Today, pretty much all keyboards and drum machines are digital, and can basically be thought of as keyboard controlled CD players.
And like a CD player, the output from a digital synthesizer is relatively powerful when compared to an electric guitar or an old Rhodes piano.
Because they’re so “loud” they needed headroom to operate, meaning that the old active DI box that may have been a boon to the low-output Rhodes piano can no longer keep up.
The headroom is limited by the internal battery or limited by the low current afforded by phantom power.
To make matters worse, unlike a CD that is processed and compressed before it is mass produced, digital samplers are raw.
They can generate huge transients that will overload most active DI boxes, and end up distorting horribly.
The problem is further exacerbated with digital pianos. These full-range devices are not only very dynamic, they have a frequency range that starts way down low and goes up forever.
To handle modern keyboards, there are two choices:
1) Send the keyboards into a mixing console where the internal rail voltage is sufficiently ample that it is able to handle the range.
2) Send the signal to a passive direct box where the headroom is not limited by the current afforded to them. Passive DI boxes are different- they use transformers.
Phantom Of The Power
Replace the diesel engine inside a dump truck with a 4-cylinder car engine and fill the truck with gravel. What will happen? Nothing. The engine will be unable to handle the load.
The same applies to phantom power. Folks tend to “believe” that if it’s active, it must be good. But the truth is, phantom power was never designed to power direct boxes.
As noted in my previous article, phantom was invented by Mr. Neumann as a means to charge the capsules on his microphones. He needed a lot of voltage (48 volts) and very little current (5 milliamps).
A quality preamp requires +/- 16 volts (32-volt swing) and about 50 milliamps of current. With 1/10th the current, it’s like trying to run a dump truck with a motorcycle engine.
Passive direct boxes are not power limited. They’re old-fashioned devices that basically combine a couple rolls of wire (coils) with a chunk of metal (the core).
A DI has the task of converting a high-impedance signal to a low-impedance balanced signal where it can be managed by the mic splitter and mixing console’s preamp.
With keyboards, current enters the transformer where the conversion occurs. But instead of overloading. like an active circuit, transformers distort gradually. More precisely, they don’t so much distort as saturate.
We often say that transformers sound “vintage” or have a limiting quality about them. This is because good quality transformers generate warm sounding even order harmonics, or what is commonly known as a warm Bessel Curve.
Thus the reason highly dynamic buffered signals like digital keyboards sound great when they are used with a passive direct box.