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Re: You must mean milliohms

Posted by Soundmind (A) on February 18, 2005 at 07:11:27

In Reply to: Re: You must mean milliohms posted by Mahatma Kane Jeeves on February 18, 2005 at 06:06:41:

Thevenin's theorum and Norton's Theorm are a duality. Thevenin says a source can be replaced for mathematical anylitical purposes with an ideal voltage source having zero internal impedence in series with an internal complex impedence (resistance plus reactance). Norton's theorem says it can be replaced with an ideal current source having infinite internal impedence in parallel also with an internal complex impedence. When people talk about the output impedence of audio amplifiers, they invariably are talking about the series impedence of the source referred to in Thevenins theorem. The two theorems are interconvertable because they are two different ways of looking at exactly the same thing although in the amplifier you specified, it is more convenient to look at Norton's theorem to analyze it. It is an unusual and possibly unique exception because of the method of controlling the output. In theory for example, if the load were to be disconnected, the output voltage would go to infinity trying to continue to drive current through an infinite load. Any amplifier (or any other circuit for that matter) having an internal (series)impedence (Thevenins equivalent) which is ten times the load impedence, cannot have an efficiency of delivering power to the load of more than 10%, that is 90% or more of the electrical power delivered by the power supply will be converted to heat internally. The example points out an intresting thing about so called "power" amplifiers. They don't actually amplify power, they amplify voltage and their final stage is capable of delivering a lot of current (and voltage) to a loudspeaker so it is a powerful signal. In the case of your amplifier, it actually amplifies current, not voltage and the output stage can also deliver both adequate current and voltage to power a loudspeaker system. In a sense it is a tradeoff between the advantages of voltage versus current amplification. Because the load is reactive, neither can produce a transfer of power to the load independent of frequency and as pointed out in the original technical paper, special consideration has to be given to the design of the loudspeaker drivers and crossover networks to exploit the advantage current controlled amplifiers have to offer. They need series rather than parallel networks. Most loudspeakers on the market fail to qualify by this criteria very badly.

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