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Audio Amplifier Design
Warren Mar
January 11, 2011
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Design Criteria
The human audible range is 20 Hz - 20 kHz.
2
Power Supply
To drive the 8 Ω, we need a supply voltage of at least 28.3 V.
RL = 8 Ω
1 2
50 W(RMS) = Vpeak
RL
2
Vpeak = 28.3 V
Ipeak = 3.6 A
To get the DC voltage to power the LM4780 chip, we need to convert the
120 VAC 60 Hz building supply into ±40 V by using a AC to DC converter
composed of a center tapped transformer with some rectifying diodes and a
voltage regulator. The transformer turns the 120 V to 40 V. The rectifying
diodes, convert the AC into bumps, which gets smoothed out by large capacitors. This smoothed out bumping signal then gets further regulated into a
stable supply. Noise in the power supply can couple into noise in the output.
A stable power supply is a must. A dual-polarity power supply will give us
what we need as seen in Figure 1. A sample netlist is provided in Listing 1.
1
Figure 1: Dual-polarity power supply [3]
Listing 1: Dual power supply netlist
Dual Power Supply
VIN 1 0 SIN ( 0 120 60 0 0 )
RS 1 2 0 . 1
LP1 2 0 90m
LP2 2 0 90m
LS1 4 0 10m
LS2 0 5 10m
K1 LP1 LS1 0 . 9 9 9 9 9
K2 LP2 LS2 0 . 9 9 9 9 9
D1 6 4
D2 4 7
D3 6 5
D4 5 7
∗ 7 v+ 6 v−
RL 7 6 50
C1 7 0 10m
C4 0 6 10m
∗ analysis
.TRAN 1m 0 . 1
. PRINT TRAN V( 1 ) V( 4 ) V( 5 ) V( 6 ) V( 7 )
.END
2
2.1
Transformer
Toroidal geometry results in less lost form stray fields.
V1
V2
=
N1
N2
, where V1 is the voltage and N1 is the number of windings. To go from 120
V to 40 V.
N1 = 3N2
, so the primary windings will be triple that of the secondary windings.
Since it is difficult to get the core and laborious to wind a toroidal transformer yourself, buy you can easily buy one such as the one seen in Figure 2.
500 VA means Vrms Irms < 500 for safe operations.
Figure 2: Avel Y236804 500VA 40V+40V Toroidal Transformer from partsexpress.com [2]
2.2
Rectifying Diode Bridge
For the rectifying diodes used in the bridge, we need ones that have a low
turn on voltage to minimize losses and able to support enough current.
Minimum current around 10A. There are rectifying bridge modules that
can be purchased, but they usually are designed to higher requirements for
voltage and current, so it will be cheaper to go with individual diodes to
make our bridge.
3
150
Line Voltage
Secondary Positive
Secondary Negative
100
Voltage (V)
50
0
−50
−100
−150
0
0.005
0.01
Version0.025
of MATLAB0.03
0.015Student
0.02
Time (s)
Figure 3: Center-tapped transformer
4
Figure 4: Avel Y23 transformer series connection diagram [?]
5
Figure 5: Rectified positive supply output
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2.3
Regulation
The rectified voltage needs to be smoothed out for it to be DC. At minimum
capacitors are needed. A voltage regulator can also be added, but some
efficiency is lost, because it regulates by dumping excess current to ground.
Q = CV
I=C
3
dV
dt
Amplifier Circuit
Humans can hear from 20 Hz to 20 kHz. The ability to hear higher frequencies gets lost as one ages. The audio source is mostly a CD, so that’s
44.1 kHz at 16-bits, which gives you 22500 Hz as your highest frequency
and 20 ∗ log(21 6) = 96.3dB of dynamic range. Having a noise level less
than 96 dB, won’t make much of a difference. LM4780 is composed of two
LM3886. Output Power/Channel ??with 0.5% THD+N, 1kHz into 8 60W
(typ) THD+N at 2 x 30W into 8 (20Hz - 20kHz) 0.03% (typ)
The amplifier can be thought as a simple opamp. Since we want to use
the two opamps in different configurations, we need the proper switches to
wire it up.
Mute and soft turn on.
The PCB traces will be designed with ExpressPCB. The LM4780 datasheet
has a lot of information.
When selecting resistors, measure them and choose the ones that are the
closest.
7
Figure 6: Stereo circuit diagram [1]
Figure 7: Mono circuit diagram
8
Figure 8: Bridged circuit diagram [1]
Figure 9: Mute soft start [1]
9
Figure 10: LM4780 PCB
10
4
Chassis
Metal chassis to shield against electromagnetic interference acting as a faraday cage. The power supply should be on the opposite side of the incoming
signal lines to reduce noise for EMI and capacitive coupling. The front panel
will be designed with front panel express.
Figure 11: a) Front Panel b) Back Panel
References
[1] Lm4780 datasheet.
[2] www.parts-express.com.
[3] Paul Horowitz and Winfield Hill. The Art of Electronics. Cambridge
University Press, 1989.
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