The TL084 opamp has a 3MHz gain-bandwidth product. The 150K resistors in the circuit are, for this topology, about 10X the value for which stability would be assured. Users should check IC2 pins 7 and 8 for signs of ringing or oscillations at about 1MHz. If so at pin 8, add a few pF in parallel with R11. If oscillation is seen at pin 7, try a few pF in parallel with R8 and R9, but check for non-linear oscillation due to interaction with the diodes.
Also, the TL084 only has proper specifications for +/– 5V or greater supplies.
