Audio Research M300 Tube Monoblock Amplifiers - 300W!

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Serious 300W monoblocks featuring auto bias, so you never have to calibrate or fuss over the tubes.  Perfect for driving a difficult load like ribbon or planar speakers. 

So rare we could not find another set for sale anywhere. 

Featuring a great American warm tube sound from decades ago, back when good gear was still hand manufactured.

Both amps are in super nice condition and were well cared for, with no faded aluminum or scratches, and are from a smoke free home.

All fans work properly and the tubes have lots of hours left in them.  Fully tested in our lab.

Click here for a PDF of the full M300 Manual, Specs, and Schematics 

Our Testing Process for Tube Amplifiers at SkyFi:

We start with a visual inspection of all internal components to make sure there are no signs of stress or aging. Capacitors are checked for telltale sings of bulging or leaking, resistors are checked for signs of overheating or cracking, and tubes are inspected for signs of stress overheating or cracking.

We then power up the unit carefully and run a simple 1k sine wave while monitoring the output on an oscilloscope for signs of distortion or noise. Finally we run the amplifier to its full rated power into the appropriate resistive load provided by our Sencore amplifier tester. If there is an issue, it will usually be noticeable at this point. If it all looks clean we will often test for THD (Total Harmonic Distortion) measured by our Keithley distortion meter, and compare it against the original manufacturers specifications. The amplifier the remains powered on to "burn in" on our test bench for a few hours to make sure there are no heat related issues. If there are balanced inputs available they will also be tested.

We also test all tubes for output gain and leakage on our calibrated B&K 747 tester against the original manufacturers specifications. If needed, the tubes will be replaced from our vast NOS (New Old Stock) vintage or modern inventory.

Final testing involves pushing actual music thorough the amplifier. We've learned over time that some issues are only noticeable to a trained ear while listening to familiar source material. Our test bench has reference vintage Kef speakers that we're very familiar with which quickly reveal any discrepancies. Some amplifiers will then move into our listening room where they are tested with our in-house reference speakers of choice.

Technical Specifications:

Rated power:
300W, 16Hz-20kHz into 2, 4, or 8 ohms, at below 0.8% THD

Power bandwidth:
10Hz-60kHz.

THD:
0.05% at 1W output

Clipping:
330W at 1kHz, nominal

Input impedance:
60k ohms

Feedback:
20 dB

Slew rate:
25V/us

Rise time:
3us

Sensitivity:
0.9V in for 300W out

Damping factor:
About 30

Hum & Noise:
Less than 2mV RMS; 90dB below full output

Power Consumption:
130W on standby, 450W quiescent (no signal), 900W at rated output power

Dimensions:
19" W by 10½" H by 16" D

Weight:
110 lbs.

Please click here for detailed specifics regarding our specialized packing process that separates us from the rest.