Page 1
m Pure Class A operation delivers quality power: 45 watts × 2 into 8 ohms
m Power MOS-FET output stage features 6-parallel push-pull configuration
m Instrumentation amplifier principle m Further improved MCS+ circuit
topology m Current feedback combines stable operation with outstanding
sound m Bridged mode allows upgrading to true monophonic amplifi er m Large
high-efficiency toroidal transformer rated for 600 VA m 4-step gain control
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Pure Class A power amplifi er uses power MOS-FET devices to deliver 45
watts per channel (8 ohms) – Fully balanced signal paths as found in highquality instrumentation amplifi ers. Further refi ned MCS+ topology and
current feedback result in superb sound quality and excellent ratings for S/N
ratio, THD, and other parameters. Strong power supply and power MOSFET devices in six-parallel push-pull confi guration sustain an amazing 360
watts per channel into ultra-low impedance 1-ohm loads (with music signals).
Pure Class A power amplifi ers from Accuphase
have long been blending the purity of class
A operation with the superior performance of
power MOS-FETs. Acclaimed by audiophiles
the world over, these products refl ect a level
of technical know-how that is second to none.
Completing the A series lineup, the A-45 is ideal
for systems that demand not quite the high
power levels of the A-60 but require more power
than the A-30 provides. The A-45 of course also
demonstrates the same unwavering dedication
to sound quality and outstanding design policy
which are the hallmark of Accuphase. This pure
class A stereo power amplifi er with its distinctive
external heat sinks is built for the true enjoyment
of music.
The so-called instrumentation amplifi er principle
is used throughout. The signal handling stages
feature further improved MCS+ topology and
the renowned current feedback approach.
This results in electrical characteristics that
surpass even the demanding standards set
by its predecessors. Employing only highest
grade materials and strictly selected parts, the
A-45 realizes the two most important goals of
an amplifi er: very low output impedance and
constant drive voltage.
The output stage features power MOS-FETs
renowned for their excellent sound and superior reliability. For each channel, six of these
devices are arranged in a parallel push-pull
arrangement. Because MOS-FETs have
negative thermal characteristics, there is no
danger of thermal “runaway” as exists with
bipolar transistors. Operation remains totally
stable even when the amplifi er is running hot.
Driving these devices in pure class A assures
high-defi nition sound that brings out the fi nest
nuances in the music.
The heavy-duty power supply features a massive power transformer rated for a maximum
of 600 VA, plus two smoothing capacitors of
47,000 μF each. This easily sustains output
levels of as much as 360 watts per channel into
1 ohm (with music signals). If even higher power
is required, bridged mode turns the A-45 into a
high-grade monophonic power amplifi er.
n Power modules with 6-parallel push-pull arrangement of power MOS-FETs
deliver 360 watts per channel into 1 ohm (music signals only), 180 watts
into 2 ohms, 90 watts into 4 ohms, or 45 watts into 8 ohms.
n Amply dimensioned power supply with high-efficiency toroidal
transformer and large fi ltering capacity.
The high-effi ciency toroidal transformer is rated for 600 VA, and the two extra-
large 47,000 μF aluminum electrolytic capacitors were selected for best sonic
performance.
Toroidal power transformer
n Bridged mode allows upgrading to monophonic amplifi er with 720 watts
into 2 ohms (music signals only), 360 watts into 4 ohms, or 180 watts
into 8 ohms.
Bridged connection increases the output by a factor of 4, creating dramatic power
reserves in a monophonic confi guration.
n Four gain control settings minimize residual noise.
The gain control switches gain in the fi rst stage of the instrumentation amplifi er.
Four settings are available (MAX, –3 dB, –6 dB, –12 dB).
n Fully balanced input stage shuts out external noise interference.
n
Mode selector for easy switching between dual mono/stereo/bridged
operation.
n PCB copper foil and all major signal path components are gold-plated.
n Large analog power meters with off switch and sensitivity selector.
n Oversize speaker terminals accept also Y lugs.
Meter and input buttons
Power MOS-FETs
Large speaker terminals Parts selected for sound quality and reliability Assembly with meter and protection circuitry
Balanced input connectors Gold-plated parts
Filtering capacitors
Gain selector
Instrumentation amplifi er
+INPUT
–INPUT
Gain control
MCS+ (Multiple Circuit Summing-up)
Bias stabilizer
circuit
Bias stabilizer
circuit
Bias stabilizer
circuit
Bias stabilizer
circuit
Fig. 1 Circuit diagram of amplifi er section (one channel)
Bias stabilizer
circuit
Power MOS-FETs
6-parallel push-pull
Output current (A)
Output voltage (V)
1-ohm operation possible with music signals only
Fig. 2 Load impedance vs. output power
(output voltage/output current)
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Instrumentation amplifi er and further refi ned MCS+ topology
Instrumentation amp confi guration allows
fully balanced signal paths
The newly adopted “instrumentation amplifier”
principle ensures that all signal paths from the
inputs to the power amp stage are fully balanced. This results in excellent CMRR (common
Signal input stage Power amplifi er stage
+ INPUT
– INPUT
Instrumentation amplifi er confi guration
n
Power amplifi er assembly
Power amplifier assembly
with six parallel push-pull
power MOS-FET pairs per
channel mounted directly to
large heat sink, MCS+ circuitry, and current feedback
amplifi er.
mode rejection ratio) and minimal distortion.
