Why would someone use a DI box if they're recording directly into the external sound card

Question

It has been recommended to me to connect my bass directly to a DI box and then into my external sound card.

The sound card is MOTU 896.

I don't understand why this is the case and the person who suggested it is no longer around for me to ask.

I was under the impression from other posts like What does a DI box do? that you'd use a DI Box for live performance, where the signal was going into a console, but in this case, it's going directly into the sound card, over a short distance (no more than 3 metres).

I understand a DI box may add some character/flavour, but, is there any real need to use a DI Box when recording bass guitar (dry) and the intention is to use an bass amp simulation (VST).

Answer 1

The job of a DI box is not to colour the sound at all. Some may, but that is not the original purpose. The original purpose is impedance matching.

You only need a DI box when you are going into what would otherwise be a mis-matched input, for instance a mic or line input. Most computer interfaces these days have a third option - instrument - which would be suited to plugging a bass or guitar straight into.
Sometimes there is a hardware or even software switch on a dual-purpose line/instrument jack, sometimes it just has broad enough characteristics that it can accept both.

There are secondary/tertiary purposes, but not really relevant to this particular application - that of balancing a signal so it can be pushed a long way; to a studio patch-board or over long distances for live work.
There's also the potential for a ground-lift to isolate earthing/grounding issues. In a home studio, you may run into grounding issues, but any hum in a home environment is most likely to be induction from nearby computer monitors, lighting etc. It's always worth testing to see if a ground-lift DI may fix it, but it won't fix induction, only true ground issues.

Answer 2

No reason for that. A DI box will change the impedence of the signal, and also convert it to a balanced output; if you need to be over 20ft or so from the mixing console, a DI box is essential.

Some DI boxes also model the sound for you - e.g SansAmp, Tonehammer etc - but as you already know, that's not essential.

In short - plug into the sound card, and enjoy playing the bass. Worry about a DI box once you get to playing out live.

Answer 3

When recording to a sound card, a reason to use a DI box is input impedance. (DI boxes have many other uses in live sound)

EDIT: Looking at the manual of the MOTU 896 interface, I'm not sure if it is an issue. The manual says the quarter-inch jacks will "accept" a guitar, and that the inputs are "high impedance", but I couldn't find numerical figures about what that actually means. As a warning example, the Echo product demonstrated below was advertised as "high impedance", but the numbers were like 1/10 of a normal guitar amplifier and the sound was bad. It might be an issue for you. Owning a good-quality DI box never hurt anyone.

Some sound cards may be relatively bad in this regard, despite the manufacturer's claims. The lower the input impedance (i.e. alternate-current resistance) is, the more work the guitar's pickups have to do in order to create a voltage between the poles of the input. (Mainly talking about passive electronics, with no battery power on the guitar or anything.) If there's very little resistance, it's easy for the electrons i.e. electric current to start moving through the wire. But moving electrons is not what you want to do, you want to move the signal and that's done by creating a voltage i.e. electrical pressure or tension in the wire. Through the magnetic pickups, the motion energy of the strings pulls an electronic "signal rope" (for the lack of a better analogy), and you don't really want the pull to succeed, you just want there to be lots of tension. The receiving end (input) detects the tension, and so signal is transmitted. But the rope shouldn't move. The more it moves, the more you have to pull it in order to keep the level of tension, and that means work, which eats energy. You want the vibration of the strings to generate a voltage (tension), not work as an electric power supply. If the energy is transformed into electrical current, high frequencies are attenuated and the notes lose sustain, because the energy is "leaked" to unnecessary things.

Check out this comparison between an old Echo AudioFire 8 audio interface's "guitar/bass" input called "Hi-Z", which the manual says is 102 kOhm, and the same single-coil Squier strat played through a Countryman Type 85 DI box that's 10 MOhm according to the manufacturer. The standard for guitar amps is said to be 1 MOhm.

I think that's like two completely different instruments. The 102 kOhm value is not very good for a "guitar" input, when the standard for guitar amps is 10 times higher, 1 MOhm. But hey, just call it "Hi-Z" and "guitar/bass" and everyone is happy? ;)

A similar phenomenon or rule applies to amplifiers and speakers. The lower the impedance of the speakers is, the more "watts" you have to have on the amp side, because the amp will have to do more work to create the electric tension i.e. voltage. The ultimate low-impedance load would be a short-circuit, which will (if it's not protected) cause the amp to overload and burn a fuse or itself. This is why amps have a specified minimum speaker Ohm value, so you shouldn't use anything below 4 Ohms, for example.

Answer 4

Theoretically, there is no need to run into a DI and then into an external soundcard. In practice... maybe.

The job of a direct input box is to convert a guitar's or bass's high-impedance signal circuit to a low-impedance mic/line circuit to send "directly" to the mixer, instead of capturing the instrument's sound by putting a mic in front of the speaker cabinets. In live sound, a direct box gives the sound engineer a cleaner bass sound with less fiddling to mix in to the PA mains; a cabinet mic will pick up other sound including the crowd, and mic positioning has an important effect on tone so sound checks will take longer as a stage hand futzes with the cab mic to give the sound engy what he wants. A DI also reduces the required size and power of a bassist's "stage rig", often to just the DI itself, as the DI pumping his sound into the PA should give the band all the bass they need through the system's mains and monitors.

While designed for these stage uses, they're also useful for recording, as they allow inputting instrument signals directly into studio equipment like multitrack recorders, with no microphone needed. This again gives you a cleaner input without, say, the sounds of your three elementary-age daughters banging around upstairs doing God knows what (just a hypothetical, in absolutely no way whatsoever related to my personal experience laying down tracks). It also allows you to lay down guitar riffs or bass lines into your DAW at any time of the day or night, without needing a mic'ed amp blaring your bass tone at performance levels into your SM-57 (and the rest of the house).

Now, your average external soundcard (aka "audio interface", to further differentiate from the internal soundcard hardware of your PC) will commonly support "Hi-Z" inputs by incorporating a DI transformer circuit into the interface design. That increases the value of this piece of gear, and reduces your initial investment, by reducing the need for additional equipment in your home studio to make the interface useful. Almost all the major brands and models of USB audio interface allow plugging in at least one instrument. So, you don't need to have a separate DI in the signal chain.

Now, here's where theory meets practice. These audio interfaces are built on a budget, to meet a pricepoint that's attractive to the average music hobbyist. As such, they usually cut a corner somewhere, and one option is that DI circuit. To avoid making the interface too large (and heavy and expenive), a much smaller transformer coil is typically used for each input, and that has an effect on sound quality. A dedicated DI box will usually have a more substantial transformer, that more faithfully reproduces the upstream signal. That signal, now balanced and running into the interface via an XLR or TRS cable, will bypass the DI circuit in the soundcard, giving you cleaner tone and less noise in the DAW tracks.

Dedicated DIs can also have other circuitry, ranging from a simple gain boost ("Active" DIs boost the gain and reduce transformer impedance to reduce signal attenuation over a long run through the snake to the sound desk) to a full tone-shaping preamp (the LR Baggs Paracoustic DI is a combination DI/preamp designed for acoustic guitars; the Tech21 SansAmp is a more well-known general-purpose preamp for electric guitar and bass). If you get your live performance tone through one of these devices, there's a lot to recommend keeping it in the signal chain in your home studio at least as a starting point, and very few reasons to skip it.