Burn-in time for testing op amps ?

[KlausB]

Hi everybody,

for my phono stage I use the OPA627 and OPA603 in a composite topology as shown in the schematic below and I must admit that I am very pleased with its sonic performance.

Unfortunately the CFB OPA603 is out of production. In search for a suitable substitute I tried three alternative CFB op amps (LM6181, LT1206, AD811). Every op amp was allowed a burn-in for 24 hours prior to testing (connected to +/- 15V supply voltage for 24 hours). After first tests I chose feedback resistors of 820 Ohm instead of 1000Ohm for the three op amps to be tested.

The LM6181 and the LT1206 sounded harsher compared to the OPA603. On the other hand the AD811 seemed to lack some air and detail in the sonic image. To my disappointment none of the three op amps tested could qualify as a convincing substitute for the OPA603.

My question:

Were time and/or method for burning-in insufficient to test the op amps ? Did anyone made similar experiences ?

Regards and thanks

KlausB