From: John Woodgate
Subject: Re: Audio opamp shoot out
Date: Mon, 28 Oct 2002 22:13:32 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Mon, 28 Oct 2002 22:31:45 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Winfield Hill
wrote (in ) about 'Audio opamp shoot out',
on Mon, 28 Oct 2002:
>> John Woodgate scribbled:
>>> I wouldn't take BD as an authority ...
>> Maybe there is a grain of truth in what you say!
>> But on the face of it, I can't see much wrong with the data he
>> provided here, although the scaling of the plots is a bit weird IMHO
> Martin, I agree.
> The issue that concerns me is the 600-ohm load he used. When
> driving current into such a load, the test really becomes one of
> balanced on-chip cancellation of thermal feedback into the input
> transistors and 1st stage current mirrors, etc. Which although
> interesting, is not that interesting.
> That's because anyone who truly wants optimal low-distortion will
> isolate the gain amplifier from the output driver, viz.,
>. ---/\/\--+-|\___|\__|__ 600-ohm
>. gnd ---|/ |/ load
> In this way a small unavoidable error in the output driver
> is reduced to nothing by the inverting stage feedback. The
> inverting stage drives a high-Z. It would be useful to see
> a set of opamp distortion measurements driving such a high Z.
600 ohms has NOTHING to do with modern audio. It's not appropriate to
mount a critique of BD's approach here, because AFAIK he does not
participate in this group. But my impression of some of his research
reports is that the required result was decided before any test were
done, and the tests designed to achieve it. So, in this case, the
ability of op-amps to drive low-impedance loads (cable capacitance?) may
have been in question.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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