Question

QUESTION 1 Anna the Audiophile has asked for your help to build an amplifier and filter to take small signals from her hifi system and amplify them so that she can drive her new subwoofer. The hifi system produces AC signals at varying frequencies with 250mVrms maximum magnitude Her subwoofer requires the signals to be 20 Vrms maximum magnitude. The signals that Anna is interested in are below 200 Hz. She would like the filter to attenuate signals at frequencies above 200 Hz. Subwoofer Amplifier | Va 1. Swi | C1=1000 μF an 250 mV Filter 4Ω Hifi our Vin system Speaker (a) Design the amplifier and filter circuit to meet Anna's requirements. The circuit must produce the required gain, and also attenuate the signals above 200 Hz. Show calculations for the selection of components where practical. Show a circuit diagram for the complete amplifier and filter circuit. (15 marks) (b) Anna reads the subwoofer manual and notes the value of DC blocking capacitor C1 = 1000 μF. She wonders how the capacitor will affect the frequency response of the subwoofer. What kind of filter is formed by Ci and the speaker resistance? Calculate the cut-off frequency of this filter. Describe the behaviour of this filter in words. (5 marks) (c) When first turned on, Anna's amplifier fails and applies +15V DC directly to the output (va). After 30 milliseconds, the speaker protection circuit operates Sw, to disconnect the speaker and connect t to ground (Sw, changes from position ‘1' to position ‘2) Taking the time the amplifier was turned on as t-0 seconds, calculate and plot the voltage across the speaker for the first 60 milliseconds. Ensure your drawing is to scale and has labelled key values (12 marks) (d) The amplifier fault has now been fixed and the system works normally (Sw, is irn position l'). Perform frequency response analysis for the amplifier and filter you designed in part (a) to calculate the gain in dB and phase in degrees when the hifi system sends a signal at 10 Hz, 100 Hz and 1000 Hz. (15 marks) (e) Calculate the output voltage of the amplifier and filter circuit (va) when a 250 mV 100 Hz signal is applied at its input (Vin).

Answer 1

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