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Understanding Audio Connections

Discussion in 'Recording' started by NCdan, Mar 2, 2009.

  1. NCdan

    NCdan Guest

    I believe the main reason you would want to use a direct box would be to get a your signal louder with less noise, which really isn't a bad idea.
     
  2. RemyRAD

    RemyRAD Guest

    The only reason why you want an active DI box, is for its tubelike, high impedance input. Any preamp you purchase that indicates it has high impedance DI like inputs is designed for your electric guitars, electric basses & acoustic guitar pickups. Most electronic keyboard instruments have lower impedance, emitter follower transistorized output stages. Those can look at slightly lower impedance inputs such as those 50,000 ohm transformer inputs. But I have regularly used those on electric bass, passive pickups, with a great deal of success. Still doable with other flat wound & humbucker pickups. But only you can determine if the loading effect, from a transformer input adversely affects the sound of your axe. Many don't find it objectionable. But others will want a more open sounding quality, which will require a higher input impedance from an FET input section. All guitar amplifier inputs are generally 1,000,000 to 2,000,000 ohm inputs as are active DI boxes.

    It's really unnecessary to balance a guitar output when going to its dedicated guitar amplifier since those are all high impedance, unbalanced. The instrument inputs on your audio interface are designed to mimic those of guitar amplifiers. A keyboard output, on the other hand, can directly feed a 10,000 ohm line level input such as the line input on most any mixer/console without adversely affecting sound. HumBucker guitar pickups are in a sense balanced pickups, which cancels out spurious noise & RF susceptibilities. Of course they are designed as unbalanced outputs since that's what guitar amplifiers have. An active direct box on the output of your guitar, could feed a quality input of a fine microphone preamp. When taking guitars direct, that's the way I have to feed my Neve, if I take a guitar directly. The Neve doesn't have high impedance musical instrument unbalanced inputs. So in your situation, it's a needless expense. Is that making more sense for you?

    Unbalancing the world, one transformer at a time.
    Ms. Remy Ann David
     
  3. MadMax

    MadMax Well-Known Member

    oh boy... the can of worms is now in session.....


    As with 95% of ll things currently audio....



    It depends.


    What we're really talking about here is signal connectivity.

    How you pass audio signal from one device to another is in itself a good exercise in impedance and load matching. If not a proper jumping off point for sanity.

    OK, jg, You know what voltage and current are.... I'm going to guess you also have an idea of what resistance is... e.g. the opposition to current and where voltage is developed.

    As one holds any single variable constant and you vary 2nd value, the corresponding 3rd value MUST also change at an equivalent rate to the 2nd variable.... simple physics.

    As you transfer energy (V, I, R) from one circuit to another, you transfer at essentially one of 3 ratios or conditions... L to H, H to L, or equal energy transfer. With me so far?

    Depending upon components that make up the circuitry, it will have either a neutral, capacitive, or resistive loading effect on the next circuit.

    These characteristics, along with the audio signal, can be used to "color" the signal... or not.

    DI boxes are no different...

    I use a combination of different DI boxes. And all primarily for the desired function I want from the transfer.

    If I want clean... PRISTINE clean, I use a passive DI that will pass as much bandwidth as possible. Of course there's no free lunch, so I'm gonna have to likely gas the pre a right good bit before I can commit it to tape/media/etc... Which can introduce noise if you use too much gain.

    If I want sumpin' kinda' nasty and stinky, kinky like a Memphis blues screamin' chug... You better believe I'm gonna get some toobs involved in the signal path to get as much harmonic distortion as I can thick black mud out of the Mississippi delta, and smear it over it like a peanut butter sandwich.

    If I just need a faithful reproduction, I'll use a solidstate active DI.

    When you wanna get into phase translation over impedance coupling... we'll be in good shape.
     
  4. jg49

    jg49 Well-Known Member

    Yes in regards to guitars that makes perfect sense. I get the keyboard idea as well.

    I am still wondering if my ART T8 transformer(s) is considered a passive DI? http://www.artproaudio.com/downloads/manuals/omt8.pdf

    Another question from the previous topic is Davedog said
    "In rack mounted gear, the better quality stuff will have a chassis ground lug on the outside of the case and all of these should be tied together and earthed somewhere in reference to the power supply for the rack."

    So where typically is this grounded to? My rack systems are portable for mobile recording and how important is establishing this type of ground in those situations?
     
  5. MadMax

    MadMax Well-Known Member

    You ain't never seen a ground spike, have ya?

    WICKED COOL....





    Unless it's your gear....
     
  6. jg49

    jg49 Well-Known Member

    So I gather this second ground stops ground spikes?

    But where is it established?
     
  7. Codemonkey

    Codemonkey Well-Known Member

    To tell the truth I have never seen a ground spike either.

    I'll assume it's a physical spike in the ground that allows for discharge.
     
  8. jg49

    jg49 Well-Known Member

    Sotra like have you seen any Irish bull?
    Because you have to listen for it.
     
  9. MadMax

    MadMax Well-Known Member

    No... a ground spike is VERY real...

    It usually happens when a genny is un-grounded or is set for 3 phase and the ground is hot... or the most typical... a lightning strike.

    Blows the smoke out of everything on the 110 bus....

    I've seen it one time on an X-Ray generator, about 10 different computers and printers, a couple of breaker boxes, and 3 homes where TV's, radios, microwaves, hot water heaters, and the rest of the household electronics are toasted... but have heard plenty of stories about it in audio, lighting and other commercial gear.

    If you don't ground the chassis, you're usually ok... but when the unexpected can happen at the worst possible moment... it ain't pretty.

    If nothing else, open the gear, and find a suitable ground point... scrape away the paint... drill a hole and install a good brass lug connection.
     
  10. jg49

    jg49 Well-Known Member

    I am sorry I guess I was not being clear I understand where the ground lug is on the chassis I meant what to ground it to. Obviously the plug has a ground it seems a little reduntant to ground it to the outlet but...

    I have never experienced a ground spike but I am aware of the phenomenom.
     
  11. Link555

    Link555 Well-Known Member

    Any modern and decent design the chassis ground will be coupled to the earth ground inside the box. Some gear includes aground lift switch which will disconnect the internal circuit from this ground.

    Rane does a good job explaining the issue. They Messed up on their earlier designs, so they learned the correct grounding methods the hard way…….I am very thankfully learning from their mistakes ;)
    http://www.rane.com/note151.html
     

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