From: "Phil Allison"
Subject: Re: Class B amplifiers: what are the large low-value resistors for?
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Date: Fri, 15 Nov 2002 10:18:42 +1100
NNTP-Posting-Date: Fri, 15 Nov 2002 10:08:54 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Paul Burridge" wrote in message
> On Thu, 14 Nov 2002 17:34:21 +1100, "Phil Allison"
> opined thusly:
> > ** 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
** Then why did you not put this in your OP ???
> > Using 1 ohm resistors will reduce the available power -
> >by a large factor if the amp has "VI limiting" built in - most do.
> This one hasn't: it's just a breadboard experiment.
** More left out info.
BTW, there's no reactive load, just a loudspeaker.
** What has reactive load got to do with the matter ??
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
** Not bloody likely ........... apologies to GBS
> What I'm outlining here is class B *audio* of a few watts. Sorry if I
> should have made that clearer.
** I figured you were repairing a commercial amp.
> > 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!
** Your problem is overbiasing or no heatsink.