With the
remarkable advance in audio technology achieved within recent years, and the
ceaseless striving of an elite team of engineers devoted to the task of
producing perfect sound reproduction equipment, Technics has succeeded in
achieving very high standards in its audio equipment, as can be seen in the
case of the ST-8080 stereo tuner. This high standard will inevitably become the
norm in future years, but that is hardly welcome news for audio enthusiasts
looking for the best their money can by now.
The ultimate
aim in any top grade stereo tuner is the perfect reproduction of the original
waveform of the radio signals transmitted from the broadcasting station. This
is certainly no easy task, since, besides other things, this involves the
complete banishment of all interference and noise signals and the ability to
mainta in stereo separation at exactly the same level as in the original
signals.
The waveform
of the signals comprising any musical expression is usually very complex, and
cannot be used effectively to point out the differences between original and
reproduced signals. In normal analysis, the tuner's ability to reproduce these
signals is shown by using simple square wave and tone burst signals.
The results
shown here illustrate the marked differences between the ST-8080 (in which the
ultimate design priority has been placed upon waveform fidelity) and a
conventional tuner (conspicuous for the lack of emphasis on waveform).
Photograph
A.
The results
shown in this photograph highlight the difference in response to a composite
signal of a 50Hz square wave signal plus 1kHz sine wave signal. The very steep
"sag" in the square wave output signal from the conventional tuner is
indicative of the poor amplification phase characteristics in the low frequency
region (which introduces distortion into the signal), while the depressed
sections at the start of the 1kHz sine waves signify a very small dynamic range
at lower frequencies.
Photograph
B.
5kHz and
15kHz sine waves are modulated and demodulated independently, but are shown
here (upper half of photograph) on the same time axis in order to reveal any
change in time relation between the waves. The output of the ST-8080 shows no
time change whatsoever, while the 5kHz and 15kHz signals passed through the
conventional tuner have arrived at the output at different times. lf these 2
signals are then combined( lower half of photograph) the loss of waveform
fidelity in the conventional tuner is even more evident.
High
Sensitivity Front-End (10.8 dBf)with 2-Stage RF Amplifier and 4 -Ganged FM
Variable Tuning Capacitor.
The adoption
of two low-noise 4-pole dualgate
junction FETs and linear 4-ganged FM variable tuning capacitor in the 2-stage RF amplifier stage is largely responsible for the very high sensitivity in the front-end.
Usable
sensitivity is 10.8dBf (or 1.9 µV according to IHF '58) at 300 Ω antenna input impedance, while the 50dB quieting
sensitivity (stereo) is 34.3dBf (or 28.4 µV
IHF '58) at 300 Ω. With this
kind of sensitivity, together with the greatly reduced interference signal levels, even very remote FM stations are
received with great clarity and fidelity.
Of special
importance is the suppression of the interference caused by RF intermodulation,
and the consequent improvement in selectivity in the more crowded sectrons of
the FM band. The RF lM interference rejection curves are shown here in graph
form.
Naturally,
the higher the values( in dB) the better, but such values are very difficult to
obtain without sacrificing sensitivity in reception of distant stations.
Albeit, as can be seen in the graph, Technics has succeeded in attaining avery
high standard indeed, and this without compromising sensitivity at all.
New
Developments in Local Oscillator and Mixer Stage Further lnprove Stability and
S/N Ratio.
The greater
overall stability of the ST-8080 tuner is also due to the improved stability of
the local oscillator and mixer circuits which incorporate special low-noise
transistors, and a recently developed aluminum core oscillator coil. And
circuit sections prone to the effects of changing humidity, etc. Have been
further stabilized by the use of special point-to-point wirrng techniques.
The use of
special low-noise transistors designed to eliminate noise in both high and low
frequency regions, and the extremely strict selection of circuit components,
have greatly attributed to the very high S/N ratio, and the noticeable lack of
frequency drift. Special care has been
taken to separate oscillator's hot side from the foil on the printed circuit
board, another innovation resulting in drift-free performance.
