Fast attack, slow release envelope detector
This may work for you. We are not a design house per se, but this would get you a functional minimum and maybe all you need. Your option to use a single ended op-amp but it needs to be a JFET/MOSFET type because the second op-amp (2 per package) is a very high impedance buffer so you can have a long decay time.







simulate this circuit &ndash; Schematic created using CircuitLab