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Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
References: <firstname.lastname@example.org> <email@example.com> <3DKC9.467$Lq.firstname.lastname@example.org>
Subject: Re: Class B amplifiers: what are the large low-value resistors for?
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Date: Wed, 20 Nov 2002 13:51:08 -0000
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Phil Allison wrote:
> "Kevin Aylward" wrote in message
>> *Pure* class B dose not require temperature compensation. Temperature
>> compensation is to compensate the *bias* current. If there is no bias
>> current, there is nothing to compensate. The reason for this
>> misunderstanding could be that is usually to say class B as
>> shorthand, when in fact one rarely uses class B, it is always class
>> AB, i.e. there is at least some standing current.
> ** Many commercial power amps produced since the late 1960s
> operate with the main output devices completely cut off at idle - or
> pure class B. The Crown DC300 and DC300A were early examples along
> with most later Crowns and Phase Linears and many clones.
I meant *real amplifiers*:-)
Crown DC300's were crap, despite their reputation. In fact, Crown have a
current offering of a switching amp which has an amazingly high 0.5%
distorting at 1kz. I dread to think what the IMD is as they don't even
> There was temp compensation built in to make sure that the
> output transistors did not become biased on at high temperatures due
> to their Vbe dropping.
I agree that technically this is temperature compensation, but the
phrase temperature compensation when applied to amplifier output stages,
usually refers to protecting against thermal runaway. I don't see why
they would bother to prevent the outputs being slightly on. Its very
unlikely that that could cause thermal runaway, and the more bias the
less x-over distortion. It sounds like a right silly idea, probably
because it was done 30 years ago when they didn't know any better. I
still remember using 100 watt transistor amp modules that used a driver
transformer because the designers were too daft to realise that the
product did not have any glass in it.
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
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