These gadgets might not be the most glamorous devices, but at some point in your recording adventures you’ll probably find you need each and every one.
Whether you’re running a live‑sound system, working in a professional studio, or recording yourself in the spare bedroom, sooner or later you’ll come across a situation where you either need to track down a fault or connect two ‘incompatible’ devices — and you’ll need a ‘problem solver’ to do it. This article takes you through the most common types of audio problem solvers, explaining what they do, how they work and when and how you should use them — and sometimes how you might creatively misuse them, too!
Probably the most frequently used problem solver is the direct injection or ‘DI’ box, which is designed to accept an unbalanced, instrument‑level signal and provide a balanced ‘mic‑level’ output. The main advantages are that the mic‑level signal can travel happily down long cables unaffected, and the XLR cable from the DI box’s output can plug into any stage box without scaring anyone! This allows a guitar (or almost any instrument or electronic signal) to be connected safely to a distant mic preamp, such as on a mixing console or audio interface.
Naturally, there are variations on the theme, and prices range from $20£20 to several hundred, the variance being mostly due to features, sound quality, and robustness. It’s perhaps also worth pointing out that ‘sound quality’ can refer both to the technical performance and to the sonic characteristics, and some of the most expensive are as much amp simulator or saturation processor as they are DI! Choose well, and a DI box should last, literally, a lifetime — and in that context, the higher‑priced units might offer rather better value than you initially think.
DI boxes are available in single, dual‑ (stereo) and multi‑channel models, and in either ‘active’ or ‘passive’ forms. Passive versions are often less expensive than active types, but while they can do the job they may be technically and audibly inferior in some situations, particularly when used with electric guitars, as I’ll explain shortly. But passive DIs can be more versatile than active ones, and I’ll come back to that later.
A passive DI box contains little more than a specially designed transformer with a fairly high ‘turns ratio’ of 10:1 or 12:1, which means the output is a 10th or 12th the size of the input. That means it reduces the signal voltage from instrument level (nominally ‑20dBu) to mic level (around ‑40dBu). The way the transformer is wired allows it to convert the unbalanced input to a balanced output, too. The design of the transformer allows it to present a moderately high input impedance of around 150kΩ, and that is fine for anything with an electronic output like electronic keyboards, DJ rigs, drums machines, and so on. However, it is often too low to work properly with electric guitars and basses, which typically need a load of at least 250kΩ and ideally much higher. For these kinds of instruments, an active DI box is a better choice because it can provide a much higher input impedance.
A very important benefit of the transformer is that it provides electrical (galvanic) isolation between the source and destination equipment. This means that there is no direct electrical connection between the input and output connections — the signal is passed from the transformer’s primary winding to the secondary winding only as a varying magnetic field. This isolation means that passive DI boxes inherently protect the input source from phantom power, which may well be present on the mic cable connected to the output of the DI box — nobody one wants their shiny new keyboard or DJ console fried by 48V phantom power the moment it’s plugged in on stage!
The same isolation is also a safety feature. It protects the source or destination in the event of a serious electrical fault in the other, and can be — indeed it has been, literally — a lifesaver. That’s why I always advocate using DI boxes at live gigs, especially so when working with ‘unknown’ equipment, to electrically isolate the performers’ gear from the live sound equipment.
If seeking a DI box for keyboards or other electronic sound sources, a good quality passive DI is a good choice. The Palmer PAN 01 and ART Passive Direct are good single‑channel options, while for stereo applications the Mackie MDB‑2P, Klark Teknik DI20P and Radial ProD2 are solid choices. There are many more on offer from other manufacturers, of course.
The alternative form of DI box, as I mentioned earlier, is the active DI box, so‑called because it contains active electronics which provide a much higher input impedance than is possible with a passive transformer. Most active DI boxes present an input impedance of around 1MΩ (1000kΩ), which allows electric guitars and basses to perform and sound as their designers intended — their pickups are not unduly ‘stressed’. Active electronics need power, of course, and that comes either from an internal battery or from phantom power over the mic cable. The latter is generally considered the most secure and reliable arrangement as it can’t run down mid‑gig!
