At first I used a TL082 for the first and third opamp and an ne531 for the 2nd opamp ( the actual rectifier ) Due to eneven frequency response, I switched to an NE5534 for the 2nd opamp and reduce the compensation capacitor from 100pF to 33pF. There were still too many bumbs in the frequency response so I decided to connect the input of the rectifier stage directly to the signal generator, this improved things considerably, finally I changed the 3rd opamp to TL071, such that any DC offset could be trimmed. I will need to work on the implemetatiin of the input preamplifier, possibly do it in 2 stages of x10 using an ne5532. The diodes were BAT48 schottky diodes. IMO schottky are the best choice for this circuit, Germanium diodes leak too much current in the reverse direction and silicon places additional demands on the opamp particularly with small signals.
