Stupid N00b can't build an AB box

[Colonel Angus]

 
I needed a utility box for my band. The goal was to have essentially and A/B/C switch so 1) no cord switching is needed for the Aux Guitar and Aux Keyboard (they are not used at the same time), but also 2) be able to use the Supro as a "country stack" (2nd amp for the main guitar) when the Aux is not in use. Here is the design I came up with:



The 3PDT are being used as 2PDT, I built this in a pinch with parts on hand

It doesn't look as crappy IRL, I swear...

All the sound goes where it  needs to be, but this @#$%ing thing hums like crazy!!! Only the keyboard input is quiet, I've tried multiple guitars and all the jacks and they are really hum-infused. This box is a paper weight currently.

I'm sure it is ground noise, but I wired the ground scheme to the best of my meager knowledge. Help, please!

Can someone please tell me how/why I am stupid? Also, any advice on fixing the hum problem would be clutch  

[iccaros]

did you use shielded wire?
try a star ground instead of everything being in series

[Colonel Angus]

No shielded wire. I'll try some. Should all the wire involved be shielded? Then connect the shielding to ground? I'll read up on star grounding and try that first. I don't have any shielded wire currently, just prebond stranded PVC coated, of the standard hookup variety.

Thanks iccaros, I hope to have this sorted for our show tomorrow. First one in 6 months, so the pressure is high. We have really been practicing hard and I think a clever switching scheme will put the icing on the cake.

[R.G.]

Whenever I see "A/B box" my mind internally says "hum".

It's not necessarily that you did anything wrong, just that hum is everywhere, and any time you connect multiple pieces of equipment, it has a door to get into the audio path.

Pretty much all simple A/B boxes will hum in some circumstances.

You may be able to patch around it by having the 3PDT switches switch both signal and grounds so that the active path only has the active input and output devices, including their grounds.

[iccaros]

No shielded wire. I'll try some. Should all the wire involved be shielded? Then connect the shielding to ground? I'll read up on star grounding and try that first. I don't have any shielded wire currently, just prebond stranded PVC coated, of the standard hookup variety.

Thanks iccaros, I hope to have this sorted for our show tomorrow. First one in 6 months, so the pressure is high. We have really been practicing hard and I think a clever switching scheme will put the icing on the cake.

Shield audio wire, no need to shield ground
advice, 1 listen to R.G.
Advice 2, buy or barrow a A/B/C box for the show, too easy to let a simple mistake interrupt a good show. You can build this for your next show you got because of this one.

but in any case, make runs as short as possible.
A star ground has ground come from one location and run to each ground. This way you have no loops.
As R.G. Noted, some noise is unavoidable, stage lights are horrible 

Look in to a buffer for each input, this would be an active switch, all inputs would present the same load after the buffers

[Colonel Angus]

Thanks guys!

I think we can do the gig without a box, really the impetus was being able to use the supro on everything (it makes you sound like jimmy page  ).

I'm interested in the technique RG described, but lack the mental faculty to visualize the layout (it was hard enough to visualize this flawed scheme)

I'll start by star grounding the thing. From which point should the star ground originate? Output Jack?

The buffer idea makes sense, especially considering that the box will be switching between the Nord Electro 2 and an active Ovation 12 string electric.
Something like the Jack Ormann buffer? Just build one on perf for every input and wire it after the jack, before the switch?

Some noise is acceptable in this situation, but this is an insane amount of noise. Especially considering that there is no noise from the keys, makes it pretty jarring (and im sure doubly so for a soundman) when you switch to the guitar.

[iccaros]

I'll see if I can draw up what R.G. Wrote.. I could be wrong but I think he is says that you could switch the ground and the input, which separates the ground making it point to point.  I think you need another switch and they are on or off, so you could end up with a/b/c  ab/ ac /bc /ac /abc..  in which case mixing resistors would be needed.

[Colonel Angus]

Oh got you, that makes sense. 3 separate switches. Makes sense.

I'm gonna fiddle with it when I get home, start by changing the ground wiring and work from there. I don't have any more switches but I just ordered a 10pk of china blues (24.99 shipped from US on Ebay) so I can mess with that idea later.

[iccaros]

try it with the two switches you have (two inputs) and see if that works.

For a star ground I would use a point I create in the center attached to the chassis of the pedal as my star point.

[Colonel Angus]

Cool. That makes sense. Duty called last nite so no project time, looks like sunday will be the day.