my external-input amp (revised)

[Matt Haines]

OK, here's my revised external-input amplifier schem. The goal is to bring
audio signals from the outside world, such as samplers and other synth, up
to the levels best suited for a modular synth input. So I'm looking to
bring audio of about +/-0.5v up to +/-5v, for a 10:1 gain. The changes are
based on what folks here on DIY have suggested as improvements. I've quoted
the comments below, and have tried to incorporate the ideas into the new
version.

Please feel free to comment.

                          __________
                          |        |
                          \       GND
                          / 220k
                          \
                          /
                          |       30pF
                          +-------||--+
                          |     /     |
                          +--/\/\/\---+ 2M2 pot
                          |   /       |
                          |  |\       |
                          +--| \ (-)  |
                 +V          |  \     |       1k
                  | (diode)  |   \----+-----/\/\/\---+--- out
                  ^          |   /  LF351            |
        0.22uF 1k |    47k   |  /                    v
input o-||--/\/\/\+--/\/\/\--| / (+)                 -  (reverse zener pair,
                  |          |/                      ^   5.1v each)
                  ^ (diode)                          |
                  |                                 GND
                 -V




Don Tillman says:

>And lose the gratuitous polarity inversion.  (That's considered tacky
>in both audiophile and engineering circles.)

Juergen Haible says (based on my original (-)input op-amp):

>(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.

Don also said ("pot" referring to the previous horrible design):

>I meant input proection like this: Input jack to 1.0k resistor, other
>side of that resistor goes to the anode of a diode to +15v, and to the
>cathode of a diode to -15v, and to the actual input (I guess that
>would be the pot in this case).

John S. said:

>     I would put a shunt series reverse pair of zeners right on the output
>     of the op amp to ground.  You would pick the max voltage you want
>     (hard limit) by adding the forward voltage drop of a diode (0.7V) plus
>     the zener voltage of the diodes.  For example, picking a limit of
>     6.8V, you would pick 5.1V zeners.



If I didn't quote someone or forgot to include a good suggestion, my apologies.

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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.