|
The AX901 is a high quality 2 channel amplifier designed for a wide
variety of applications in the video studio. With quality audio (ie.
VHS HiFi) becoming increasingly important in video productions, it has
become essential that the video producer use audio monitoring equipment
of a higher grade than is found in typical monitor/receivers. However,
traditional "professional audio" gear
has been beyond the budgets of many video producers, and they have opted
for consumer HiFi gear normally sold in HiFi shops and department
stores.
Now, JVC would like to bridge this gap by offering the right products at
the right price--through its Professional Video Dealers.
AX-901 Integrated Amplifier
AX-901 affords a wide dynamic range of 100dB. JVC Dynamic
Super-A and three Gm Circuits together ensure that you'll
attain life-like reproduction from any speaker system, with
extremely low noise at all volume levels. The AX-901 is a true
high-performance amplifier that is a perfect match for digital
programs or video post-production.
The JVC design philosophy for amplifiers.
In amplifier engineering, our goal is linear signal transfer for
superior in-use performance. We achieve this by concentrating on
three design aspects. They are:
1. Ensuring a "clean" signal - Direct-in/out connection
shortens the signal path for purer sound, Super-A removes harmful
switching distortion, and simplified 2-amp construction keeps the
number of active stages to an absolute minimum.
2. Reducing power-supply impedance - this is achieved by the
Direct Power Supply and double-locked power supply.
3. Improving actual performance - Gm Circuits expand usable
dynamic range considerably. Dynamic Super-A reduces (1) distor-
tion caused by speaker motion (by rendering counterelectromotive
force harmless), and (2) TIM (Transient Intermodulation) Distor-
tion.
JVC Dynamic Super-A and Gm Circuits:
The secrets behind the great sound and performance of our amp.
JVC completely eliminated transistor switching distortion with
Super-A; the result is smooth sound. Then we tackled the
speaker-amplifier interface problem by introducing Dynamic Super-A;
interaction between the speaker and amplifier is no
longer of concern. This year we have improved the real-life
in-use performance of this amplifier with our revolutionary new
Gm Circuits. The details of this amazing innovation are discussed
later.
Simplified Two-Amp construction: An important step in reducing
noise and distortion.
Two-amplifier configuration with class-A operation in both stages.
The basic consideration in the design of any audio amplifier is
to minimize the degradation of sound quality inevitable when
sound signals are amplified. One way this can be done is by simplifying
the signal path. Ordinary integrated amplifiers have
four amplification stages between input and output, starting with
the MC head amplifier if one is provided.
In contrast to this, the AX-901 employs a two-amplifier configur-
ation, the simplest possible, taking advantage of integrated
amplifier construction as far as possible. This consists of an
MC/MM DC servo equalizer amplifier and a high-gain Dynamic
Super-A DC power amplifier. In any operation mode, there is a
maximum of two amplifiers, even with MC input, between the input and the
speakers even when the tone control circuit is in line.
This arrangement, the simplest circuitry possible, makes a major
contribution to the ultimate in high-fidelity sound reproduction
achieved by the AX-901. With inputs via the TUNER, TAPE DAD, and
AUX jacks, the AX-901 is a pure DC power amplifier using a single
amplifier.
The amplifier features simplified construction using only two
active-stage blocks. One is the power amplifier with its
Gm Circuit (Gm Driver), while the other is the phono equalizer
amp.
This two-amp design means that no separate tone control amp
is used, since the tone control circuit is incorporated into the
high-gain power amp. As a result, audio signals retain their
clarity and crispness throughout their journey within the amp.
Direct-Power Supply and direct-in/direct-out connection:
A shorter signal path means a purer signal.
Careful planning has let us place the power supply (transformer and
capacitors) close to the amplifying circuits (this we call our "Direct
Power Supply"). In combination with high-capacity
transformers and high-density capacitors, it feeds abundant low-
impedance electrical energy to all amplifying circuits for tight,
rich sound reproduction. Our direct-in/direct-out connection
also contributes to better sound quality by trimming connection
leads to the barest minimum.
Gm Circuit
The integrated amplifier in our lineup features our new
Gm Circuits.
The Gm Circuit is essentially a noise reducer. Its electronic
circuit design converts voltage into current for amplification,
then reconverts the amplified current back into voltage. This
unique new process is best understood when you consider that a
transistor is inherently a current amplifying device, even though
it is usually used for voltage amplification. It is thus apparent that
when we use transistors in the manner in which they
perform best - as current amplifiers - a tremendous improvement
in actual performance is possible.
