A question about effect routing and impedance

[diablochris6]

I am interested in building a multi-effect box with parallel effects. I know about buffering the signal before splitting it and that a mixer is usually used for the output, but what if you don't want/need a mixer? I would rather use the volumes of the pedals themselves. Can you plop the two effect outputs onto one jack or pcb? I understand that impedance is a big issue here; if the pedals were the same output impedance, it wouldn't be a problem, but if they are different, another buffer would be needed? I rummaged around the forum but couldn't find much (maybe I don't know what to look for?), so if someone could point me to an overlooked, existing topic or offer advice, I am all ears.

Also, what is a good reference to learn about impedance in general?

[R.G.]

For that special case, you can use something like a resistor in series from each effect to the output jack. This will mix the outputs and let you have control of each "channel's" volume from the effects volume.

However, there will be some signal loss. You lose half your signal with two pedals, and more with each pedal you add. This is because each added pedal acts like the ground end of a voltage divider for all the other resistors adding at the output jack. Still, if each pedal has enough volume, it may be workable.

The simple thing to do is to do an inverting mixer with one opamp from a dual, then another inverter with the other opamp to get it back to not inverted. This lets each pedal use its volume control independently with no volume loss to any of them. Issues with passive mixing is why active mixers came to be.

There is a topic addressing this and having a schematic for a simple dual opamp mixer, but it's been a while.  I think it was over a "blend" issue; that's the two-input version of what you're trying to do for "N".

It also contains an explanation of some facets of impedance as it affects mixing issues.  I'll see if I can find it.

[R.G.]

http://www.diystompboxes.com/smfforum/index.php?topic=107666.0

http://www.diystompboxes.com/smfforum/index.php?topic=104070.msg930754#msg930754

[diablochris6]

Thanks for all the info, RG. It is giving me plenty to read as I am trapped underneath my sleeping toddler right now! There is plenty of meat to dig into with these posts, but I want to verify my initial findings. The easiest way to do this is with a simple voltage divider on the outputs, but another option is to use the mixer on GGG.com, keep inputs 1 & 2, and swap out the pots for fixed resistors? The two opamp sections invert the signal, so by going through 2 of them, the signal is non-inverted. Would this also alleviate the need for a phase inverter circuit like I see on the ROG splitter-blend?