DI Boxes, active onboard preamps and mic grounding?

[JamalPiper]

Hey So I've got a few questions.


In a passive DI circuit used for guitar/bass what is the ratio for the transformer? I've heard 10:1 and I've heard 1:1... 1:1 doesn't make sense to me because of impedance, but i keep seeing it come up...



I want to put an active preamp and DI circuit into my bass. (currently have a passive DI and no preamp). Is there any inherent problems with running the DI off phantom power while running the preamp off battery? Or anything that I would need to be aware of or cautious about?



Are mic casings generally grounded (connection to XLR pin 1)? What can happen if they aren't/are? (READ: I just built a mic and I didn't ground it, will it kill someone)



In XLR Connectors there is a solder tab for the metal housing. Should this be soldered to ground? What happens if it isn't? (READ: I made a dozen cables a year ago and I didn't ground them, could they kill someone)



thank you much.

[Minion]

If you are going to put an Active preamp and a Passive DI in your bass then you don"t need phantom power for the DI because it is Passive and needs no power , the Ratio of the Transformer used in the DI will depend on if you have an active preamp in your Bass , this is because the Ouput impedance of a active preamp is low but passive pickups have a high output impedance .....

So if you put in an active preamp in your Bass the DI Transformer can be a 1:1 , if you don"t have an active preamp then a 10:1 or even a 20:1 would be better ..... I don"t see how you are going to get an active preamp with a battery and a audio transformer in your bass but I guess it is possible , I would simply build an active preamp that has an active DI output all running off the same battery and it is easier to design a Preamp to run off of a battery then off of Phantom power .....


Cheers

[dickinsonc]

First.... If your going to use a DI (or build it in the bass?) I assume you are doing it to 1) cut the noise and/or 2) put it into a mixer with balanced input.
<i> Note: Running the bass directly into the board will give you a nice clean sound.... there will be no "amp" coloring.... is this what you want (most don't... they want that Marshall/Vox/Fender/ whatever "sound" (distortion/coloring/harmonics.. whatever you want to call it)</i>
I'm not sure why you need a pre-amp... Running a balanced input out of the DI would allow you to run a LONG ways with minimum noise.
IF you do want some amplification.... I would look into some of the combo DI & amp modelers... (like Line6 makes). Would give you some control over your sound (like running through an amp).
Also... <i> I have done this one for years.... I think it works (although the engineering types might blast me on this one)... I do it for keyboards... don't know why it wouldn't work on a guitar or bass. </i> I make my 1/4" "guitar" cords out of 2 condutor shielded. I run the signal through the 2 conductors and ground only ONE end of the shield (this is how we do it in instrumentation to avoid ground loops/noise).
re: DI's... I've good luck with a Samson active DI (well built, not terribly expensive). I use it kinda like a splitter... it has balanced (XLR) and unbalanced (1/4 TS) outputs which are both hot at the same time... I run one to the monitors (1/4) and one to the board (xlr) (in stereo.. the sound guys really hate me)
cl

[Nigel Goodwin]

I use a cheap Behringer twin active DI - usually just plug guitar and bass straight-in and off to the mixer/amp on two channels. If we happen to be using combo's as well, I use a twin DI on each, switched to single mode, this allows the normal loop-through for the combo and two XLR outputs (of which you only use one). There's also a ground lift switch on them, but I've never had occasion to use it.

The body of a microphone is normally grounded, it's made out of metal in order to screen the wiring and insert (from hum, noise, radio signals etc.), if you don't ground it, it doesn't screen at all.

[JamalPiper]

Hey thanks everyone for your responses.


I realize that my second question was awkwardly phrased. What I meant was that I would have an active DI circuit powered by phantom power and a preamp powered by battery. I've always heard that active Di's are better for low frequencies.



Right now I have a 1:1 transformer in the DI circuit and it's having impedance issues. I was unaware that preamp had low impedance outputs, so I think I will stick with the 1:1 passive DI and a put in an active preamp.



I'm going to try the cable idea, a friend of mine recently asked me to fix a few cables for him. I have a spool of 2 conductor + shield cabling. So I'll try it. With these it's important to label a amp/guitar end right? Because the grounded end needs to go to the amp correct?

[dickinsonc]

I'd say probably yes... the shield ground at the amp, only because the amp is grounded to the mains (with the keyboards, it doesn't make any difference... both units are grounded... which is why de-coupling the shield would, I'm assuming, keep the ground loops out. The two units are still hooked together through the 2 wires inside the shield.
Be sure & post whether this has reduced the noise, and if the shield ground end makes any difference
cl

[utterdisaster]

I want to record accoustic guitar w/fishman pu. I use adobe duet to interface to Apple's Logic Pro. No problem when I plug in Strat,(wound pu), but with the fishman I get hideous 60hz hum. I know about z mismatch but that usually only effects signal levels and a DI box would fix that. Would a DI with grnd lift eliminate the hum? And would passive work or would I need active?