Question about OPA power reference in mixer designs

[Bo Deadly]

I think this was alluded to but you might consider making each channel AGND a differential input at the mix amp. This is done in Mackie mixers as described in here:

  https://groupdiy.com/index.php?topic=74613.msg944764#msg944764

Meaning the grounds are summed like the signals but passively and so with much smaller resistors.

Just because Makie's are cheap doesn't mean they're not designed well. That "trick" probably results in a huge improvement in crosstalk at least. And it probably wouldn't be that hard to retrofit into an API style console like yours as long as you have all signal grounds referenced to it's AGND. If you so much as accidentally connect a shield wire to the ground of another circuit it would ruin the whole scheme.

 

[JohnRoberts]

I think this was alluded to but you might consider making each channel AGND a differential input at the mix amp. This is done in Mackie mixers as described in here:

  https://groupdiy.com/index.php?topic=74613.msg944764#msg944764

Meaning the grounds are summed like the signals but passively and so with much smaller resistors.

Thanks for another example of using differentials.

That is pretty much a standard differential summing bus... The summed local grounds feed a virtual earth + input. 

There is vanishingly little benefit from making the impedance of the + and - bus the same (not much common mode noise inside a metal box), but a real practical benefit from making the ground summing Rs small (like lower Johnson noise that that would be boosted by N+1 ).

Just because Makie's are cheap doesn't mean they're not designed well. That "trick" probably results in a huge improvement in crosstalk at least. And it probably wouldn't be that hard to retrofit into an API style console like yours as long as you have all signal grounds referenced to it's AGND. If you so much as accidentally connect a shield wire to the ground of another circuit it would ruin the whole scheme.

Not a trick (IMO), and both Greg Mackie and Rick Chen (his lead design engineer) know their way around basic mixer design.

Don't get me started on the actual tricks they used..    They kicked my ass in the market place (mostly with advertising) but at this point it's ancient history. The millions they invested in advertising "made in USA" came back to bite them when they moved production to China (karma?).

JR