EN 1449.FM23, FM24, FM33Circuit Descriptions, List of Abbreviations, and IC Data Sheets
In order to guarantee this good bass performance, the closed By comparing the input voltage to a triangle wave, the amplifier
box has to be airtight. This is achieved by a closely fitting foam increases duty cycle to increase output voltage, and decreases
ring between the front and back part of the cabinet.duty cycle to decrease output voltage.
The output transistors (item 7365 on diagram A3) of a Class D
9.5.2 Supply (Diagram A7)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
The supply voltage is a symmetrical voltage of +/- 14.5 V_dc,
transistors have a low "on" resistance (R_DS(ON)), little
generated by the main supply via L5002.
voltage is dropped across them, further reducing losses.
V_SND_POS on connector 0302 pin 5/6, and
A Low Pass Filter at the output passes only the average of the
V_SND_NEG on connector 0302 pin 1/2.
output wave, which is an amplified version of the input signal.
In order to keep the distortion low, negative feedback is applied
9.5.3 Filter (Diagram A2)
(via R3308). A second feedback loop (via R3310) is tapped
after the output filter, in order to decrease the distortion at high
Electrical filtering is needed for following reasons:frequencies.
Limiting the cone excursion, thereby reducing the
distortion.The advantage of Class D is increased efficiency (= less heat
Increasing the power handling capacity (PHC).dissipation). Class D amplifiers can drive the same output
In this amplifier panel, active second order Sallen-Key filters power as a Class AB amplifier using less supply current.
are used, with crossover frequencies of 1 kHz for the low pass The disadvantage is the large output filter that drives up cost
filter, and 3 kHz for the high pass filter.and size. The main reason for this filter is that the switching
waveform results in maximum current flow. This causes more
The audio signals are filtered before the amplifier. There are loss in the load, which causes lower efficiency. An LC filter with
some reasons for doing this: a cut-off frequency less than the Class D switching frequency
It is now easy to do active filtering, and (350 kHz), allows the switching current to flow through the filter
At less costs (no expensive coils and capacitors).instead of the load. The filter is less lossy than the speaker,
which causes less power dissipated at high output power and
Low Pass Filter (LPF)increases efficiency in most cases.
For L and R separately, a Low Pass Filter (IC7238A and B) is
processing L_LOW and R_LOW.9.5.5 Mute (Diagram A2 to A6)
The output signal of this filter is then fed to the audio amplifier
(identical for right channel).
A mute switch (item 7302) is provided at the PWM inputs (item
7315, LM311). This switch is controlled by the
High Pass Filter (HPF)AUDIO_ENABLE line, which checks the availability of the
For L and R separately, a High Pass Filter (IC7260A and B) is +9V_STBY voltage.
processing L_HIGH and R_HIGH.
The output signal of this filter is then fed to the audio amplifier
9.5.6 Protections
(identical for right channel).
Short-circuit Protection (Diagram A3)
9.5.4 Amplifier (Diagrams A3 to A6)
A protection is made against a too high temperature of
transistor 7355 in case of a short-circuit of output FET 7365-1.
Each speaker has its own 15 W class-D amplifier. These so-Transistor 7340 is sensing the current through transistor 7355
called SODA (Self Oscillating class-D Amplifier) amplifiers via R3355, and activates the DC-protection line (see figure "DC
combine a good performance with a high efficiency, resulting in protection") in case the current becomes too high. This is the
a big reduction in heat generation.same for all four amplifier parts.
PrincipleDC-protection (Diagram A7)
Audio-power-amplifier systems have traditionally used linear
+9V_STBY
amplifiers, which are well known for being inefficient. In fact, a
5753
linear Class AB amplifier is designed to act as a variable
resistor network between the power supply and the load. The
OUT_LHOUT_LLVCC_10_POS
transistors operate in their linear region, and the voltage that is 27533752
7735
dropped across the transistors (in their role as variable
3770377137753751
resistors) is lost as heat, particularly in the output transistors.3754
77517761
3750
Class D amplifiers were developed as a way to increase the DC_PROT
A
efficiency of audio-power-amplifier systems.
7755
3780378127603765
3760
+V
OUT_RHOUT_RLVCC_10_NEG
CL16532099_001.eps
200801
Figure 9-20 DC Protection
Because of the symmetrical supply, a DC-blocking capacitor,
between the amplifier and the speaker, is not necessary.
-VHowever, it is still necessary to protect the speaker for DC
CL16532099_002.epsvoltages.
200801
The following protections are therefore implemented:
Via R3765 and R3775, each stabilised supply voltage line
Figure 9-19 Principle Class-D Amplifier
(via items 7735 and 7745) is checked on deviations.
Via R3770/3771/3780/3781, each amplifier output is
The Class D amplifier works by varying the duty cycle of a
checked for DC-voltage.
Pulse Width Modulated (PWM) signal. |
ACEC