Advanced
Type Flat Group Delay Filters Solve the Conflicting Demand for High Selectivity
and Low Distortion in lF Stage.
A
selectivity rating as high as 85dB, and adistortion ratio as low as 0.15% (100%
modulation, MONO) in the ST-8080 has been accomplished by some very clever
electronic engineering. The four flat group delay filters (one narrow band
type, and three wide types) feature 4
-resonator type ceramic filters of very high selectivity characteristics. In
addition, the amplification stage consists of 3-stage and 2-stage
differential amplifiers (in two separate
lCs) connected to a quadrature detector.
At ±400kHz, a very impressive
alternate channel selectivity of 85dB has been obtained. And even at ±200kHz, selectivity is still in
excess of 10dB, which means that no matter how crowded the band may be, the
desired signal can still be singled out with a high degree of clarity. These
filters also assit in maintaining excellen stereo separation.
Here again
is ample evidence of the very careful attention that Technics pays to the
performance of the audio equipment it designs, not only in terms of audio requirements, but also in terms of
performance as communications equipment. No matter how low harmonic distortion
may be suppressed, it will have little significance if the system is still subject to other forms of interference. Only
when all performance requirements are fully met, can the equipment be honestly
be considered as a genuine FM stereo tuner.
Super Flat
Frequency Response from 20Hz to 18kHz (+0.2dB,-0.8dB)
Needless to
say, waveform fidelity would be impossible
to attain in a tuner suffering from phase inversion and poor frequency
response. In the ST-8080, the 19kHz
pilot signal, required for stereo demodulation in the MPX curcuit, is cut out
with razors harp precision by the pilot cancel circuit (a Technics invention
now copied by many other manufacturers). Besides improving frequency response,
it also reduces distortion, and escapes the problem of phase inversion
associated with low pass filters (which also inavoidably cut off some of the
high frequency portions of actual FM programs).
Excellent
linearity is also obtained, the response curve being incredibly flat (+0.2dB; -0.8dB) all the way trom 20Hz to 18kHz.
Together
with the improvements gained by the flat group delay filters in the lF stage,
an extremely high degree of fidelity in waveform has been accomplished.
Furthermore the 38kHz subcarrier signal
has also been cancelled by a similar circuit, reducing the possibility of
interference from this source to zero.
Superb
Stereo Separation (45dB at 1kHz) Due to New PLL
IC in MPX Circuit
Although the
PLL (Phase Locked Loop) system is now quite common in the MPX circuit (where
the left and right stereo signals are retrieved from the transmitted
"multiplex"signal), the ST-8080 has improved upon this system by
accommodating it in a newly developed IC (AN363). The advantages include
greater stability and reliability under varying external conditions, decreased
distortion especially in the high frequency region, and higher stereo
separation. The improvement in waveform transmission is quite marked (see
photograph A and B), not only because of the greatly reduced higher harmonics,
but also because of the reduction of
unwanted components caused by beating.
This IC
employs a voltage controlled oscillator to generate a 76kHz signal, which is
then divided into 38kHz, and 19kHz signals.
This 19kHz signal is synchronized perfectly with the pilot signal,
passed out of the IC via pin no.12, and undergoes waveform shaping into a
pure19kHz signal, which is then mixed with the composite signal at the input of
the MPX circuit, resulting in the cancellation of the unwanted pilot signal.
This further eliminates the occurrence of poroblems in the succeeding nonlinear
circuits.
Low-Noise
Amplification IC in AF Stage
The
thoroughness of Technics pursuit for waveform fidelity is further emphasized by
the quality of the circuitry employed in the AF stage. The IC adopted here includes a first-stage
differential SEPP output for audio frequency amplification, and is operated by
a stabilized power supply. Besides the variable level outout control, the
ST-8080 also features a separate output for tape decks. The level volume
control is the precision adjusted low gang error type. The fixed output
terminals (for recording) include a
special type of lowpass filter of
excellent phase delay characteristics, and which suppresses carrier leak to
less than -70d8.
