[sdiy] Circuit purpose?

[David G Dixon]

It looks like an AC coupler with two electrolytics back-to-back to make a
bipolar electrolytic, with a resistor to ground to shunt any DC.  I don't
understand the 680R resistor, though.

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of 
> David Ingebretsen
> Sent: Monday, March 17, 2014 6:23 PM
> To: Synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] Circuit purpose?
> 
> 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
> 
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