Hitachi HMA-8300 Service Manual page 7

DESCRIPTION OF THE NEW CIRCUIT
1. Dynaharmony (Series E Amplifier) output circuit
The level of the music source changes momentarily, the
percentage of high level (over 1/2 of peak value) is very
small, it is only less than 2% of the total music signals.
Large output amplifiers have been used conventionally
to playback at high levels without distortion; however,
in this case, high voltage is impressed to the output tran­
sistor, so the power consumption is large.
Studies have been made, how to obtain high output
powers without the increase of the output transistors'
consumption in order to improve the efficiency. Thus,
the new dynaharmony output circuit (Series E Am­
plifier) was developed.
The principle circuit is shown in Fig. 10. Q1 — Q4 are
drivers at the output stage, and Q6 and Q7 operate
with normal small input signals. When the input signal
is large, Q5 and Q8 operate in addition. When the posi­
tive input signal at normal level is impressed, Q2 and Q6
turn ON and current flows from the +Bi power source.
When the input signal level increases and reaches more
than +Bl , Ql and Q5 also turn on. At this time, since
the emitter potential of Q5 exceeds +Bl, the current
does not flow from +Bi but only from +B2. When the
input signal level decreases, Ql and Q5 are turned off,
current does not flow from +B2 and current is supplied
from +Bi again.
When the input signal is negative, Q2 and Q6 turn off
and Q3 and Q7 turn on. The operation, when input
signal is negative, is the same as that of positive.
2. Current mirror circuit
This set contains a current mirror circuit which drives
the output stage in push-pull operation to keep dis­
tortion low.
The current mirror circuit is composed of Q703, 704
705 and CR703 as shown in Fig. 11.
When a positive signal is impressed to Q704 and a ne­
gative signal to Q705, the collector current of Q704
and Q705 become as follows:
out the push-pull operation and can drive the output
stage with low distortion.
3. Constant current circuit
This constant current circuit stabilizes the bias of the
main amplifier circuit. In Fig. 11, Q706 functions to
make the sum of the emitter current of Q701 and Q702
constant and Q701 and Q702 operate as a complete dif­
ferential amplifier.
Assuming that the current flowing to R705 increases due
to fluctuation of the power voltage, etc., this current
is divided in two by Q701 and Q702, and the voltage
drop increases in R707 and R708. Next, the base po­
tential voltages of Q704 and Q705 increase, the sum of
the emitter current of Q704 and Q705 increases and the
voltage drop of R716 increases. The increase of the
voltage drop of R716 is fed back to the base of Q706,
the collector current of Q706 increases, and the col­
lector current of Q706 absorbs the surplus current from
R705 through R706. Also, when the current flowing to
R705 decreases, the base potentials of Q704 and Q705
decrease, whereby the collector current of Q704 de­
creases and the current flowing through R706 decreases.
The sum of the emitter current of Q701 and Q702 is
kept constant in this way. In addition, CR716 is the
diode used for temperature compensation of the idle
current; this is attached to the output transistors to
sense the temperature and compensate the idle current.
4. Idle current compensation circuit against power supply
voltage fluctuations
The circuit shown in Fig. 11 compensates the idle
current by feeding the power supply voltage through
R717 back to R716, the common emitter resistor of
Q704 and Q705, negatively.
Assuming that the power voltage begins to rise, the
current which flows to R716 through R717 increases
and the voltage drop in R716 increases.
This increase is fed back to the base of Q706, decreases
the emitter current of Q701 and Q702, and decreases
the base potential of Q704 and Q705. Accordingly, the
collector current of Q704 and Q705 decreases, and then
decreases. When the power supply voltage in­
Vidie
creases, the heat generation of each semiconductor
element increases because of increasing power con­
sumption, and then the voltage between base and e-
mitter lowers and the idling current increases even if
is constant.
Vidie
This circuit, however, lowers the Vidie as well as the
V as described above, so the idle current is kept con­
be
stant. When the power supply voltage drops, the circuit
operates in the reverse of the above to keep the idle
current constant.