Q. Will my audio interface reduce the sound quality of a good external ADAT converter?

By Hugh Robjohns

Many interfaces have ADAT connections that allow you to connect external mic preamps and converters — and as ADAT is a digital format, a cheap interface will not compromise the quality of a more expensive external unit connected to it via ADAT.

I have a fairly cheap and cheerful audio interface that has ADAT ports to allow me to add more inputs and outputs. If I were to use these to hook up a more expensive interface with better mic preamps, will I see an increase in quality, or can the cheap interface’s converters still compromise the sound quality anyway?

SOS Forum post

SOS Technical Editor Hugh Robjohns replies: Assuming you connect a higher-quality device you should see an improvement because you’re not at the mercy of any A-D/D-A conversion! Inside your cheap (or any other) interface, there are a bunch of A-D and D-A converters that convert between digital and analogue signals for the unit’s onboard analogue inputs and outputs. These have nothing to do with the interface’s digital (typically AES, S/PDIF and/or ADAT) inputs and outputs.

The digital signals to and from these converters and all of the digital ports are routed through some form of DSP, which handles the task of data reformatting needed to translate between the computer’s USB / Firewire / Thunderbolt ‘language’ and the ‘language’ required by the interface’s converters and digital ports. Think of it not as ‘conversion’, but as ‘reformatting’ or ‘repackaging’. The digital audio data from the ADAT socket is completely unmolested; it’s just repackaged differently as it is passed on to the computer. It is most definitely not converted to analogue and then re-encoded back to digital!

So, in the scenario you describe, the cheaper audio interface is providing the interface between the incoming ADAT data from the higher-quality device and the cheaper device’s USB or Firewire (or whatever other protocol) connection to your computer. The digital audio from the source remains as digital audio throughout that ‘translation’ process, and the only ‘conversion’ that could potentially be incurred is a sample-rate conversion, if the interface provides that facility — and even that remains entirely within the digital domain. Few interfaces offer that facility anyway, which is why it is important to set the clocking options correctly on the interface and external ADAT source!

Ideally, assuming you’re using the preamps on both your cheap interface and the ADAT-equipped mic preamp box, the cheap audio interface would be set as the clock master, the expensive (external ADAT source) one as the slave, and a synchronising word clock signal taken from the cheap interface’s clock-out port to the expensive interface clock-in. That assumes that the A-Ds and D-As in the cheap interface perform best when running on their internal crystal clock, while the external ADAT source has a decent clock-recovery system. If you’re not using any of the analogue inputs on your cheaper interface and your interface allows, you should set the higher-quality unit as the clock master. 

Published December 2017

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Q. When used live, how should I protect an audio interface from phantom power damage?

By Hugh Robjohns

Phantom power shouldn’t be delivered to line outputs but you can never guarantee that a FOH mixer hasn’t been modified. A passive DI box or transformer isolator, such as the ART DTI, can protect audio interfaces from potential catastrophe when used on stage!

I recently worked at an event where the sound engineer blew up the sound card on a computer, apparently because the sound desk it was connected to sent phantom power to all channels. As I have a small gig coming up where I need to run sound from a computer, keyboard player and a drum pad using a small mixer that also sends phantom power to all channels, I don’t want that kind of disaster! So is there a way that one might prevent equipment damage from phantom power?

Is a DI box the answer, and if so, what type? I would also like a good DI box for my guitars too, so can I kill two birds with one stone? And would the DI box help reject interference? Finally, someone told me that phantom power does not run through jack lead connections. Is this correct and would that negate the need for a DI box?

Karl Addy

SOS Technical Editor Hugh Robjohns replies: Very few electronic line-level devices (computer sound cards, electronic keyboards, MP3 players, DJ mixers, etc) like having 48V phantom power applied to their outputs! Some are designed to cope, but many will be damaged or even destroyed, so some form of electrical protection is definitely advisable, if only to protect against accidental mis-plugging on a dark live stage!

A basic requirement is to introduce a means of electrical isolation between the source equipment and live sound mixer, and the conventional options are either a DI box or a ‘line isolator’ box. Both essentially use a transformer to provide the electrical isolation and thus prevent the mixer’s phantom power from reaching the source equipment.

The main practical difference between these two devices is that DI boxes are designed to transform the unbalanced line or instrument signal down to a balanced mic-level output signal, while a line isolator doesn’t change the signal level at all and so provides a balanced line-level output from an unbalanced line-level source. The choice of one over the other really depends on what signal level the mixer’s mic inputs can cope with, but in most live-sound cases I would generally go with a DI box.

When working with electronic sources (sound cards, keyboards, etc), a ‘passive’ DI box would be fine (I often use a Radial ProD2, for example), but if you are likely to want to use the DI box with an electric guitar I’d recommend buying an ‘active’ one instead, simply because the input impedance is usually higher and is less likely to affect the guitar tone in a negative way. Two popular ‘industry standards’ are the Radial J48 and the BSS AR-133. Although initially expensive these devices both sound very good indeed, and will easily last a lifetime; most cheap alternatives don’t and won’t!

