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From: email@example.com (N. Thornton)
Subject: Re: Class B amplifiers: what are the large low-value resistors for?
Date: 19 Nov 2002 15:35:20 -0800
NNTP-Posting-Date: 19 Nov 2002 23:35:20 GMT
Paul Burridge wrote in message news:<firstname.lastname@example.org>...
> On 14 Nov 2002 17:14:24 -0800, email@example.com (N. Thornton) opined
> >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.
> It was *supposed* to be pure class B but for some reason it's
> definitely well into the AB region. :-\
You say you have biasing / temp compensating there, yet its meant to
be class B: how is that so? For real class B you don't need to apply
any bias chain, so there should be no need for temp compensation. If
the tr bases are connected directly and driven from an opamp, it
should work OK, as well as Class B does at least.
I am wondering if your circuit actually might not be Class B. Could
you clarify on that for us?
> >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
> >quickie amps.
> >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
> Interesting. Yes, it hadn't occurred to me until you mentioned it that
> the lower barrier height of germanium devices would better in respect
> of the non-linearity of the crossover region.
Also germaniums don't have the hard 0.6v knee that silis have, so an
opamp driving a class B germanium pair has a lot less work to do than
one driving a silicon output pair. As you know the LM324's specs are
quite sluggish: with silicons it is entirely unable to keep up with
the necessary corrections. But with germaniums all is well.
I feel I still don't know your particular circuit, nor do I know yet
why you're using that one in particular.
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