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Ask Jonah: Answering Reader Questions About Direct (DI) Boxes

Going deeper with the passive versus active DI (direct input box) conundrum, summing, and the use of tube and FET models.

By Jonah Altrove January 16, 2019

The Countryman Type 85 direct box

Hi, Jonah:

What’s the deal with tube DIs like the Groove Tubes Ditto? I have an active pickup in my acoustic guitar and I never know what kind of DI to use. – Peter Bailey

Ah yes, the old DI (direct input box) conundrum. Here’s the skinny: All DIs fall into two categories: active and passive.

Passive DIs provide complete signal isolation due to their transformer and can operate without a power source. Active DIs need power (generally either phantom or batteries), can supply gain to the signal, and can have an extremely high input impedance, which is a must for certain sources such as piezos.

Both types can handle the important tasks of balancing a signal and better matching (technically “bridging”) the impedance between the source and the mic preamp.

When choosing, consider whether the specific application requires full isolation and/or gain, but you will be well served by the general rule of using active DIs on passive sources and passive DIs on active sources. This practice will help you keep you maximize SNR (signal-to-noise ratio) while reducing extra gain stages.

So what’s all this ado about tubes? At least one manufacturer of tube DIs states that the tubes are used to achieve much higher input impedances. This works, but a tube is not the only way to achieve a high input Z. An FET (field effect transistor) can work just as well – for example, the classic Countryman Type 85 achieves a 10 Megaohm input Z using an FET input stage, so the tube isn’t the only way to skin this cat.

However, I’d wager that tube DIs are usually chosen for artistic purposes. A DI is often used in place of an instrument amplifier, and since the amp itself contributes to the sound of the instrument, a plain ol’ DI’d bass or guitar can sound a bit odd.

Tubes are a good way to add back some of the natural distortion which is part of the characteristic sound of the guitar amp. This subtle distortion is part of why some people still prefer vinyl and analog tape. I scanned through a bunch of online reviews for popular tube DIs, and virtually all of them included the word “warmth.”

At the end of the day, some musicians opt for a tube DI simply because they like the way it sounds. As an engineer, I discuss this with artists before making any changes to their carefully sculpted tone once their signal is in my custody.

But because in this case you’re the one playing the guitar, my advice is to head to your local music shop and arrange for a test drive to see if the tube sound is a good fit for the tone you’re looking to achieve. If I were mixing you, as long as your acoustic’s active pickup is putting out a healthy level, I’d start with a well-made passive DI.

Read all of the “Ask Jonah” articles on PSW here.

Hey Jonah:

Can you settle something for me? I often use the two 1/4-inch jacks on a DI to easily sum both sides of a stereo input into mono for a single board channel. My boss insists this is bad practice. Who’s right? – Anonymous

Bad news, amigo.

The two 1/4-inch jacks on a DI are generally wired in parallel – literally, they’re wired together – so plugging two different outputs into the two jacks is electronically equivalent to plugging those two devices straight into each other.

Each low-impedance output sees the other output as more or less a short to ground, and the two outputs try to drive each other. (Think about what would happen if you wired two power amplifiers into the same set of loudspeaker terminals. The concept is the same.)

This may work just fine, and it often does, which might be why you’ve never noticed an issue, but it can also cause bad things: overloading the output op-amps can cause nasty distortion or in some cases cause the device to simply shut off.

These outcomes are less common with modern, competently designed devices, but I have still seen them occur and see no reason to take a gamble. If you need to combine inputs, use a passive summing box, which is a great thing to carry in your toolkit.

The rule for Y cables is easy: splitting an output to multiple inputs is fine, but trying to combine multiple outputs into a single input is a no-go. For more on why, see the seminal “Rane Note 109: Why Not Wye?” on ProSoundWeb.

Send your questions to [email protected].


About Jonah

Jonah Altrove
Jonah Altrove

Veteran Live Audio Professional
Jonah Altrove is a veteran live audio professional on a constant quest to discover more about the craft. Send him your "Ask Jonah" questions at [email protected]

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Andrew Broughton says

The Radial JDI passive DI (and some other products by Radial) has a "merge" function where you can sum the 2 1/4" inputs via a resistor network internally so as to not "short out" the connections.
http://www.radialeng.com/product/jdi

Joe Dombroski says

My favorite is the BNC T adaptor! Not sure why that connector was made:) On iPods you can actually get them to cut out if you crank them up and short left to right without two resistors to simulate a set of ear buds.

Charlie Post says

Just buck up and use a stereo DI. Most have a "mono" function which, as Andrew suggested, merges the signal from both channels. Later when you have enough inputs and need to run it in stereo, you will have that option available.

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