There are three different types of Gm Circuits - Gm Driver,
Gm Volume and Gm Selector. Three Gm Circuits are highlighted
in the AX-901.
Gm Volume
A typical digital program offers a signal-to-noise ratio of more
than 90dB, or a dynamic approaching 100dB. It's true that there
are a large number of amplifiers that clear the 90dB hurdle.
But in real-life performance, few amplifiers are able to maintain
the ratio at such a high level under all conditions: the ratio tends to
shrink as the volume is reduced from maximum to more
usual listening levels, because residual noise remains constant.
The culprit is the volume control, that is, a variable resistor
(potentiometer). It changes the signal-to-noise ratio depending
on its setting. The ratio is highest only at its maximum; at
other settings - including those most frequently used - the ratio
is poorer.
Gm Volume has the ability to amplify signals and adjust their levels
from minimum to maximum. It lets you enjoy the highest
signal-to-noise ratio at the maximum setting. Yet at other
settings it actually reduces noise as the volume is reduced.
Since Gm Volume allows the amplifier to deliver always optimum
gain, it provides a signal-to-noise ratio that is about 25dB
better than that provided by norma volume control designs.
Moreover, the volume control itself is not in the signal path,
therefore sound quality cannot be degraded by distortion and
crosstalk caused by poor mechanical contact resulting from
corrosion.
Gm Selector
Gm Selector cuts the redundant gain (hence residual noise too) of
the power amplifier in three steps -0dB, -6dB, -6dB and -12dB.
The loss in gain results in a substantial gain in performance:
the signal-to-noise ratio is improved by 5 to 10dB. Hum and
noise do not pose a problem even if the overall volume is set at
a very, very low level.
Gm Driver
Gm Driver improves the real-life performance of the amplifier by
driving the power stage at constant voltage to reduce output
impedance and achieve flat frequency response. As a result, the
effects of counterelectromotive force and variations in speaker
impedance are made harmless, while at the same time, distortion
due to the inherent nonlinearity of power transistors is reduced.
Dynamic Super-A
Dynamic Super-A: For low-distortion performance throughout the
audible frequency range.
Dynamic Super-A is a refinement of our revolutionary Super-A, the
design that offers the distortion-free sound of a class-A amp,
and the efficiency of a class-B amp.
Popular class-B amps offer high efficiency, but they achieve it
by switching the output transistors on and off. This results in an
increase in distortion - switching distortion. Unrelated
harmonically with music, switching distortion adds a harsh,
metallic or gritty quality to the sound you hear. The alternative is
the class-A amp which offers smooth low-distortion
switchless sound, yet it requires its output transistors to be
always on. This means low efficiency, bulky size and high cost.
JVC Super-A is the answer to this dilema - it combines the
positive aspects of both class-A and class-B amps. It does not
generate switching distortion (like class-B amps). It gives
sound a smooth, yet powerful quality.
Dynamic Super-A was developed with the singular purpose of
catching up with vast improvements that have recently been made in the
dynamic range of new digital technologies.
1. Isolated drive circuit
Many conventional amps use a "constant-current dirver" to
interface the predriver with the power output state. However, the high
impedance of the predriver creates current distortion.
It's possible to reduct its impedance (and current distortion), but only
at the expense of open-loop gain. This means less
negative feedback is applied, which results in increased
distortion.
JVC has found a way to reduct current distortion and improve
open-loop response; the answer is the isolated drive circuit.
It electrically isolates the predriver from the power output
stage by means of an FET. This allows the predriver to drive the
power stage at constantly low impedance, thereby eliminating
current distortion.
The influence on the power amp on counterelectromotive current fed back
from the speakers is reduced.
The power amp should be able to render insignificant the effect of
counterelectromotive current, current created by the motion of
the speaker cone that is fed back to the amplifier. This is because
counterelectromotive current decrease the amp's ability
to effectively control speaker cone movement, thereby making
sound dull and lifeless.
Our power amp has the capacity to reduce the effects of this spurious
current, because its isolated drive circuit drives the amp with low
output impedance. Further, thanks to the Super-A bias control circuit
and the wave correction processor, linearity is much improved.
2. Wave correction processor
The basic function of the wave correction processor is to improve the
open-loop response of the amplifier. In combination with Super-A, it
successfully reduces open-loop distortion in the power amplifier.
|
