How to implement a ground lift switch for a buffered A/B/Y pedal?

[joulupukki]

Instead of using one of those 120 VAC power adapter plugs that takes 3-prong to 2-prong on one of my amplifiers, I’d like to be able to have a switch on my buffered A/B/Y box. The problem is that without that adapter I get hum in one of the amps. Is it possible to prevent the hum by adding a switch like this?

The original schematic is available here: https://generalguitargadgets.com/pdf/ggg_ic2buf_lo_aby.pdf

 

[58Bassman]

You don't want to lift the ground- it's not safe for the electronics, or you.

Have the electric service repaired or if you can't, use something like a HumX.

 

[joulupukki]

With both amplifiers plugged into the wall with the 3-prong plugs (where ground is connected to both), isn’t the reason for the hum being the fact that when you connect both amps to the ABY switch, there’s not a clear path to a single ground, hence the hum (indicating a ground loop)? Each amp on its own doesn’t have hum.

 

[joulupukki]

Maybe I should be using an audio transformer to isolate the ground loop instead of lifting it? I guess I’m trying to build something like this: https://www.dpeffects.com/2016/03/isolated-aby-splitter.html

Like, could I simply add this to output B?

 

[58Bassman]

Maybe I should be using an audio transformer to isolate the ground loop instead of lifting it? I guess I’m trying to build something like this: https://www.dpeffects.com/2016/03/isolated-aby-splitter.html

Like, could I simply add this to output B?

View attachment 1098704

As long as this is used at line level, it would isolate the amps. For that matter, you could go to a car stereo shop and use one of their isolators- they're not expensive and I have used them for distributed audio systems in homes.

Jensen Transformers are considered to be one of the best but if you want to spend less, Edcor is another very good brand. Check their site for items and cost.

 

[Bill Moore]

I think you would be fine isolating one of the outputs, even using isolating washers with your existing jack. Seems I have a Boss pedal switch that has an option to lift the signal ground/shield.

 

[joulupukki]

Maybe I should be using an audio transformer to isolate the ground loop instead of lifting it? I guess I’m trying to build something like this: https://www.dpeffects.com/2016/03/isolated-aby-splitter.html

Like, could I simply add this to output B?

View attachment 1098704

Just following up on this. I added this to the B output today and it works perfectly. No more hum and the sound of the guitar signal sounds the same as before. Now each amp can keep their ground and no more hum when I’ve got both amps connected through the ABY switch.

 

[joulupukki]

Ok, another follow up on this…

Turns out it works amazingly well with a 9V battery only. As soon as I connect it on my pedalboard or with an independent 9V power supply, the sound out of the isolated output (B) has horrible ground loop noise. Using the 9V power supply there’s no problem out of output A, which is grounded to the pedal chassis.

What could be going on and is there a way to fix this?

 

[Annies Dad]

What kind of pedalboard power supply are you using? Most of the nicer ones have isolated 9V outputs. Like the Voodoo Labs stuff for example.

 

[joulupukki]

What kind of pedalboard power supply are you using? Most of the nicer ones have isolated 9V outputs. Like the Voodoo Labs stuff for example.

Well, I don’t have anything that expensive, but I’ve tried all of my different 9V transformers and get the same problem from every single one of them. I think it’s probably something I’ve misunderstood about what can be done.

Here’s how I currently have this hooked up. Works great on 9V battery, just not a 9V AC Adapter. What am I missing?

 

[joulupukki]

Reporting back on this…yep, I just upgraded my board’s power supply to a 1 Spot CS6 and the noise doesn’t happen. There is still a very slight hum on the non-isolated channel and so I’m going to probably use the other little isolation transformer I bought and add it there as well … if I can find room inside of the enclosure.

 

[joulupukki]

I added a TY-141P (ordered from Mouser) audio transformer to isolate the A output and it seems to work even better than the 42TM018. The 42TM018 has a very slight hum still no matter where I place the ABY pedal on my board. So, I may order another TY-141P next time I make an order with Mouser. All in all though, the project is a success and it's nice to have when comparing a couple of amplifiers.

 

[Peegoo]

@joulupukki

Most all wall-wart/plug-pak power supplies are isolated from ground anyway; they're "double insulated" on the AC/primary side, so they require only two prongs.

I suspect your issue may be different ground potential between the two amps you're using. Try this: plug in your amps and switch them on, but don't connect anything to their inputs. Set your DMM to read AC volts (if it's a manual meter, set it to the lowest AC setting on the dial) and touch one probe to a screw on the rear of the chassis of amp A, and touch the other probe to a screw on the rear of the chassis of amp B.

Are you seeing any voltage? If so, your mother's brother's name is Robert.