audio research

Tube stereo line stage.

From the ARC website

The LS25 vacuum tube stereo line preamplifier offers sonic performance, functional flexibility and value second only to the world-renowned Reference One preamplifier. It is unprecedented measured by any performance criteria at its price.

Many will perceive the LS25 as a more compact REF1 -- and to be sure, there are many similarities in the mechanical design. Chassis construction is like the REF1, with finished slab sides and heavy top and bottom covers; chassis height and depth are slightly reduced, however. Front-panel control layout is also familiar: microprocessor-controlled rotary devices are used for overall gain set (High, Medium and Low), volume, balance and input selection. Two-way toggle switches control Power On / Off, Stereo / Mono, BAL / SE, Processor / Input, Monitor / Input and Mute. The selectable overall gain function allows a wider range of volume-control operation for input sources having different output levels (for example: a phono stage vs. a high-output D/A converter). The A-V processor to control overall system volume level instead of the LS25's volume control when used in this mode. Thus, the LS25 fits smoothly into any high-performance installation which combines audio and video playback functions.

The rear panel offers a full complement of audiophile-grade balanced and single-ended connectors: seven inputs (including tape monitor), a tape / record out and two sets of main outputs. Balanced or single-ended optimization for every input is selected via the front-panel toggle, and is automatically remembered (along with previously selected gain level) by the LS25’s internal memory. All functions (except gain and SE / BAL) are addressable via the supplied handheld IR remote control. A.C. power is supplied at the rear panel via a three-pin IEC socket and 14-gauge detachable power cord.

Inside, the LS25 is pure Class-A operation Audio Research: gain is supplied via four 6922 twin triodes, with fully regulated massive power supplies and Infinicap coupling caps at the output for maximum dynamics and transparency. Performance is smooth and detailed, with good soundstage focus and plenty of deep bass impact. The overall presentation is refined, yet with power that extends effortlessly into the frequency extremes. Like the REF1, the LS25 never meets a sonic challenge it can’t handle with aplomb -- and it does so while offering all the features needed to fit into today’s multipurpose entertainment systems.



LS25 MKII (2000)

From the ARC website

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.


MSRP (US new retail)
$4,995 (LS25)
$4,995. Increased to $5,495 on Feb 5 2001. (LS25 MKII)


Frequency response ±.5dB, 1.0Hz to 100kHz -3dB pionts 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. Inputs (8): Tuner, Phono, CD, Video, Aux 1, Aux 2, Monitor, Processor (XLR and RCA connectors)
Output impedance 650 ohms Balanced, 325 ohms SE Main (2), 20K ohms minimum load and 2000pF maximum capacitance. Outputs (3: 2 Main, 1 Record (XLR and RCA connectors)
Output polarity  
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: (4) 6922/E88cc dual triode
  LS25 MKII: (2) 6H30 dual triode
Power requirements 100-135VAC 60Hz (200-270VAC 50/60Hz) 45 watts maximum
Other 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 5.25" (13.4 cm) H (standard rack panel) x 11.75" (29.8 cm) D Handles extend 1.50" (3.8 cm) forward of front panel.
Weight 18 lbs. (8.2 kg) Net; 28 lbs. (12.7 kg) Shipping.