Odp: [sdiy] 1/8th inch questions

[Theo]

The resistors are there for two reasons (maybe more but I don't know the
others)

1) Opamps don't like capacitive loads, like capacitors or patch cords.
The resistors help prevent oscillation of the opamp.
In many schematics you will see 100R...1k series resistors all over the
place for this reason.
For more see application bulletin AN-257 on the Analogue devices website
(recommended read!)

2) accidentally patching two outputs together can go well beyond short
circuit.
So you still want some current limiting as protection.

Theo


From: Seb Francis <seb at is-uk.com>

> 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
>