op-amp mixer configuration

[oolfur]

okay - might seem like a dumb question, but i'f rather check and double check with someone before fabricating my synth design!

so - i want to sum 4 different signals together, coming from a voltage controlled signal selector...

the output has to be unity gain, so my question is this: is there any practical difference between using an op-amp in a "buffer" configuration versus using a non-inverting amplifier in this case?

(in a unity gain application, what is the difference anyway? i'm having trouble wrapping my head around this...!)

right now i have each signal connected to the positive input of the mixer op amp through a 10k resistor respectively. a 10k resistor is going from op amp output to negative input.



any reason to believe this could be unstable?

would a non-inverting mixer perhaps make more sense? it would require one more resistor going from negative input to ground...!

any thoughts greatly appreciated,
thank you...

[blue hell]

The circuit you propose above will probably give disappointing results as the actual circuit that is before the mixing resistors will mess with the signal levels of the other inputs as well (when the driving circuit would have a non-zero output impedance, like when you would switch it on / off).

Mixing on the inverting opamp input does not have this drawback, but it would be an inverting mixer then. So you may want to add a 2nd inverting opamp after the mixer to get the signals go in the right direction again.

See http://electronicdesign.com/ideas-design/efficiently-design-op-amp-summer-circuit for some explanation of this.

[Grumble]

The reason why the schematic I show in my reaction is usually used as a mixer and yours is not is as follows:
An opamp will amplify signals until both the inputs have the same potential.
In the diagram in this post the positive input is at ground level, and the opamp will amplify until the negative input is at ground level (when possible!) and that is why you can use a number of inputs that will not influence each-other.

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[oolfur]