Audio Research M300

M300 Hybrid Monaural Power Amplifier - Audio Research Corporation developed and patented new circuitry allowing hybrid utilization of Fets with vacuum tubes in ways which can bring out the best in both devices. This circuitry is used in the relatively new, but already widely-acclaimed SP-11 preamplifier. Drawing from this technology, Audio Research has designed a new hybrid power amplifier, designated the M300 - a monaural unit rated at 300 watts.

Hybrid Monaural Power Amplifier

Most hybrid power amplifier designs use small signal-input vacuum tubes, then rely on FETs for the output stage. Audio Research has chosen instead to eliminate these low-level input tubes - tubes which can be prone to problems with hum, noise, microphonic, drift and, of course, require periodic replacement. The new FET "front end" is combined with Audio Research's long-famous, patented cross-coupled circuit to provide a design offering exemplary stability, reliability (low maintenance), and superior performance.

These design advances have been coupled with yet another Audio Research patent; an output stage utilizing vacuum tubes, and partially cathode-coupled (as all previous Audio Research vacuum tube amplifiers have ben), but with the screen (or accelerating) grids operating with a signal voltage precisely in phase with the cathode voltage. True "pentode" operation and efficiency is thereby achieved; however, all thr advantages of partial cathode coupling are maintained .

Advantages include:

Efficiency up to 60% above. (Class "A" designs achieve 25% to 50% typically 30% to 35%)

Inherently better cross-coupling

Output transformer with lower "turns" and "impedance" ratios, resulting in greater bandwidth.

Partial cancellation of saturation effects of the "DC" inn the output transformer's core

Switching transients essentially eliminated, rendering the question of class of operation ("A", "AB", etc.) irrelevant. The virtues of Class "A" operation without the usual drawbacks

Bias adjustment is essentially non-critical.

Output tubes need not be matched, although slighty better performance will result if they are.

Lower internal impedance improves handling of complex speaker loads.

Operationsl Features:

Front-panel replaceable line and screen voltage fuses, with LED indicators.

Two integral, low-noise fans for cooler operation, longer tube life. Front-panel switch allows "Hi" and "Lo" speeds.

"Standby" switch for lower power consumption and less heat during warm-up periods or telephone interruptions.

Automatic muting circuit provides silent shutdown upon power shut-off or interruption. No "thump" from speakers.

Gold-plated input connectors connect "ground" before "hot".

1, 2, 4 and 8 ohms output taps.

Front and rear handles for easier handling

Tube Saver protection Circuit

The M300 Power amplifier contains a remarkable new circuit called the :Tube Saver" it's a small secondary circuit board located on the back side of the main board in the amplifier. Primarily, the "Tube Saver" reduces stress to tubes and associated driving Fets. As an overload protection circuit, it is triggered only when abnormal current is sensed from a cause like those listed  below. Under even extreme conditions of normal use (i.e. extremely high playback levels), the circuit is designed to remain inactive. When triggered, the circuit immediately shuts down th eoutput stage and indicates protection mode by dimming the "Screen" LED. At this point, the amp should be manually switched off, then restarted after a period of 30 seconds.

Specifications

Power Output:  300 watts minimum continous at 8 Ω, from 16 Hz to 25 kHz with less than 0.5% T.H.D. (typically below .005% at 1 watt).

Approximate Actual Power Available at Clipping:  330W at 1 kHz, nominal.

Power Bandwidth:  10 Hz - 60 kHz (-3 dB Points)

Input Sensitivity:  1,5 V RMS for rated output

Input impedance:  60 k Ω, nominal

Output Regulation:  Approx. 0,3 dB, 8 Ω load to open circuit (Dampping factor approx. 30)

Negative Feedback:  20 dB

Slew Rate: 25 volts/microsecond

Rise Time: 3 microseconds

Hum and Noise: less then 2 mV RMS 90 dB below rated output (broad-bandwidth unweighted, inputs shorted)

Power Supply Energy

Storage:  approx. 350 joules

Power Requirements:  105 - 125 V AC 60 Hz (210 - 250 V  AC 50 Hz); 900 watts at rated output, 450 watts at "idle" - 130 watts at "standby"

Dimensions (W x H x D):  480 x 270 x 410 mm (19" x 10,5" x 16")

Weight:  50 kg (110 lbs) net

Tubes required:  8 - 6550 Power Output (matched pairs will yield marginally better performance)