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

[Harry Bissell]

If the resistors are through-hole, you might arrange them in vertical mount (with the input
lead short to the board)  or like cordwood (one row against the board, one mounted in between
and over the top). Or you could use small SMT parts and get the resistors closer...

THIS is one instance where a ground plane underneath would be a definite plus...

BTW, if you are summing so many inputs, the output must be able to source (sink) the sum
of all the currents... so the resistors need to be fairly high value, or the opamp needs
a lot of output current. Here's another vote for sub-mixing !

H^) harry

----- Original Message -----
From: Oscar Salas <osaiber at yahoo.es>
To: 'Synth DIY' <synth-diy at dropmix.xs4all.nl>
Sent: Mon, 09 Aug 2010 15:35:14 -0400 (EDT)
Subject: Re: [sdiy] Length of a summing node (was Re: AC coupling caps on	MS20 clone)


> 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.



      

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-- 
Harry Bissell & Nora Abdullah 4eva