Mixer opamp, which one?

[Ingo Debus]

Mark Havard-Brown wrote:
> mixer I presently have, the builder has used a TL081. The datasheet I have
> on the TL071 specifys the devices noise distortion, whereas there is no
> mention of the subject on the TL081 datasheet, (both sheets are from
> Motorola). Question, which is the better to use?

TL071. Less noise, almost the same price.

> Additionally, the designs I have do not use more than six inputs. Are there
> any issues with the number of channels one just "bolts" onto the input of
> the opamp"?

In my mixer there are 12 inputs summed with one standard summing opamp
circuit, no prob at all. I use resistor arrays (one common pin for
several resistors) for the summing resistors, they are handy for PCB
layout.
But: although the gain from one input to the output of the summing amp
is low (1 in my mixer) the noise gain is pretty high when there are many
inputs. It makes a difference if a TL072 or a NE5532 (less noise) is
used here.

Ingo