my external-input amp (revised)

[Matt Haines]

>   Date: Thu, 13 Jun 1996 01:27:19 -0800
>   From: haines at apc.net (Matt Haines)
>
>   OK, here's my revised external-input amplifier schem.
>   Please feel free to comment.
>
>Accch!

"Accch!" yourself! :)


                          __________
                          |        |
                          \       GND
                          / 82k
                          \
                          /
                          |       30pF
                          +-------||--+
                          |           |
                          +--/\/\/\---+  1M resistor
                          |           |
                          |     /     |
                          +--/\/\/\---+  5M pot
                          |   /       |
                          |  |\       |
                          +--| \ (-)  |
                 +V          |  \     |       1k
                  | (diode)  |   \----+-----/\/\/\---+--- out
                  ^          |   /  LF351            |
        0.22uF 1k |    47k   |  /                    v
input o-||--/\/\/\+--/\/\/\--| / (+)                 -  (reverse zener pair,
                  |          |/                      ^   5.1v each)
                  ^ (diode)                          |
                  |                                 GND
                 -V

 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. With
the pot at max resistance, the combined resistance will be 833k. With the
82k resistor to ground taken into consideration, we get a gain of 11 or so.
With the pot acting as a short, we get a gain of 0. If the pot goes dead
momentarily (i.e. it's scritchy), we have an effective resistance of 1M,
which would give us a gain of 13. So scratchy pots aren't going to be all
that noticeable.

So far so good?

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

Thanks.


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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.