Passive A/B/Y Hum Question...

[railhead]

First off, I know the best way to get around what I'm going to ask about is to build an "active" ABY -- but let's not go there for now... 

That said, I'm getting some hum and high-pitched squeal from a passive ABY I built, and my wiring is like the one from SingleCoil.com (http://singlecoil.com/docs/aby.pdf). I'm putting it all in a polished shell, so I'm wondering if I should be using isolated/insulated jacks? I tried removing the 2 outputs from the shell so that they were free-floating, but the hum was still there. Should I isolate the input jack as well?

TIA

[Andi]

The hum will be from the ground loop, not shielding. That's only really fixable with a transformer (or that fancy thing RG did with op-amps).

[railhead]

The bugger wit that, is that Morely has their ABY schematic online, and they don't use transformers -- it's totally passive. How on earth could they make a pedal that could have such nasty hum? 

[railhead]

Also, anyone have a schematic with an ABY+transformer?

Lastly, pointers for killing the ground loop in this circumstance? Anything I can do with the guts?

[Andi]

Geofex has a transformer isolated ABY - you'll need to buffer too though.

Otherwise you could try the shoddyfix; isolate one output from the casing and lift its ground connection. Or try a small value resistor in its ground connection. Not nice but it might help.

[Paul Marossy]

The hum will be from the ground loop, not shielding. That's only really fixable with a transformer (or that fancy thing RG did with op-amps).

+1

Also, anyone have a schematic with an ABY+transformer?

Lastly, pointers for killing the ground loop in this circumstance? Anything I can do with the guts?

You can "kill" the ground loop by using a direct box if you're going straight to a soundboard or amp/effects unit that allows an XLR input. Or physically seperate the signal grounds from eachother in the things connected together that are causing the problem. However, doing so opens you up to a potential, but not real likely safely hazard. NEVER defeat the ground at the power cord, though. That is really asking for trouble...

[trixdropd]

I'd love to know a passive solution as well. radial has a transformer isolated A/B/Y that needs no power. Anyone?

[Andi]

Presumably they have a higher impedance transformer that the guitar pickups can drive. Dunno.

[trixdropd]

Presumably they have a higher impedance transformer that the guitar pickups can drive. Dunno.

That's what i figured too.

[R.G.]

OK, let's cut through the suppositions.

First off, I know the best way to get around what I'm going to ask about is to build an "active" ABY -- but let's not go there for now... 

That said, I'm getting some hum and high-pitched squeal from a passive ABY I built, and my wiring is like the one from SingleCoil.com (http://singlecoil.com/docs/aby.pdf). I'm putting it all in a polished shell, so I'm wondering if I should be using isolated/insulated jacks? I tried removing the 2 outputs from the shell so that they were free-floating, but the hum was still there. Should I isolate the input jack as well?

The shell must be grounded to one of them at least, and input jack is a good place to do it. The two output jacks MUST be connected to the input jack ground by wires as shown, or the hum will be intolerable.

What's not properly understood is that the AMPS which are connected to this thing can cause hum all on their own, by either leaking AC or not having a three wire grounding AC cord. Nothing in the ABY box can make up for that if it's wired as suggested in the drawings.

The hum will be from the ground loop, not shielding. That's only really fixable with a transformer (or that fancy thing RG did with op-amps).

...or both!

The bugger wit that, is that Morely has their ABY schematic online, and they don't use transformers -- it's totally passive. How on earth could they make a pedal that could have such nasty hum? 

The hum from only amps is noticeable, not disastrous in most cases. Sometimes it's quite small, but it's relying on the amps, not the ABY to work.

Also, anyone have a schematic with an ABY+transformer?

Geofex.com

Lastly, pointers for killing the ground loop in this circumstance? Anything I can do with the guts?

Do this. Take your DMM, set to the lowest ohms range, and touch one probe to the ground sleeve on the input jack. Now touch the shell and the grounds on the two output jacks. If you have zero ohms to all of them, you've done all you can in the guts if you insist on a purely passive box and not losing treble to a transformer.

I'd love to know a passive solution as well. radial has a transformer isolated A/B/Y that needs no power. Anyone?

I've never found a transformer that a guitar can power that will cause no treble loss. It's one of the tradeoffs you make when using a transformer and inductive guitar pickups.

[Paul Marossy]

I've never found a transformer that a guitar can power that will cause no treble loss. It's one of the tradeoffs you make when using a transformer and inductive guitar pickups.

Did you ever try one of those Jensen audio transformers? I've always wondered about those because the one they show on some of their passive schematics for use with a guitar use are listed as being $70!

