Why are most live sound engineers so afraid for +4dbu balanced line outputs? As a double bassist, I use a Grace M101 preamp during live gigs. This preamp offers +4dbu balanced line outputs. However, many sound engineers do not seem to be able to cope with this kind of line outputs, and instead insist on putting a DI between the preamp and the mixing board. So, they convert a preamped signal into one that can be preamped again through their boards. Wouldn't it be far more sensible to connext the output to a line input, or use the pad function that most professional live mixers have? Or am I overlooking some other critical aspect?

Any info would be greatly appreciated!
Cheers,
Vincent

I'll try to reply based on observation in experience.

I think its mainly to do with adding a precaution in the chain. A DI will help minimise the risk to the desk should the signal jump rapidly.

I may be wrong on this but I can't see any other need to add a DI between the heat of the balanced outs and the desk.

ken

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I'm All Ears.

It comes down to painful experience....

A **LOT** of the line outs fitted to bass guitar heads frankly suck, either pin one problems, or inadequate protection from phantom power (or both), we just find that sticking a DI box in there is cheap insurance.

Experience is that if you try sticking the line out on any random head straight up the snake, you stand an excellent chance of getting annoying buzzes, putting a transformer in the way is just good sense.

Besides, galvanic isolation from whatever the band turns up with is a GOOD thing for all concerned.

Regards, Dan.

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Audiophiles use phono leads because they are unbalanced people!

Actually there are two reasons(On top of what Dan said;):

One is habit. Many amps(I might even venture to say most) do not have the right output impedance for a preamp input on a console, which to be honest even if you are providing a line level output many engineers will go into the preamp for multiple reasons. Making sure that the signal is matched to the input impendance on the console(And by matched I don't mean equal to) allows you to do long runs without a degredation in signal quality. As a result many engineers are in the habit of using a DI to ensure this just in case.

The second reason has to do with ground loops especially. This depends on the system and the venue involved, but seeing as how especially smaller venues you can develop a ground loop between the stage and the outlets there and the console and the outlets there, putting a DI inline will allow you to easily lift the audio ground to deal with this.

And while not a direct answer to your question...
Finally if a good powered DI is used, the signal loss will be extremely minimal, and since many such DIs can be powered from Phantom Power, it means they can be put there without another power supply, and can often look like a passive DI.

Seablade

One more possibility not yet mentioned. Not every designer has allowed for the possible application of phantom power across the outputs of their unit. I think 48v killed my Aphex 1401 and early Memesis acoustic pickups came with express instructions not to apply PP to your system (?) for fear it will fry your preamp. Given the history it's not surprising S/Es are cautious. By the way, what is the nature of your objection?

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Ohm's Law states, "Your PA isn't as powerful as you think it is".

Interesting thread. As my guitar rig (prog. 19") isn't fully finished yet, this is a good time for me to get it just right..

As an electronics technician, I'll build a finalizing module with manual tone controls to adapt to the venue, plus suitable linedrivers with both single ended and balanced outs, all designed to be able to properly drive cables, short-circuit protected et al..

What would a sound engineer want to see? :
Transformer coupled outputs?
Which (switchable?) output level(s)?
Which output impedance(s)?
Name anything..

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/van

Thank you all for your replies, your suggestions make sense. I do not have a clear objection against using a di between my preamp and the mixing board. It was just something that surprised me. To be clear: the Grace is not a bass preamp, but a studio mic preamp (in essence a single channel and simplified version of the Grace M201 that Hugh raved about in SOS), since I use a mic only (a Neumann KM 185). I thought the sound, as well as the consistency and reliability of the signal, would be good enough to rely on. But perhaps many engineers are not familiar with the Grace brand and do not want to take any risks...

Thanks again,
Vincent

I'm sure many if not most Live Sound engineers will know about Grace mic preamps. But while such superb designs bring distinct advantages in a recording, no one is going to appreciate the difference in a live sound event between a Grace and the 'standard preamp' within the console. They just won't be able to hear any difference -- just as they won't hear the difference between pluging your mic straight into the FOH console, or going via your Grace, a DI and then the console preamp again.

In some circumstances it is practical to arrange a balanced line level feed to the console's line inputs -- thereby avoidinghte console's preamp stage -- but few 'standard' rigs are set up to operate that way. Most expect only mic level signals from the stage. Plus, sending a balanced line level signal down still risks ground loop problems, and a line isolating transformer (which is often more expensive than a DI box) would still be necessary.

If it was my rig, I'd leave the Grace at home for recording, and allow the Live Sound guys to plug your KM185 straight into their system.

Hugh

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Technical Editor, Sound On Sound

Again, thanks for all your replies. So, the general consensus is that the preamp is a bit redundant when going through a PA. I guess that makes sense. The preamp will remain useful when playing without a PA, though (together with a Flying Mole power amp and a Glockenklang Acoustic Art MK I cab).

Thanks again,
Vincent

On the principle of keeping it simple, putting every channel through a mic input on the mixer is as straightforward as it gets. Although adding DI's is in theory more complicated, you know everything arriving at the mixer is mic level. So if you suddenly need to switch channels on the mixer it's safer than having a mixture of line and mic levels sources. Plus, as has been pointed out, engineers like to know there's isolation in the chain between their valuable kit and a band's stuff, which probably includes all sorts of strange gear with funny outputs.

I've also run into problems with some venues having cheap monitor consoles (*cough* behringer) that only accept mic level on the XLR inputs.

My MOTU 896HD is also in getting fixed right now, as the outputs started dying one by one. Only thing I could put it down to was phantom power being applied.

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Visit my band's website

Wouldn't be at all surprised. It is quite possible and not that uncommon.

hugh

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Technical Editor, Sound On Sound

I've had several guitarists who've helpfully unplugged my DIs from the stage end and gone straight out of their amp into the desk 'because it's got an XLR output, see, and I don't want a DI polluting my signal', only to find their pride and joy wasn't expecting 48v and goes poof.

The other favourite is talent with their own jack-xlr bodge cable; yes, you might use it at home, no, all desks _aren't_ the same, yes, exposing it to PP _has_ let the smoke out of your expensive keyboard's output stage