Subsystem 2 – Volume Control
The circuit to control the volume is shown below.
The variable resistor is the control, when its resistance is set to zero the output signal should be the same power as the input signal and the amplifier will be operating at full volume. When the variable resistor is at its highest resistance the output should be very low and the amplifier will be operating at lowest volume. To get my resistor values comfortable for use I will need to experiment with them, the easiest solution being to replace the 90 Ω with a variable resistor for ease of adjusting.
Testing:-
These tests have provided the desired results, proving that this subsystem is working properly.
Subsystem 3 – Bass and treble controls
Using the circuit simulating program, ‘Livewire’ I will test that this circuit does decrease the frequency cut off points:
< This shows that the low pass cut off point has been decreased from 25khz to 20khz, decreasing the treble.
This shows that the sub system doesn’t > change a 200hz signal. This is as it
should be.
< This shows the high pass cut off point has been increased from 15hz to 50hz. This decreases the bass.
This subsystem is working as it should.
Subsystem 4 – Pre Amp
This is the pre amp circuit diagram
The amplification stage is split into two parts, the pre amp and the power amp. This decreases the amount of unwanted noise in the output signal. Each amplifier has notch filters to cut out sounds beyond the human hearing range (15hz – 25Khz). These notch filters are created by C2 and R1 (high pass) and C1 and R2 (low pass).
This amplifier is set to give a gain of 10 (220k/22k=10). The gain is set by R1 and R2. A gain of 10 means that it multiplies the input by 10.
Test:
I will test that this circuit is giving a gain of 10:
This shows the output voltage (red line) 10 times bigger than the input voltage (blue line). This subsystem is working properly
Subsystem 5 – Power Amp
This is the power amp circuit diagram
This circuit is identical to that of the pre amp except that it is designed to give a gain of 15 (330k/22k=15) (R4/R3).
I will now test that this circuit is giving a gain of 15:
This shows the output voltage (blue line) 15 times bigger than the input voltage (red line)
This subsystem is working properly.
In developing my pre amp and power amp I had to calculate the values of the resistors and capacitors in the notch filters:-
I can use the equation to give me the cut off frequency of the RC filter. The equation actually gives me the 3dB point, or the point at which the filter reduces the volume of that frequency by half.
I used a circuit simulation program called livewire to simulate high pass and low pass notch filters at different frequencies. I have used a signal generator to provide the input and an oscilloscope to measure the output.
A high pass filter: this high pass filter is designed to cut off at below 15hz. I have tested it with a range of 40hz down to 5hz and measured the voltage of the output.
To calculate the resistor and capacitor values for thforis filter I manipulated the formula to read:
Capacitance = 1
2π x Resistance x Frequency (cut off)
I decided on a value for my resistor, 220kΩ.
So: Capacitance= 1
2π x 220k x 15
Capacitance = 48nF
Tests:-
High pass filter at 40hz:-
The output is still 5v, as it should be.
High pass filter at 20hz:-
The output is just under 5v, as it should be if it is to be cut down to 2.5v at 15hz.
High pass filter at 15hz:-
At 15Hz it has cut the voltage down to about 3.5v, not quite half but close enough – it is cutting the voltage down.
High pass filter at 5hz:-
At 5hz the filter has cut the voltage down to a little over 1v, the filter is doing what its meant to be doing.
This shows me that my high pass filters are working, giving me a cut off point of 15hz. I will now test my low pass filter. This filter should give me a cut off point of 25khz. To achieve this I will use a 220kΩ resistor. To calculate the capacitor I used this formula:-
Frequency (cut off) = 1 _
2π x Resistance x Capacitance
So: Capacitance = 1
2π x 220k x 25k
Capacitance= 28pF.
I will test this filter with frequencies from 10khz to 30khz.
Low Pass filter at 10khz:
It is giving an output at 5v, as it should
Low Pass filter at 20khz:
It is still giving an output of 5v, as it should.
Low Pass filter at 30khz
This is now cutting the output down to 3v. This is working properly.
