amateurs not welcome (was: my external-input amp)

[Matt Haines]

>   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.

It causes it to jump to the maximum desired gain, but not to +V as before.
Only a partial fix, but I *did* come up with it myself, and it would have
avoided disaster at least. Oh well.

  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.

No matter how I configure a non-inverting amp, I'm going to have a minimum
gain of 1. But your solution is a good one.

>   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.
>

That's what I thought. But since you hadn't specified the location of the
zeners, I was double-checking.

>   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?
>

Just trying to figure out what you were talking about, and incorporating
other folks' ideas as well.

>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.

Oh I'm sorry, I didn't realize I was subscribed to "pro-DIY". I thought
this was "regular-DIY".

Don, I never asked you for help. I asked *DIY* for help. You offered your
comments, as did a number of other folks, and I'm thankful for the
knowledge I've gained. I never asked you (or anyone else) to write a
textbook on the subject. I never asked YOU SPECIFICALLY, for anything. You
volunteered the info. Thank you. If you don't want to help any more, just
don't answer. But DO NOT treat me in this manner.

Sure this is basic stuff...I'm a beginner, so whatdaya expect? I'm not
looking to become an engineer or even a full-time tech-jockey. I want to
PLAY this stuff, and building is a nice little
hobby/tax-write-off/career-enhancer. All I know about electronics I've
learned from a couple of books, ElectroNotes, and the good graces of AH and
DIY members. I haven't had the benefit of a classroom where questions can
be answered, so DIY/AH takes the place of that. My knowledge is admittedly
sketchy, and always will lag behind the pros such as yourself. If the
requirments of being a member of DIY were "knowledge „ Don Tillman" then
there might not be many people subscribed.

FYI, I went to the largest bookstore in my area last week, and searched for
books on op-amps. Nothing. (Although they had plenty on digital filters and
repairing microwave ovens and fluid dynamics.) They couldn't find anything
by the author you suggested. I took your hints, and have so far been unable
to do anything with them. Should I give up learning in the meantime?

This is stupid. I'm asking for help, and I'm getting slagged for it by
someone who didn't even need to answer at all. Is there anyone out there
who wouldn't mind discussing this subject with me, other than Don Tillman?
I'd normally ask Ric Miller these questions, but he's indisposed at the
moment and so I turned to DIY. Perhaps I should keep my questions private
until I've obtained my EE degree.

Oh and by the way, I can BUILD an amplifier circuit. Done it a number of
times already, for filter inputs, VCA's etc. But I want to build an amp
circuit best-suited for my particular needs based on real-world conditions.
Something textbooks don't always address well. Believe me, if there were
something in my copy of ElectroNotes called "external input amplifier" I
would have built it already and not bothered anyone.

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Matt Haines  haines at apc.net         .       .       .       .
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The above opinions are not my own, but those of my employer. I take no
responsibility for any opinions stated by my employer in this message.