Where all stupid comes to congeal neatly in one handy tub.
Friday, August 27, 2010
Wednesday, August 25, 2010
Monday, August 23, 2010
Friday, August 20, 2010
Wednesday, August 18, 2010
Tuesday, August 17, 2010
Thursday, August 12, 2010
Wednesday, August 11, 2010
Sunday, August 01, 2010
DI How to
"What does a DI do, and how should I use it?" - Dave Muddiman's MySpace Blog |
REPLY FROM DaveMIX
"What does a DI do, and how should I use it?" is a question I get a lot, and at first it was hard to answer (in English, especially). And I'm too busy at a gig to explain this fully, so here is the big mother; everything I know about DI boxes for everyone to read - at their own pace.
DI BOX DEFINED
A "Direct Input" box, or "Direct Injection" box does three things at once: (1) Impedance matching, (2) impedance conversion, and (3) balancing.
Transmission of signal involves not only the input and output, but also the cable with its inherent losses. As runs become longer, the approach to reducing losses changes. In modern equipment this cable loss is managed, but if the inputs and outputs are out of range, you need a DI (or something) to address the mismatch. Were not talking volume matching (voltage, or power), but impedance matching (the amount of current required to transmit a voltage).
1) Impedance Matching
A DI box has inputs and outputs that match what they should be hooked up to. Most instruments have a high impedance output (electric guitar, 500-3000 ohms), and should be plugged into a high impedance input (guitar amp input, 2000-10000 Ohms). The mic input at a console is low impedance (400-800 Ohms) and should be feed with a low impedance source (a microphone, 20-300 Ohms).
A DI has the appropriate impedance for those connections (input greater than 10,000 ohms, output less than 300 ohms).
2) Impedance Conversion
The DI box must change the impedance. This is traditionally done with a transformer (passive) or an amplifier stage (active). Active DIs were very popular when they were introduced in the 80s.
3) Balancing
The DI box must take the unbalanced signal from a guitar and make it appear as a balanced signal at the console.
The familiar unbalanced guitar cable has a single signal or "hot" wire and a wire braided "Shield" wire which completes the circuit (current return) and distributes a "signal ground" to refer the signal. Also the grounded shield completely surrounds the hot wire and blocks out noise (RF).
A mic output has a shield, a hot (+), and a cold (-) where the cold is the exact opposite voltage as hot. This reduces wire related problems in several ways. One way is that the signal is referred between hot and cold, and the shield is not part of the audio signal but is just for isolating the hot and cold signal from noise. Equal but opposite voltages leaves a sum of zero, reducing losses, and most importantly you get all the advantages of "twisted pair".
DI INPUT TYPES
The countryman Active DI has an input impedance of 1 million ohms. Adding an active DI will, not alter this way the tone of the guitar/amp reaction. Using a DI on an instrument not hooked to an amp, will also work, but you have barely completed the circuit, and could have some tonal changes or even background noise.
Imagine you are measuring the temperature of a glass of hot water with a thermometer. If the thermometer is large, it will cool the water - the process of measuring would skew the results. Likewise an active DI will skew the sound the least - 1 million Ohms is like a small thermometer.
Transformers, meanwhile, have improved. Jenson transformers are quite good, and the new Lundahl brand of transformers is even better. You can now have less distortion and less phase error with the new transformers than you will ever get with active. The input impedance is lower than a Countryman Active, but it is well within reason with the new transformers.
Active DIs require power for the electronics. This can be from a battery, phantom power, or both. Batteries will wear out, phantom power makes a pop when engaged, and not every PA has phantom power. Also some if not all active DIs pop when turned on, which happens a lot since most active DIs turn on when plugged in. Reliability of the power, and that popping sound are the downsides I see to active DIs.
Cheaper passive DIs have lesser transformers which have an audible effect on the sound of the instrument in both the PA and the instrument amplifier.
DI OUTPUT
The passive DI has a transformer which creates a "transformer balanced" output; a great method for balancing. Active devices use amp stages to generate equal but opposite "active balanced" signals on the hot and cold output, this method and another electronic method called "servo-balanced" are also very good.
Some active DIs still have a transformer for balancing the output. Cable Factory makes a DI that is switched either passive or active, but always has transformer balanced output.
A third method of balancing an output is not as good, but fortunately is never used for DIs, because avoiding this shortcoming is what DIs are for. "Impedance balanced" outputs have an active amp for hot, a passive resistor to ground for cold, and a shield connected to ground - no longer the true definition of balanced. Most mixers, Mackie for example, have impedance-balanced outputs for XLR outs. Using a DI after the mixer would give you the improvement of a true balanced output.
GROUND AND SIGNAL GROUD ISOLATION
Before and after a DI, ground is used for a lot; grounding the shield for noise suppression, signal reference in unbalanced systems, connecting the chassis of audio gear together, and safety. The way all DIs work, the unbalanced input ground doesn't have to be connected to the output ground if a simple "ground-lift" switch is included. This allows an unbalanced signal shield to "float" while still having a signal reference.
If you connect a DI and get a hum from a "ground loop", then this ground-lift switch could be exactly what is needed. At the DI's output, the console is a main distributor of signal ground and should always be connected to the electrical earth ground. The DI chassis and output sections are always signal-grounded to the console. At the DI's input you may have a grounded system (like a bass amp) that creates a loop with the signal ground. As a rule of thumb, if the device doesn't have an electric plug with ground, then you cant have a ground loop. (do not disable a grounded outlet - if your devices were designed to have them, keep them grounded, and have a ground tester for mystery locations).
