m Pure Class A operation delivers quality power: 30 watts × 2 into 8 ohms

m Power MOS-FET output stage features triple parallel push-pull

configuration m Instrumentation amplifier principle m Further improved MCS+
circuit topology m Current feedback combines operation stability with
outstanding sound m Bridged mode allows upgrading to true monophonic
amplifier m Massive transformer rated for 400 VA m 4-step gain control
 Pure Class A power amplifier using power MOS-FET devices Fully
balanced signal paths as found in high-quality instrumentation
amplifiers. Further refined MCS+ topology and current feedback design
result in even better S/N ratio, distortion, and other performance
parameters. Robust power supply and power MOS-FET devices in triple
parallel push-pull configuration sustain an amazing 150 watts per
channel into ultra-low 1-ohm impedance loads (with music signals).

Pure Class A power amplifiers from Accuphase amplifier is running hot. Driving these devices in Instrumentation amp configuration allows fully
have long been blending the purity of class A pure class A assures high-definition sound that balanced signal paths
operation with the superior performance of power brings out the finest nuances in the music.
MOS-FETs. While building a string of outstanding In pure class A operation, the power supply delivers
amplifiers, Accuphase has accumulated a store a steady current, regardless of the presence or The A-30 features a new “instrumentation amplifier”
of technical know-how that is second to none. The absence of a musical signal. This means that the principle whereby all signal paths from the inputs to
latest in this series, the A-30 is a pure class A amplifier remains unaffected by fluctuations in the power amp stage are fully balanced. This results
stereo power amplifier based on the advanced voltage and other external influences. On the other
Signal input stage Power amplifier stage
technology of the model A-60. hand, it also means that the output stage generates
The A-30 employs the so-called instrumentation considerable thermal energy. In the A-30, this is + INPUT +
amplifier principle throughout. The signal handling dissipated effectively by large heat sinks which –

stages feature further improved MCS+ topology provide ample capacity to remove the heat NFB
NETWORK

and the renowned current feedback approach. This produced by the internal circuitry. GAIN CONTROL
+
OUTPUT
has resulted in electrical characteristics that The heavy-duty power supply easily sustains CIRCUIT
–
surpass even the demanding standards set by its output levels of 120 watts into 2 ohms, 60 watts NFB
NETWORK

predecessors. Employing only highest grade into 4 ohms, or 30 watts into 8 ohms (per channel). –
materials and strictly selected parts, the A-30 This linear progression demonstrates that the – INPUT +

pursues the two most important goals of an amplifier will be capable to drive even speakers
Instrumentation amplifier configuration
amplifier: very low output impedance (Note 1) and with very low impedance ratings or with
constant drive voltage (Note 2). pronounced impedance fluctuations. Stability
The output stage of the A-30 features power MOS- remains excellent at all times. The amplifier also in excellent CMRR (Common Mode Rejection Ratio)
FET devices renowned for their excellent sound has the necessary reserves to handle musical and minimal distortion. Another significant advantage
and superior reliability. Because they have negative transients that require considerable power in an is that external noise and other external influences
thermal characteristics, there is no danger of instant. If even higher power is required, bridged are vir tually shut out. The result is a drastic
thermal “runaway” as exists with bipolar transistors. mode turns the A-30 into a high-output improvement in operation stability and reliability.
Operation remains totally stable even when the monophonic power amp.
Further refined MCS+ topology for even lower
noise
Note 1: Low amplifier output impedance
The load of a power amplifier, namely the loudspeaker, generates a
Accuphase's original MCS (Multiple Circuit Summing)
counterelectromotive force that can flow back into the amplifier via the
NF loop. This phenomenon is influenced by fluctuations in speaker
impedance and interferes with the drive performance of the amplifier.
The output impedance of a power amplifier should therefore be made
as low as possible by using output devices with high current capability.
This absorbs the counterelectromotive force generated by the voice coil
and prevents the occurrence of intermodulation distortion.
Output current (A)

