AW: Re: external input (again)

[Haible_Juergen#Tel2743]

>
>Thanks Gene! If I *were* being audiophile-minded, what would I do 
differently?
>
>Left in for reference.... |
                          |
                         \ /
                          `

>>
>>  input o----+
>>             |                          3.3M
>>             |                      +--/\/\/\---+
>>             /                      |           |
>>     100k    \      0.1    100k     |  |\       |
>>      pot    /<-----||----/\/\/\----+--| \ (-)  |
>>             \                         |  \     |       1k
>>             |                         |   \----+--+---/\/\/\------ out 1
>>            GND                        |   /       |    1k
>>                                       |  /        +---/\/\/\------ out 2
>>                                    +--| / (+)     |    1k
>>                                    |  |/          +---/\/\/\------ out 3
>>                                    |
>>                                    \
>>                                    /
>>                                    \  100k
>>                                    /
>>                                    |
>>                                    |
>>                                   GND
>>

Ok, here we go:

As Gene pointed out, high impedance -> much noise.
To make it worse, the 3.3meg will cause a serious bandwidth
limit if you just take the ever-present stray capacitance into
account.
Having full gain, but attenuating before the amplifier, is a waste
if you have strong signals.

There are many ways to improove the design, but I'll choose
you one that will use as few components as yours, but has much
better performance in case you don't crank the pot fully up,
and still slightly better performance, if you do.

(Asuming you need 50k minimum input impedance, as you have
in your circuit.)

(1) connect the input directly to the capacitor.
(2) change the capacitor value to 0.22u or greater
(3) change the input resistor from 100k to 47k
(4) replace the 3.3meg resistor with a log potentiometer
      of 2.2meg. Ccw goes to opamp input, Cw *and* taper
      goes to opamp output.
(5) Use a FET input opamp (like TL071 or LF 351) and discard the
      100k resistor to the positive opamp input.

This way you only have the gain, noise gain, and bandwidth
limit which you actually need for a certain input signal strength.

JH.