From: martingriffith@SPAMbtopenworld.com (Martin Griffith)
Subject: Re: Audio opamp shoot out
Date: Mon, 28 Oct 2002 22:57:24 +0000 (UTC)
Organization: BT Openworld
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NNTP-Posting-Date: Mon, 28 Oct 2002 22:57:24 +0000 (UTC)
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On Mon, 28 Oct 2002 22:13:32 +0000, John Woodgate
>I read in sci.electronics.design that Winfield Hill
>wrote (in ) about 'Audio opamp shoot out',
>on Mon, 28 Oct 2002:
>> 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
>Interested in professional sound reinforcement and distribution? Then go to
>PLEASE do NOT copy news posts to me by E-MAIL!
I know 600R is ancient telecom, and nobody in their right mind would
use a 600R load in practice, these days (75R is much better!)
Terminating an IC with 600R load when testing something on the bench
will give you a "feel" of how the device will perform in the field.
Loads of IC's are spec'd in the data sheets typically 2k2 or 10k load.
Now I consider this as too light a load.
If you've got to distribute a signal to several users, say 4 users at
10Kohm each,any you forgot to bring an audio DA, thats enough to annoy
many GP opamps
I havent heard of a "standard" test load for line level pro audio
equipment, but everyone (in pro audio land) seems to accept lumps of
equipment should be able to deliver a decent performance into 600R
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