Ad797

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Is it for an MM or an MC phono amp?

There has been some discussion recently on this in various
threads, and it seems many or even most would advice against
it for MM, but vote for it or a similar one for MC.

Op amps like AD797, LT1028 and LT1115 have very low voltage
noise. However, they are all bipolar input op amps so they have
a much higher current noise than, for instance, JFET input
op amps. For MC, the source impedance is usually so low that
the voltage noise is dominant, but for MM the source impedance
is much higher so the current noise can cause a noise voltage
many times higher than the op amps own voltage noise.
Basically, check the current noise of the op amp and multiply
with the source impedance and compare it to the voltage noise.
That is somewhat simplified, but gives you the idea of how a
particular op amp will perform.
 
Peter Daniel said:
Hi Peter,
In the good old days when John Curl and Tom Colangelo were with the Mark Levinson company they made only discrete circuits. At some point in time probably Madrigal decided to go for integrated circuits. It is my opinion that a better sounding circuit can be built discretely but it is more "hard work". This applies particularly to phono stages.
Why would Sonnya and I go through all the trouble to design a discrete opamp?
http://www.diyaudio.com/forums/showthread.php?s=&threadid=1794&highlight=
 
Has anyone perchance used the AD797 with TL431-based shunt regulation (either with or with auxiliary current booster)?

I've previously used the 797 mainly with series IC regulators (for instance, 317/337, LT1086/LT833) without noting any stability or sonic problems (assuredly with local capacitive bypassing at the load), and now I would like to try IC-based shunt regulation.

However, studying the data sheet of the 431 reveals that the output impedance rises steadily above 50kHz until it hits about 13~14 ohms in the 1~5MHz region. I am interested in knowing if this impedance will pose problems for the AD797, and likewise if local capacitive bypasses to ground at the 797's power supply pins will be sufficient to keep it stable and well-mannered.

Any experience or thoughts will be appreciated.

regards, jonathan carr
 
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