Here's an example, a passive guitar splitter - http://www.jensen-transformers.com/as/as013.pdf
I've heard that this is what Allan Holdsworth built to split his guitar signal, but he has to use a booster pedal with it because the signal loses some strength thru that passive splitter circuit. Anyhow, $70 for an audio transformer seems like a crazy amount of money to me...

[MarcoMike]

Excuse me... why should a A/B box introduce hum?! ground loops for the metal enclosure?! really!?! it never happened to me... and always use metal jacks on metal enclosure and ground wires to them to be sure of the contact...
if that is the reason just remove the internal ground wires, or use insulating jacks... you chose..

I'm reading your first post again... <<I'm putting it all in a polished shell>> ... this means it is not in a "closed" enclosure yet? it could be environmental interference... pc monitor, mobile phone... the squeal sound like electronic stuff from the outside to me...

[railhead]

Excuse me... why should a A/B box introduce hum?! ground loops for the metal enclosure?! really!?! it never happened to me... and always use metal jacks on metal enclosure and ground wires to them to be sure of the contact...
if that is the reason just remove the internal ground wires, or use insulating jacks... you chose..

I'm reading your first post again... <<I'm putting it all in a polished shell>> ... this means it is not in a "closed" enclosure yet? it could be environmental interference... pc monitor, mobile phone... the squeal sound like electronic stuff from the outside to me...

I'm not talking about an A/B box -- those have no issues. It's an ABY box with 2 amps connected at once.

[Andi]

What's not properly understood is that the AMPS which are connected to this thing can cause hum all on their own, by either leaking AC or not having a three wire grounding AC cord. Nothing in the ABY box can make up for that if it's wired as suggested in the drawings.

Aye - when I originally drew that diagram I made it clear that it wasn't suitable for going to two amps.

I'm actually struggling a bit to think what it is suitable for...

[MarcoMike]

I'm not talking about an A/B box -- those have no issues. It's an ABY box with 2 amps connected at once.

Ups.... sorry! 

[R.G.]

I'm not talking about an A/B box -- those have no issues. It's an ABY box with 2 amps connected at once.

Here's the deal -
Amps connected to the AC line will have their signal "grounds" at slightly different voltages. This comes from two sources, one being that all amps leak a little AC into the signal path, and another being that the AC sources they're connected to are at slightly different AC "ground" voltages.

When you force the signal grounds to be at the same potential by shorting them together, currents flow, sometimes large currents. This has the effect of forcing the signal circuits to perceive the AC "ground" difference as signal because ordinary guitar amps have none of what's usually called 'common mode rejection'. Common mode rejection is why pro audio/signal gear is 600 ohm balanced - it's to reject hum and noise.

To get rid of two-amp hum, you have to either (a) eliminate the difference in signal grounds between the two amps or (b) isolate the grounds in some way that prevents the ground hum. (A) is possible, but not without modifying either the amps or the AC power wiring; this is very impractical on most stages. Sometimes making sure both amps are plugged into the same AC power socket  and are both three-wire power cords can get you to low enough hum to be OK. (B) is what can be done easily; it must be done in or around the ABY box.

(B) is done by lifting one of the grounds, breaking the connection of both grounds together. You will hear people telling you to use a ground lift adapter on one of the amps. Do not do this. You are literally risking death by electrocution if you do this. The right way is either to use a transformer isolator or to use an active circuit to subtract away the difference in ground voltages to one of the amps. Both circuits are available at GEOFEX.

Transformers are a good way to isolate ground hum. However, to preserve signal fidelity (that is, translating into guitar speak, not suck tone) they must be very carefully designed. In the transformer world, "carefully designed" along with "wide bandwidth" translates to "expensive".  Those $70 Jensen transformers are not overpriced for what you get. However, they are subject to some problems of treble loss caused by the oddities of the guitar pickup.

The transformer isolated ABY at GEOFEX uses an active circuit to force cheap transformers to produce good results.

The opamp ABY at GEOFEX uses an active circuit to detect the difference in ground levels and subtract them away from the signal going to one amp so the result is no hum.

[Paul Marossy]

To get rid of two-amp hum, you have to either (a) eliminate the difference in signal grounds between the two amps

Yep. And what I hear a lot of people doing is defeating the ground on the power cord on one of the amps to get rid of the hum. Not a good idea...

In the transformer world, "carefully designed" along with "wide bandwidth" translates to "expensive".  Those $70 Jensen transformers are not overpriced for what you get. However, they are subject to some problems of treble loss caused by the oddities of the guitar pickup.

Ah, now I can understand the price tag a little better. Well, I had a vague idea of that being the reason why, but this is confirmation.