The amplifier design was orginally kept as basic as possible so that the amplifier could be optimized for a single configuration. Given this design, flexibility was required in the power supply circuitry so that a consistent +/-30VDC was provided to the amplifier regardless of power input. Given the required power, the audio amplifier provided the required power gain via a pair of TIP115/117 transistors. Previous to the power amplification stage, the audio signal is passed through a high input impedance buffer, graphical equalizer, volume/balance control, and a peak limiter protection circuit.
The Preamp section of the Amplifier is made up of the Buffer, Graphic Equalizer, Volume and Balance Control, and Peak Limiter. The Preamp section conditions the audio signal before the Power Amplification occurs.
The Buffer circuit is used so that the current that is drawn from the source (i.e. CD player) is not large. The gain of the Buffer is set so that when at maximum volume and balance control set in the middle, the peak limit begins to work when the input to the Buffer is set at 1 VRMS.
The Graphic Equalization is made up of six stages. Q301, R305, R306 and C302 in combination make up a gyrator. A gyrator is an active circuit that uses a capacitor to simulate an inductor. The value of the inductor is calculated by:
L along with C1 make up a tuned circuit which is connected to the pot so that the tuned circuit can be moved from the positive to negative input to vary the gain at the tuned frequency. The frequency that the circuit is tuned for can be calculated by:
The Volume and Balance control is just a simple circuit. R332 is there so that when the Balance control is used to ensure that the sound level of the output remains relatively constant when the Balance is adjusted. R332 also conveniently makes the adjustment more slow when close to the center of the Balance Control. The Volume Control is just a simply voltage divide and because of that an op-amp is used to give a large impedance so that it does not load the voltage divider. The resistors around the Volume Control cause a linear pot to appear approximately logarithmic which is good since general linear pots are less noisy then logarithmic pots.
The Peak Limiter works in such a way that when the voltage on the input exceeds the drop across the zener diode and diode it locks the maximum feedback voltage at that value causing the gain to decrease as the input voltage increases. This introduces odd harmonic distortion but that is unavoidable with all peak limiters. The distortion only occurs when the circuit is limiting the voltage since when the voltage is less then the zener that path is an open circuit. To limit the distortion R336 and R337 are kept small.
Power Amplifier Circuitry
The Power Amplifier is a basic class A/B design. We went with a class A/B amp because of its low distortion and high efficiency compared to a class A or class B amplifier. The difference between a class B amp and a class A/B is the current source that is realized by Q402, D401 and D402. This is similar to a class A design and is used to ensure that Q403 and Q404 do not completely turn off to reduce the distortion caused by the turn-on transients in the transistors. The constant current source ensures that Q401 is always on so that it acts like a diode, which keeps Q403 biased properly at all times.
The signal is easy to trace through the circuit. When the signal goes positive the voltage on the base of Q403 increases which causes more current to flow through Q403. This creates a larger voltage drop across R404, which drops the voltage on the base of Q404, which increase the current through Q404. Q404 is a high power transistor that supplies the current gain to drive the loudspeaker. The negative half cycle works in the same manor but the lower transistors are used.
The LM1876 is used to supply the voltage gain for the Power Amplifier, which is set by the feedback resistor R409. The LM1876 was chosen because it is capable of handling the +/- 30V rails and also provides protection circuitry. The chip provides both under voltage protection during power up, as well as a mute function and standby mode.
There were no major design problems encountered in the building of the Amplifier. One problem that did occur was that suitable pots for the equalizer circuit could not be found. We did manage to due some preliminary testing with a small 10k trim pot, but in a final design another pot would be required. It was also difficult to find the proper values of capacitance needed. The other issue, which arose, was the peak limiter. Originally the circuit was design so that R336 and R337 were 100W which ended up overdriving the LM837. The peak limiter has not been tested in anyway since it requires a full load test, which is not possible with the available power supply.
Some improvements that we would like to make on the Amplifier are insert transistor switch to control mute that is available on the LM1876. Also it would be nice if there were some switches that could remove the balance control and equalizer if the listener does not want to affect the sound in anyway. This could be easily accomplished with a few switches. To really spice it up a microcontroller could be added to control the transistor switches and also a LCD or LED display unit. An inferred detector and remote control could also be designed to interface with the microcontroller. Using low noise op-amps instead of transistors could also make improvements to the quality of the Equalizer.
The Amplifier still needs to be tested in a more structured sense to get an idea of the distortion levels, RMS output power, peak power, and efficiency but it does work which is a good start.