From: Paul Burridge
Subject: Re: Class B amplifiers: what are the large low-value resistors for?
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Date: Thu, 14 Nov 2002 14:40:25 +0000
NNTP-Posting-Date: Thu, 14 Nov 2002 16:09:18 GMT
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On Thu, 14 Nov 2002 17:34:21 +1100, "Phil Allison"
> ** They *are* necessary, with no emitter resistors the amp would
>probably self destruct when it got hot by a process called "thermal
>runaway" - kind of a vicious cycle that overheats the silicon.
But I'm getting thermal runaway anyway even with 2 * one ohm resistors
> Using 1 ohm resistors will reduce the available power - possibly
>by a large factor if the amp has "VI limiting" built in - most do.
This one hasn't: it's just a breadboard experiment. BTW, there's no
reactive load, just a loudspeaker. Okay, I know even a loudspeaker is
technically a reactive load, but we're only talking low audio
frequencies here, so I guess it'd be purely resistive, yeah?
What I'm outlining here is class B *audio* of a few watts. Sorry if I
should have made that clearer.
> You may parallel connect several 1 ohms to get 0.33 or 0.25
>ohms - that should work fine.
AFAICS, reducing the inter-emitter resistances would only exacerbate
the thermal problem!
"I was going to help him up, but then I thumped him."
Michael Head's plea in mitigation to Weybridge Magistrates, Sept. 1975