National opamp inflation

[contravalent]

Hello, Whaleman: I would suggest you post a picture of your completed prototype so that layout could be considered. One thing about your design is you have very very high bandwidth. This means that input-output couplings that are very small can cause oscillations. The first thing I would do would be decrease the bandwidth of your amplifier chain by a factor of 10 or so. It is likely the hiss you hear is actually some kind of high frequency oscillation, not noise.

 

[tiefbassuebertr]

I tried all these suggestions. There was a good deal of hiss, enough for the pre to be unusable. I remember now that the 32.5K input resistor was necessary to eliminate the hiss, I tried 220 and 1.2K ohms at the input of the 2604 buffer and tried different combinations of input/FB resistors at 49713. No workie...

There are no details concerning your application and PCB layout. Additional there are only few advices concerning this and no simplified circuit in the datasheet under
http://80.93.56.75/pdf/0/5/5/2/1/05521640.pdf
This makes it difficult to mention helpful advices for perfect operating.