video record
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The programme helps students with the practical aspects of building their home experiment kit hi-fi amplifier and revises the principles behind it.
Metadata describing this Open University video programme
Module code and title: T283, Introductory electronics
Item code: T283; 06
First transmission date: 09-07-1980
Published: 1980
Rights Statement:
Restrictions on use:
Duration: 00:24:12
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Producer: John Stratford
Contributor: David Crecraft
Publisher: BBC Open University
Keyword(s): Amplifiers; Circuits; Distortion; Electronics
Footage description: Exterior shots of the Venables Building. David Crecraft, in one of the Technology labs inside the Venables Building, introduces the programme. Crecraft looks over the H.E.K. hi-fi amplifier and points out some of its parts. He then draws a graph to illustrate why the frequency response conponent from the pre-amplifier in the 'operational amplifier' one area has to be shaped to compensate for the frequency response of the magnetic pick-up. Crecraft, holding a loudspeaker cone, explains why the output circuit of an amplifier must have very little offset current and at the same time provide a substantial signal current to drive the low impedence loudspeaker. Using a circuit board containing a simple emitter-follower circuit connected to an oscilloscope, Crecraft explains and demonstrates that this is a suitable candidate for the above output circuit. Crecraft does the same with another simple emitter-follower. This one reproduces the negative half-cycles rather than the positive half-cycles of the other circuit. Neither circuit induces a standing current. Crecraft suggests that the above two emitter follower circuits be connected in such a way that both positive and negative half-cycles are combined. He demonstrates such a circuit and notes that while it does work, there is a large amount of cross-over distortion. With the aid of an animated circuit diagram, Crecraft demonstrates that cross-over distortion in the above circuit can be eliminated by incorporating it into another, more complex circuit. The technique used to eliminate the distortion is called "current dumping". Crecraft demonstrates the circuit and examines the output on an oscilloscope screen. In order to get a measure of cross-over distortion, Crecraft connects a spectrum analyser to the circuit. This analyses harmonic products due to distortion. Crecraft goes on to use a distortion meter to measure the smaller amounts of distortion which a spectrum analyser alone cannot pick up. He tests the home kit amplifier and finds that it compares favourably with commercially produced amplifiers. Crecraft then tests a high quality commercial amplifier which also works on the current dumping principle. He finds that this amplifier is about ten times as good as the home experiment amplifier from a distortion viewpoint. Finally Crecraft tests both the high quality commercial amplifier and the home experiment amplifier by applying a technique called 'overload recovery'. He explains what this involves and then demonstrates.
Master spool number: OU 3383
Production number: FOUT056N
Videofinder number: 2647
Available to public: no