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

[David G. Dixon]

>From what I can gather, most of this is only an issue above 100kHz.  For
audio-frequency stuff, we should be fine.


> > 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?baseLiteratureNumbe
> r=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.