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Circuit Descriptions, Abbreviation List, and IC Data Sheets
9.6.5
Audio Amplifier Panel (Diagram A)
Introduction
SSP
TWEETER LEFT
TWIN-CONE SPEAKER LEFT
AUDIO AMPLIFIER
7225-A
L-POS
L-NEG
7225-B
R-POS
R-NEG
AUDIO
ENABLE
63Hz
Figure 9-8 Block diagram Audio Amplifier
This panel houses the audio filters and amplifiers necessary for
driving the speakers. The differential audio inputs (for common
mode immunity) come from the SSP (via connector 0388).
The PSU delivers the positive and negative supply voltage of
12 V_dc, as well as the +5V2 (standby) voltage.
After being filtered and amplified, the signals go to the speaker
section, where the (twin cone) low/mid range speakers and the
tweeters are driven (load impedance is 8 Ω).
The headphone amplifier is a straightforward OpAmp amplifier
(IC7A07-A, MC33178D). It is supplied with +11V_AUD.
Supply (Diagram A7)
The supply voltage is a symmetrical voltage of +/- 14.5 V_dc,
generated by the main supply via L5002.
•
V_SND_POS (+12 V_dc) on connector 0302 pin 5/6.
•
V_SND_NEG (-12 V_dc) on connector 0302 pin 1/2.
Filter (Diagram A2)
Electrical filtering is needed for following reasons:
•
Limiting the cone excursion, thereby reducing the
distortion.
•
Increasing the power handling capacity (PHC).
Active second order Sallen-Key filters are used, with crossover
frequencies of 1 kHz for the low pass filter, and 3 kHz for the
high pass filter.
The audio signals are filtered before the amplifier. There are
some reasons for doing this:
•
It is now easy to do active filtering.
•
Less costs (no expensive coils and capacitors).
Low Pass Filter (LPF)
For L and R separately, a Low Pass Filter (IC7238A and B) is
processing L_LOW and R_LOW.
The output signal of this filter is then fed to the audio amplifier
(identical for right channel).
PSU
TWEETER RIGHT
AUDIO
AMPLIFIER
TWIN-CONE SPEAKER RIGHT
7260-A
L
L-HIGH
HPF
HIGH LEFT
MUTE
7238-A
L-LOW
LPF
MID/LOW LEFT
MUTE
7260-B
R
R-HIGH
HPF
HIGH RIGHT
MUTE
7238-B
R-LOW
LPF
MID/LOW RIGHT
MUTE
MUTE
1kHz
3kHz
6dB/OCT
LOW
HIGH
(LPF)
(HPF)
CL36532053_081.eps
High Pass Filter (HPF)
For L and R separately, a High Pass Filter (IC7260A and B) is
processing L_HIGH and R_HIGH.
The output signal of this filter is then fed to the audio amplifier
(identical for right channel).
Amplifier (Diagrams A3 to A6)
Each speaker has its own class-D amplifier. These amplifiers
combine a good performance with a high efficiency, resulting in
a big reduction in heat generation.
Principle
Audio-power-amplifier systems have traditionally used linear
amplifiers, which are well known for being inefficient. In fact, a
linear Class AB amplifier is designed to act as a variable
resistor network between the power supply and the load. The
transistors operate in their linear region, and the voltage that is
dropped across the transistors (in their role as variable
resistors) is lost as heat, particularly in the output transistors.
Class D amplifiers were developed as a way to increase the
efficiency of audio-power-amplifier systems.
170703
Figure 9-9 Principle Class-D Amplifier
The Class D amplifier works by varying the duty cycle of a
Pulse Width Modulated (PWM) signal.
By comparing the input voltage to a triangle wave, the amplifier
increases duty cycle to increase output voltage, and decreases
duty cycle to decrease output voltage.
The output transistors (item 7365 on diagram A3) of a Class D
amplifier switch from 'full off' to 'full on' (saturated) and then
back again, spending very little time in the linear region in
between. Therefore, very little power is lost to heat. If the
transistors have a low 'on' resistance (R_DS(ON)), little voltage
is dropped across them, further reducing losses.
A Low Pass Filter at the output passes only the average of the
output wave, which is an amplified version of the input signal.
In order to keep the distortion low, negative feedback is applied
(via R3308). A second feedback loop (via R3310) is tapped
after the output filter, in order to decrease the distortion at high
frequencies.
The advantage of Class D is increased efficiency (= less heat
dissipation). Class D amplifiers can drive the same output
power as a Class AB amplifier using less supply current.
The disadvantage is the large output filter that drives up cost
and size. The main reason for this filter is that the switching
waveform results in maximum current flow. This causes more
loss in the load, which causes lower efficiency. An LC filter with
a cut-off frequency less than the Class D switching frequency
(350 kHz), allows the switching current to flow through the filter
instead of the load. The filter is less lossy than the speaker,
which causes less power dissipated at high output power and
increases efficiency in most cases.
Mute (Diagram A3 for "Left High")
A mute switch (item 7302) is provided at the PWM inputs (item
7315, LM311). This switch is controlled by the AU_EN_NOT
line, which is controlled via the POR signal (mute at start-up)
and via the SOUND_ENABLE line from the OTC (mute during
operation). This circuitry is the same for all four amplifier parts.
FTP1.1E
9.
+V
-V
CL16532099_002.eps
EN 105
200801
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