3D Audio: AD, Pre & Mic CDs

Are you paralysed with indecision as to what gear to buy? What if you could try out 49 mics, 33 preamps and 29 A-D converters, all within the comfort of your own home? Well, we can all dream, but here's the next best thing for those unrelated to King Croesus...

Most of us rarely enjoy the opportunity to compare lots of similar pieces of audio equipment side by side, let alone audition serious high-end equipment against more affordable products, in order to judge just how diminishing the 'law of diminishing returns' really is. The process has now been made much easier by Lynn Fuston of 3D Audio, a digital audio editing and mastering facility located in Franklin, Tennessee. The story of 3D Audio's series of comparison CDs begins with Lynn having the simple idea one day of getting a bunch of his fellow recording engineers together for a listening party to try to identify the 'best-sounding' preamp — each engineer bringing their own favourites.

However, this simple plan quickly grew, and in April 2000 Lynn ended up with a stack of 33 different mic preamps ranging in price from about $62 (the approximate proportional cost of a single mic channel in a Mackie 1604 VLZ Pro mixer) up to $8000 for a vintage Neve 1081. Such an impressive array of preamps gathered in one place is not an everyday occurrence, and so as well as hosting his listening party in the studio, Lynn also arranged to record the output from each preamp being auditioned. This material was subsequently collated to produce a pair of CDs enabling the rest of us to listen and make our own minds up individually as to which preamp sounds best, and what the sonic differences are between them.

Obviously, in comparisons of this kind there are a lot of possible sources of error or unfairness, but Lynn and his colleagues have gone to considerable lengths to try to make these tests as consistent as possible: an acoustic noise source positioned a precise distance from the microphone was used to ensure consistent gains through each preamp being tested; the recording chain was identical throughout (a Mytek 8X96 A-D converter, an Audiomedia II soundcard and Sound Designer II software); and the only processing between the original recording and the production CD was some editing and word-length reduction from the 24-bit/44.1kHz sources to the 16-bit/44.1kHz CD format.

Mic Preamp Shoot-out: The Pre CD

The signal sources for these auditions comprise individual performances by vocalists Marabeth Jordon and Shane McConnell, acoustic guitarist David Cleveland, and drummer John Wheeler (playing only a snare drum). The testing starts with the female vocal tracks (using a Manley reference mic), and moves on to a stereo acoustic guitar (McPherson redwood guitar captured with a Neumann KM84 towards the base of the guitar and an Audio-Technica AT4033 looking at the neck). The second disc of the pair contains the male vocal test tracks (first on an AKG C414B ULS, and then using a few of the preamps again with the Manley reference mic), and finally snare drum strikes (using a Shure SM57).

The list of tested preamps (see The Preamps Tested box) is not exhaustive, but does cover a very interesting range. The majority are relatively costly high-end designs but there are several mid-range and budget units thrown in too, and the comparisons are often quite illuminating. A large proportion of these preamps have graced the review pages of Sound On Sound over the years, and every common circuit topology is represented here. Both vintage and modern valve designs are included along with discrete transistor and FET circuits, plus all-IC (integrated circuit) and hybrid implementations, with some using transformers and some not.

This pair of CDs makes fascinating listening in a number of different ways. Obviously, the first point of interest is the straight A/B comparison of preamps, which may be useful in creating a shortlist for potential upgrades, or in highlighting the strengths and weaknesses of your current favourite preamp in comparison with some aspirational high-end unit. But a second, and arguably more relevant use of these discs is to help in training your hearing — learning to recognise the sonic characteristics of different kinds of preamps with different sources. In many cases the sonic differences are pretty obvious and predictable, but I found there were a few surprises too — I won't spoil your fun by spilling the beans here, though!

For me, while the preamps on test here certainly exhibit some very different characteristics, I wouldn't like to have to identify just one as an overall winner. Personal preferences play too big a part, and any decision would depend on what kind of microphone is being used and what kind of tonal flavour is necessary to enhance the source in the desired way. Having said that, I found that there was a very clear subset of preamps which stood out as being special in various ways — and they certainly weren't all megabucks designs.

