Josh Hayes wrote:
A bit of a noob question, but could one just plug in an amp's speaker out and run with it like that?
Sorry for hijack... ?
A question is a question. If i had the guts to ask noob questions as you call them, i could saved myself a lot of time! Which is really smart.
With tubes you definetely need the cab plugged in or a load of whatever the amp expects to "see". So the DI having an impedance of 10kohm in parallel with the cab load of let's say 16ohm will be:
1/Rtotal = 1/R1 + 1/R2
1/Rt = 1/16 + 1/10000
1/Rt = 0.0625 + 0.0001
1/Rt = 0.0626
Rt = 15.974ohm
So It won't have any (negligible) effect on the amp plugged in with the speaker cab.
Just for fun:
a load of 10kohm, in parallel with 16ohm: 15,974 ohm (50watts in the cab will give +-5mv signal into the DI)
a load of 1kohm, in parallel with 16ohm: 15.748 ohm (50watts in the cab will give a 50mv signal into the DI)
a load of 100ohm in parallel with 16ohm: 13,793 ohm (50 watts in the cab will give me a 500mv signal in the DI)
a load of 10ohm in parallel with 16ohm: 6,154 ohm (20watts in the cab will give me a signal in DI of +-2V)
So thanks to your question, now I'm thinking that Id rather go with a DI impedance of somewhere between 200ohm and 500ohm after looking at how much I'd have to amplify the DI signal. Thanks!
And a ground lift, removes the ground as a reference for the signal.
So lets take a "normal" studio problem.
Electric guitar plugged into guitar amp, Amp has 3 prong grounded plug (required for safety!) and you mic it up.
No funny noises.
Now you want to track the guitar before it hits the amp (maybe to re-amp or to use Guitar Rig later, etc)
So you Plug guitar into DI, DI to amp and DI to soundcard.
All of a sudden there is a ****load of noise hum buzz. You have created a "ground loop"
The signal into the amp uses the shield of your cable as negative BUT now that shield (via DI to soundcard to Computer Plug ground) has 2 grounds its trying to use as negative. So now 3 signals are passing though that cable:
guitar positive to amp ground (1), guitar positive to computer ground(2), computer ground to Amp ground(3 NOISE!)
solutions: remove the Ground from (a) the amp or (b) the Computer. Technically you are touching those strings which are tied directly to ground, so not the smartest of moves to actually disconnect any grounds from equipment. But if you have a ground lift switch on the DI (c), you can choose which output from the DI is grounded and which is floating. Now all equipment stays safe and grounded, but your strings are only connected to the ground in your amp, while the signal from the DI to computer etc is a copy without a ground reference. nice and quiet like it was before you connected a DI.
of course the next question is: "why have a switch at all?"
if you don't plug into the amp then you would, for noise reasons, rather ground your strings to the computer ground than nothing at all. So by flicking the switch. All is well again.
wow. long winded. looks like I'm the hijacker...