On Tuesday, October 25, 2011 1:40 PM, "Bob Ammerman" wrote:
>=20
> > Plugging an audio cable between two devices that are powered by AC can
> > put a lot of stress on opamps, whose inputs can sometimes be almost
> > directly connected to the input jack.
> >
> > Is there a device that is preferred for clamping audio inputs, like a
> > bidirectional surge absorber is used for power input, but suitable for
> > this application?
> >
> > A reasonable amount of capacitance can be tolerated, say up to 100 pF.
>=20
> Capacitively couple it on both the ground and "hot" sides to avoid common=
=20
> mode problems?

Hi Bob,

It's the transient that occurs during the plugging-in. Typically an RCA
or phone plug contacts a signal to ground or a signal to signal before
the ground to ground connection is made. Imagine connecting the signal
input of a grounded device to the ground of a laptop computer, one that
has a two-prong power plug. It already is capacitively coupled, the EMI
circuitry of the laptop's power adapter has a capacitor from AC center
tap to DC ground. So you are shoving 60 VAC (120 in 240 volt lands) into
a poor little opamp. It's very lively.

Eventually the grounds of the two systems make contact and things
usually are fine from then on. Usually.

Best regards,

Bob


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