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A-B Switch: Reason to Get Complicated?
Old 2 weeks ago
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A-B Switch: Reason to Get Complicated?

Hi all, long time lurker first time poster. I finally signed up so I could ask about the best way to approach a very simple project: An A-B switch for my guitar. Here's what I'm thinking:

  • 1 guitar input to be selectable between two outputs.
  • Silent operation (no pop during switch, no hum).
  • An LED indicator to show active channel.
  • I'd prefer the LED be bi-color (polarity controlled) so it can tell me if the pedal is on, and which channel is selected.
  • Would be cool if it accommodated TRS cables as well as TR.

Some time back I built a box that used a mechanical relay to switch the audio signal as well as an indicator LED, but the pop is unbearable and I want to create a dead silent one. Testing the one I made, I have found that part of the pop is due to the relay, and part of it is due to the switch I use to drive the relay. I'm assuming it may also be due to something else, but it's definitely true that these components are clunky, noisy things that should probably never have been used for audio.

I have in mind that I can accomplish what I'm after using a 3PDT switch with this design (it won't let me post images yet, so please click it):

A couple questions do come to mind though.

The switching aspect of this design would work regardless of whether it's got power, which I appreciate. However, I noticed in the one I made previously that if did not ground the case (which was connecting to all shanks) on the (-) terminal, I got a hum on both amplifiers. With the above schematic, if I didn't have it plugged in, that would be the same scenario, I believe. Is this an example of a 'ground loop?' Is there a good way to deal with this?

Also, looking around for A-B switch schematics, I'm seeing examples that connect the tip terminals to ground when not in use. For example, the below link, which includes pulldown resistors as well. Are these two features (pulldowns and tip grounding) advisable?
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