Gain pot in an inverting op amp stage, DC issue

[Bo Deadly]

Nice catch, I guess the OP could use something like an OPA2134, or  even an OPA627 if you can afford it. the 4562 looks like a reasonable choice.

Actually the LM4562 would not work either. The optimal source impedance is voltage noise / current noise right? So it looks like optimal Zsrc for the LM4562 would be even lower than the NE553x by quite a bit. So one would  pretty much have to use a JFET op amp.

 

[abbey road d enfer]

The optimal source impedance is voltage noise / current noise right? So it looks like optimal Zsrc for the LM4562 would be even lower than the NE553x by quite a bit.

That is true, however the final performance depends on the actual values of noise voltage and noise current.  As long as the impedance seen by the opamp is close enough to the OSI (i.e.not more than 2x and no less than 0.5x) the lower noise voltage of the 4562 will make it a winner against the 5532.

So one would  pretty much have to use a JFET op amp.

The OSI of FET opamps is in the megohm range, so the actual noise voltage is the dominant parameter; most FET opamps have a significantly higher noise voltage than bipolars. OPA134, with 8nV/sqrtHz, is about twice noisier than 5532.

 

[saint gillis]

BTW thank you John Roberts, it works very well !