Pure class-A power amplifiers with rich expressions are
irreplaceable products; they take audiophiles' hearts andwon't let go.
Accuphase has been developing over 20 models of pureclass-A power amplifiers reflecting the latest technologies
on a moment-to-moment basis.
The A-300 is the ideal power amplifier that aims tosurpass the sound quality and electrical performance of
the A-250 launched in 2017.
The A-300 achieved the unprecedented low noisecharacteristic and high output power with the latest
technologies and accumulated knowledge cultivated in
the flagship model development.

1Unit dimensions and weight are the same as the former
model A-250.

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The digital power meter shows the actual output power
without any effects of speaker impedance.
An LED bar graph displays output power calculated fromthe output voltage.
The gain selector on the front panel allows the user toselect the appropriate gain level from four settings: "Max,-3dB, -6dB, and -12dB ".
This gain switching is achieved by controlling the gain of
the input amplifier section rather than attenuating theinput signal with an attenuator.
Therefore, the A-300 can reduce the residual noise by
selecting a lower gain position.
Two pairs of large speaker terminals offer beneficial biwiring connections with loudspeakers.

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It contains an intense power supply, a massive specialmade high-efficiency toroidal transformer, and two
sizeable 100000μF special-made filtering capacitors in
the unit's center.
The major difference between the A-300 and the A-250 is
the position of the power transformer and filtering
capacitors. The A-300 placed the power transformer
away from the input amplifier, which helped to minimize
the influence of leakage flux from the power transformer.
Transformer
Capacitor

A-250
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The A-300 enhanced the class-A operation area by 25%,
as 125W into 8Ω load.
Rated output power is double when the load impedance
becomes 1/2; it reaches 1kW into a 1-ohm load.

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The former model A-250 has excellent noise
performance; however, the A-300 achieves even 20%
lower guaranteed output noise voltage than the A-250.
The A-300 is the lowest noise power amplifier in theformidable Accuphase power amplifier lineups.

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The A-300 achieves extraordinary noise characteristics
through the following technologies.
Two amplifiers, consisting of balanced input amplifierparts, are operated in parallel, which contributes toideally decreasing the output noise to 70%.
Discrete amplifiers with low-noise devices are also
effective for low-noise characteristics.
The Balanced Input amplifier is placed as close to the
input terminal as possible and succeeds in minimum
external noises.

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The A-300 has a guaranteed Damping Factor of 1000. It
is the same as the A-250, but the actual measured value
is over 2000, which is 43% higher than the former model.
*Damping-Factor, DF:
An index of speaker driving ability. A Higher DampingFactor amplifier has higher speaker driving ability.
DF = 8Ω / Output-impedance8Power MOS-FET devices in a ten-fold parallel push-pull
configuration halve the output impedance.
The gold-plated glass cloth fluorocarbon resin print circuitboard helps to improve lower impedance and reliability.
The gold-plated big bus bars are on the paths wherelarge current flows. They contribute to the lower
impedance for the circuits and allow ample output powertransmission to the loudspeakers.
For the A-300, a more heavy-duty MOS-FET device isused in the final power amplification stage than the A250 one.
*Current capacity of power MOS-FET
A-250 MOS-FET: 10A
A-300 MOS-FET: 33A

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Remote sensing is the technique to lower the amplifier's
output impedance by the negative feedback with signal
sensing from nearby the speaker terminals.
Balanced remote sensing is also the technique to lower
the output impedance by both the signal sensing and theGND senses, that is, the negative feedback of the GND
level.
Balanced remote sensing improves not only the Damping
Factor but also the Total Harmonic Distortion and theIntermodulation Distortion.

10A mechanical relay is a typical speaker protection
component, but its contact resistance is higher thanpeople think.
Therefore, Accuphase has chosen the MOS-FET switch
instead of conventional mechanical relays for speakerprotection. Thanks to this MOS-FET switch, the damping
factor, reliability, and sound quality are all improved.
The A-300 adopts a new MOS-FET, with a very low onresistance of 1.6mΩ. (A-250 used 2.0mΩ on-resistance
MOS-FET.)
The A-300 also employs carefully-selected very lowimpedance components such as large speaker terminals,
rectangular wire coils, etc.
Making signal paths thick and short helps to attain low
impedance as well.
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The newly designed output protection circuit can detect
any short-circuiting of the speaker terminals with due
consideration for product safety.
The A-300 adopts temperature sensors that detect theheatsink temperature; thanks to this, the unit accurately
ascertains the high-temperature alarm in the poweramplifier module.
Keeping the negative influence to a minimum, the hallelements detect the excessive current when the output isshort-circuited.
*When these protection circuits are activated, the unit
completely interrupts the speaker output and makes thepower meters flash to indicate the abnormal condition.

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