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Old 20th January 2013, 11:19 AM   #33051
PMA is offline PMA  Europe
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AD797 needs careful el. design, which was not followed in the JC2 phono stage, attached by Richard. Especially supply rail bypassing. As it is an input stage and BJT, I would definitely add at least some RC filter, though there might be a small noise increase.. The BJT input connected directly to input connector is WRONG. I also do not think much about output loading/bias by the JFET. The further stages are adding unnecessary noise.
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Old 20th January 2013, 12:38 PM   #33052
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In an other thread, L.C, (a talented designer) tried to improve its nice SSA (A current feed-back power amp). He raised the VAS gain by 10/15dB. This increase, of course, the local distortion a lot, but increase the global feedback ratio. He report an obvious improvement in listening experience of the same amp.
One more testimonial in favor of global feedback advantages with fast devices.
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Old 20th January 2013, 01:02 PM   #33053
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Quote:
Originally Posted by PMA View Post
As it is an input stage and BJT, I would definitely add at least some RC filter, though there might be a small noise increase.
Anti-RFI? Why not a small inductor? Less noise.

Yes, high bandwidth stages (IC or not) need good local bypassing.
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Old 20th January 2013, 01:30 PM   #33054
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Originally Posted by SY View Post
Anti-RFI? Why not a small inductor? Less noise.
Yes, if the whole circuit stable. Bypassing - the recommendation for AD797 is in the datasheet.

Why RFI? Detection and demodulation in high gain BJT stages. Quite often in phono preamps with BJT input opamps.
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Old 20th January 2013, 01:41 PM   #33055
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Quote:
Originally Posted by jan.didden View Post
... at -10zilliondB even from 5 miles out ...


Some 7th in a "real world" circuit:
Some years ago I tried different opamps in the input stage
of a Sony A6 DAT recorder.
Except for the LM4562 (second picture) all opamps tried (AD8620 here as an
example) showed clearly visible 7th.
Attached Images
File Type: png ad8620.png (10.0 KB, 181 views)
File Type: png lm4562.png (14.7 KB, 181 views)
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Old 20th January 2013, 02:01 PM   #33056
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Quote:
Originally Posted by SY View Post
no matter how much dust you try to kick up
SY
Although there is some noise raised over some minor issues, Ed is not acting the way you said. I refuse to assign such intentions to him.

Besides, he is not gifted enough for that.
Trust me. I have gained vast experience living in this country where the art of camouflage dust spreading has been refined to a high degree.

PSUD2 is indeed a fine SW tool.
PSUD2
I have verified (when modelling is performed with care to details) it’s accuracy to uV level on dynamic Vs and Is through measurements on actual circuits.

For me the noise numerals is not an issue when noise is real noise (no certain frequencies raising their head and it’s level is relative stable).
It can turn into a problem if and when it’s amplitude and/or frequency envelope changes dynamically with (modulated or excited by) the signal.
I would appreciate if some of you knowledgeable participants can list possible mechanisms for such an effect.
For my level of capabilities, discussing effect levels below say –130db rel signal will be of academic interest only but still welcome.

George
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Last edited by gpapag; 20th January 2013 at 02:13 PM.
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Old 20th January 2013, 02:15 PM   #33057
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Originally Posted by gk7 View Post
Except for the LM4562 (second picture) all opamps tried (AD8620 here as an example) showed clearly visible 7th.
So you listen with your eyes?
Sorry, couldn't resist that one...

I am puzzled by your post though.
I agreed that it exists. I questioned the audibility at extremely low levels.
Now you come along to proof it exists.
So do you actually agree with me or what?

jan
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Old 20th January 2013, 02:36 PM   #33058
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Quote:
Originally Posted by Esperado View Post
In an other thread, L.C, (a talented designer) tried to improve its nice SSA (A current feed-back power amp). He raised the VAS gain by 10/15dB. This increase, of course, the local distortion a lot, but increase the global feedback ratio. He report an obvious improvement in listening experience of the same amp.
One more testimonial in favor of global feedback advantages with fast devices.
I agree with B. Putzeys wrt GNFB - if you are gonna use it, use a lot of it.
I prefer not to
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Old 20th January 2013, 02:41 PM   #33059
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Originally Posted by jan.didden View Post
... I questioned the audibility at extremely low levels.
I do not want to enter a discussion about the audibility of the 7th harmonic, but just
wanted to show that in real circuits (as opposed to datasheets with measurements at
unity gain) it´s obviously there. If -105 dB (not "-10zilliondB") for high order harmonics is an "extremely low level" or already to high is debatable of course. A line stage I´ve built based on Johns
JC-2 (ML-1) circuit doesn´t show anything above the third as a matter of fact.

Last edited by gk7; 20th January 2013 at 02:44 PM.
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Old 20th January 2013, 02:53 PM   #33060
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Originally Posted by jan.didden View Post
I questioned the audibility at extremely low levels.
Read your own bookzine?
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