Q: why include the option to lift ground?

[darron]

hey guys. just another quick question (:

i've noticed with the more expensive high voltage circuits like DI boxes and AB switches (to switch amp heads) there are switches that give the options to life the ground to isolate ground loops. i understand why that's a good thing, but why is there the option? what could someone stand to gain from not lifting the grounds?

hope it's an easy answer (:

thanks. darron.

[d95err]

If you connect your guitar to two amps using e.g. a splitter cable or a simple A/B/Y box, you get a ground loop which causes hum. This happens because the signal ground will see two different paths to the amplifier. For amp A, the shortest path is through the guitar cable to the amp. But, ground can also go through the cable to amp B, through B's chassis to the safety ground. Through amp B's power cord to the wall socket. Then from the wall socket through amp A's power cord and to amp B. The second path is longer and will have a higher resistance. This can cause the ground to be not exactly 0 volts, or flutter a bit. This causes hum.

To remedy this problem, you can isolate the signal ground going to one of the amps, usually by using an audio transformer. The isolation breaks the ground loop so there is only one path for ground between guitar and each amp.

Do NOT (as some people suggest) try to remedy this problem by taping off the safety ground on one of the amps. This is putting your and other peoples life in danger and it is worth a few extra bucks to avoid that hazard.

[darron]

If you connect your guitar to two amps using e.g. a splitter cable or a simple A/B/Y box, you get a ground loop which causes hum. This happens because the signal ground will see two different paths to the amplifier. For amp A, the shortest path is through the guitar cable to the amp. But, ground can also go through the cable to amp B, through B's chassis to the safety ground. Through amp B's power cord to the wall socket. Then from the wall socket through amp A's power cord and to amp B. The second path is longer and will have a higher resistance. This can cause the ground to be not exactly 0 volts, or flutter a bit. This causes hum.

To remedy this problem, you can isolate the signal ground going to one of the amps, usually by using an audio transformer. The isolation breaks the ground loop so there is only one path for ground between guitar and each amp.

Do NOT (as some people suggest) try to remedy this problem by taping off the safety ground on one of the amps. This is putting your and other peoples life in danger and it is worth a few extra bucks to avoid that hazard.

hey d95err. like i said, i know why you should isolate the grounds, but i just don't know why people give the OPTION to do so. why don't people just make devices with grounds always isolated? why would you want the switch to 'unlift' (drop?) the grounds?

thanks (:

darron

[Paul Perry (Frostwave)]

why don't people just make devices with grounds always isolated? why would you want the switch to 'unlift' (drop?) the grounds?

Well, if you have TWO ways for the ground circuit to be completed, you have a loop - but if you have NO way, then you have an even worse problem

[darron]

why don't people just make devices with grounds always isolated? why would you want the switch to 'unlift' (drop?) the grounds?

Well, if you have TWO ways for the ground circuit to be completed, you have a loop - but if you have NO way, then you have an even worse problem

yeah. of course. there has to be a ground connection to the desired path. i was thinking about when you are given an option to use isolated grounds or not, like in this pedal.
http://www.zzounds.com/item--RADBSABY

perhaps my understanding is completely wrong.

i made a thread a few weeks ago for a schematic i came up with for an AB amp selector:


i can't think of any reason why i should put in a switch to reconnect the grounds together....

[d95err]

Sorry, I misunderstood the question.

As someone above said, if you've got no path to ground, you have a bigger problem. If for instance you are using two devices that both have isolated output (or if one of the amps is not connected to safety ground), there is a risk that there will be no ground connection. In that case, you'll want to remove the ground lift to get one connection to ground.

[darron]

Sorry, I misunderstood the question.

As someone above said, if you've got no path to ground, you have a bigger problem. If for instance you are using two devices that both have isolated output (or if one of the amps is not connected to safety ground), there is a risk that there will be no ground connection. In that case, you'll want to remove the ground lift to get one connection to ground.

ahh! that's a better explanation. so would you suggest that i include a switch in the layout above to have the option to have grounds lifted or not lifted? in the layout above though, only the ground that is unused is lifted at the moment. the way that i see it though from the layout above is that the box will act only like an extension of the instrument (guitar) cable and the other one will be totally disconnected from both signal and ground. unlifted grounds could give a quieter switching.

thanks for your answers. it's helping me (:

[d95err]

Disconnecting ground between the input and the unused output should be fine and avoid ground loops. However, you need to make sure the unused amp input is grounded, otherwise you'll get a lot of noice and hum in the unused amp.

