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Flat Amplifier
The first stage of this 2-stage direct-coupled
flat amplifier is equipped with a FET for higher
input impedance. Ample NFB also permits an
lower output impedance. As a result, there is little
change in noise level when the VOLUME control
resistance is altered, and there is also sufficient
power to drive the NFB-type tone control circuits
connected to the following stage:
NOTE:
Since the input
impedance of the NFB tone control
circuits is relatiuely low, there will be a certain amount of
change when the uariable resistor is rotated.
Tone Controls
The SX-1280 features two pairs of tone control
circuits (twin tone controls), each with a different
turnover frequency. The main tone controls are
similar to other regular tone controls, while the
sub tone controls permit variation of the frequency
response at te very high and very low ends. Both
main and sub tone controls apply C-B feedback to
a single transistor amplifier, with frequency selec-
tion provided within the feedback loop, and the
frequency response altered when these frequencies
are changed.
All of these circuits may be by-passed by simply
putting the TONE switch into the OFF position.
sx-1 eao
Filter Circuits
The filters employed in this receiver are -12dB/
octave NFB-type active filters. By coupling the
emitter-follower to RC passive filters, peaks have
been obtained in the region of the cut-off fre-
quencies, thereby sharpening the "knee charac-
teristics" of the RC passive filters.
Power Amplifier (Fig. 6)
The first stage consists of a PNP dual transistor
differential amplifier (Qz ) which loads the current
mirror circuit (Qs) to obtain high stable gain even
in the high frequency range. The predriver stage
(Qa ) operates with the constant current circuit
(Qr ) as the load, amplifying the voltage to the re-
quired level. With a 3-stage Darlington connection
in the power stage, and a parallel push-pull circuit
in the final stage, a power output level of 185W +
185W/89 (20H2- 20kHz, T.H.D. less than O.OgVo\
has been obtained.
The purpose of Qt is to equalize temperature
changes in Qr. By keeping the input base potential
of Qz at a constant 0V, the Qz output neutral
point potential will remain steady despite changes
in temperature. Since this power amplifier is a DC
amplifier, any DC voltages appearing at the Q, in-
put base will
be amplified and passed to the
output. In order to avoid this, the drop in poten-
tial across R1 due to the Qz input base current, is
designed to be absorbed by the Qr base current,
thereby maintaining the Qu input base potential
constant at 0V.
OUT
To protection
circuit
+ B r
+ B z
Constant
current
load
circu it
Current
m i r r o r
circuit
F i g . 6 P o w e r a m p l i f i e r c i r c u i t
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