An alternative source of extremely good and highly cost-effective active DI boxes is Orchid Electronics: (www.soundonsound.com/reviews/orchid-electronics-di-boxes). Technically, these don’t use an isolating transformer (this contributes to the lower cost and smaller size) but they will still keep the phantom power safely away from the source equipment. I have several and use them frequently.

The notion of reducing the signal level through a DI box only to boost it again in the mixer’s mic preamp isn’t ideal, so if the mic preamp has the headroom to cope with a line-level signal (many do), then it might make more sense to use a line isolator box instead, providing a balanced line-level output from a line-level source. My usual recommendation is the ART DTI box, simply because it’s quite affordable and it has a plethora of different connection formats, making it very versatile indeed. Again, there are higher-quality alternatives, including the Radial Twin Iso that uses Jensen transformers, but they cost considerably more and it’s unlikely that the slightly better sound quality will be noticed in most live-sound contexts.

Moving on to your other queries, it’s generally true that phantom power is not present on the line inputs of most mixers, and so shouldn’t appear on jack leads, either. However, I have come across mixers and preamps that do send phantom power over the line input connections — and in any case, you never know if someone may have used an adapter or bodge lead to convert between a phantom-powered mic input and a line socket, so it would be unwise to assume that a jack lead connection doesn’t have phantom power on it, especially when working with unknown equipment and sound engineers in the dark confusion of a live sound stage! My advice is to always protect your valuable equipment with a DI box or line isolator!

Finally, there can be a beneficial reduction in interference when using DI or line isolation boxes, but only because placing the box next to the unbalanced sound source means the long cable run back to the mixer will be balanced and thus able to reject interference far better in comparison with running long unbalanced cables directly between the source and mixer. 

Published July 2018

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Q. How can I expand my Roland Quad Capture via S/PDIF?

By Hugh Robjohns & Matt Houghton

The dual-channel ART Digital Pro VLA II mic preamp has an on-board A-D converter that caters for both RCA and optical forms of S/PDIF, as well as the AES3 format.

I have a Roland Quad Capture [audio interface], which I’m very happy with. I would like to use the digital inputs in addition to the two analogue inputs, but there’s nothing I know of on the market that will allow me to do this. Can you offer any ideas or suggestions — other than buying another interface?

SOS Forum post

SOS Technical Editor Hugh Robjohns replies: The digital inputs on the Roland interface come in the form of a coaxial S/PDIF input, so if you want to route analogue sources into the interface via that port you will need an analogue-to-digital (A-D) converter, and if it’s specifically extra mic channels you want then, if you want a single product to do it all, what you really want is a dual-channel mic preamp with built-in A-D converter and a coaxial S/PDIF output. You could then connect the two using a standard RCA phono cable that’s intended to convey digital audio or video signals. (Note that standard RCA phono cables, intended to connect analogue hi-fi gear, tend to be too ‘lossy’ and can be unreliable in this particular role).

There are a number of dual-channel mic preamps on the market which offer digital outputs, and these range from the remarkably good ART Digital Pro VLA II at around $299, up to the stunning Grace Design m201 with digital card at an eye-watering $3499.

The only potential ‘gotcha’ to watch out for is in the digital clocking. In any digital audio system, there can only be one master clock — everything else must slave to that. In your situation, that will mean setting the converter in the external mic preamp to run on its internal clock as the system master, and configure the Roland interface to use the S/PDIF digital input as its clock source, so it works as a clock slave.

SOS Reviews Editor Matt Houghton adds: One thing you didn’t mention was your budget — so it’s worth adding that if you only need to add line-level inputs on a budget (or mic-level but you already own some preamps), it’s worth checking whether you already own another product with an A-D conversion stage built in.

For example, in the past, I’ve pressed my TC Electronic M300 external effects processor into service as a line-level A-D converter. Just plug your line-level source into the effects box’s analogue inputs, and connect the effects box’s S/PDIF output to your audio interface. Obviously, it has to be an effects unit for which the effects themselves can be bypassed! They won’t be the very best converters or clock in the world, but assuming it’s not a decades-old device, they should sound more than clean enough to get you up and running.

Published August 2018

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Q. Which PC laptop should I buy?

By Martin Walker

Dell's notebooks offer good specs at decent prices, but you may find that they are not ideal for audio.

I'm considering buying a laptop for various audio tasks, including recording school choirs, community music workshops, live DJ'ing (using Ableton Live), and writing my own material. I'm looking at a Dell Inspiron 9400 and think I've made my mind up but I was just wondering if you had any advice on this particular machine and if I should be considering any others?

Lee Perry

PC Music Specialist Martin Walker replies: Thanks for the compliment, Lee — we do our best! As one of the first models featuring Intel's Centrino Core Duo processor range, Dell's Inspiron 9400 will certainly be fast enough to run lots of plug-ins and soft synths and perform plenty of audio manipulations in Ableton Live, whether you choose the 1.86GHz or 2.0GHz model. For recording school choirs and community music workshops I suspect you'll want a quiet laptop, so that no cooling fan noise is picked up by the microphones, and once again a Centrino model should be ideal.