Another significant advantage is that external
noise and other external influences are virtually
shut out. The result is a drastic improvement in
operation stability and reliability.
Further refi ned MCS+ topology for even
lower noise
Accuphase’s original MCS (Multiple Circuit
Summing) principle uses a number of identical
circuits connected in parallel to achieve superior
performance characteristics. MCS+ is a further
refined version of this approach. By extending
parallel operation to the class-A drive stage of
the current/voltage converter, the noise floor
has been lowered further.
Current feedback principle assures excellent phase characteristics in high range
As shown in the illustration, the A-45 uses the
output signal current rather than voltage for
feedback. Since the impedance at the current
feedback point is very low, there is almost no
phase shift. A minimal amount of NFB therefore
results in maximum improvement of circuit
parameters.
– Input
Buffer
+ Input
Buffer
Principle of current feedback amplifi er
Current
adder
I-V
converter
Trans-impedance
amplifie
Current NFB
network
Amplifie
Output
Page 4
Connection example for bi-amping setup
Left loudspeaker
In a bi-amped setup, the speaker units for the
LOW frequency range and HIGH frequency
range are driven by separate amplifi ers, for
optimum sound quality.
The speakers must have a built-in crossover network
and separate inputs for LOW and HIGH range.
Two A-45 units are required.
Connection example for bridged setup
Right loudspeaker Left loudspeaker Right loudspeaker
In bridged mode, the A-45 becomes a monophonic amplifier with even higher power
output.
For stereo operation, two A-45 units are required.
A-45
for left
channel
Mode selector
of A-45 (DUAL
MONO position)
Preamplifi er
n Front Panel
n Rear Panel
Left/right channel output power meters
(dB and % scale)
Function indicators
METER –20 dB, UNBALANCED, BALANCED, BRIDGE
Meter operation/illumination button ON OFF
Meter sensitivity selector button (–20 dB)
Input selector button UNBALANCED BALANCED
Power switch
Gain selector
MAX –3 dB –6 dB –12 dB
A-45
for right
channel
m Use the LEFT (BALANCED
or UNBALANCED) input
connectors for both A-45
units.
Unbalanced inputs
Balanced inputs
Ground
Inverted (–)
Non-inverted (+)
Left/right channel speaker output terminals
Mode selector
DUAL MONO NORMAL BRIDGE
AC power supply connector
A-45
for left
channel
m Use the LEFT (BALANCED
or UNBALANCED) input
Mode selector of
A-45 (BRIDGE
position)
Preamplifi er
connectors for both A-45
units.
GUARANTEED SPECIFICATIONS
[Guaranteed specifi cations are measured according to EIA standard RS-490.]
m Continuous Average Output Power (20–20,000 Hz)
Stereo operation 360 watts per channel into 1 ohm()
(both channels driven) 180 watts per channel into 2 ohms
90 watts per channel into 4 ohms
45 watts per channel into 8 ohms
Monophonic operation 720 watts into 2 ohms()
(bridged connection) 360 watts into 4 ohms
180 watts into 8 ohms
m Total Harmonic Distortion Stereo operation (both channels driven)
0.05% with 2 ohm load
0.03% with 4 to 16 ohm load
Monophonic operation (bridged connection)
0.03% with 4 to 16 ohm load
m Intermodulation Distortion 0.004%
m Frequency Response At rated output: 20 ~ 20,000 Hz +0, –0.2 dB
At 1 watt output: 0.5 ~ 160,000 Hz +0, –3.0 dB
m Gain
(in stereo and monophonic operation)
m Gain Selection MAX, –3 dB, –6 dB, –12 dB
m Output Load impedance Stereo operation: 2 to 16 ohms
Monophonic operation: 4 to 16 ohms
m Damping Factor 200
m Input Sensitivity (with 8-ohm load, GAIN selector in MAX position)
Stereo operation: 0.76 V for rated output
0.11 V for 1 watt output
Monophonic operation: 1.51 V for rated output
0.11 V for 1 watt output
m Input Impedance Balanced: 40 kilohms Unbalanced: 20 kilohms
m Signal-to-Noise Ratio (A-weighted, input shorted)
115 dB (GAIN selector in MAX position)
120 dB (GAIN selector in –12 dB position)
At rated output
m Output Level Meters
–50 dB to +3 dB (indication in dB and %)
m Power Requirements AC 120 V/230 V, 50/60 Hz
(Voltage as indicated on rear panel)
m Power Consumption 235 watts idle
430 watts in accordance with IEC 60065
m Maximum Dimensions Width 465 mm (18-5/16")
Height 211 mm (8-5/16")
Depth 464 mm (18-1/4")
m Mass 30.9 kg (68.1 lbs) net
38.0 kg (83.8 lbs) in shipping carton
Note: Load ratings marked () apply only to operation with music signals.
28.0 dB (with GAIN selector at MAX)
With music signals only, 1-ohm loads are permissible for
stereo operation and 2-ohm loads for monophonic operation.
Logarithmic scale, with defeat switch, and sensitivity switch (–20 dB)
A-45
for right
channel
Remarks
This product is available in versions for 120/230 V AC. Make sure that the voltage shown on the rear panel matches the AC line voltage in your area.
The shape of the AC inlet and plug of the supplied power cord depends on the voltage rating and destination country.
n Supplied accessory m AC power cord
m
Specifi cations and design subject to change without notice for improvements.
http://www.accuphase.com
K0605Y PRINTED IN JAPAN 851-0158-00 (AD1)
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