PASSIVE AND ACTIVE DIRECT BOXES EXPLAINED
At its core every direct box has one job, to accept an audio-input and transform it into something that looks and acts like the output of a low-impedance balanced microphone, for input into a sound or recording system. It sounds simple enough but there are many ways to construct a direct box and different designs deliver different sounding results.
|AN INEPENSIVE A.R.T. DIRECT BOX
Direct boxes come in a lot of shapes and sizes but the main job description remains similar across all of them, create an XLR balanced audio output that fits in the preamp gain staging of a typical low impedance microphone like the Shure SM-57.
Back in the 70’s if you wanted a good passive direct box you had to order a transformer from Deane Jensen and build your own according to his schematic. The transformer component was a 12:1 step down type packaged in a MuMetal® can. Fortunately it’s a lot easier to obtain a high quality direct box today and there are plenty to choose from.
BASIC PASSIVE DIRECT BOX
|HOSA PASSIVE DIRECT BOX, UNDER $30 ON AMAZON
They are called passive direct boxes because no external power source is needed to function. No battery and no phantom power is required to operate a passive direct box. These are the most simple and typically lowest in price. A transformer is usually employed to step the input-signal down to microphone level, create a balanced output which comes out through a male XLR connector and maybe most important, galvanically isolate the source from the destination. A ground-lift switch which disconnects pin #1 on the XLR output is usually provided to eliminate ground-loop hum caused by bridging AC power distribution systems.
|WHIRLWIND PASSIVE DIRECT BOX, $60 ON AMAZON
Most direct boxes have a built-in Y-cable on the input side, it is typically a 1/4” input jack and a second 1/4”-thru connection jack to feed a stage amp. This allows a musicians instrument to go into the direct box and feed thru to the XLR output and simultaneously use the 1/4-inch ”thru” connection to drive an amplifier on stage or in the studio.
The drawback to a passive direct box is what it does to the sound. The transformer used in a purely passive direct box design must steal a little bit of power from the signal passing through it and it affects tone negatively.
One night I was setting up to do a live location recording and I connected an inexpensive passive direct box on the bass players instrument cable. He immediately started asking me what I’d done to his tone. I switched out the passive direct box for a Countryman active model and the problem was solved (he stopped complaining). I did not hear the degradation because I was completely unfamiliar with his sound but the bass player sure heard it. He heard the tone difference and it made his regular live bass amp feel different in the way it responded.
From that experience I developed the following guidelines for direct box usage:
Active source = Passive or Active direct box
Passive source = Active direct box
A high quality active direct box can be used in any situation, there is no inherent advantage to a passive design other than simplicity.
There is nothing wrong with a passive transformer design but it will always degrade the instrument audio signal if the instrument is also passive. Passive direct boxes work great with active sources such as keyboards or active bass guitars like my Jackson 5-string.
BASIC ACTIVE DIRECT BOX
From the outside an active direct box looks the same as a passive model. They have the same 1/4” input / thru jacks and XLR male output with ground-lift switch. A tell-tale glowing LED may be present to indicate the box is receiving phantom power or possibly has a battery inside.
|RNDI ACTIVE DIRECT BOX
Inside an active direct box the 1/4” input feeds an amplifier which isolates the signal-source instrument from the impedance of the transformer which connects to the XLR output. Using an active circuit allows the direct box designer to set the input impedance very high causing the least sonic degradation to the audio passing through.
DIRECT BOX TRANSFORMERS
High quality direct boxes have an audio transformer component involved because transformers provide 100% galvanic circuit isolation and high immunity to induced noise. No other design or circuit component can do what a transformer does for isolation of electrical systems. With a transformer in the circuit electrical-grounds from different power sources don’t matter as much, the transformer isolates the two (or more) electrical ground connections so that hum and noise are eliminated or minimized.
|RADIAL PASSIVE DIRECT BOX
It is not uncommon for more expensive active direct boxes to include additional circuitry and functional features.
