Audio Research LS25 MKII Tube Preamplifier With Remote

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This preamp came to us from a great client that upgraded to a REF piece from ARC.

It has been well cared for and is working to factory specs. Tubes were tested and working strong.

This is more desirable MKII variant featuring a bunch of performance and reliability updates from the factory (specifics below from ARC).

All we did was repair an LED issue and clean the contacts on the remote. While all remote control buttons are tested and working, the volume does require a firmer press than the rest but works consistently.

SkyFi Tube Notes:

Both tubes are Russian Reflektor 6N23P. They have both been tested on our modern and professional Amplitrex AT-1000, and both test strong with good triode balance.

SkyFi Test Notes:

• All balanced and single-ended inputs and outputs tested and working properly.
• Very good channel balance.
• Remote functions all work consistently, but Volume UP and DOWN both require a firm push.

Your purchase includes the original box, remote manual, and a power cord.

Click below to add our recommended matching cables from Kimber Kable, all brand new as SkyFi is an official Kimber dealer.

Kimber Kable - RCA Interconnects - Better

Kimber Kable - RCA Interconnects - Best

Kimber Kable - BALANCED XLR Connectors - Better

Kimber Kable - BALANCED XLR Connectors - Best

Kimber Kable - Power Cords - Better

Kimber Kable - Power Cords - Best

More from Audio Research:
LS25 MKII (2000)
Having held a firm position in our product lineup for two-and-a-half years, the LS25 has proved highly popular for both its musicality and its many features and system flexibility. With recent engineering breakthroughs at Audio Research, however -- especially stemming from the development of the VTM200 input stage technology -- it became increasingly clear that the venerable LS25 was the perfect preamplifier candidate for these latest performance enhancements.

Because it already has so many useful control functions, the LS25MKII remains almost entirely unchanged in its exterior configuration -- meaning original units can be updated at the factory to the MKII spec. Inside, however, you will find a new motherboard architecture that incorporates a JFET input stage similar to that used in the VTM200, followed by a cathode-follower / output driver stage using two 6H30 twin triodes.

The 6H30 is a new tube for Audio Research ideally suited to its function in the LS25MKII. It is a Russian military tube in current production and use, and is rated by its manufacturer to have an estimated service life of up to 10,000 hours. It has shown highly stable and consistent in its operating characteristics. While its pin configuration is similar to the 6922, it is not a direct replacement. It has higher current requirements for the heater filaments, so it requires a different power transformer than used in the original LS25, as well as power supply adjustments.

Finally, like the VTM200, the LS25MKII also incorporates a rear-panel 12V on/off trigger for even easier incorporation into audio-video installations, or simply for installations where the customer wants more convenient system control. This is the only external modification to the original. You will see this feature appear on future models from Audio Research as well.

The complete MKII changes are available to original LS25 owners as a factory update. New 6H30 tubes with damping rings are included. Contact the Audio Research Customer Service department to schedule any LS25 updates. Units sent in without prescheduling may encounter substantial delays in turnaround.

In performance, we believe the LS25MKII blows by the original in every respect. It is faster, more dynamic, more open-sounding and presents a more tightly focused soundstage. Bass response is cleaner, quicker, with more apparent impact, while high frequencies are much purer sounding than the original. You will love the coherence of the new MKII -- vividly alive from top to bottom, with an immediacy that is stunning when driving really capable power amplifiers like the VTM200's!

Once again, Audio Research brings new life and improved performance to an existing model, enhancing value for every owner.

SPECIFICATIONS:
FREQUENCY RESPONSE
±.5dB, 1.0Hz to 100kHz -3dB points below 0.2Hz and 400kHz.

DISTORTION (THD)
Less than .01% at 2V RMS output.

NOISE & HUM
14µV RMS residual IHF weighted balanced noise output with gain control at minimum (103dB below 2V RMS output).

GAIN
Main Output: Selectable for each input: 18db, 12dB, 6dB Balanced output. (12dB, 6dB, 0dB SE output). Record output: 0dB.

INPUT IMPEDANCE
120K ohms Balanced, 60K ohms SE.

OUTPUT IMPEDANCE
650 ohms Balanced, 325 ohms SE Main (2), 20K ohms minimum load and 2000pF maximum capacitance.

MAXIMUM INPUTS
28V maximum, BAL, 14V maximum SE (6dB gain).

RATED OUTPUTS
2V RMS 1Hz to 100kHz into 200K ohm balanced load (maximum balanced output capability is 40V RMS at less than 0.5% THD at 1kHz).

POWER SUPPLIES
Electronically-regulated low and high voltage supplies. Automatic 40 sec. warm-up/brown-out mute. Line regulation better than .01%.

TUBE COMPLEMENT
LS25 MKII: 2 x 6H30

POWER REQUIREMENTS
100-135VAC 60Hz (200-270VAC 50/60Hz) 45 watts maximum

OTHER
• Vacuum tube audio circuit, solid-state power supply.
• Inputs (8): Tuner, Phono, CD, Video, Aux 1, Aux 2, Monitor, Processor (XLR and RCA connectors).
• Outputs (3: 2 Main, 1 Record (XLR and RCA connectors).
• Controls: Volume (104 steps), Gain, Balance, Input.
• Toggle switches: Power On/Off, Stereo/Mono, Bal In/SE In, Processor/Input, Monitor/Input, Mute/Operate.

DIMENSIONS
19" (48 cm) W x 5 ¼" (13.4 cm) H (standard rack panel) x 11 ¾" (29.8 cm) D Handles extend 1½" (3.8 cm) forward of front panel.

WEIGHT
18 lbs. (8.2 kg) Net
28 lbs. (12.7 kg) Shipping.

Audio Research LS25 MKII Tube Preamplifier - Owner's Manual

The SkyFi Testing Process for Preamplifiers:

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 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.