The Inspiron 9400's 17-inch widescreen display and high-end graphics with 256MB of dedicated RAM is generous, to say the least. However, choosing a model with such a large screen does have one disadvantage — weight. At 3.6kg (4.2kg with the AC adaptor) the Inspiron 9400 is generally regarded as too heavy for regular travel which, judging by your list of proposed activities, may influence your final decision.

Don't forget also that you've left out the most important gear item in your query — the audio interface. Recording school choirs and community music workshops could require plenty of microphones, so in a way you're putting the cart before the horse — perhaps choosing a suitable multi-channel interface should come first. Indeed, were you only to be carrying out these activities, many people wouldn't recommend a laptop at all, instead favouring some sort of hard disk recorder or digital multitracker with built-in mixer and mic preamps, so you'd end up with less boxes to cart around.

However, with your DJ'ing and own music composition in mind, let's return to the proposed laptop. Many musicians would look down their noses at one aspect of its performance — the hard drive. At 5400rpm, this seems out of sync with the rest of the specification. Although I'm perfectly happy with the performance of the Seagate Momentus 5400rpm hard drive in my own laptop, which is capable of recording and playing back at least 30 audio tracks at 24-bit/96kHz, most musicians nowadays tend to look for laptops with 7200rpm hard drives, particularly if they intend to record using the more demanding 24-bit/96kHz audio format.

Of course, like lots of other musicians, you could add an external 7200rpm hard drive, but that's yet another box to transport to your live recording sessions. This brings us to another hurdle for PC-music laptop performance — the Firewire controller chip found on the laptop's motherboard that's in charge of Firewire duties, which can be crucial to achieving glitch-free Firewire audio recording and playback.

There are three main audio interface formats suitable for laptop use: USB, Firewire and PCMCIA. Since the Dell Inspiron 9400 features one of the new Express Card 54 slots rather than a PCMCIA one, you're restricted to the first two, but fortunately buying a Centrino-based laptop guarantees you an Intel USB controller for maximum compatibility with USB audio interfaces. However, there are still very few multi-channel USB 2.0 audio interfaces available, so the majority of musicians who need multi-channel recording capability opt for a Firewire audio interface.

Specialist laptops like this one from Nu Systems are custom-made with audio in mind.

It's generally accepted that Texas Instruments' controller chips are more widely compatible with Firewire audio devices, and most interface manufacturers provide some sort of list of compatible chips on their web sites. However, it can be difficult to find out which Firewire controller chip a particular laptop uses, unless its manufacturer is prepared to tell you — and a mistake here will be expensive to rectify.

One final factor can prove very frustrating for musicians when choosing a laptop: ground loops. Many manufacturers use three-wire earthed power supplies for their laptops, particularly if they feature metal cases, and this extra earth connection is notorious for causing ground-loop problems such as background buzzes, whistles and other noises in your audio, which change during hard drive activity, when you move your mouse, and during graphic redraws. I'm afraid Dell are one of the most notorious in this respect, and I specifically mention the Inspiron 8200, 8500 and 8600 models as causing such problems in my PC Music FAQs forum, atwww.soundonsound.com. You can cure such problems with a systematic approach and DI boxes between your audio interface and other gear, but it's added frustration and expense.

For this reason, I generally recommend one of two approaches when choosing a PC laptop for music. The safest is to contact a specialist music retailer (many of whom advertise in the pages of SOS) and discuss your requirements with them, as they not only choose their laptop components carefully for maximum compatibility and minimum likelihood of ground-loop problems, but can also advise on the most suitable audio interface to go with the laptop, depending on what tasks you want to perform.

You may pay a little more than you would for a mass-market model, but you'll get technical backup from experts who know all about music hardware and software — after all, most mainstream laptop manufacturers have little or no idea of the particular technical requirements of the musician, which are rather different from most ordinary users. I bought my Centrino laptop from a specialist music retailer, as have members of SOS staff, simply to avoid all the potential pitfalls and possible strife.

The second approach is to post your suggested laptop model on the SOS Forums and ask if any other musician has bought it, and, if so, what experience they have had. Such answers are, of course, no guarantee of a problem-free purchase, and you're likely to have the added confusion of a host of other models being recommended as alternatives, and others to avoid, but at least this approach is better than buying blind. Whatever model you decide on, choose your audio interface at the same time, and try to buy from a dealer who is prepared to give you a refund or exchange on the laptop in the (hopefully unlikely) event of the two ending up in conflict.

Ultimately there are hundreds (possibly thousands) of different PC laptop models available, all including a slightly different combination of components such as motherboard, CPU, RAM, hard drive, graphics, screen, selection of ports, and so on. Such competition keeps prices keen, but it does make choices more difficult for the musician! 

Published July 2006