RG Keen mixer, or stereo summer, or something.

[sbm]

Hi all

Been a long time, and built half a dozen really cool pedals out form here.  Thank you to all.  Now that life has cleared up somewhat, and am re-learning how to speak solder again, I finally have something I'd like to build, which is essentially a stereo summing mixer that will accept XLR inputs.

I have some effects that have stereo XLR outputs, and would like to ultimately combine one L/R pair input into a mono 1/4" output (i know, I know).  I have been studying various schematics as examples that do variations on the theme, like with this https://www.diystompboxes.com/smfforum/index.php?topic=125388.msg1194940#msg1194940.  But I have also looked at some other examples, which scale from simple to more elaborate.  So in potentially reverse order....

For example, the Rane SM26S (page 30) I suppose I could take each mono for each of left and right:

https://www.ranecommercial.com/legacy/pdf/sm26s_manual.pdf

The really interesting example from the Eventide H3000, particularly interesting how if I read this right + and - appear to get summed on each separate input (page 60):

https://cdn.eventideaudio.com/uploads/2021/09/H3000-Service.pdf

I guess with all those opamps they are doing something souped up!  But maybe baby steps, starting with some simpler examples:

http://www.all-electric.com/schematic/simp_mix.gif

and,

http://generalguitargadgets.com/pdf/ggg_mixer_sc.pdf

I guess what I'm asking is if starting and keeping it simple will get me there by cobbling bits and pieces together until I arrive at something that sounds good.  Any guidance on where to start appreciated.  Thanks!

[antonis]

Any guidance on where to start appreciated.

https://user.faktiskt.io/rikkitikkitavi/SELF/Small%20Signal%20Audio%20Design%20(2010).pdf
Chapter 12 & Chapter 14.. 

[sbm]

Wow, now that is cool!  Thanks for that.  I'm going to keep looking at other schematics and thinking about this, maybe even breadboard bits and pieces of various circuits to try out.

[antonis]

Just take into account that the author of the above superlative dissertation could be characterized as "ultra-low-noise obsessed" with serious impact on resistor values and transistors/ICs selection..

[Elektrojänis]

The really interesting example from the Eventide H3000, particularly interesting how if I read this right + and - appear to get summed on each separate input (page 60):

It's not actually summing. Summing would cancel out the signal and sum the noise. The circuit actually takse the difference of the + and - signals so it will get a bit hotter signal than either of them alone and cancel out the noise.

If you are going to work with balanced signals (XLR:s usually mean that), you might want to read up on balanced connections. There is a good series of articles on Elliot Sound Products site:
https://sound-au.com/balance.htm
https://sound-au.com/articles/balanced-2.htm
https://sound-au.com/articles/balanced-io.htm
https://sound-au.com/articles/balanced-4.htm

You might want to browse other articles on that site too.

Then again if you just want to combine two balanced XLR-outputs to drive one unbalanced mono input, a few resistors from the hot pins of the XLR's to the tip of the plug might work just fine. Or just take one of the outputs and leave the other unconnected, Mixing the outputs could produce results that are worse depending on the effect.

[momo]

"Stereo Summer"

Nice band name...

:-)

[sbm]

@Elektrojänis: Thanks for those links, most helpful, and good additional reading for the weekend.  Speaking of Stereo Summing.....I came across this Rane note (109 I think it was) which talks about exactly that, using various methods:

https://www.ranecommercial.com/kb_article.php?article=2106

That same manual I found for the SM26S also discussed the methods to come up with adapter cables too, so that was cool.  Much to chew on.