Built-in REC
Level Oscillator for Level Setting Accuracy
This
oscillator generates a 440Hz sine wave siqnal whose level corresponds to an FM
radio signal of 50% modulation. This enables recording enthusiasts to adjust
recording levels to a very high degree of precision.
High Grade
Signal Strength Meter for Greater Precision and Versatility
Linked to
the quadrature detector circuit, this extremely li near signal meter (fl at
right up to 65dBf) is fully qualified to measure signal strengths to very high levels. This will prove very
useful in finding optimum antenna position and direction for weak and strong FM
stations alike.
Two-Way
Muting
With muting
circuitry in the reed relay of the output stage, and in the quadrature detector
IC, 2-way muting is provided to suppress inter-station and tuning noise to a
high degree.
MPX Hi-Blend
for Noise-Free Stereo Reception of Distant Stations
The
multiplex hi-blend switch betters S/N ratios in reception of weaker and more
distant FM stations. The ST-8080 also features multipath measuring terminals to
which an oscilloscope may be connected. Optimum antenna position and direction
can thus be determined with great accuracy in situations troubled by multipath
interference.
Also High
Quality AM Reception
Nor has
there been any skimping in design considerations for AM reception.
Incorporating a high-Q ferrite bar
antenna, an IC developed specifically for AM, and the very high
selectivity of the ceramic filters in the lF stage, exceptronally high
reception standards have been achieved.
Technical
Specifications
(DIN 45 500)
FM TUNER
SECTION
Frequency
Range: 88 -108 MHz
Sensitivity: (± 40 kHz deviation)
S/N 30
dB : 1,8 µV (300 Ω); 0,9 µV (75 Ω)
S/N 26
dB : 1,6 µV (300 Ω); 0,8 µV (75 Ω)
S/N 20
dB : 1,4 µV (300 Ω); 0,7 µV (75 Ω)
IHF
Usable Sensitivity: 10.8 dBf (1,9 µV,
300 Ω
IHF '58)
S/N 50 dB
Quieting Sensitivity:
Mono :
13,6 dBf (2,6 µV, 300 Ω IHF '58)
Stereo
: 34,3 dBf (28,4 µV, 300 Ω IHF '58)
Total
Harmonic Distortion
Mono : 0,15
% (1 kHz)
Stereo :
0,3% (1 kHz)
S/N (40 kHz
deviation)
Mono : 70 dB
(75 dB, IHF)
Frequency
Response:
Variable
: 20 Hz - 18 kHz (+0,2 dB; -0,8 dB)
Fixed : 20 Hz - 15 kHz (+0,2 dB -0,8 dB)
(DIN) : 20 Hz - 18 kHz (±15 dB)
Alternate
Channel Selectivity: 85 dB
Capture
Ratio: 1,0 dB
Image
Rejection at 98 MHz: 85 dB
IF Rejection
at 98 MHz: 100 dB
Spurious
response rejection at 98 MHz: 95 dB
AM
Suppression: 55 dB
Stereo
Separation: 45 dB (1 kHz), 35 dB (10 kHz)
Carrier Leak
:
Variable
: -60 dB 919 kHz); -65 dB (19 kHz IHF)
Fixed : -65 dB 919 kHz); -70 dB (19 kHz, 38 kHz IHF)
Limiting
Point: 1,2 µV
Bandwidth :
IF Amplifier
: 250 kHz
FM
Demodulator : 820 kHz
Antenna
Terminals: 300 Ω and 75 Ω
AM TUNER
SECTION
Frequency
Range: 525 - 1605 kHz
Sensitivity
(S/N =20dB): 30 µV
Selectivity: 25 dB
Image
Rejection at 1000 kHz: 45 dB
IF Rejection
at 1000 kHz: 40 dB
GENERAL
Output
Voltage
Vriable
: 0 - 1,4 V
Fixed : 0,6 V
Power
Consumption: 14 W
Power
Supply: 110/120/220/240 V 50/60 Hz
Dimensions
(W x H x D): 450 x 140 x 367 mm
(17-23/32" x 5-1/2" x
14-7/16")
Weight: 8,4 kg (18,5 lbs)
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