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.STEREO POWER AMPLIFIER
. l30W/30W/ch. (PURE CLASS»A)
.MOS FET OUTPUT STAGE
.OPERATION CONTROL SWITCH
The Accuphase P7260 is a highly perfected stereo power amplifier,
which is the result of Accuphase's mind dedicated to eager pursuit of
sound quality and of the combined aggregation of Accuphase audio
MOS FETs, which are recognized as the most promising power
amplifier device, are employed in the output stage. In addition to
superior inherent characteristic of MOS FETs that there is no trace
whatsoever of notching distortion, it IS capable of swttching over operating
mode to pure Class-A operation in which crossover distortion is
It has a power output of 130 Watts per channel (Srohm load,
20#ZU,OOOHZ, 0.005% distortion) at Normal Operation, which provides
ample sound pressure even for lowest efficiency speakers, Despite the
output power of 30 Watts per channel at ClassVA operation, it will still
supply sufficient sound pressure for ordinary, medium efficiency
speakers rating of more than QOdB/W/m.
Pure Class-A operation of the P260 most strikingly exhibits its
effectiveness in high grade multi~amplification systems. For example,
superior quality reproduction will immediately become apparent when
it is utilized together with high efficiency horn speakers to handle
amplification of the mid and high frequency range channels of such a
Two pairs of MOS FETs are used in the parallel push~pull output
stage and the P-260 employs the extravagant "Accuphase Original
complementarysymmetry, push-pull driven DC amplifiers in every
stage so that they may possess further superior open-loop character-
istics, inherent characteristics before NFB application. The P260
promises highest quality reproduction with minimum sound coloration.
and especially transitional distortion is effectively prevented because of
these careful circuit design approach.
I CLASS-A OPERATION
The P-260 offers a choice oiitimiiotv
between Normal and Class-A opera-
tion with front panel switching.
Pure Class-A operation was pro-
vided in response to the endless
Search of dedicated audiophiles for
lurther sound perfection, and in
view of the fact that the electrical
characteristics of recent power
amplifiers have approached the
theoretical performance limitations
for normal Class-B or ClassAB
operation. (which is the commonly accepted practice for practically all
audio equipment on the market todayli
The P-260's Class-A operation is achieved in the orthodox manner
which calls for the operation slopes of pushrpull amplification devices
to overlap each other in perfect alignment. Current flow from the
power supply to the amplification devices is constant, and stability,
both operational and thermal, is maintained at all times,
Switching of the operational control system is achieved electronical~
ly as opposed to mechanical switching of bias resistors. The system is
built around 0, and Q| ,, as shown in the amplifier block diagram
of Figure i. O is an Opto-Coupler that controls transistor 0,
which controls the bias resistor R.
ClI , , which contains a photo transistor and a Light Emitting Diode
lLEDl does not operate and remains OPEN during normal operation.
Under this condition the Q, is ON and shorts the bias resistor R that
determines the normal operation of FETs,
When the Operation selector is switched to Class~A, current flows
through the Light Emitting Diode and activates the photo transistor
which turns 0, 0 OFF, and releases the short of the bias resistor R. This
causes ClassA operational current flow through all the transistors and
FETs that follow 0, 2, and, also at the same time, reduces the voltage
supplied to the power output MOS FET devtces by cutting the B
voltage of the power transformer. As for O, to 0,, , ClassrA operation is
used for them at all times.
High stability is achieved since the signal path line is not wired over
long distance for switching two operational modes, because of this
remote switching way. The OPERATION switch on the front panel
controls the selection of Normal or Class-A operation, and the LED
lamps will be coincidently switched over to indicate operation mode.
Fig.1 MOS FETs' SYMMETRICAL PUSHvPULL DC POWER AM(' >
2 M08 FETs USED IN POWER OUTPUT STAGE
MOS FETs, which are considered the most promising devices for
power output applications, are used in a parallel push-pull circuitry in
the final output stage. MOS FETs have many advantages over bipolar
and/or SIT (V-FETl devices, and greatly improve the high frequency
range characteristics by eliminating notching distortion. Also they have
a wide lrequency range and are very effective in minimizing harmful
transient intermodulation lTIMl distortion, Moreover, higher reproduc-
tion quality is possible when MOS FETs, which have high gain and are
voltage controlled devices, are used in the final stage because Class-A
operation for the driver stage, as well as all preceding stages, can be
adopted more readily.
COMPLEMENTARY-SYMMETRY PUSH-PULL DC
AMPLIFIER CIRCUIT IN EVERY STAGE
Figure 1 shows the block diagram of the P»260 which is built around
complementaryrsymmetry push~pull DC amplifiers in every stage. This
circuitry assures high stability amplification because of its superior
inherent characteristics before Negative Feedback (NFB). As a result,
very little NPR is required, which accounts for its very low distort'
ratio. The high stability of the coleementarysymmetry push
circuit also effectively helps to reduce TIM distortion, and contrib
greatly to improved sound quality.
Distortion data are shown in the last page. and Fig. 2 shows the IM
Distortion data under new IHF Standard Methods of Measurement
defined in 1978.
When plural signals closed each other in frequency cause non-linear
distortion, as well as harmonic distortion of each original signals, a third
signal of difference in frequency between each signals appears and
deteriorates the reproduction sound. This is the process of the IM
Distortion stated in the new IHF. This distortion is occasionally called
as BEAT DISTORTION. Even if original signals are out of the audible
range, such a distorted third signal comes to appear in the audible range
and it results in a deterioration of sound quality. This became a great
issue recently This kind of Ilvl Distortion is apart from the former
lntermodulation Distortion which is measured with the frequencies of
SOHz or 60Hz vs 7.000Hz and is called as SMPTEVIM,
Refer to Fig.2. It shows the distortion spectrum of the P-260 which
was measured under the condition at rated output, with input signals in
frequencies of 19kHz and 20kHz at the rate of 50 to 50. If the IHF-WI
Distortion has been caused in the P-260, a spectrum appears at the
frequencies of 1kHz,2kHz,3kHz and so on. The Fig. 2 attests the truth
that none of IM Distortion have been detected and any distortion are
out of measuring limit of 0,0027%
In addition, virtual elimination of DC drift at the output by the
utilization of dual transistors in an input differential circuit and rational
layout of components according to thermal considerations has resulted
with very stable, and pure DC amplifier characteristics of the P-260.