my external-input amp (revised)

[Don Tillman]

   Date: Thu, 13 Jun 1996 10:48:36 -0800
   From: haines at apc.net (Matt Haines)

   > You can't put the pot between the output and (-) input like
   >that; any lack of connection as you turn the pot will open
   >the feedback loop causing the output to slam to one of the power
   >supplies.  Ie., it'll be *really* scratchy as you turn the pot.

   Good point. OK, I've changed the value of the resistor going from (-)input
   to gnd, and changed the pot value, and added a resistor in parallel. 

Umm no, that doesn't solve the problem.  A continuity issue with the
wiper will still cause the gain to jump to its max.  A far better
approach is to simply place the pot from the (-) input to a resistor
to ground, so that when there's a continuity issue with the wiper, the
gain just falls to one.  (Ie., it turns into a follower.)

That does leave you with a minimum gain of 1, which may or may not be
a problem.  If so, I'd recommend using a volume pot at the input and
a gain range switch in the feedback loop, again from the (-) input to
ground.

   >Also, the point of the clipping zeners is to keep the opamp from
   >raw clipping, but that won't happen when the zeners are after the
   >opamp.

   So where would they go? Before? In the feedback path? 

Yes, the feedback path.  That way the diodes will clip the signal far
more gracefully than overloading the various internal opamp stages.

							 You had mentioned
   diodes (apparently of the non-zener variety) in some of your comments,
   which I added at the input stage and quoted you. 

Yes.

   Should these be zeners instead? 

No.  Think about their purpose; they keep the signal from going more
positive than Vcc or more negative than Vee.

   As for the ones on the output...won't these protect my other
   modules in case something dire goes wrong? 

The 1.0k output resistor already limits the output current to 15mA.
What additional protection were you thinking of?

Y'know, this is pretty basic stuff and it's covered in many books much
better than I can possibly explain it tapping at a keybaord.  I don't
have the time to write up a serious textbook on how to design preamps
so instead I've been trying to just slide you a couple hints in the
hope that it would spur you into researching the issues yourself.
That's really how one learns this sort of thing.

  -- Don