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Basically I'm in need of a DI box and I'm not really sure what to get. I have a performance in January where I'm using Ableton Live 9 alongside a guitarist. The guitar will be DI'd straight into Live for live processing. I'm not a guitarist myself and the only experience I have had with DI'ing, the DI box has already been there and ready to use.

I'm not sure what and passive means for DI boxes, so breakdown of the different types of DI boxes and their use would be beneficial.

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The most important difference between passive and active DIs is that the latter have a much higher input impedance: usually something like 200 kΩ, whereas a passive DI "inherits" the low impedance of the mic input you're feeding (around 2kΩ), or transforms it up a bit to something in the 20 kΩ range. That makes hardly a difference for instruments with low output impedance (e.g. keyboards, acoustic guitars with build-in preamp): if the output impedance, e.g. 300 Ω, is much lower then the input impedance, you're in "voltage saturation mode", then it makes no difference whether the input impedance is ten times as high or a thousand times as high. So for "powered" instruments you'll always be pretty ok with a passive DI.

The story is very different for instruments that are themselves passive. In particular, guitar pickups have a resistance up to some 10 kΩ even in DC, and the inductive impedance is much higher. So if you plug an electric guitar into a passive DI, the transmission becomes current-saturated. In this mode, the pickups operate completely different from voltage-saturation, the resonance almost vanishes and the volume pot becomes very quiet much quicker. So if you're DI-ing a guitar and don't want to alter the sound even before processing, you must use an active DI, whose input impedance is close to that of an electric-guitar amp (those range between 50 kΩ and 500 kΩ).
It's even more drastic for acoustic guitars with passive piëzo pickups: those have a high capacitive impedance. If you connect them to a low ohmic impedance, you're building a high-pass filter, cutting away all bass frequencies (often in fact everything below 1 kHz). That's why acoustic instruments, unlike electric guitars, almost always have dedicated preamps with input impedance > 1 MΩ: because even standard active DIs have too low impedance to get an even frequency response.


I should add a bit more detail on impedance matching in a passive DI. As I said, passive DIs can also increase the input impedance: by transforming down, i.e. using a transformer with more windings on the primary side than the secondary. The cost is that the output voltage is reduced. Not all DIs do this, for some models I've used I'm sure the output was actually in the same range as the input. (Strangely, it isn't usually specified on a passive DI what winding ratio is used.) But Wikipedia quotes ratios between 10:1 and 20:1 as typical, which is definitely more than I would have reckoned. With such high ratios, voltage loss becomes an issue, but good mic preamps should have enough SNR so this should still be ok; after all, dynamic microphones also offer rather poor voltage. At any rate however, even if you transform a 2 kΩ input up to 40 kΩ, this is still rather on the low end for electric guitar, so my point stands: the main issue with passive DIs is low input impedance.

Now, it would certainly be possible to design a passive DI with transformer ratio as high as perhaps 100:1. Then the input impedance would be high enough to connect guitars, perhaps even a piëzo – but the output voltage really wouldn't be satisfying anymore, no matter what instrument you connect. So I think the current situation is pretty fine for practical purposes:

  • Active DIs can easily offer enough input impedance for most instruments, without sacrificing gain (though they normally offer a switch to do that on purpose, if your inputs are overloaded). If you need high impedance, use an active DI.
  • Passive DIs don't aim at very high input impedance, so as not to sacrifice too much gain either – and that's ok, because for active instruments impedance is not really an issue. Hence, for those instruments a passive DI is a cheap, simple, reliable alternative. Also, you may use them to deliberately dampen a guitar's pickup resonance; perhaps that's worthwhile sometimes. (A few guitar effects pedals have low input impedance, best known the Fuzz Face, and the thus changed guitar response makes up a good part of many fuzz sounds you can hear on old records.)
  • I think I've got my head around that haha thanks for the extensive answer. I've ordered a passive DI for now, if I don't like the sound I'll go ahead and get an active DI as well. Can't have too much equipment ha. Very useful information though thankyou – Perceptic Dec 11 '14 at 10:34
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    Thanks for this. You might want to contribute to the Wikipedia article on DI Units, which is missing this information. I think my answer doesn't deserve all its upvotes, because of this omission, but I don't want to plagiarise yours, nor remove my post altogether because I think it has other useful info -- so I'm leaving it as it is. @Perceptic I would encourage you to accept this answer instead. – slim Dec 11 '14 at 10:44
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A DI ("Direct Input") box converts a signal on an unbalanced lead into a signal on a balanced lead (that is, usually a lead with XLR connectors).

The advantage of a balanced lead, of course, is that you can run it for long distances without hearing interference.

With a passive DI, the signal on the XLR lead is at the same level as the input signal. So, if you plug a guitar into it, whatever's at the other end needs to be able to handle instrument-level input (which is similar to mic-level; most mixers and sound interfaces will be OK with this).

An active DI contains a pre-amp, which you would normally use to bring the output signal up to line level. That's a big difference, and brings with it a lot of factors. It needs a knob to control the gain. It needs power -- from battery or phantom power. The quality of the amplifier is a factor. You need an active DI if your signal source is instrument-level, but the receiving end requires line-level.

It's more common for active DIs to have extra features such as passthrough, than for passive DIs.

If you are running the XLR lead a very long way, it may be beneficial to have a line-level signal on it. Even though balanced lines compensate for interference, a stronger signal relative to interference can only be helpful.

If you're not running the signal a long way, perhaps you don't need a DI box at all. From what you've described, I imagine you and your guitarist standing next to each other on stage, with your computer and audio interface in front of you. If that's the case, just plug his guitar lead directly into your audio interface. People use the term "DI" for any situation in which an instrument that's traditionally played through an amp and mic'd, is instead connected electrically to whatever's doing the mix. There doesn't have to be a DI box or an XLR plug involved.

  • Thanks for the reply. I was thinking I wouldn't have to use a DI, as yes, he will be playing probably no more than 4ft from the computer. I had a practice session and I was getting huge amounts of hum. I understand that this could be the pickups picking up interferance from somewhere. As we cant practice at the venue until the day, I figured it would be best to try and prevent it in any way possible, so I'm going to get one just to be sure. The interface will be a Scarlett, which accepts instrument level input, so would you reccomend a passive DI? – Perceptic Dec 10 '14 at 16:25
  • I'd go passive if money is an issue. Or borrow whatever you can. However hum sounds more like a ground loop or a dodgy lead (or a fault in the guitar's electrics). You wouldn't expect "massive amounts of hum" from a 10ft guitar lead. – slim Dec 10 '14 at 16:40
  • Okay thanks for the info. I'll go ahead and get one. It may not solve it if it is an issue with something else but at least I'll have one for the future. Thanks again! – Perceptic Dec 10 '14 at 18:33
  • Sorry, but that's totally not the crucial difference between a passive and active DI. – leftaroundabout Dec 10 '14 at 23:52
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    I guess I was being a bit harsh – this answer does, of course, still make some correct points, it just doesn't get to the gist IMO. In particular, the preamps in active DIs generally don't boost the signal voltage at all ("bring the output signal up to line level" – in fact they rather bring the signal down to mic level!), though they do in a sense boost the current. That, in turn, actually means that the signal is more immune to interference, though when the original source was low-impedance then the balanced connection after a passive DI is plenty stable for any realistic XLR connection. – leftaroundabout Dec 11 '14 at 13:12

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