Audio Systems

Introduction Audio systems are designed to give an output frequency within the audible range for a human being (20 Hz to 20KHz). Below is a block diagram to illustrate the different stages involved in transferring multiple audio input signals to a speaker after being mixed and amplified. Extra Fact Human speech is in the range of 30Hz – 3.5Khz Input Source Input Source Input Source Pre-Amplifier MixerTone ControlsPower AmpSpeaker

1. Input source

2. Pre Amplifier See chapter 2 page 4

3. Mixer Draw diagram and explain formula Summing Amplifier Chapter 2 page 5

4. Tone Control Explain that this will be done in next Chapter with Passive and Active filters

5. Power Amplifiers Imagine you are in a large venue and you need everyone to be able to hear the sound coming from the stage. The sound must come from the speakers which will usually have a low impedance (from 4 to 8  typically). The output of the power amplifier must transfer as much power to these speakers as possible. We need to use a theorem called the MAXIMUM POWER TRANSFER THEOREM. This ideally requires the output impedance of the power amplifier to be equal to impedance of the speaker.

Voltage Followers When the conditions for good voltage transfer are not met then a VOLTAGE FOLLOWER (below) can be used. The conditions required for good voltage transfer are:- –the input impedance of the driven stage must be a great deal higher then that of the voltage source. An Op Amp is ideal for creating this difference in impedance as we have seen previously it has an infinite input impedance. This creates excellent voltage transfer. Very often the voltage gain for the Op Amp will be unity – V in = V out this is why they are often called VOLTAGE FOLLOWERS V in V out Op Amp Voltage Follower 0V

DC Emitter Follower 0V When good Power Transfer is required (i.e. high current flowing through means good power), then. Very Important Input impedance of Emitter follower is equal to =V IN or = h FE x R L R V in V out IBIB