You should always hook up the DI with ground on. It's the right way and the safe way. If you have a hum, try switching it off. Sometimes the hum disappears completely, sometimes it gets louder. If it gets louder, something is wrong beyond the scope of the DI. If the hum goes away; that's what the switch is for.
Again, if it's an acoustic guitar and no amp, then you should always be grounded - you can't have get a ground loop from a battery.
SUPERFULOUS FLUFF
The manufactures make DIs with added features. Ground is required. But there is more, which I don't advise using.
Something that would destroy most DIs, would be hooking up to an amplifier output (speaker jacks) to get the tone of the amp driving a speaker. But some DIs come with that option; you must select a switch first and then hook a speaker cable into the inputs - make sure the amp is off Sparky! The Countryman Active DI does this, some like the "redbox" also had the beginnings of cabinet emulator technology also built into them. Radial makes a red DI box that does what the red box did.
Some DIs offer bass cutoff switches and EQ (such as the Sans amp DI). Having musicians set EQ for the house mix is can bring up some weird results, a better and more straightforward way is described next.
WHERE TO ADJUST WHAT
When you want to change your volume or EQ, it does make a difference where you do it. If the DI is hooked up as standard to the bass amp input, then any changes to the bass amp will not be heard in the PA, while changes to the bass guitar or the effects will be heard in the PA as well as the bass amp.
So if the adjustment is to meet the needs of changes in the music then do it on the instrument, and if you need to adjust because your perception isn't right on stage then do that at the bass amp. This is true for any instrument with an amp and standard DI connection.
POST AMP DI
If the DI is after the amp because you have a DI with the ability to be connected to the speaker jacks, then the amp adjustments will be heard in the PA. This is a downside to this configuration.
Another way is to connect to a line out of an amp. Again all amp adjustments will be heard in the PA, not good. Still I'd connect the DI there for certain situations, but do you need a DI? Read on.
NO DI, THE HARD-WIRED OPTION
Of the three jobs of a DI listed in the beginning, number three can be done for under $20. With a cable and two connectors, you can convert unbalanced to impedance-balanced but you will not have corrected for the impedance mismatch.
Connect the unbalanced hot to the balanced hot, connect unbalanced shield to balanced shield, and connect the unbalanced shield to balanced cold. You can not ground-lift. The mismatched impedance will draw too much current, perhaps causing distortion or tonal changes - but the output impedance of today's new gear is dropping, and many instruments can take the load (current draw). Not recommended to long term use, but they are handy when someone asks for more DIs than you can spare.
YOUR CASE
You could run from lineout into the mixer without using a DI. If line out is from an unbalanced 1/4" connector, then run an unbalanced cable with 1/4" connectors at both ends to your mixer. All mixers have 1/4" inputs; some are more for line level and some are more for mic or instrument level, but it will work and be fine.
Unbalanced instrument cable should not be too long, the upper limit is between 20 and 40 ft, but at line level you are okay. Balanced line level is the output mixing consoles return from a mixing board down a 300 ft snake to the amps, so unbalanced line level to your mixer can go across a stage, or even down a snake.
BRING A SPEAKER
For Rock n Roll, a DI is not enough for bass or keyboards, the musician must also have an amp and speaker cabinet. I see your Hartke B900 amp also has a speaker which is good, since otherwise you will only be in the main PA speakers. You should not expect to be in stage monitors with most PAs since bass guitar makes notes much deeper that the voice. Not only does it expose the vocalist's monitor to damage, it also reduces perceptibility for the singer.
If you're doing a small acoustic-type show, you can get away with only being in the PA and maybe a bit in the monitors. Reducing your volume will reduce how much you have to carry - but can you refuse the urge to rock?
DI RECOMMENDATIONS
I currently am buying only Cable Factory passive DIs with Lundahl transformers. The basic model, Cable Factory DI-PRO-P1, cost under $200, sounds great, and always works. I recommend it as the best overall DI. They also make an active/passive DI, the DI-PRO-AP1. They have other stuff too (they make my snakes) at CableFactory.com (1-888-383-4883), ask for Bob and tell him I sent you. They only sell direct.
Radial also makes good DIs with Jenson transformers. They make many with feature options galore, many with features not conducive to reliable use in concert in my opinion, but useful in studios for example. They make a good one for stereo laptop connections. They also make one with cabinet emulation, and a very basic top-notch. They make a few budget DIs, which I recommend against.
Countryman's "Type-85" active DI is very good sounding, and I still use them live for any instrument I want to be as pure and distortion-free as possible. They operate on both 9 volt battery and phantom power supplied for a mixing board. BSS also makes a great active DI, as well as Radial and Cable Factory.
I would avoid the cheaper DIs if at all possible. With the best DIs, it's hard to measure any error even with a very clean a quite measurement machine. With the cheaper ones it becomes easier to measure and even perceptible.
THERE IS ANOTHER WAY
A good mixing board has good balanced outputs, it also can take an unbalanced input- Hey it's a DI! Sound engineers are picky about quality so, this is a big move up. Focusrite, Neve, SSL, and other top live and studio console designers make "channel-strips" where all the features of a single channel are put in a rack-mount unit with transformer-balanced outputs. You'd get a very good preamp, very effective EQ and maybe compression or limiting. Some even let you adjust the input impedance to allow tonal variations based on the input circuit. You will lose out on the ground-lift switch, and $1000 - $3000 for the unit.
Now, that was a lot, let me know what you're thinking.
-DaveMIX
Subscribe to:
Posts (Atom)