Most active DI boxes also contain an output transformer, so that they offer all the same safety benefits as discussed for the passive DI, but some omit the transformer for cost and quality reasons — while they’re perfectly capable of converting the signal level and performing the balancing, and can be a good cost‑effective option in your own studio, do check before buying if you’re concerned about electrical safety. Popular active DI boxes include the BSS AR‑133, Klark Teknik DN100 V2, Warm Audio WA‑DI Active, and the Radial J48, amongst an ocean of others.
As well as performing their basic functions, the vast majority of DI boxes of both types incorporate some user‑configurable facilities, such as selectable attenuators (pads) to allow them to accept different input levels (commonly instrument, line, and/or speaker levels). Almost all DI boxes also have a switch that separates the input ground from the output ground, to avoid ground loops — typically this is called a ‘ground‑lift’ switch. Most also have a ‘link’ or ‘thru’ socket (same thing, different name), which is wired directly in parallel with the input socket so that the source signal can, if required, be passed both to a backline amplifier (via this output) and the mic preamp (via the XLR).
More elaborate models might also feature an extra, electronically‑buffered unbalanced output to feed a tuner or similar, or might be able to mix stereo inputs to create a single, mono output signal. Some are designed specifically to accept inputs directly from a laptop computer or even a vinyl record pickup cartridge, while others might include guitar amp/cab emulation circuitry or other musically‑beneficial tone shaping facilities, such as in the A Designs REDDI and Tech 21 SansAmp Para Driver.
Although the 1MΩ input impedance presented by most active Di boxes might sound a high number, it’s still not really high enough to work properly with the piezo pickups found on some acoustic guitars and other acoustic instruments. These devices typically prefer to see 10MΩ or more and, consequently, there are special active DI boxes intended specifically to cater for piezo pickups — examples are the Radial PZ‑DI, Caveman AP1 and Fishman Aura Spectrum DI.
I mentioned earlier that some problem solvers can be ‘misused’ in a creative way, and one option is to use a passive DI box backwards! Obviously, the electronic circuitry within an active DI box means the signal can only flow from input to output, but the transformer in a passive box doesn’t really care which way the signal goes. That means that you can provide it with a balanced mic‑level signal at its output (via a suitable adaptor cable) and generate an instrument‑level unbalanced output. Okay, so this bodge isn’t an ideal solution for various technical reasons, but it can be a useful get‑out‑of‑trouble fix in an emergency. The proper tool for the job is a ‘re‑amper’, of course, which I’ll cover below.
A line isolation box is very similar in concept and implementation to a passive DI, but it is intended for use with line‑ or instrument‑level sources and it provides the same level at the output as the input, and since the intention is not to change the signal level the transformer has a ratio of 1:1. Instead, it’s there to do two things. First, it provides electrical (galvanic) isolation between the source and destination, this time principally to prevent ground‑loop noise (hence the common alternative name of a ‘hum killer’), but also to act as an electrical safety barrier between source and destination equipment. Second, it acts as a format converter, providing a balanced output from an unbalanced input, or vice versa, depending on the connection options.
Most line isolation boxes are two‑channel units and, like passive DI boxes, prices vary considerably — the more expensive models generally have better‑quality transformers, which introduce less distortion (especially at low frequencies) and can handle higher signal levels. As already suggested, typical applications for a line isolation box include breaking ground loops between computer interfaces and active monitor speakers, or connecting keyboards or DJ rigs to a PA mixer’s line‑level inputs (instead of using a DI box to feed mic inputs). Popular models include the ART CleanBox‑2 and DTI, and the latter is particularly versatile because it has multiple connections in different formats. Behringer’s MicroHD 400 is another affordable option.
