Low-Pass Filter - Sony SCD-1 Technical Background

2.10. Low-Pass Filter.

After I/V conversion, a simple low-pass filter is
all that's required to produce an analog output.
Conventional low-pass filters use transistors, op
amps or other active circuit elements in the sig-
nal path. Because Sony was intent on preserv-
ing the simplest possible signal path, we chose
a different design. Our General Impedance
Circuit (GIC) low-pass filter positions the active
circuit elements outside the signal path, to
preserve the integrity of the original sound.
Fig. 11 GIC Type Low-Pass Filter
Standard VCVS Type Active Low-Pass Filter
-
+
Signal Flow
Signals pass through
active elements
GIC Type Low-Pass Filter
Signal Flow
-
+
The GIC low-pass filter takes transistors,
op amps and other active circuit elements
out of the signal path.
10
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+
Signals do not pass through
any active elements
+
-
Super Audio Compact Discs can have frequency
response to 100,000 Hz. As such, the cut-off
frequency and characteristics of the low-pass
filter have a direct impact on the audio
specifications—and sound quality—of the player.
Refining the sound means balancing the various
design goals: flat amplitude response, maximum
high-frequency extension, low noise, optimum
phase characteristics and suitability with
downstream audio equipment.
The GIC circuit of the Sony SCD-1 and
SCD-777ES take all these factors into account.
Frequency response is flat to 50,000 Hz (some
two and a half times the response of a CD player).
Above 50,000 Hz, response begins to roll off
slowly. The result is smooth, open, non-fatiguing
sound with exceptional definition and dynamics.
Fig. 12 Characteristics of Low-Pass Filter Used in SCD-1
0
-50
-100
20 100 1k
Frequency [Hz]
Sony's filter design achieves
flat response to 50,000 Hz.
The GIC filter is followed by a discrete
component buffer amp, for optimum coupling
to a preamplifier.
Analog circuit with GIC low-pass filter.
10k 100k