ABY switch - is it more complicated than I had anticipated?

[Mattxorz]

Ok, I'm working on an 2 button A/B/Y switch so that I can select one, the other, or both of my amplifiers from my pedalboard with the push of a button. I'm using two 2PDT on/on switches and three mono (TS) phone jacks, like the ones you plug your guitar cable into.

As far as signal flow goes, I thought I had it all worked out. It does switch the path of my guitar's signal, but it also creates a good amount of hum noise. I was thinking maybe it was the two amp's inputs fighting to pull some current or something? Do I need to add some diodes? I really don't know. Oh, I also shielded the inside of the plastic casing I'm using with aluminum foil and have it grounded- I wouldn't expect this much noise from something like that anyway.

I'm attaching a quick diagram of my wiring. I apologize for now knowing schematic symbols. I tried to be as clear as possible.

Any ideas as to what could be wrong are greatly appreciated.

[J M Fahey]

To know where the problem comes from:
1) connect either to one of the amps, *or* the other, not both at the same time.
If you still have hum, you have a shielding problem.
2) now connect to both.
If now hum appears, or increases appreciably, you have a ground loop problem.
Can be solved but becomes more complex.

[phatt]

 What about this?
*This assumes both Amps are grounded back to the mains*

Cut the common to *One* of the outputs (won't matter which one)

Depending how complex your signal path is it may fix the hum.

If the ABY box is all metal you will need to use a plastic socket to lift ground or use an isolated socket of some sort.
Phil.

[joecool85]

If the ABY box is all metal you will need to use a plastic socket to lift ground or use an isolated socket of some sort.
Phil.

He said it was plastic but that he added tinfoil as shielding and grounded that.  Maybe he should remove the tin foil.

[Mattxorz]

I severed the ground connection from one of the outputs. That did seem to get rid of the amps loop humming, but it also un-grounds one of my outputs, so the amp plugged into that output hums even louder now.

[Enzo]

You have the classic ground loop.  Try this.  Turn on both amps.  No instruments.  Now plug a cord into the inoput of one and the other end into the input of the other.  Obviously no music comes out, but I bet it starts humming.

The loop occurs because the two amp chassis are being connected to gether by the shield/ground of the guitar cord.  In your case, the two amps have guitar cord to the AB box, where they are connected together.  Probably hums whenever the AB box is connected to both amps, and probably doesn't matter if a guitar is plugged into the box or not.

They make commercial ground loop busters.  I have made them with small transformers providing the isolation.  Ungrounding one of the amps will break the loop, but it is not the way to do it.  We need to break the ground path betwen the two amp chassis.

[Mattxorz]

Ok- before I address Enzo's solution, I changed a few things. I had some bleed between the amps when one was selected but the other was not, so I routed the output signal of each to ground when they're not selected. I also disconnected these grounds with the other switch when it is set so that both amps are selected. That way Neither will be muted (if one was, it would mute both because of their connection). I attached an up-to-date wiring diagram.


Enzo, that makes perfect sense. And you're right. The scenarios you described are exactly what's happening. Instead of transformers, could I break the ground loop with a diode right before the output jack ground connection of each output? That way each amp's ground would only let juice through one way? Let me know what you think.

[Mattxorz]

Nope- tried it. Apparently I don't have that great of a grasp on these things.

[phatt]

Hi Mattxorz,
                You need to wire it so that when both Amps are on (Y) then one ground is lifted,,,but when (A) or (B) is used and you ground the unused signal path then it matters not.

I'm guessing this as I've never done it the way you are attempting it but I'm familiar with ground lifting issues.

ENZO's right, you really need to ground lift the Amps internals but that may require a fair bit of work depending on the design and layout.

The internal Amp trick requires a pair of back to back diodes a cap and a 10 Ohm resistor between chassis and circuit common.

Go here for clues;
http://sound.westhost.com/project27.htm
Figure 3 down the bottom of page.

OR invest in an iso box which is made for the job.

Frankly I'd just google around as you can often find some gems.

Phil.

[Mattxorz]

I'm not going to do anything internally to my amps- I've got them for sale atm. I just decided to forget the "Y" functionality, and be content with an a/b switch for now. I got that working. Thanks for the advice, all.

[Enzo]

One way to bust the loop is a small 1:1 transformer.  You branch your Y as usual, one branch feeding directly into the input of one amp, the other branch feeds the 1:1 transformer primary.  The transformer secondary then fed the input of the second amp.  DO NOT connect a ground between the two sides of the transformer.  Now the transformer sends the signal on through, but there is no ground connection between the two amps through the signal cables.  Loop busted.

This is totally external; to the amps, nothing whatever is changed on them.

SOme pairs of amps will not create ground loops.  It is a matter of individual amps ground potentials.  This is not a defect in either amp.