Odp: [sdiy] 1/8th inch questions

[harrybissell]

Opamps may be 'short circuit protected' but they will get DAMN hot
if you allow them to limit their own current... and dual / quad units might
die with more than one output shorted...  resistor is cheap insurance.

H^) harry

Seb Francis wrote:

> So why use a resistor on the output at all if the buffer opamp can withstand indefinite short circuits?  Is it something to do with not glitching the power bus?  I guess the normal short circuit situation is simply plugging and unplugging jacks.
>
> Seb
>
> John Blacet wrote:
>
> > I believe that this refers to the practice of including the output
> > resistor (1K) in the feedback loop of the op amp. For example, if you
> > have a 100K feedback R and a 1K output R, the actual output is taken
> > from the junction of the two, not the output pin of the amp. Of course,
> > this changes the actual gain, so you have to figure the Rs for that. I'm
> > not sure if this is something you can do with a buffer configuration.
> >
> > In practice, we have opted to use a 100 ohm resistor for the output. The
> > 074X series for example, is rated for indefinite short circuits to the
> > supplies and ground. The 100 ohm is just extra current limiting, but
> > gives us 10X the "fanout" of the 1K typically used.
> >
> > --
> > Regards,
> > --/////--
> > John Blacet
> > Blacet Research
> > http://www.blacet.com