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)
No comments:
Post a Comment