noise on sound system

[inetdog]

I'm confused. The 1/4" connectors on a DI are just connected in parallel; lifting ground won't do anything to that connection. Lifting the ground just separates the shield on the balanced side from the shield on the unbalanced side. You cant separate the shields on an unbalanced connection because the shield is part of the circuit.

So, if you use a DI box which does not provide the feature of isolation via a transformer between 1/4" input and output (which would allow you to separate the grounds but would use insulated 1/4" jacks in the box), you can use the 1/4" or RCA (unbalanced) to XLR (balanced) feature of the box, run an XLR cable, and use a simple XLR to 1/4" adapter (balanced to unbalanced) at the other end, and then use the "ground lift" feature of the box to completely isolate the two systems. If all you were concerned about was avoiding the presence of ground loop voltage on the signal leads, you could just use an XLR to 1/4 adapter at each end of the balanced cable. But that would still allow the ground difference to cause problems inside the circuitry of the computer.

[ggunn]

So, if you use a DI box which does not provide the feature of isolation via a transformer between 1/4" input and output (which would allow you to separate the grounds but would use insulated 1/4" jacks in the box), you can use the 1/4" or RCA (unbalanced) to XLR (balanced) feature of the box, run an XLR cable, and use a simple XLR to 1/4" adapter (balanced to unbalanced) at the other end, and then use the "ground lift" feature of the box to completely isolate the two systems. If all you were concerned about was avoiding the presence of ground loop voltage on the signal leads, you could just use an XLR to 1/4 adapter at each end of the balanced cable. But that would still allow the ground difference to cause problems inside the circuitry of the computer.

On all the passive DI's I have seen, the two 1/4" jacks are simply in parallel and have no isolation between them. The ground lift only affects the balanced output.

You could certainly use a pair of adapting transformers as you say, but you'd also need a ground lifting adapter or you could just cut the shield connection in one end of the mic cable which connects them. Otherwise the shield is contiguous and the ground loop is still there.

[H2SO4_guy]

In my case I always hooked the laptop to a mixing console with XLR (Balanced) inputs with the ground lifted at the direct box. When I only used a 3.5mm trs (The standard laptop audio out jack) to the dual 1/4" phone plugs the ground was there if using the power supply. If I was on the laptop battery the buzz was not there. Are you wanting to use an XLR type input or unbalanced RCA / Phone 1/4" type?

Skip

[inetdog]

In my case I always hooked the laptop to a mixing console with XLR (Balanced) inputs with the ground lifted at the direct box. When I only used a 3.5mm trs (The standard laptop audio out jack) to the dual 1/4" phone plugs the ground was there if using the power supply. If I was on the laptop battery the buzz was not there. Are you wanting to use an XLR type input or unbalanced RCA / Phone 1/4" type?

Skip

Either balanced XLR or *balanced* 1/4" phone should work as long as the connection between the laptop and the mixing console is balanced and there is no connection between the grounds. But to get a balanced output from the laptop will take a transformer. Either in the form of a 1/4 to XLR Adapter or a DI box.