My RNDI direct box has the ability to receive high-level speaker signals and it can step them down to standard XLR microphone output level.
My “White Knight” passive direct boxes have an extra switch which optimizes them for either line-level input or the much smaller instrument-level input signal size. This in-built attenator can be a big source of problems and loud noises if it is switched off while in use.
|TRANSFORMER IN MuMETAL® CAN
The finest direct boxes provide a MuMETAL® can around the transformer to help prevent magnetic interference. This is especially important for low level instrument signals from passive instruments. MuMETAL® is not inexpensive so its use is more limited to premium components. Jensen transformers have provided MuMetal® shielding cans on their sensitive components like microphone inputs and direct box transformers since the company began back in the 70’s.
If you have a Direct Box that has a level switch or attenuator on it this can lead to a very dangerous performance situation. Let me explain, if you have the Direct Box set to attenuate a very high level speaker connection down to microphone level, what happens if you flip that attenuator out of the circuit while it is in use? You get a massive loud signal blasting through your PA system. We are talking horn-diaphragm popping, speaker-blowing, horrible. Don’t do it! As an engineer you need to keep untrained people from adjusting any attenuators on your direct boxes while they’re in use.
Direct boxes that have level controls or switchable attenuators should be kept out of reach from curious musicians with wandering hands. The last thing you need is a musician pawing around in dim light, groping your direct box switches and creating a massive audience killing blast of sound by accidentally switching off an input-attenuator. If this sort of accident ever happens while you’re the engineer every eye in the venue will be on you.
When engineering live sound I prefer single purpose direct boxes with only a ground-lift switch so that accidents can be eliminated. Direct boxes with switchable attenuator circuits can be placed in small cardboard cartons and sealed with tape to prevent someone from accidentally turning off the attenuator.
STEREO DIRECT BOXES
What is a stereo direct box? It’s two separate direct box circuits in one enclosure. Unlike some stereo circuits, direct boxes can be running entirely different signals on the left and right channels or they can be just passing a stereo signal. The actual direct box carrying the signal does not care or know about different sources because it is two completely separate and isolated direct box circuits housed in one enclosure.
There are only two advantages to a stereo direct box over using two separate direct boxes.
Advantage 1: Lower Cost Per Channel: RNDI single channel = $269 / channel, RNDI stereo = $249 / channel. Stereo model = lower cost per channel.
Advantage 2: Physical space: A two-channel direct box takes up virtually the same amount of space as a single channel direct box. Having two D.I. boxes in one enclosure can help keep cables and connections organized, logical and more compact.
WHY ARE THEY CALLED “D.I. BOXES”
What does D.I. stand for? Back in the 70’s D.I. meant “Direct Inject” or Direct Injection of signal. I was never comfortable with the whole “injection” theme, it always seemed like more of a connection than injection. Direct Connection or D.C. was already taken for the power distribution system so the term “Direct Injection” stuck and became a defacto standard term.
Rupert Neve was the first guy I heard say that D.I. means “Direct Interface”, to me, this makes a lot more sense than direct injection.
Whatever you call it a D.I. box is used to make a high quality direct connection between a musical instrument and a mixer or microphone input.
A direct box is a tool that converts a high or low impedance, unbalanced signal to balanced / isolated / low impedance / microphone-level output. Other important design criteria are the external enclosure and the design of the transformer. Steel blocks noise more effectively than aluminum so a steel outer enclosure will protect sensitive components from R.F. better than an aluminum enclosure.
A MuMetal® “can” surrounding an audio transformer is capable of reducing induced-noise by 30 dB or more compared to identical open frame transformer designs. MuMetal® is expensive due to the cost of the alloy components and limited applications for the unique shielding capability. Deane Jensen would never have sold a direct box transformer without a MuMetal can surrounding it because of his fanaticism for making the highest quality components.
THIS IS THE END OF PART 1, Click the link below to continue reading and discover how to use direct boxes.
Thank you for reading High on Technology, Good Music To You!
©July 2021 by Mark King, it’s not ok to copy or quote without written permission.