Odp: [sdiy] 1/8th inch questions

[Seb Francis]

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