Note 2: Constant drive voltage principle

Even in the presence of a load with wildly fluctuating impedance, the
ideal power amplifier should deliver a constant voltage signal to the
load. Figure 2 shows the results of actual output voltage/output current
measurements at different load impedances for the A-30. It can be clearly
seen that output voltage is virtually constant at various loads, which
means that current increases in a linear fashion. Actual measurements Output voltage (V) principle uses a number of identical circuits
of clipping power have yielded the following figures, which impressively ★1-ohm rating is for music signals only.
connected in parallel to achieve superior performance
demonstrate the more than ample performance of the A-30: 1 ohm: 175 Fig. 2 Load impedance vs. output power
watts, 2 ohms: 142 watts, 4 ohms: 94 watts, 8 ohms: 58 watts.
characteristics. MCS+ is a further refined version of
(Output voltage/output current)
this approach. Improvements in the bias circuitry of
the input-stage buffer amplifier result in greater
stability. This in tur n makes it
possible to extend the parallel
MCS+
( Multiple
Summing )
Circuit
Bias stabilizer operation approach to the class A
circuit
+ B3 drive stage of the current/voltage
REGULATOR + B1 converter, thereby further lowering
Q1
Q5 Q9 Q 13 the noise floor.
+ B2
Q 17 Q 19 Q 21 Q 23 Q 25
+ INPUT + Q7 Q 11 Q 15
Power MOS-FET output stage
Q3
– with triple parallel push-pull
power units delivers 120 watts
NFB
NETWORK Bias stabilizer
circuit into 2 ohms, 60 watts into 4 ohms,
or 30 watts into 8 ohms with
NFB
OUTPUT
GAIN CONTROL
CIRCUIT
NETWORK
outstanding linearity
Bias stabilizer Bias stabilizer
circuit circuit

NFB
The output stage (Figure 1) uses
NETWORK
Q2
power MOS-FETs. Three pairs of
–
Q6 Q 10 Q 14
Q 18 Q 20 Q 22 Q 24 Q 26
these devices are arranged in a
– INPUT + – B2 parallel push-pull configuration for
Q8 Q 12 Q 16
each channel. The result is stable
Q4
operation with ideal power linearity
REGULATOR – B1
– B3
even down to ultra-low impedances.
Bias stabilizer
circuit
The maximum power dissipation of
one MOS-FET is 130 watts, but the
Fig. 1 Circuit diagram of amplifier section (one channel)
actual power load per pair is much
 coupled with housed in an
utterly natural enclosure filled with

(Large)
e n e r g y a material that
b a l a n c e. transmits heat and
Figure 4 shows absorbs vibrations.

Gain
frequency This completely
response for prevents any
Frequency (High)
different gain adverse influences
Fig. 4 Frequency response with current feedback
lower, so that each device is driven only in its low- settings of the (Response remains uniform also when gain changes) on other circuit
power range where linearity is excellent. c u r r e n t parts.
A music signal consists of a continuous succession feedback amplifier. The graphs demonstrate that Two aluminum
of pulse waveforms. To prevent clipping on occasional response remains uniform over a wide range. electrolytic
momentary high-level pulses, the maximum clipping capacitors rated for
level of the A-30 is set to 50 watts per channel into 8 Robust power supply with large power 47,000 µF each
ohms (sine wave output). transformer and high filtering capacity serve to smooth out
the pulsating direct
current from the
Current feedback circuit topology assures In any amplifier, the power supply plays a vital role
rectifier, providing
excellent phase characteristics in high range since it acts as the original source for the output
more than ample
delivered to the speaker. The A-30 employs a large
filtering capacity.
and highly efficient 400 VA power transformer. It is
In the A-30, the signal current rather than the more
conventionally used voltage is used for feedback.
Since the impedance at the current feedback point n Power amplifier assembly with three
parallel push-pull power MOS-FET
(current adder in Figure 3) is very low, there is almost pairs per channel mounted directly
to large heat sink, MCS+ circuitry,
and current feedback amplifier

Current
– Input adder Output
Buffer I-V Amplifier
converter
Trans-impedance
+ Input amplifier
Buffer
Current NFB
network

