From: "Trevor Wilson"
Subject: Re: Class B amplifiers: what are the large low-value resistors for?
Date: Fri, 15 Nov 2002 05:32:43 +1100
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"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
**Presumably you have adequate heat sinking? Bias current is suitably low?
Do you have some kind of thermal feedback, from output devices to some kind
of bias control element? All these things are pretty much essential with a
typical Class A/B BJT amplifier.
> > 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. 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?
**Not in this universe. There is only one brand of commerically available
loudspeaker, which APPROACHES a resistive load. That brand would be Magnapan