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Old September 15th, 2004, 06:46 PM   #1
Fred Retread
 
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Stealing from mixer bus inserts

I'm scheming to take balanced main L and R stereo signals from an Electro-Voice mixer via the EV's unbalanced 1/4" TRS main bus inserts.

In each of the two bus insert jacks I propose to use an unbalanced 1/4" TRS to 2 1/4" TS adaptor cable with both TS plugs going into the two parallel-connected inputs of a Rolls Matchbox passive direct box. This will interrupt the bus internally but reconnect it externally such that the Rolls transformer's primary coil can access it in parallel. Another way to look at it is that the board would get its return as if from an external processor, and I would get a balanced XLR output for a cable run to my waiting BeachTek.

Does anyone see anything wrong with this plan?
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Old September 15th, 2004, 09:23 PM   #2
 
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in theory it should work. No different really than a normaled patch bay, but that can go awry sometimes too...
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Old September 16th, 2004, 11:37 PM   #3
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Agree, in theory it will work fine. But occasionally you'll find something unexpected that can dash your theory to bits, so you'll want to test it first. :)

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Old September 17th, 2004, 06:57 PM   #4
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I take it you have either two Rolls or a stereo Rolls DI?
If not you'll have a problem. Mono instead of stereo and
chances of what we call shunting the signal(s).

The only other thing to watch for would be ground buzz/hum. I hope those
DI(s) have a ground lift switch which can come in very handy
for getting rid of or reducing hum.

I would also plan on using
battery power for your camera which should greatly diminish
the chances of hum. If you must use wall power on the camera,
bring along a ground lift (or a couple) because sometimes
lifting the ground on things can eliminate buzz.

Also, check on the pin configuration of the console. Most times
tip is hot, ring is return and sleeve is ground, but NOT always.
Some gear has the ring as send and the tip as return. Whatever
your set up wants, you have to supply the correctly wired
TRS to Two 1/4" adapter.

As mentioned, testing before the gig is always a very good idea.
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Old September 18th, 2004, 08:47 AM   #5
Fred Retread
 
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Thanks, all.

Point taken on always testing, always listening. But yes, I would use a separate Rolls for left and right, and yes, I normally run the camcorder on battery.

That last point raises a few questions which may sound naive to you experienced hands:

1. Rightly or wrongly, I gather from what I've read here and in promotional material that active direct boxes have better signal handling qualities than passive boxes (specifics not clear), but poor ground isolation. Is this correct?

2. If I never have any AC line powered devices at the camera end, do you think I am free enough from ground loop concerns to choose active boxes over passive?

3. Can I assume that the output ground on the direct box is common to the input ground when ground lift is not used? I hope so, or it would seem that I would have to provide a ground for the boxes. Otherwise the XLR shield along with the connected BeachTek and camera grounds will be floating.

4. But come to think of it, if ground lift is used at both ends of a balanced XLR run and the shield is truly isolated and floating, what are the consequences? I visualize the shield acting as an antenna for EMI in the room, and small currents sloshing back and forth in it, but how would that hurt me if I have balanced conductors?
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Old September 19th, 2004, 04:55 PM   #6
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<<<-- Originally posted by Fred Retread
1. Rightly or wrongly, I gather from what I've read here and in promotional material that active direct boxes have better signal handling qualities than passive boxes (specifics not clear), but poor ground isolation. Is this correct?>>

Active DIs *can/might* sound better than passive DIs, BUT in your case
I don't see the need. When you split a microphone's signal,
sometimes a much as 4 ways, the loading on the preamp
can distort the signal in bad ways. Since your signal is line level,
I wouldn't bother with the expense. And yes, since an active DI is an
electrically powered device in the chain, (usually powered
with the sound boards phantom power)
you can get zizzes from active DIs that passives won't have.
The real nice actives actually have a microphone preamp built in them.

<<<2. If I never have any AC line powered devices at the camera end, do you think I am free enough from ground loop concerns to choose active boxes over passive?>>>

As previously stated, since you are splitting line level one way, I don't
see the reason. Now, if you go with something like the Klark Technik
box, that is head and shoulders above the Rolls,
it might sound a little better, but at a cost. I would not spend more money
on active Rolls (even though I have never used Rolls DIs, but their other
gear is pretty marginal imo).

<<<3. Can I assume that the output ground on the direct box is common to the input ground when ground lift is not used? I hope so, or it would seem that I would have to provide a ground for the boxes. Otherwise the XLR shield along with the connected BeachTek and camera grounds will be floating.>>>

Actually, when you lift the ground on the DI, the ground is actually
what we call "pushed". Meaning, there is one break in it, not a break on
both ends (floating). A pushed ground is connected at both ends
but broken in the middle.

<<<4. But come to think of it, if ground lift is used at both ends of a balanced XLR run and the shield is truly isolated and floating, what are the consequences?>>>

The ground would not be floating unless you lifted it at the
beachtek end too. Don't do that.


<<<I visualize the shield acting as an antenna for EMI in the room, and small currents sloshing back and forth in it, but how would that hurt me if I have balanced conductors? -->>>

What would happen in an area with high EMI using a floating shield XLR
that acts like an antenna would be to have that EMI's signal induced
into the other wires and you'd most likely hear radio or any other EMI
that the shield is trying to shield.
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