[sdiy] Length of a summing node (was Re: AC coupling caps on MS20 clone)

[Oscar Salas]

> I've also heard that a summing node
> at the -ve input of an op-amp should be kept fairly small to
> limit noise.
> Is this true, and if so, to what extent?

I found it on the "IC Op-Amp Cookbook" by Walter G.Jung. (pag 158.)  He says that the inputs of any op-amp should be minimum length conductor  as possible because stability. And on "Audio IC Op-Amp Applications" (pag. 19) adds that in audio circuits the minimum-lead-length and ground quality are good for low noise amplifiers.
Also I found these application notes with other subjects but pointing to short traces in op-amp inputs:
www.ti.com/sc/docs/apps/msp/journal/aug2000/aug_09.pdf
http://focus.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=slyt190&fileType=pdf&track=no

> For example, in a multi-voice synth, I could have one mixer
> resistor on each voice board, and then have one bus wire
> which takes all those resistors to the summing node. This is
> supposed to be "bad" because the summing node is "stretched"
> along the whole bus wire.
> The "good" way to do it would be to run 16 wires (it's a 16
> voice synth, ok?!) back to the mixer resistors which are all
> next to the op-amp. That way the summing node is small and
> won't pick up noise.

As I understood input resistors should be close op-amp input. Better, so,  travel along the PCB with the low impedance traces as the op-amp outputs.
If 16 resistors on line in a op-amp input will make the node so long what about sub-mix them in groups of 4? you will need but four op-amps more.

Oscar.