[sdiy] Circuit purpose?

[David Ingebretsen]

I've run across a circuit bit a couple of times now that follows the output
of an op-amp, and I don't understand its purpose. It consists of two 10uF
electrolytic capacitors connected negative to negative with a 680 ohm
resistor between the two negative sides of the capacitors. There is then a
100k resistor on the positive side of the output capacitor to ground.

                         +              680R             +
OP-AMP  >--------| |------/\/\/\/------| |--------------->output
                                                                 |
                                                                  \
                                                                  /
                                                                  \    100k
                                                                  /
                                                                 |
                                                                 |
                                                               ----
                                                                --    GND

Why have this? What is it doing for the circuit?

David