AW: my external-input amp (revised)

[Haible_Juergen#Tel2743]

> 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?

No. The additional resistor is not necessary, and it will bend your pot
courve. Simply connect the pot as I originally suggested:
ccw end to opamp input, tap *and* cw end to opamp output. This way
the opamp will see the maximum pot resistance in its feedback loop at
least, if the tap looses contact.


>>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?

Depends on what you want: If your intention is to prevent the opamp
from saturating, put them into the feedback path. If you want the zeners
to prevent the circuit from overvoltage from the output, leave them
where they are. If you want both, you probably need two zener pairs.


JH.