John Curl's Blowtorch preamplifier part II

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#76,042
You are arguing all these days over what exactly? (“For the poets don’t like to be understood”)
If you would spare 1:17:57 away from this forum and notice what is been said here
https://www.youtube.com/watch?v=xdZMXWmlp9g

George
 

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#76,047
gerhard said:
I have found my note, he said 0.5V. Just so in passing, it was no bragging contest.
Click to expand...

Sorry, I simply meant I have never seen any effect that would qualify as huge. For instance the 5534 does not have a reputation for huge 1/f noise difference with common mode voltage. Most op-amps will have cascodes and operate at ~0 Vcb, but AFAIK the 5534 and LM833 do not and neither has a high 1/f corner.

EDIT -reference with measurements of opposite effect, http://tf.boulder.nist.gov/general/pdf/1139.pdf
 
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#76,048
OPA1642

Speaking of common-mode effects, has anyone looked carefully at the TI OPA1642? From the simplified schematic on the datasheet, it appears the input stage is a rather simple NJFET pair and a ballasted PNP current mirror --- no cascoding etc. So I would expect the common-mode distortion to be fairly high.

However, they may have omitted some crucial details.
 
#76,049
bcarso said:
Speaking of common-mode effects, has anyone looked carefully at the TI OPA1642? From the simplified schematic on the datasheet, it appears the input stage is a rather simple NJFET pair and a ballasted PNP current mirror --- no cascoding etc. So I would expect the common-mode distortion to be fairly high.

However, they may have omitted some crucial details.
Click to expand...

Please see the attached file, OPA1641 was measured.
 

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#76,050
bcarso said:
Speaking of common-mode effects, has anyone looked carefully at the TI OPA1642? From the simplified schematic on the datasheet, it appears the input stage is a rather simple NJFET pair and a ballasted PNP current mirror --- no cascoding etc. So I would expect the common-mode distortion to be fairly high.

However, they may have omitted some crucial details.
Click to expand...

The input common mode range is only 3.5V to the + rail so possibly the schematic is over simplified as there might be room for a cascode.
 
#76,056
I was looking at the part for a budget MM phono pre application, and noted the 5.1nV/sq rt Hz noise density, the preceding part having been the OPA2134.

But I think I will allow a trifle higher parts count and use an AD797 or LME49990 preceded by a BF862 cascoded source follower and servo. The overall input voltage noise will still be acceptably small, although could be reduced by a more elaborate series feedback arrangement for the JFET. Things rapidly spiral into complexity, and I'd like frequency compensation to be simple for a change.
 
#76,059
bcarso said:
I was looking at the part for a budget MM phono pre application, and noted the 5.1nV/sq rt Hz noise density, the preceding part having been the OPA2134.

But I think I will allow a trifle higher parts count and use an AD797 or LME49990 preceded by a BF862 cascoded source follower and servo. The overall input voltage noise will still be acceptably small, although could be reduced by a more elaborate series feedback arrangement for the JFET. Things rapidly spiral into complexity, and I'd like frequency compensation to be simple for a change.
Click to expand...


cascoded source follower? 😕


< http://www.analog.com/library/analogDialogue/archives/47-10/discrete_amplifier.pdf >

regards, Gerhard
 
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