08-11-2012, 10:55 AM
Final Project Report Stereo Audio Amplifier
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ABSTRACT
This report covers the design and implementation of a multi-stage stereo
audio amplifier with its own voltage regulator, LED display, and digital volume
control for each channel. The input can come directly from a CD audio player or
similar device, and typical 8 Ohm speakers are driven. Digital volume control is
implemented using a dipswitch, but this control device could easily be replaced
by electronic components. The signal output is clean with no distortion up to
certain audio levels, but some distortion occurs at higher levels.
The unit is designed with five distinct components. The power supply
provides positive and negative 12 Volts. The digital volume control is
implemented using a summing amplifier, and there is one for each channel.
Amplification is done in two stages, each of which is centered on Bipolar
Junction Transistors. The first stage is a Common Emitter Circuit that functions
to amplify the voltage of the signal. This stage has a large output resistance
and can not effectively drive 8 speakers. The second stage is a Class A-B
Power Amplifier that provides the Common Emitter stage with a larger load, and
is itself able to drive the low-resistance speaker with an acceptable current gain.
LED output was achieved using multiple comparators, which compare the
output to predetermined levels and light the LED’s when different levels are
reached.
Specifications
The product is to be suitable for in-home use. The input can be stereo or
mono from any low impedance audio source such as a compact disc or MP3
player. The input must not exceed the maximum voltage level of 250mV peak,
since audio quality can not be guaranteed at such levels. The system must
drive two 8-Ohm speakers with a minimum gain of –3dB and 20dB. This is
0.7V/V and 10V/V in terms of amplitude. Additionally, there can be no more
than a +/- 1 dB gain difference over the audible range of 300Hz to 10kHz
without distortion.
The volume must be digitally controllable with 3 bits on each channel and
there must be 4-stage LED indicators corresponding to 0.25V, .5V, 1V, and 2V
output levels for each channel as well.
The system is run from the normal household wall socket supply of 120
Vrms at 60Hz. The design can be adapted to accept input from any type of
audio plug. Output is adaptable to normal speaker wire.
The Power Supply
The power supply takes the large AC signal from a household wall socket
and reduces and rectifies it to the +/- 12 Volt DC signals required to operate the
circuit. The first step is to pass the signal through a transformer with a ratio of
approximately 6:1. The secondary of the transformer outputs an AC signal with
30V peak waves. The signal is then fully rectified1 into positive and negative
only signal sweeps using a standard bridge rectifier configuration (See figure 2).
The prototype used a pre-built bridge rectifier for space considerations and costeffectiveness,
but assembly out of individual components is an alternative.
By taking the center tap of the transformer as ground, the rectified output
sweeps of the bridge rectifier nodes are defined as +15 and –15 Volt peak half- waves. The waves are then smoothed out by placing large (2200 uF) capacitors
between the positive/negative outputs and ground. The capacitors charge
during the output peaks and discharge when the waves are low. This smoothes
the signal into almost DC with some ripple. Voltage regulators are then used to
further smooth the signal and to provide a more reliable output.
It is important to note that interference and feedback in the power supply
was a significant problem once the entire circuit was connected. This problem
was alleviated by connecting large capacitors from the positive and negative
nodes to ground at multiple sites of the circuitry. The large capacitors act as
shorts to AC signals only, and so any distortions propagating in the supply lines
are grounded through them.
The Volume Control
There were several alternatives for digitally controlling the volume of the
signal. One way was to alter the state of the amplifying CEC circuit by using the
dipswitch to change resistance values. This method introduces no noise, but
there are several obstacles that make it impractical. Most importantly,
specifications require exact decibel volume control, corresponding to 0.707 and
10 V/V gains.
The gain of the CEC is most easily controlled by changing the value of
Rc in the circuit. However, such changes have far reaching implications. By
changing Rc, the bias of the entire circuit is altered. Furthermore, the output
impedance of the circuit depends directly on Rc, so these changes also affect
the transfer of the signal to the Power Amp. All of these interconnected
reliances make precise calculation of overall gain levels impractical.
The circuit was designed with gain control happening at the input of the
circuit. Since whatever goes into the circuit was multiplied by 10, the maximum
input gain was 1 and the minimum was 0.07. These gains were achieved
precisely by using an operational amplifier in the weighted summer configuration
(see figure 3).
The LED Display
The concept behind the LED display is a simple array of comparators,
one comparator for each light. An integrated bar display having 10 lights was
used to provide a cleaner looking circuit, and it was decided to make the middle
two lights power indicators, and for the level indications to extend to the left for
the left channel and to the right for the right channel. The overall effect was
very professional.
Since an operational amplifier is required for each comparator, it was
decided to use comparator chips, each containing 4 op-amps. This made the
stage more compact and required a smaller number of connecting wires.
However, this compactness also made the wiring very crowded and difficult to
navigate.
Spice Simulation
Spice simulation resulted in inexact, yet satisfactory behavior. Generally,
capacitors in spice created much more impedance to the AC signal than they
did in real tests. Because of this, all capacitors in spice were set to 100mF for
simulation purposes. The layouts in the previous section were used for
simulation. Lack of available parts made it necessary to substitute LM324 opamps
in place of the LM741C models that were actually used. Furthermore,
center-tapped transformers and dipswitches were not available in pspice.
Voltage regulators are also not found in pspice, and so the output signals are
seen without their effects.
Conclusions
The circuit was an overall success, but things did not turn out as
originally planned. The initial intention was to build the preamplifier stage using
a CSC Mosfet design, but satisfactory performance could not be achieved.
Additionally, the experimentation with power amplifier design resulted in the
destruction of many NPN and PNP BJT’s. Design progress was delayed while
additional parts were on order.
There is distortion in the circuit output at high levels and this could have
been avoided if it had not been for a miscalculation in the gain requirements.
The circuit was mistakenly built for the highest possible gain, while a lower gain
would have been sufficient with less distortion.
The gain at frequencies higher than 500 Hz is 21.3 dB. At 300 Hz the
gain is 20.6 dB. This makes the maximum difference over the range 0.7 dB,
which is within specifications. The circuit produces full amplification at
frequencies up to 250 kHz. The loss of gain at the lower amplitudes is most
likely due to impedance from the capacitors used as DC blockers. The loss at
higher frequencies may be due to interferences or the internal capacitances of
the BJT’s. See Appendix 1 for printouts of the resulting frequencies.