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Old 18th January 2013, 02:23 AM   #32841
fas42 is offline fas42  Australia
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Originally Posted by john curl View Post
Well fas42, 'kinky sound' is what I hear in digital as well. You apparently like it.
My last exposure to 'analogue' sound, at the recent Sydney hifi show, left me rather non-plussed: polite and lethargic are perhaps the best adjectives to use. I have heard better, much better in analogue -- perhaps that crowd have given up ...

It is a shame that people can't point to a reference sound, as one can for colour or temperature, say -- it's very hard to convey the qualities of sound, as an object in itself, using words ...

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Old 18th January 2013, 03:05 AM   #32842
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If i made no mistake, the open loop bandwitch of this AD797 is 5500Hz. Not fast enough for me.
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Old 18th January 2013, 03:28 AM   #32843
a.wayne is offline a.wayne  United States
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Originally Posted by john curl View Post
This is not a useful test. We KNOW that the AD797 measures very, very good, with static distortion. Slew rate should be adequate as well.
If any and everyone of you wants to use an AD797 as a line amp, be my guest.
I personally would use an AD825, myself, and use the AD797's in my test equipment.
20v/us is sufficient ....?
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Old 18th January 2013, 04:07 AM   #32844
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Originally Posted by Esperado View Post
If i made no mistake, the open loop bandwitch of this AD797 is 5500Hz.
The way I read the DS (fig16), the OL gain is already falling at 100Hz 6dB/8ve so the OL BW must be lower than this. They don't show below 100Hz.
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Old 18th January 2013, 04:20 AM   #32845
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Originally Posted by john curl View Post
It is almost impossible to measure the actual nonlinearity of an AD797 at low levels (typical working levels) because the high negative feedback and the residual noise floor hides it.
John do you mean this kind of low-level? Looks pretty kinky to me - anyone know what happens when we close the loop around such a kinky TF?

(Pic taken from DS of OP177)
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File Type: png OP177.png (12.3 KB, 197 views)
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Old 18th January 2013, 04:37 AM   #32846
jcx is offline jcx  United States
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well you could put a R to a rail to SE Class A bias the output

AD797 has ~500 uA output bias so should supply ~+/-1 mA in push-pull

and that's plenty to drive a real buffer or CFA op amp input of MegOhm||pF if you need more output

you really don't have to only use just one op amp if you really want superior performance

DSL driver CFA op amp for instance are designed, speced to compete on distortion at 100 kHz- few MHz while driving W into 25 Ohm - audio cross over distortion is not a problem

wrap a "good" "audio" input op amp feedback loop around one of the DSL drivers to clean up thermals and what's left to complain about?

Last edited by jcx; 18th January 2013 at 04:45 AM.
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Old 18th January 2013, 04:51 AM   #32847
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Exactly, Abraxalito! That is sort of the thing that I used to measure with my Tek curve tracer accessory. What IC was that?
The AD797 can't properly drive its own feedback loop. jcx
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Old 18th January 2013, 04:51 AM   #32848
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I take it PMI fixed this without resorting to classA biassing the output stage - the schematic still shows the typical complementary pair, the total supply current is 2mA and the plot of the device I've attached. The gain linearity problem would appear to be elsewhere.
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File Type: png OP177-fig25.png (11.9 KB, 165 views)
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Old 18th January 2013, 05:03 AM   #32849
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What IC was that?
They don't say - its only presented (in the DS for OP177) to say 'look how ours is better!' - great marketing

Incidentally John, have you come across this paper before - you might find it interesting? In a nutshell its about how opamp LTP input stages can act rather like balanced mixers...
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Old 18th January 2013, 05:08 AM   #32850
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Originally Posted by abraxalito View Post
They don't say - its only presented (in the DS for OP177) to say 'look how ours is better!' - great marketing

Incidentally John, have you come across this paper before - you might find it interesting? In a nutshell its about how opamp LTP input stages can act rather like balanced mixers...
Very interesting paper.
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