Layouts like the picture above are very difficult to read, so I can't see if it looks OK or not. It would help if you could draw a schematic.

[darron]

Disconnecting ground between the input and the unused output should be fine and avoid ground loops. However, you need to make sure the unused amp input is grounded, otherwise you'll get a lot of noice and hum in the unused amp.

Layouts like the picture above are very difficult to read, so I can't see if it looks OK or not. It would help if you could draw a schematic.

yeah. i understand how it could be a real pain to read. i only get it right away because i drew it. the unused output are indeed muted by shorting it to its own ground, but that unused ground is not connected to the shielding, through, or used output's ground. so in the end there's no purpose in giving the option of isolating grounds or not? why is this done by others? is it perhaps to at least demonstrate what the unit does?

[CGDARK]

yeah. i understand how it could be a real pain to read. i only get it right away because i drew it. the unused output are indeed muted by shorting it to its own ground, but that unused ground is not connected to the shielding, through, or used output's ground. so in the end there's no purpose in giving the option of isolating grounds or not? why is this done by others? is it perhaps to at least demonstrate what the unit does?

It is there, because there are some instances that two amps connected together with and ABY box do not hum at all. I have found this a lot of times in my experience. So with this feature you have the choice "to lift or to to lift; that is the question". On the other hand I think is a form of saying that their units are better than the others, because they have this "feature". Just a commercial thing.

CG

[darron]

yeah. i understand how it could be a real pain to read. i only get it right away because i drew it. the unused output are indeed muted by shorting it to its own ground, but that unused ground is not connected to the shielding, through, or used output's ground. so in the end there's no purpose in giving the option of isolating grounds or not? why is this done by others? is it perhaps to at least demonstrate what the unit does?

It is there, because there are some instances that two amps connected together with and ABY box do not hum at all. I have found this a lot of times in my experience. So with this feature you have the choice "to lift or to to lift; that is the question". On the other hand I think is a form of saying that their units are better than the others, because they have this "feature". Just a commercial thing.

CG

awesome. sort of what i was thinking. there wouldn't be hum a lot of the time. so there'd be no benefit from leaving the ground loop except maybe getting less switching noise. i'm not after the commercial appeal. thanks guys!

[Claus H]

The Only problem is if you have groundet the amp and what comes before if (connected to the through jack) to the same ground wire the leds will both all the time  fun feature (no harm intended) you might even get a little light in them if they are really high bright and the only ground is you your guitar and fingers

Claus H

[Pushtone]

To answer you question:

Q: Why include the option to lift ground.

A: Because the manufacturer does not know how their product will be incorporated into an equipment setup with regards to the electrical/electronic connections. Therefore they opt to ensure their product works in all possible situations.

That example of two amps, one with a two prong AC Mains and the other with a three prong AC Mains may not work if the ABY box is lifted and could even be an electrocution danger. So they give you the option. Another example I ran into on stage was using a DI to interface a laptop with a PA. Something about the laptop being powered by a battery broke the audio signal ground when the DI was LIFTED.

Important to emphasize that using old vintage amps without a proper ground connection in ABY setups like your drawing can be dangerous. Be careful.

I have that ABY box you linked to.
In the manual is an electrocution warning about connecting amps with two prong AC mains.
The transformer does its job but can load down the signal. A buffer before the ABY is a good idea in my situation.

In the case of the Radial ABY linked above, The transformer is lifting the ground by isolating it which is a better way to do it than switching open grounds.
In the manual it says the lift switch only works when the transformer is switched into the signal path.
No transformer, no lift.

When using a transformer the LIFT switch could alternatively be labeled GROUND RECONNECT.


Checkout this schematic
http://www.jensentransformers.com/as/as007.pdf

The ground lift is shown as a path AROUND the transformer.
When the switch is open the input signal ground is isolated through the transformer.
If the switch is closed ground bypasses the transformer and its isolation.




