From: Kevin McMurtrie
Subject: Re: Class B amplifiers: what are the large low-value resistors for?
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Fri, 15 Nov 2002 06:08:08 GMT
NNTP-Posting-Date: Thu, 14 Nov 2002 22:08:08 PST
In article ,
firstname.lastname@example.org (N. Thornton) wrote:
>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
>> > 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'm thinking you're not using class B here :) More like AB, which is
>somewhat different. Class B would be simpler, and can in fact be made
>to work without any problematic level of crossover distortion.
>If you're willing to use geranium output trs you can do a much simpler
>cct that's totally stable and sounds nice. I always do that for
>Use an LM324 opamp to drive 2 small 1 watt Ge output trs, no bias.
>Just connect the bases together and hook them straight to the LM324
>output. A few Rs for the opamp feedback, one output cap, and thats it!
>It only works with geraniums though: silicons give terrible crossover
>dist. And its runaway proof, despite the lack of any obvious runaway
>Theres no thermal or short protection, and no capacitive load
>proofing. Them is extra!
The LM324 itself has terrible crossover distortion! It's meant to
deliver current in only one direction so there's a couple volts of slack
in its push-pull transistors. That's what "Single Supply" means.