It’s often necessary to share the output of a microphone between two or more separate destinations. A live‑sound application, for example, might be to split the stage mics between the front‑of‑house (FOH) and monitor consoles, or between the FOand a separate recording setup. In a studio, it might be to share the mic between different preamps for comparison purposes. The usual technology for this application is, once again, entirely passive, but it uses a special transformer, which typically will have a single primary winding (as usual) but two or three secondary windings, configured in 1:1:1(:1) ratio, meaning that all the outputs are at the same level as the input.
Because phantom power is a DC voltage it can’t pass through the transformer, so all mic splitters have a ‘direct output’ socket which is wired directly across the microphone input socket.
Because phantom power is a DC voltage it can’t pass through the transformer, so all mic splitters have a ‘direct output’ socket which is wired directly across the microphone input socket (in exactly the same way as the ‘link’ or ‘thru’ socket of a DI box). This direct output is always connected to the ‘main’ destination, which is the one that provides phantom power to the mic if needed. The transformer’s primary winding is also wired to the microphone input socket, so independent (electrically isolated) outputs can be provided through the secondary windings.
Each secondary winding normally has its own Faraday shield within the transformer connected to the corresponding output XLR’s pin 1, and usually there are separate ground‑lift switches for each output — these tie the separate output shields to the microphone/direct output ground, if needed (or ‘lift’ them to avoid ground loops).
Again, the more expensive mic splitters have better‑quality transformers which introduce less distortion and can handle louder signals. Some also include an input pad to avoid overdriving the transformer when the microphone is placed in front of very loud sources. Typical examples include models from ART, EMO, Palmer, and Radial.
As with DI boxes, there are active versions of mic splitters too, but these are typically only available in large multi‑channel formats and most act like simple mic preamps, providing phantom power (if needed) to the mic, and something approaching line level at the transformer‑isolated balanced outputs.
Re‑amping is a popular music production technique, whereby a cleanly recorded (for example through a DI box but not an amp) instrument track is sent back out to an amplifier/speaker in the studio, and then the acoustic sound from that amp is re‑recorded. Alternatively, it may be desirable to route the cleanly recorded instrument through a chain of stomp effects pedals and re‑record the output of the effects chain. The technical problem to be solved here is that the outputs of most mixing consoles and audio interfaces are balanced, line‑level signals, whereas the guitar amp or stomp pedal expects unbalanced, instrument‑level signals.
The best solution is a re‑amper or re‑amp box and, again, there are many variations on the theme. The simplest form employs a step‑down transformer which performs both the signal level and connection format changes. Most also have facilities to adjust the output signal level and to isolate the input and output grounds, to avoid ground‑loop problems (ground lift). A proper re‑amper is much easier and more convenient to use than the ‘backwards’ passive DI box bodge that I mentioned above.
But some re‑ampers are pretty elaborate and can be expensive. Often, they have DI circuitry in the same case, so that the device can be used both for recording and re‑amping — and can both send to and receive a signal from guitar pedals, without involving an amp. But more conventional examples include the Orchid Amp Interface or the Radial Reamp.
An alternative re‑amping solution, which can be very convenient for stereo signal paths, is to (mis)use a ‘consumer equipment interface’ (more on these below). This provides the appropriate level and connection format changes, but does so through active circuitry rather than transformers — this means there’s a risk of ground‑loop issues in some circumstances. Nevertheless, if you have such a device available and lack a dedicated re‑amper, this approach can prove convenient when wanting to process recorded signals through stomp boxes.
A very common requirement in all pro‑audio installations is to be able to connect consumer equipment into a professional system. Going back a few years, the most frequent example was to hook up a cassette machine to record the output of a studio console and play back demo tapes, and later the cassette gave way to the CD burner. Today, we’re more likely to be connecting laptops, MP3 players, smartphones or video machines, or perhaps plumbing a secondary consumer monitoring system (hi‑fi amps or active speakers with unbalanced inputs) into a monitor controller or audio interface.
