A/B/Y Pedal

[hejo0807]

I'm attempting to put together an A/B/Y pedal as my first project.
I have been doing some research and have discovered the ground hum issues that are associated with these pedals.
I have yet to find a schematic that would eliminate the ground hum.
Also, if I'm planning on running two amps in unison would I need a buffer to help with the impedance, or would the
need for that be eliminated along with the ground hum.


Thank you

Josh

[trixdropd]

I'm attempting to put together an A/B/Y pedal as my first project.
I have been doing some research and have discovered the ground hum issues that are associated with these pedals.
I have yet to find a schematic that would eliminate the ground hum.
Also, if I'm planning on running two amps in unison would I need a buffer to help with the impedance, or would the
need for that be eliminated along with the ground hum.


Thank you

Josh
Geofex.com/ Improved hum free a/b/y

[joegagan]

yes, a simple buffer is needed to drive both amps.

the standard morley a/b/y  schematic is what i used, but i put a simple buffer ahead of the circuit. Can't think of where to point you in the direction of a standalone buffer schematic like i used, but it is very common in a lot of circuits.

i use two amps a lot. in my experience, ground issues are not a problem most of the time, if i do get a hum i use a simple ground lifted cable to one of the amps signal line. solves the problem almost every time. for the rest of the rare cases, see geofex humfree splitter.

one other issue to keep in mind is the phase of each of the amps. if they are out of phase with each other , the sound will be thin. in that case, a phase reversal switch on one of the lines is needed. more info to come.

[Paul Marossy]

Here's a simple way to get rid of ground loops when using an A/B/Y box: http://www.youtube.com/watch?v=4YM1iwC6vhg

[joegagan]

the transformer design of the geofex unit is good, my only point was to say that i have gotten away with using a transformer-less splitter for a long time, been lucky i guess.

[Paul Marossy]

the transformer design of the geofex unit is good, my only point was to say that i have gotten away with using a transformer-less splitter for a long time, been lucky i guess.

Yes, I think you have been lucky! 

[familyortiz]

Here's a ton of info.
My version was a splitter with no switching. These things work perfectly.

http://www.diystompboxes.com/smfforum/index.php?topic=64924.0

[wavley]

Yeah, I used a variation of RG's Hum Free too, it works quite well.

[ashcat_lt]

Here's a simple way to get rid of ground loops when using an A/B/Y box: http://www.youtube.com/watch?v=4YM1iwC6vhg

WTF???!!!???

This is all wrong!  He talks in two different places about the importance of good grounding for safety reasons, then goes and lifts the safety ground on one of the amps?

NO.  Bad monkey.  Get back in your cage.

[MikeH]

I've never liked the idea of using a 2-prong adapter to eliminate a ground loop.  At that point you're basically relying on the cables connecting the 2 amps together to be the safety ground path for one of the amps (Oh yeah, not to mention you're connected to that ground too).  I have often wondered if it would be best to lift the ground connection on one of the cables going from the splitter to the amp when in Y mode.  You'd have to use isolated jacks, and some clever switching to only lift that ground when using Y mode, but it could be done.


Or just build the geofex hum free splitter.   

[ashcat_lt]

Yeah, and that thing is NOT a "ground lift adapter" or whatever he called it.  It's an adapter to plug a 3-prong cable into a 2-prong outlet.  The little ring thingy should be screwed to the (properly grounded) outlet box. 

[modsquad]

Technically it does lift the ground from the wall plug circuit...I think we are picking nits.   I wouldn't recommend it for stuff you don't want to ruin with a surge.

[familyortiz]

I wouldn't even consider a ground lift. The transformer coupled designs have been thoroughly tested and have an excellent frequency response with no hum issues at all.

[joegagan]

my house was built in 56. two wire system. ground is neutral, all my amps are still connected by the two wires.

[Paul Marossy]

Yeah, and that thing is NOT a "ground lift adapter" or whatever he called it.  It's an adapter to plug a 3-prong cable into a 2-prong outlet.  The little ring thingy should be screwed to the (properly grounded) outlet box.  

Yes, and it can also be used to disconnect the ground on one amp so the amps aren't competing with eachother for a path to ground. Both amps are still connected to ground via the cables that are connecting the inputs together. It's not the best way to do it, but it does work. If you'll notice in the video he does verify that ground is available. If there wasn't one, I'd agree that it's a dangerous situation. The copper shielding conductors in the typical guitar cable collectively approx. equal the ground wire in a cheap 3-conductor power cord. But it does rely on the ground connections in the cable to be good. I know, it's not the best way to do it. If I thought it was dangerous, I would not have posted it here.

BTW, that ground adapter plug will only work if your receptacle has a grounded METAL enclosure. For example, in my house, the receptacle enclosure is PLASTIC, not metal. Kind of a moot point because I know my house is properly grounded, but other people may use these things thinking that they are doing something when in fact it is not really.

Technically it does lift the ground from the wall plug circuit...I think we are picking nits.   I wouldn't recommend it for stuff you don't want to ruin with a surge.

Your typical power strip won't help with that anyway. I've had one power power surge at my house that KILLED my computer EVEN THOUGH IT WASN'T RUNNING AT THE TIME, and it was plugged into a decent for real surge protector power strip thingie. It flat out killed the motherboard.

[PRR]

> that ground adapter plug will only work if your receptacle has a grounded METAL enclosure. For example, in my house, the receptacle enclosure is PLASTIC, not metal

Well, if everything is done "right", a receptacle with groundING (3-conductor) feed in a plastic box has the bare wire to the receptacle green screw, receptacle frame, and the cover screw hole.

Of course a "ground" is NOT a ground until YOU verify that it really IS connected correctly.

My last kitchen had metal boxes, and 3-wire cable, but the prior idiot CUT-OFF all the annoying green wires.

This house, the builder often wrapped the bare on the device screw, but often forgot to tighten it. This is extra fun, because a touch will light the 3-lamp outlet tester, but burn/oxidize if large fault currents want to flow.

[Paul Marossy]

Well, if everything is done "right", a receptacle with groundING (3-conductor) feed in a plastic box has the bare wire to the receptacle green screw, receptacle frame, and the cover screw hole.

Of course a "ground" is NOT a ground until YOU verify that it really IS connected correctly.

My last kitchen had metal boxes, and 3-wire cable, but the prior idiot CUT-OFF all the annoying green wires.

That's not surprising. In my house, all of the boxes are plastic and the only metal on the receptacle is the screws, the contacts inside and the wire itself. Point is that you could be in an old home where a previous owner messed with stuff and you think you are getting a connection to ground by using that adapter thing, but aren't because you have a plastic replacement box that is not grounded, etc.

What bothers me is that there is a double standard in the U.S. when it comes to electrical systems in buildings. In residential, they can use Romex wiring nailed directly to a stud, plastic receptacles, etc. but in commericial buildings EVERYTHING has to be run in a conduit - even low voltage wiring, phone lines and CAT5 cable. IMO, any building anywhere should be built to the same standards when it comes to the electrical wiring. Oh well, as is customary, I fret over nonsense I can't change.