiss that may not have shown up in earlier testing. We also check that all of the tone controls and filters perform as expected.

If the preamplifier has remote control functions these are also tested. Preamps with tube circuits or complicated power supply topologies are connected at our long term test rig for extended stress testing under real world conditions.