Consumer equipment typically has unbalanced connections operating at a nominal level of ‑10dBV (which is roughly ‑8dBu), whereas professional equipment is balanced at a nominal +4dBu. This near‑12dB level difference and connection format incompatibility is routinely addressed with some active electronics in a dedicated ‘consumer equipment interface’, with most providing two channels in each direction: unbalanced consumer signal at ‑10dBV to balanced line level at +4dBu, and vice versa. Most consumer equipment interfaces provide fixed ±12dB gain/attenuation, though some allow the gain/attenuation to be adjusted to optimise signal levels to and from the connected equipment.
As I hinted above, these boxes are also handy solutions when re‑amping signals through stompbox effects, since the latter generally operate with unbalanced connections at levels that are not too dissimilar to consumer equipment. Note, though, that these interfaces tend not to employ transformers, which means there is no galvanic isolation between the source and destination devices. Thus, ground loops are a possibility if using any (Class I, grounded) mains‑powered pedals. Battery‑powered effects pedals or those powered from Class II isolated PSUs shouldn’t present any problems at all.
The low cost, ubiquity and convenience of Cat 5E (and related) cabling has, in recent years, made it a popular option for lightweight analogue and digital audio ‘snakes’. The four separate twisted pairs within a standard Cat cable can be employed to carry four separate balanced audio signals, and the cable’s characteristic impedance is also suitable for conveying AES3 digital audio as well as conventional analogue audio over distances up to 100 metres.
Most Cat adaptors are configured to send four independent balanced signals in the same direction, so a matching pair of male/female Cat adaptor boxes are needed. One box, as the input unit, is typically fitted with female XLRs and the other, the output unit, with male XLRs. Radial’s Catapult system is one such example, and Whirlwind’s Catdusa another. Each of the four twisted pairs within the Cat cable is wired to one of the four XLRs, although note that since there’s no universal wiring standard exactly, which pair is allocated to which connector may vary between manufacturers! For example, input 1 on one manufacturer’s input box might appear on output 3 on a different manufacturer’s output box.
Some manufacturers offer bespoke breakout boxes which cater for two signals in both directions. This allows, for example, two mic feeds to be sent to a preamp rack and two headphone monitor feeds returned into the live room. Cranborne Audio’s CAST system works this way, for example. The same Cat cable is used in both applications; only the break‑out box wiring differs.
The cheapest standard Cat 5E cable is ‘UTP’ (Unshielded Twisted Pair) but it’s not a great idea to use unscreened cable when working with audio signals over long distances. Instead, Cat 5 FTP (Foil‑screened Twisted Pair) cable, which has an overall screen, is the minimum standard cable for use in this kind of application. In practice, most Cat adaptor systems employ Cat 6 FTP cables, which have an overall screen, although Cat 7 S/FTP (Screened Fully‑shielded Twisted Pair) cables are even better, as they are guaranteed to have separate screens over each individual pair — a feature which could be important to minimise crosstalk between channels, such as when working with multiple microphone signals of different levels.
One problem solver I find particularly useful is a ‘phantom‑checker’, used to test for the presence of a phantom power voltage on each wire in an XLR cable; I’ve carried one on my keyring for decades! Several mic manufacturers used to make these in various forms, but when writing this article the only commercial unit I was able to find which remains currently available is Canford Audio’s. The most technical models only illuminate if the phantom voltage is within the prescribed voltage limits and/or capable of providing adequate current, but simpler versions are still useful and easily within the bounds of simple DIY if you can solder; there are several designs on the Internet. Most are built into a standard XLR plug body, making them easy to carry around for fast, efficient determination of whether phantom power is reaching a mic or DI box (and for confirming broken cables). You’ll also find other useful test gadgets built into XLR connectors, including phantom‑powered pink‑noise and tone generators.
Of course, a more comprehensive way of testing cables is with a dedicated cable tester, which will help you identify what the fault actually is! There are many such devices and they cater for many different types of connector. I’ve yet to see one that caters for everything in one box but there are several which offer most of those in use in the audio world today, and some also include useful additional features, including phantom‑power checking facilities, tone/noise generators and monitor speakers, so that connections can be tested audibly. Some, including the Hosa CBT‑500, also allow you to test not only analogue audio connections and MIDI DINs but also USB and Cat cables.