Subsystem 6 – Power Output
This is the circuit diagram for the power output stage
The op amps cannot give out enough power to power a loudspeaker so an extra stage is needed to increase the power of the output. This stage does exactly that by feeding the output from the op amps through a pair of transistors, this does not amplify the output, just increase its power.
To reduce any noise produced I can take the negative feedback of my power amp from the output of the power output stage. This will increase the sound quality.
The Final Circuit
Shown is the circuit for one channel, there will have to be two of these.
Since designing this circuit, I have improved my volume control circuit, now, instead of using a potential divider circuit I will us a single potentiometer.
How it Works
One of the four inputs is selected by one of the switches to the left, this signal is then fed into the potential divider circuit (in the actual circuit this has been replaced by a potentiometer). This sets the volume. The signal then travels through the high pass and low pass filters which set its bandwidth (the bass and treble controls). From here it goes on to the pre amp and its set on notch filters. This gives it a gain of 10, i.e. it multiplies the amplitude of the wave by x10. The notch filters ensure it does not amplify anything beyond the human hearing range (15Hz – 25KHz) in an attempt to reduce interference. The next stage it passes through is the main amp, this acts in exactly the same way as the pre amp except that it gives a gain of 15, giving an overall amplification of x150 (10x15). The last stage for the signal to pass through is the power output stage, this gives a unity gain, meaning that it does not amplify the signal. All it does is give the signal power enough to power the loudspeakers. The diodes create a constant of 0.7v at the base of each transistor so that the wave is not distorted. From here the signal passes through a capacitor to block any dc current and then into the loudspeaker.
Prototype Circuits
After designing and testing all my sub-systems on the computer I built them all separately on prototype boards.
For my input control board I checked that the notch filters correctly adjusted the frequency response and volume control adjusted the amplitude of the output.
All my subsystems behaved as I had expected them to when built and tested on prototype board. I then connected all my sub systems together and they appeared to work when tested with a signal generator and oscilloscope.
PCB Layouts
This is the PCB layout for the pre amp and the power amp, again I have 1PCB per channel.
This PCB layout is accurate but it uses more space than it needs to. While this makes very little difference cost wise if I were to mass-produce this circuit board there would be a large amount of wastage in all the boards combined, amounting to a lot of money wasted. Here’s a smaller version of the same PCB:-
This PCB is much more space efficient.
I have also designed PCB boards for the other components of my circuit:-
The input stage:-
The power output stage:-
All of the above PCB’s are single channel so I will need two of each to make up my dual channel (stereo) amplifier.
To make my PCB I will use UV reactive copper clad board and a mask.
I will print this circuit onto acetate and place this on top of the UV reactive copper clad board, I will put these upside down on top of a UV light source. I will turn the light on and leave the board on top of it for 2-3 minutes. I will then place the board (minus acetate) into a dilute Hydrogen Peroxide (H2O2) solution for 2 minutes or until the circuit tracks can be clearly seen on the board. The Hydrogen Peroxide dissolves any of the UV reactive layer on the board that was covered by the print on the acetate. The board is then placed in ferric acid to etch.
Here is my completed set of circuits:
Unfortunately, on testing the entire circuit there were some problems.
Firstly, the transformer itself didn’t work, this being the only part of my system that I hadn’t tested previously, assuming it worked as I bought it. There was an open circuit on the primary coil. I overcame this problem by using the power supply inbuilt in my prototype board. This would not supply enough power but it would be sufficient for testing purposes. After connecting this and checking all of my power connections I turned the power supply on. Within seconds one of my op-amps started smoking and emitted a small pop. Presumably it has blown, but, as I don’t have a spare one to hand I can’t check. All the power wires were correct, there was not a short circuit on the board and the op-amp was fitted the right way around. I cannot understand why it blew in the circuit as I had tested the circuit using a signal generator and oscilloscope previously, when it had worked, I had also tested it on the prototype board.
Bar the fact that the system doesn’t work I am pleased with it. All of the PCB boards were etched very well, and the soldering was good (at least there were no faulty connections or places where solder crossed between tracks).
Parts List:-
Sources of Information:-
- GCSE Electronics Booklets.
- Success in Electronics by Tom Duncan.
- Rapid Electronics Catalogue 2006
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- www.howstuffworks.com
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