Some other cool guitar schems on the Jensen site
Passive ABY
http://www.jensentransformers.com/as/as013.pdf

Active 4 way split.
http://www.jensentransformers.com/as/as014.pdf

provides Isolated Line Output from Guitar Amplifier
http://www.jensentransformers.com/as/as045.pdf

More
http://www.jensentransformers.com/apps_sc.html

[Claus H]

why don't people just make devices with grounds always isolated? why would you want the switch to 'unlift' (drop?) the grounds?

Well, if you have TWO ways for the ground circuit to be completed, you have a loop - but if you have NO way, then you have an even worse problem

yeah. of course. there has to be a ground connection to the desired path. i was thinking about when you are given an option to use isolated grounds or not, like in this pedal.
http://www.zzounds.com/item--RADBSABY

perhaps my understanding is completely wrong.

i made a thread a few weeks ago for a schematic i came up with for an AB amp selector:


i can't think of any reason why i should put in a switch to reconnect the grounds together....

Sorry could not really make head or tail of this without the qoute 

The Only problem is if you have groundet the amp and what comes before if (connected to the through jack) to the same ground wire the leds will both all the time  Smiley Smiley fun feature (no harm intended) you might even get a little light in them if they are really high bright and the only ground is you your guitar and fingers Smiley

Claus H

[darron]

To answer you question:

Q: Why include the option to lift ground.

A: Because the manufacturer does not know how their product will be incorporated into an equipment setup with regards to the electrical/electronic connections. Therefore they opt to ensure their product works in all possible situations.

That example of two amps, one with a two prong AC Mains and the other with a three prong AC Mains may not work if the ABY box is lifted and could even be an electrocution danger. So they give you the option. Another example I ran into on stage was using a DI to interface a laptop with a PA. Something about the laptop being powered by a battery broke the audio signal ground when the DI was LIFTED.

Important to emphasize that using old vintage amps without a proper ground connection in ABY setups like your drawing can be dangerous. Be careful.

I have that ABY box you linked to.
In the manual is an electrocution warning about connecting amps with two prong AC mains.
The transformer does its job but can load down the signal. A buffer before the ABY is a good idea in my situation.

In the case of the Radial ABY linked above, The transformer is lifting the ground by isolating it which is a better way to do it than switching open grounds.
In the manual it says the lift switch only works when the transformer is switched into the signal path.
No transformer, no lift.

When using a transformer the LIFT switch could alternatively be labeled GROUND RECONNECT.


Checkout this schematic
http://www.jensentransformers.com/as/as007.pdf

The ground lift is shown as a path AROUND the transformer.
When the switch is open the input signal ground is isolated through the transformer.
If the switch is closed ground bypasses the transformer and its isolation.




Some other cool guitar schems on the Jensen site
Passive ABY
http://www.jensentransformers.com/as/as013.pdf

Active 4 way split.
http://www.jensentransformers.com/as/as014.pdf

provides Isolated Line Output from Guitar Amplifier
http://www.jensentransformers.com/as/as045.pdf

More
http://www.jensentransformers.com/apps_sc.html

hey pushtone! thanks for your response and those links. they will definitely come in handy! the two way passive guitar splitter schematic is exactly what i would need to be looking at to incorporate a transformer. could you please explain to me how the safety hazard still exists? the unused 'send' to the unused amp is totally isolated from grounds and the signal path, and therefor the other amp and the guitar in every way. it's like one amp plugs into a lump of plastic and the other amp just connects like a straight guitar cable would. do you mean that it's dangerous because of the risk of arcing, or because of my wiring failing, or ... hmm. i know it's not good enough to just say "don't play in the rain"

from what you've said i'll definitely give the option to lift grounds then (unlift    )

i've been looking at transformers from my local suppliers. i'm seraching for 'audio transformer', 'isolation transformer' etc. they all seem to come up with 4 terminals except for the pcb mount ones in this style:
http://www.altronics.com.au/index.asp?area=item&id=M1005

would that be usable?

it's funny. i've been taught how a transformer can be dangerous because it's isolates ground, but here that's what we want.

thanks heapppppps

darron