Luxman L 80 L 81 V Service Manual

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L SOLID STATE INTEGRATED AMPLIFIER

SERVICE MANUAL

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Equalizer Amp. Section

Adopted is an Operational I.C., RAYTHEON RC-HSSB-DN, which is of 8-pin Dual In-Line
Package Type. .A built-in phase compensation capacitor for high frequency makes it impossible
to adjust the value even for the R 5 D works. This contirbutes so much to the various
characteristics and sonic quality. Fundamentally at the negative feedback amplifier,
especially at the equalizer amplifier, the high frequency phase compensation should be kept
in proper condition. When the compensation is too weak, the circuit becomes instable and in

many case oscillation is inevitable. In such state, the sonic quality is out of discussion.

0n the contrary, if the phase compensation is too strong, the distortion at high
frequency range is much increased and at the samt time it affects sonic quality to a great
extent. That is why the input impedance is reduced by the high frequency phase compensation
(e.g., Mirror Integration), which is indispensable to the multi-stage amplifier, and linearity
of the former stage is affected to deteriorate the distortion characteristic. The capacitor

inserted between Q6 and Q7 is for high frequency phase compensation.

To comply with the unique gain distribution of the L-80V, we considered a semi-conductor
device which offers more inherent gain, comparing with the conventional 3-stage E-E Feedback
type equalizer. The 1.0. offers more than lOOdB of inherent gain, and the loop gain at lKHz
is approximately 37dB, which ensures sufficient amount of Negative Feedback at low frequency
range. The RC-4558-dN is carefully selected to fulfill no more than l.5uV Input-Conversion
Noise Voltage. Despite that the phase compensation is included, proper compendation is realized
as well as the stability, and therefore any type of cartridge can be connected. As for the

load condition, the I.C. circuitry exceeds the conventional 3-stage E-E Feedback Circuitry.

Power Amp. Section

Adopted is the fully complementary circuit configuration, which seems to be the most ideal
one at present. Signals are supplied form the equalizer amp directly to the power amp section
via buffer stage. The rated output of 50W/ch is ralized at lQOmV of equalizer output voltage
(Input Sensitivity 2.8mV). This means the voltage gain is approximately SQdB, which is higher
by some 6dB than that of standard power amplifiers. And naturally various problems must be
considered. First, the harmonic distortion, especailly at high frequency range, tends to be
worse. In actuality, distortion at lOKHz is twice as bad as that of the amplifier having some

33dB voltage gain. This is of course in the case of using the same semi-conductor device.

To compensate the lost gain caused by applying Negative Feedback, it is necessary to
increase the inherent gain by studying the inherent characteristics. At the first differential
input stage, it is of utmost necessity to reduce the DC offset voltage at the output terminal,
and therefore required are transistors of matched hfe characteristic, and of high hfe at the
operational current area. For the L-BOV, adopted is that of SdB allowance between minimum and
maximum. The standard hfe value is 500, which is very high. Also at this stage a zener diode

is arranged to deal with the mains power fluctuation.

Second differential Amp. Stage

This stage plays an important role to decide distortion ratio, especially at high frequency
range. Fundamentally transistors of high fT and low Cob are necessary, and high load
impedance should be realized since most of the voltage gain depends on this stage. Therefore
inherent gain is obtained sufficiently up to high frequency range thanks to constant current
drive. The fT of the transistors is more than 130MHz (Ic = lOmA), and the Cob is less than
2pF, which is far above the audio frequency band. Nevertheless from the View point of fee, the
fee is 75kHz in case hfe is determined as 200. Thus such high frequency characteristic is

indispensable.