Fig. 3 Principle of current feedback amplifier

no phase shift. Phase compensation

therefore can be kept at a
minimum. A minimal amount of
NFB results in maximum
improvement of circuit
parameters. The result is
excellent transient
response and superb
sonic transparency,
Instrumentation amp type gain control minimizes
n Easy switching between dual mono/stereo/bridged
residual noise
mode
In the dual mono position of the
The gain control switches gain in the first amplification mode selector, the left-channel
signal is supplied to both speaker
stage that uses an
terminals, which can be used for bi-
instrumentation amp amping with dedicated low-range
design. Four settings are and high-range drivers.
available (MAX, –3 dB,
n Oversize speaker terminals also accept Y lugs and
–6 dB, –12 dB). With this Unbalanced and balanced input connectors
banana plugs
approach, reducing gain
The sturdy terminals can
also results in reduced handle even very large
noise. This is especially gauge speaker cable.
beneficial when using highly efficient speaker The connectors are
systems where the noise floor could be a problem. made of extruded high-
purity brass material and
are gold-plated for ut-
Bridged connection allows upgrading to a true most reliability and mini-
mum contact resistance.
monophonic amplifier with 300 watts into 2 ohms
Y lugs and banana plugs can also be used.
(music signals only), 240 watts into 4 ohms, and
n Large analog power meters Protection and meter circuit assembly
120 watts into 8 ohms
n Balanced connection prevents induced noise
Operation in bridged mode results in a monophonic n PCB copper foil and all major signal path components
amplifier with four times the power output than during are gold-plated
stereo operation. This gives effortless dynamic
n Buttons for meter on/off, meter sensitivity, and input
power. selection

High-quality, high-reliability parts

n Front panel
GUARANTEED SPECIFICATIONS
[Guaranteed specifications are measured according to EIA standard RS-490.]
m Continuous Average Output Power (20 - 20,000 Hz)
Stereo operation 150 watts per channel into 1 ohm (✽)
(both channels driven) 120 watts per channel into 2 ohms
60 watts per channel into 4 ohms
30 watts per channel into 8 ohms
Monophonic operation 300 watts into 2 ohms (✽)
(bridged connection) 240 watts into 4 ohms
120 watts into 8 ohms
Note: Load ratings marked (✽) apply only to operation with music signals.

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)
0.05%, with 4 to 16-ohm load
n Rear panel
m Intermodulation Distortion 0.005%

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 28.0 dB (GAIN selector in MAX position)

(stereo and monophonic operation)

m Gain Settings MAX, –3 dB, –6 dB, –12 dB

m Output Load Impedance Stereo operation: 2 to 16 ohms

Monophonic operation: 4 to 16 ohms
[★ With music signals, load impedances of 1 Ω (stereo) or 2 Ω (mono) can be driven.]

m Damping Factor 150

m Input Sensitivity (with 8-ohm load, GAIN selector in MAX position)

Stereo operation: 0.57 V for rated continuous average output of 30 W
★ 0.11 V for 1 watt output
Monophonic operation: 1.14 V for rated continuous average output of 120 W
0.11 V for 1 watt output
★
m Input Impedance Balanced: 40 kilohms Unbalanced: 20 kilohms
A Right/left-channel output power meters (dB and % scale) H Unbalanced inputs
m Signal-to-Noise Ratio 111 dB (GAIN selector in MAX position)
B Function indicators I Balanced inputs (A-weighted, input shorted) 118 dB (GAIN selector in –12 dB position) at rated output
METER, –20 dB, UNBALANCED, BALANCED, BRIDGE a: Ground
C Meter operation/illumination switch m Output Level Meters –40 dB to +3 dB (dB/% scale)
ON OFF b: Inverted (–)
Logarithmic scale, with switching capability for meter operation and sensitivity
D Meter sensitivity selector button (–20 dB) c: Non-inverted (+)
E Input selector button UNBALANCED BALANCED J Mode selector m Power Requirements AC 120 V/230 V (Voltage as indicated on rear panel) 50/60 Hz
F Power switch DUAL MONO NORMAL BRIDGE m Power Consumption 170 watts idle
G Gain selector K Right/left-channel speaker output terminals 260 watts accordance with IEC 60065
MAX –3 dB –6 dB –12 dB L AC power supply connector★
m Maximum Dimensions Width 465 mm (18-5/16”)
Remarks Height 170 mm (6-11/16”)
★ This product is available in versions for 120/230 V AC. Make sure that the voltage shown on the Depth 425 mm (16-3/4”)
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 m Mass 21.5 kg (47.4 lbs) net
destination country. 26.0 kg (57.3 lbs) in shipping carton

n Supplied accessories: • AC power cord

• Specifications and design subject to change without notice for improvements.

http://www.accuphase.com/ K0405Y PRINTED IN JAPAN 851-0139-00 (AD1)