3PDT Dual FX Loop...

[mg.audio]

I am building a Dual FX loop box and I would like to make it work using 3-pole stomp switches for both bypass and A/B functions. Here's a schematic, excluding the bypass section and saving one pole of the stomp switch for LEDs:

The bypass section is just going to be a typical 3PDT true bypass, in which the input of the FX loop is grounded in bypass mode.
My question is this: does hard-wiring the return jacks together like this pose any impedance or noise (or other) problems? (I'm guessing that depends on what goes in the loops.)

[tommycataus]

I have built one of these for my friend (I think it's the same) and he's had no problems so far.

http://i1015.photobucket.com/albums/af272/tomw_aus/Untitled_zps21a9a941.png

http://i1015.photobucket.com/albums/af272/tomw_aus/ABFXLoop_zps738b6f82.png

It didn't look quite as messy in real life as the version in paint, but I'm not great with schematics! Hope this helps.

[PRR]

Two returns symply Y-ed together "may" fight each other.

[Ben N]

You have an extra pole, why not use it to select a return? Or, in the alternative, put in mixing resistors, or a simple virtual ground mixer.

[PRR]

> extra pole, why not use it to select a return?

saving one pole of the stomp switch for LEDs

[mg.audio]

I have built one of these for my friend (I think it's the same) and he's had no problems so far.

Tommy - your schematic is different in two ways, 1: You have both pairs of send and return jacks wired to the switch, therefore you won't have any issues with the loops interfering with each other. 2: I want the input of the bypassed loop to be grounded, to avoid switch popping and reduce noise. Has your friend reported any popping when selecting between the two loops?

Two returns symply Y-ed together "may" fight each other.

Yep, and I can think of two ways this would happen; either the output of the "bypassed" effect loads down the signal coming from the active loop OR there's the possibility of the noise floor of effects in the "bypassed" loop being added to the active loop. I would think grounding the input fixes the noise problem unless, perhaps, there is a noisy pedal like a heavy metal distortion or a DOD Gonkulator in the loop. This may not be an issue for the guy who will be using it (hottest pedal he's got is a Keeley-modded RAT as far as I know).

[PRR]

Let me say it clearer: two outputs shorted together WILL fight.

In some cases they may "mix", but the one with the lower output impedance dominates the mix.

In cases where opamp outputs face each other, with different signals, the output will be gross distortion and hot chips.

The hiss question is less interesting (because results will be unsatisfactory for reasons above). But note that many boxes hiss even when their inputs are shorted.

> the guy who will be using it

If you made it for me, and I found that you had hard-shorted two of my precious devices' outputs to each other, I'd be upset.

[electrosonic]

Doesn't ground the unused loop, I don't see an easy way to do that without adding an active device.