SM-SX100
– 16 –
3. Application to amplifying circuit: 1-bit amplifier
3-1) Basic block and operational principle
The system classified as "Class C amplifier" among the amplification circuits switches the high voltage to control the ON
time in order to amplify the audio signal.
"PWM signal" which has the analog width in the time axis direction is generally used as the signal to control the switching.
The operational principle of Class D amplifier is developed to apply the said 7th order
modulation 1-bit signal to the
signal which controls the signal. Thus, the 1-bit amplifier "SM-SX100" is introduced.
Figure 16 1-BIT AMP. SM-SX100 BLOCK DIAGRAM
Volume
Control
Analog Signal
1-Bit Signal
Function Indicator
Function Switch
Analog Signal
Input
Line 1 (RCA PIN)
Line 2 (Canon)
SACD (Analog)
SACD (1bit)
Digital 1(ST LINK)
Digital 2(RNC)
Digital 3(RCA)
Digital 4(TOS LINK)
1-Bit Signal Input
Multi-Bit Signal
Input Terminal
Digital
Interface
Circuit
Sampling Rate
Converter
Circuit
Circuit of 1-Bit
Conversion
From Multi-Bits
Volume Indicator
LED Driver Circuit
LED Driver Circuit
Control
Microcomputer
1-Bit Amplification Signal Process Circuit
Power Switching
Circuit
Digital Driver
Circuit
7th Order
∆∑
Modulation
1-Bit Signal
Generation Circuit
∆∑
Dynamic Feedback
–22V
DC Power
Supply Circuit
DC Power
Supply Circuit
Low-Pas Filter
Circuit
Bi-wiring Speaker
Terminal
Speaker
Fuse
Power
Switch
Switching
Power Circuit
+32V
–32V
+5V +5V
+5V
+12V –12V
Digital 1 Digital 2 Digital 3 Digital 4 SACD Analog 1
Analog 2
Digital 1 Digital 2 Digital 3 Digital 4 SACD Analog 1 Analog 2
Fig. 16 shows the basic block of the 1-bit amplifier. In the block diagram, the core of the amplifying operation is "1-bit
amplification signal process circuit", being composed of the following four circuits.
1
7th order
modulation 1-bit signal generation circuit
2
Digital driver circuit
3
Power switching circuit
4
Low pass filter circuit
In the circuit
1
, the input signal is processed through the high-speed sampling
modulation circuit to generate "1-bit
signal array" which is directly coded from the input information. (Said 7th order
modulation LSI)
Though the 1-bit signal is used as the control signal to switch the constant-voltage power supply at the timing of the quartz
precision, the digital driver circuit
2
controls the drive, suppressing the delay to the minimum since the full bridge power
switching circuit
3
composed of Power MOS-FET is operated at the high speed.
Though the constant voltage is supplied to the power switching circuit
3
, the fluctuation noise included in the power supply
and the error component of the power switching are included in the output section of
3
. Therefore, the "
dynamic
feedback" loop which negatively feeds the information of the output section back to "7th order
modulation 1-bit signal
generation circuit
1
" at the high speed is provided to compensate the above fluctuation noise and error component at the
real time. The feedback operation achieves "power amplification faithful to input signal" without influence of the power
fluctuation and so on.
Since the 1-bit signal of 64fs originally catches the analog signal at the sampling frequency of 64 x 44.1 kHz = 2.8224 MHz,
the information can be transferred to the band of approx. 1.4 MHz, a half of the sampling frequency as the principle.
In the practical circuit, the band is limited at 100 kHz as the result of the following being considered to determine the pass
band.
1) The quantized noise of the 1-bit signal increases toward the high range, and it eliminates the band where the quantized
noise increases in the high range.
2) If any high frequency component of 100 kHz or more is included in the analog signal when an analog signal is taken out
of the 1-bit signal, an abnormality will occur in the circuit system and transfer system of the following step. (Example:
High frequency oscillation, unnecessary radiation, tweeter breakage, etc.)
3) The frequency component of the acoustic signal which is present in the natural is distributed in the band up to 100 kHz.
In the low pass filter circuit
4
of the last step, the components up to 100 kHz are taken out from the the switching signal
for which the voltage is converted, and are output as the analog signal for the speaker drive.
