Two amps / Ground Lifting

hotpickups

I'm planning to run two amps together at this weekend's gig. I use an Axess BS2 Buffer/Splitter for it's ISOLATED outputs etc. It has a phase inverter but not a ground lift. Is this a problem or dangerous? If so is there an adapter I can get to lift the ground on one plug

Hope you can help

robinbowes

You shouldn't need ground lift if you have isolated outputs. 

R. 

steamabacus

If the outputs are isolated that means (or should mean) that there is no direct ground connection so you should be fine.

If you do need an emergency ground lift, it's very simple to just cut/unsolder the ground connection in the plug at one end of a standard guitar cable - make sure you mark the cable to avoid mistakenly using it as a standard grounded cable in future.

There is no danger in lifting a ground connection between guitar and amp - the worst that can happen is excess noise from an ungrounded guitar. Note that, in a two amp setup, one of the amps should still be connected via a standard grounded cable - the guitar still needs to be grounded to deal with rf noise.

hotpickups

This is good news then. The reason for this thread is when I plugged my two amps in together this morning one amp was really humming in fact I thought the fan inside was the cause. I pressed the Phase Inverter switch on the Axess Splitter and the noise dramatically reduced. Didn't really make sense as that switch is a Phase Inverter not a ground lift but hey it worked. However, I was worried that I maybe putting myself at risk so I'm being cautious.

exocet

Don't disconnect any mains earth connection under any circumstances.
Pressing the phase inverter will change the relative phase of the 2 input signals. It may well be that engaging this is causing the "hum" that is present on both inputs to "cancel out" - however, your guitar signal may well be impacted in the same way.
I've always found transformer balanced splitters to be a more reliable way of obtaining two signals that are isolated from each other.

hotpickups

Just rigged it all up again it's quite now even when I toggle the Phase Inverter switch on and off. I don't what happened this morning but I know I do have a badly wired house so the electrics aren't the best i.e. Ground Loops etc. As long as I'm safe that's all I want to know.

RD Dualist and the Lazy J20 sound awesome together by the way

Thanks

hotpickups

steamabacus said:

If the outputs are isolated that means (or should mean) that there is no direct ground connection so you should be fine.

If you do need an emergency ground lift, it's very simple to just cut/unsolder the ground connection in the plug at one end of a standard guitar cable - make sure you mark the cable to avoid mistakenly using it as a standard grounded cable in future.

There is no danger in lifting a ground connection between guitar and amp - the worst that can happen is excess noise from an ungrounded guitar. Note that, in a two amp setup, one of the amps should still be connected via a standard grounded cable - the guitar still needs to be grounded to deal with rf noise.

What is the ground connection anyway @steamabacus ? Is the copper wire/shielding ?

steamabacus

Yes, the screen/shielding wrapped around the signal cable. It shunts any rf interference picked up to ground/earth before it can interfere with the signal travelling down the centre wire. It's the principle of the 'Faraday cage'.

hotpickups

steamabacus said:

Yes, the screen/shielding wrapped around the signal cable. It shunts any rf interference picked up to ground/earth before it can interfere with the signal travelling down the centre wire. It's the principle of the 'Faraday cage'.

I'm gonna buy another cable then and have that as a specific cable to only be used as a two amp set up. So just de solder and cut the shielding away will be ok @steamabacus ? Thanks for the info. 

Sporky

Yes.

Or, if there's room, cut the shield and resolder it with a 15 ohm resistor. Enough to stop ground loops, but better shielding.

hotpickups

Sporky said:

Yes.

Or, if there's room, cut the shield and resolder it with a 15 ohm resistor. Enough to stop ground loops, but better shielding.

Easy for you to say @Sporky  

Sporky

Yeah; it's a bit fiddly in some plugs.

Just cutting it back a bit is fine.

steamabacus

Yes, pretty simple really. Most of the time it makes no difference which end (the end with the screen connected or the end with the screen cut) goes into the amp socket but I've read that sometimes one way or the other may be preferable. Someone more technically knowledgable may be able to explain when and why it makes a difference.

exocet

steamabacus said:

Yes, pretty simple really. Most of the time it makes no difference which end (the end with the screen connected or the end with the screen cut) goes into the amp socket but I've read that sometimes one way or the other may be preferable. Someone more technically knowledgable may be able to explain when and why it makes a difference.

I would opt to have the signal ground connected at the "amp end" and removed from the splitter box end because the amp will (or should be) grounded properly to earth via it's chassis.

hotpickups

Apparently another switcher I've ordered from bright onion will have a ground lift so no need to alter any cables now

Thanks for the help