Did you look at the NS circuit I posted in post #4? The second Op Amp is configured as an Integrator in order to do the arithmetic & to filter the Full Wave rectified signal to provide a DC level. Thus the R6 & C5 filter in the circuit posted by the Op is not necessary.A peak detector is supposed to have a very fast attach and a slow release that cannot happen if the second opamp has a capacitor across its negative feedback resistor.