Accuphase F 15 L Brochure

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Accuphase F 15 L Brochure

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Both Balanced and Unbalanced Input/Output Circu'ts Provided,

High Effectiveness Electronic Frequency Dividing Network

The "multi-amplification system". in which the
crossover frequencies of a mum-amplification
system are generated by electronic circuits and
the speaker of each frequency range is directly
driven by a dedicated power amplifier, has been
recognized by many audio enthusiasts as the
ultimate system with which one can really relish
the zest of audio. Since its advent, it has gained a
solid foothold in the audio world, strongly
supported by people who prefer an original
speaker system of their own.

Although the multivamplification system tends to
be large and bulky because it calls for as many
power amplifiers as the number of ranges into
which the frequency is to be divided, it can ex-
tract the peak performance of the speaker,
eliminating adverse effects of the LC network on
the sound quality, Especially today, when digital
program sources are dominant, it is believed that
this system will increasingly make its superb
quality feltl

In 1976, in its pursuit of the ”ultimate audio
system", Accuphase developed a Frequency Di-
viding Network, Model F-5. which was designed
to be the heart of a multi-amplification system, to
the acclaim of a large number of audio fans. The
F-5 allowed the crossover frequency to be set
and changed by merely plugging in a printed
circuit board into a slot on the front panel, so that
the desired frequency and attenuation charac-
teristics could be consistently obtained. In addi-
tion, carefully selected circuit elements improved
the sound quality up to the virtual theoretical
limits, In 1981 , the F-5 was restyled into the F-15,
in which the filter unit amplifiers were made up of
discrete components to form push-pull circuits.
The F-15L is the result of the second model
changeoverl This Frequency Dividing Network is
provided with new balanced input/output circuits
and improvements on the features of the F-15.
The functions of the power supply have been
reinforced, and each element has been reviewed
to achieve effective balanced signal transmis-

Among its many features are fine adjustment of
the attenuation in 0.5 dB units, capability of creat-
ing a subwocfer system, and compatibility with
the crossover boards of the F-5 and F-15, to
name but a few,

Accuphase believes that this new Frequency
Dividing Network completely satisfies the needs
and demands for high quality.

Frequency Dividing Network

1 Both Balanced and Unbalanced
Input/Output Ci 'ts Provided
The F—15L Frequency Dividing Network is equip-
ped with a complete set of input/output circuits
for balanced signal transmission. Figure 1
shows the operating principle of the balanced
signal transmission. Two symmetrical signals,
each having the same voltage as the other but
opposite phase. are generated and transmitted
along a threeecore cable from equipment to
The ground line (G) only connects the ground
potential of the signals. No signal current flows
through this line. Consequently. noise compo»
nents the cable may pick up are canceled out
and do not appear in the output signals. Moreov-
er. noises from the power lines and sources can
be effectively suppressed. As a result, only pure
signals can be transmitted. This balanced signal
transmission method is widely employed in au-
dio equipment for industrial and professional
use in studios and broadcasting stations.
In addition to the balanced input/output
features, conventional. unbalanced input/output
RCA phono jacks are also provided.

Low-Noise, Wide—Band Unlt
Amplifiers of FET-Input.
Complementary Push-Pull
The main constituents of the Frequency Dividing
Network are active filter amplifiers and buffer
amplifiers. Figure 2 is a block diagram showing
the internal circuitry of the F-15L. As can be
seen, all the unit amplifiers are of the same 1-
gain configuration. In this figure, those num-
bered 2 to 5 are the active filter amplifiers that
are responsible for the crossover frequency
characteristics and are a combination of resis-
tors and capacitors,
Those numbered 1 and 6 to 11 are the buffer
amplifiers, They serve as buffers to prevent in-
terference from the circuit stages preceding and
following them.
As Figure 2 suggests, the performance and
sound quality the Frequency Dividing Network
can offer depend heavily on the quality of the
resistors and capacitors that form the unit am»
plifier and filter characteristics. Unless the quali»
ty of these elements is beyond reproach. having
a multi—amplifier system is meaningless.
A unit amplifier must have a simple signal trans-
mission path and provide superiorperformance.
The unit amplifiers of the F-15L form a pushvpull
circuit as Figure 3 shows. and its input is con—
figured as a low-noise, high~gm FET com-

Powor Amnlifiir

Fig. 1 Principle of Bullhead Typo Network

plementary, source» follower circuit. The output
is a complementary push-pull circuit consisting
of wide-band transistors.

This output circuit features no loop feedback.
while consisting of carefully selected elements.
and provides virtual upperelimit characteristics
that have been achieved by the pushvpull and
local feedback methods. The drains of Q, and
Q; are respectively connected to the emitters of
06 and 05. to prevent the Miller effect and to
suppress distortion over a wide band,

3 DC Servo Output Unit Amplifiers

The unit amplifiers of the F-15L excel in stability.
and their gain is 1, effectively eliminating "
possibility of DC drift. The occurrence of em
very slight DC drift at the stages preceding the
attenuators can cause noise while the level is
being adjusted. Moreover. drifts in the output
stage can produce DC components in the out»
put of a DC power amplifier. which must be

To prevent the DC components from generating,
and to stabilize the characteristics. the F-15L
has DC servoamplifiers as buffer amplifiers at
the stages following the filter amplifiers. Those
numbered 6 to 11 in Figure 4 are such DC
servoamplifiers. The circuit shown in this figure
is basically the same as the one in Figure 3
except that servoamplifiers IC, and ICE are ad-
ded in Figure 4. The time constants of these
servoamplifiers are determined by C, to C4, and
R. to R4.

This arrangement enables all the unit amplifiers
to operate as DC amplifiers without a coupling
capacitor, to improve the overall sound quality,

4 High-Precision Level Controls ,,
Adjustable In 0.5 :13 Units

Three level controls, LOW, MID, and HIGH, nor-
mally adjusted in units of 1 dB are provided to
each of the left and right channels. The level of
each frequency range can therefore be control-
led on the front panel of the Frequency Dividing
Network. without relying on the power amplifiers.
Whereas the attenuators can be adjusted in 1 dB
units. the —O.5 dB Shift Switch provided on the
right of each of the six level controls allows
adjustment in units of 0.5 dB. Therefore, the
attenuation can be adjusted to any of as many as
41 points: from 0 to ~20.5 dB. and —oo.

Crossover Frequency Easily

Changeable by Exchanging

“Crossover Boards“
The crossover frequency can be easily changed
by exchanging "crossover boards", each of
which has a specific crossover frequency. The
block marked "CFIOSSOVER BOARD" in Figure
2 denotes these boards. The resistors and
capacitors necessary for both the left and right
channels are integrated on a single board. For
these elements that have important influence on
the sound quality, high-precision metal film re-
sistors and high»quallty silvered mica
capacitors are employed to prevent degrada-