I find the Sonnect Audio Sound Bullet to be a particularly useful and all‑in‑one compact portable audio tester. Some cable testers come with two separate parts, so as to facilitate easy testing of continuity over long cable runs: dbx and Studiospares both offer such a device, for example. Be sure to find a model that has the connections you need to test, though. For example, there are now few (the Behringer CT‑100 being one) which can test TT bantam cables without requiring an adaptor cable.
Arguably a must‑have problem solving accessory for every engineer is the humble in‑line XLR pad. When a moderately sensitive mic is placed in front of a very loud source it can easily generate line‑level output signals, and these can overload many mic preamps. A lot of preamps have a ‘pad’ button to accommodate such situations but where this option is absent (or if, on an audio interface, it’s inconvenient to access using software) an XLR in‑line attenuator can save the day. There are many fixed attenuation options from 10 to 50 dB, but 20dB is often a good compromise value if you only want to buy one type. A few (including the Hosa ARR‑448, the Audio‑Technica AT8202 and Shure’s A15AS) offer two or three switchable settings, typically ranging between 10 and 40dB.
It’s worth noting that, with an input impedance of between 600Ω and 1000Ω and an output impedance of 100Ω to 200Ω, almost all commercial in‑line XLR attenuators are optimised for microphone applications — and all pass phantom power too. Although they’re physically capable of handling line‑level signals, their relatively low input impedance may prove too much of a challenge for some electronically‑balanced line‑level outputs to drive. A potential result of that is transient distortion, so I wouldn’t recommend their use for dropping line‑level signals down to mic level; a DI box is a safer bet for that application. That said, there are a few boxes around designed to offer variable attenuation of line‑level signals, and these can be useful if, for example, you’re deliberately driving a mic preamp which lacks its own output level control into saturation, and wish to avoid overloading the next device in the signal chain.
Sometimes, of course, the problem is the opposite — a mic’s signal is too low, rather than too loud — and the solution is an in‑line mic booster, or ‘booster preamp’. These have become very popular accessories in recent years and a lot of manufacturers now offer variations on this theme, with some models being very compact and simple, and others larger and technically far more elaborate (and costly). All are phantom‑powered, but most of them don’t pass phantom on to the microphone because they’re usually designed for use with low‑output passive mics like ribbons and insensitive moving‑coil types. Cloud Microphones’ Cloudlifter CL1 was one of the first in‑line boosters to reach the market, but other popular models include Imperative Audio’s FP1, Triton Audio’s FetHead, sE Electronics’ Dynamite and TNT models, and Royer’s dBooster and dBooster2, to name a few. Where there’s a need to boost the signal from a capacitor mic, Triton’s FetHead Phantom and FetHead Broadcast models both pass phantom power on to the microphone.
Other worthy mentions for the problem‑solving toolbox are polarity inverters and mutes. The polarity inverter is typically another in‑line XLR connector, wired internally with the hot and cold signals swapped over (so the input pin 2 goes to the output pin 3, and vice versa). This arrangement flips the polarity (wrongly referred to as ‘phase’ in some quarters) of a balanced signal, and that’s very handy if the console or DAW interface lacks this facility. For example, when top‑ and bottom‑miking a snare drum it is essential to invert the polarity of the bottom mic as the lower snare head moves in the opposite direction to the batter head. Without the appropriate polarity inversion mixing the outputs from the two mics will result in a very thin drum sound! Polarity inverters can also be handy when combining mics located in different locations — such as near and distant mics on a guitar amp, or for decoding Mid‑Sides stereo mic arrays.
Signal mutes can be in the form of an in‑line XLR connector with a rotary collar switch, or a stompbox. Either way, the idea is simply to make it easy for an artist to mute their own microphone (or other signal source) when it’s not being used — this can be very useful in ‘self‑managed’ stage shows.