Audio Research LS2B MKII Balanced Hybrid Tube and Solid State Preamplifier

General:
The Audio Research LS2 preamp came in multiple versions and variations beginning in 1991. Line stage in ARC terminology means that it does not include a phono preamplifier. To connect a turntable you'll just need a dedicated phono preamplifier, which many audiophiles prefer anyway.

This version, the LS2B MKII released in 1992, is the last and best of them all. It features balanced inputs and outputs (hence the 'B"), and remote control capability.

It's a wonderful sounding hybrid tube/solid state design which allows the internal 6922/E88CC twin triode tube to induce just the right amount of warmth into the signal path.

In overall great condition and in a rare black finish.

This preamp arrived in good working condition, and we performed a light service by cleaning the controls and detailing the chassis.

Tested perfectly in our lab and long term test bench.

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

Amplifier Recommendations:
N/A

Connections:
Mix of RCA and XLR inputs and outputs. Captive power cord

General Sound:
Accurate to the recording

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:
Just the unit and power cord.

Packing:
Will be packed using our highly developed in-house process and custom packing materials.Specs:
FREQUENCY RESPONSE
± .5dB, 1Hz to 100Hz, -3dB points below 0.1Hz and above 300kHz

DISTORTION (THD)
Less than .01% at 2V RMS output (typically less than .005% in midband)

NOISE & HUM
20µV RMS residual 1HF weighted noise at main unbalanced output with gain at minimum (100dB below 2V RMS output). xxx Balanced input noise 100dB below 1V input.

GAIN
LS2: Main Output: 18dB. Balanced output: 24dB. Tape output: 0dB (Optional 12dB gain reduction). LS2 MKII: Main Output: 18dB. Balanced output: 24dB. Tape output: 0dB (Optional 9dB gain reduction)

INPUT IMPEDANCE
LS2: 50K ohms. LS2: 50K ohms unbalanced, 3 megaohm balanced.

OUTPUT IMPEDANCE
LS2: 200 ohms main output. 500 ohms Balanced 1, Balanced 2 (10K ohms minimum load and 2000pF maximum capacitance). LS2 MKII: 200 ohms main output. 400 ohms Balanced 1, Balanced 2 (10K ohms minimum load and 2000pF maximum capacitance)

MAXIMUM INPUTS
LS2: 20V maximum. LS2 MKII: 20V maximum unbalanced, 8.5V maximum balanced.

RATED OUTPUTS
LS2: 2V RMS 1Hz to 100kHz into 60K ohm load (maximum output capability is 10V RMS (20V RMS balanced) output at 05% THD at 1kHz into a 10K ohm load). LS2 MKII: 2V RMS 1Hz to 100kHz into 60K ohm load (maximum output capability is 30V RMS (60V RMS balanced) output at 05% THD at 1kHz into a 10K ohm load)

POWER SUPPLIES
Electronically-regulated low and high voltage supplies. Two transformers (toroid for high voltage). Line regulation better than .01%

TUBE COMPLEMENT
LS2: (1) 6DJ8/ECC88. LS2 MKII: (1) 6922.

POWER REQUIREMENTS
100-135VAC 60Hz (200-270VAC 50/60Hz) 60 watts maximum

OTHER
Hybrid FET/Tube audio circuit, solid-state power supply

Dimensions:
19" (48 cm) W x 5 ¼" (13.4cm) H (standard rack panel) x 10 ¼" (26cm) D. Handles extend 1 ⅝" (4.1cm) forward of front panel. Rear chassis fittings extend ⅞" (2.3cm).

Weight:
13 lbs.

Approximate Age:
1992

Link to Manual:
Click Here

Recommended Cables:

Kimber Kable - RCA Interconnects - Better

Kimber Kable - RCA Interconnects - Best

Kimber Kable - BALANCED XLR Connectors - Better

Kimber Kable - BALANCED XLR Connectors - Best

Kimber Kable - Speaker Cables - Better

Kimber Summit Series Monocle XL Speaker Cables (PAIR) - Best

Kimber Summit Series BiFocal XL Bi-Wire Speaker Cables (PAIR) - Best If Applicable 

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 PCB’s 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 1KHz 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 1KHz 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.