Isolation transformer and ground lift questions

[Sleipnir]

Howdy,

I had posted some questions about isolation transformers and ground lift switches in a different forum a few months back but never really got answers to my questions. I hope the more technical folks at this forum can help.

It seems that many/most  ABY pedals that offer isolation transformers often also have ground lift switches. However, the way these are implemented might differ. For example, look at the block diagrams for the following two pedals

The Radial Switchbone which is active
http://www.tonebone.com/switchbone-using.php

and the Radial BigShot which is passive
http://www.tonebone.com/re-bigshot-aby-detail.htm

After reading the manual and looking at the block diagram in the link above for the Switchbone, it is evident that the second output (Out 2) always employs an isolation transformer. The manual indicates that with both amps switched on, one should engage the ground lift switch to see if there is any reduction in noise and thus choose whichever setting is the quietest. The "guitar side" of the isolation transformer is always connected to ground which I assume therefore will always allow a signal to be induced on the output side. The ground lift simply disconnects the ground (and thus cable shield) from Out 2 from the chassis of the Switchbone.

After reading the manual and looking at the block diagram in the link above for the BigShot, it is evident that the isolation transformer can be switched in or out on Out B and there is also a ground lift option. The manual states to first try the ground lift to reduce noise and if that doesn't work, to try the transformer. However, it does not indicate what will happen if the transformer is engaged and the ground lifted. The ground lift is done on the guitar side of the isolation transformer which differs from the approach taken on the Switchbone. Unless I am reading the diagram incorrectly, the when the ground is is lifted and the isolation transformer disengaged, the BigShot would behave like a very basic ABY pedal.

Some questions:
1) On the BigShot, since the ground lift occurs on the "guitar side" of the isolation transformer, would that mean when lifted, and when the isolation transformer is engaged, that you would get no sound? Don't you need the "guitar side" of the transformer to go to ground for it to work?

2) Is perhaps the BigShot ground lift there because when the isolation transformer is disengaged, the ground on the "guitar side" of the transformer could itself be causing noise since you have signal going both to ground there (when ground is NOT lifted) and also to Out B? Again, when the ground is lifted and transformer is disengaged, it seems to me that the BigShot behaves like a very basis ABY pedal.

3) On the Switchbone, since the isolation transformer is always in circuit for Out 2, shouldn't the ground at Out 2 always be disconnected? Aren't you more likely to reintroduce a ground loop when you tie the ground at Out 2 back to chassis ground and thus also to the ground at Out 1, therefore NOT isolating the grounds between the two amps? Although I am not sure why this switch is needed, it seems like it should typically be disconnected and then labeled a "ground reconnect" switch?

4) Some manufacturers of ABY pedals have warnings in their manuals stating that they cannot recommend using the ground lift feature for safety and liability reasons. I assume this mainly applies if one of the amps does not have a proper three prong plug or is not plugged into a properly grounded outlet? If both amps are properly grounded at the power outlet, then presumably there is no danger of engaging a ground lift switch on an ABY pedal?

I am sure I have misunderstood a few things here, so please let me know if you have any thoughts/comments/answers!

[Unlikekurt]

Can you provide schematic drawings for the scenarios you are describing?  It would be a lot easier to assist you.  Unfortunately after reading your post I would have to re-read it and sit down and draw schematics in order to consider the questions.

[PRR]

I suspect the BigShot diagram is drawn wrong. It makes little sense as-is. The average buyer wouldn't know enough to ask the questions you are asking.

[Sleipnir]

Thanks guys. I don't have any schematics but the links I included in the original post had block diagrams which is what I was referring to.

I contacted Radial and they confirmed that for the Switchbone, the ground lift is really only needed in those rare cases where you might want to provide a ground for an amp plugged into Out 2. Thus, it sounds like you'd typically leave the ground disconnected and in practical terms it becomes more of a ground reconnect feature. That seems to make more sense to me now.

Regarding the Radial BigShot ABY, I am still confused. PRR, perhaps you are right in that the block diagram is incorrect or not complete. The response I got was that "if you lift the ground without engaging the transformer you would have a very weak signal. The ground lift just disconnects the connection between input and output. Each still has its own ground reference....if you lift the ground without the transformer there is no return path for the ground." PRR - if this makes sense to you, could you perhaps enlighten me?

It seems to me based on the block diagram that if you're on channel B and do not have the transformer connected and also disconnected the ground, you have the input jack grounded as well as the output B jack grounded with a signal going directly between the two which to me would suggest there should be minimal loss?

For what it's worth, I also contacted Fulltone asking about the ground lift on their True-Path ABY pedals, and that evidently affects the ground shield on the isolation transformer and should normally be set to grounded. It seems like the ground lift feature of the various ABY pedals thus does differ somewhat in implementation and use.

I have to say I was impressed by both Radial and Fulltone - I got responses within a few hours of emailing them.

[PRR]

I don't want to get into that drawing of the BigShot. It is either confusing or wrong. Being Marketing Material for a non-technical audience, it does not have to be exact. As you say, the "isolated" output is not drawn isolated; and it surely must be.

The BigShot is passive. Isolation comes from a transformer. A naked guitar will not drive an affordable transformer without some change of tone. The change may be acceptable. Or you may have a buffer in front (they discuss this). If not buffered and you need perfect tone, it appears you can switch the transformer off the signal path so it is a simple non-isolated Y-box(/switch).

Have you reviewed R.G.'s ABY switch article? It will have the correct ground connections. Yes, it is active not passive. The same concepts apply, except low-price transformers with non-active/buffed guitar will tone-suck bad.
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> ground lift ... safety and liability .... I assume this mainly applies if one of the amps does not have a proper three prong plug or is not plugged into a properly grounded outlet?

Bad grounds are everywhere. Ground-differences are everywhere (otherwise we'd never need a ground-lift!). At one venue there was a variable 3VAC difference between ground on stage and ground in the booth. Lines had to be at-least balanced (not using "ground" as a signal conductor) and preferably floating. I even built a line-receiver with option to use either an active differential input -or- a transformer input. The active-diff promised more fidelity, but would funk-out on more than 10V ground difference. The transformer was actually very good quality, and would ignore hundreds of volts of ground difference (which would be VERY wrong for other reasons).