To make the listening tests as unbiased as possible, the preamps are tested in a random order which changes for each set of auditioning material. Spoken slates are appended to the end of the preceding track so that if you're accessing tracks directly you can audition the test material immediately, which makes A/B comparisons far easier to do. The sleeve notes identify each preamp with a letter code to maintain the 'blind listening' aspect of auditioning these preamps. The idea is that listeners simply note the comments for each preamp against the appropriate letter code. The sleeve notes can then be consulted after the auditions are complete to find out which models were rated highly and which may have been given a thumbs down.

The sleeve notes not only document each preamp with its identifying letter code, but also with a nominal retail price, the number of channels, any additional facilities (such as built-in EQ, dynamics, digital outputs and so on), and a company web site address. However, there is no information at all about the circuit topology or type of active devices used in each design. This is a shame, because it weakens the training aspect of these discs: unless the listener is already familiar with the preamps on test they will have to do quite a bit of web surfing to find out just what they have been listening to. I realise that ear training is not the raison d' tre of these discs, but I feel it is certainly an additional strength that has not been fully realised.

49 Vocal Mics Head To Head: The Mic CD

A logical follow-up to the preamp tests was to provide a similar comparison disc for microphones, and Lynn has chosen to compare popular vocal microphones on this disc. A selection of 49 mics are presented and comprise mainly large-diaphragm, side-address condenser mics, including not only a very wide range of modern mics, but also classic vintage designs. However, there is also one small-diaphragm condenser mic and a handful of dynamics, including some ribbons (both vintage and modern) — see The Mics Tested box on the following page for a complete list.

As with the preamp disc, the auditioning material comprises simple unaccompanied hymns from two vocalists: Marabeth Jordon again, and Chris Rodriguez. The female vocals were captured through a Great River MP2 MH preamp and the male vocals with a Grace 801 preamp, feeding a Prism AD2 A-D converter with 24-bit, 88.2kHz resolution. I guess the preamp choices reveal which units Lynn and his colleagues felt delivered the best performances for male and female vocals following the preamp testing sessions! The digital output from the converter was recorded directly into a SADiE Artemis DAW and the format conversion for CD mastering was performed internally using POW-R SRC and word-length reduction algorithms.

As with the preamp discs, each mic was positioned identically relative to the vocalist, (eight inches away for Marabeth and 12 inches for Chris) and with carefully calibrated gains to make the comparisons as consistent as possible. However, the chosen positioning — while absolutely consistent — may not have been optimal for every mic. Proximity and angle of a mic relative to the source can affect performance quite dramatically, so while the audition material makes for interesting comparisons, it doesn't necessarily always show off a mic to its best possible capabilities. An obvious example is that of the Coles 4038 ribbon mic, which sounded to me as if it was placed a little too close, resulting in rather more proximity tip-up effect than would have been ideal — this mic, being a pure pressure gradient design, suffers far more proximity effect than most tested here.

Another important issue to consider is that these examples are only of on-axis vocals captured in a quiet and well-treated acoustic environment, so consequently the off-axis characteristics of the mics are not revealed at all. This latter aspect is often the determining factor when multiple microphones are used with multiple simultaneous sound sources, of course, but is of little relevance in the context of comparing vocal mics with typical studio applications in mind.

So, this disc, like the previous set, provides some very useful comparisons of a broad range of popular vocal microphones, as well as some of their clones and updated models. Again, it serves both to help create a shortlist of possible microphone purchases or selections, as well as being another excellent ear-training aid, and it is extremely instructive to hear the different tonal characteristics of such a variety of microphones.

Battle Of The Converters: The AD CD

The final instalment in this collection — so far, at least — concerns A-D converters. The products presented for audition here comprise 29 different converters spanning prices from $700 up to $10,000, lining up stand-alone A-D converters alongside a couple of high-end computer soundcards and a few integrated digital recording systems too.

As you would expect, the converter inputs were all carefully calibrated for level, polarity and correct channel orientation, and the test tracks recorded directly to a SADiE Artemis DAW at 24-bit/44.1kHz resolution. The sleeve notes state that no dithering, noise-shaping or filtering was employed in any of the converters, but this is rather misleading, since a converter operating without dither is extremely non-linear and of little use in recording applications! What is meant, I believe, is that all the units were operated in their standard 24-bit configurations, without any internal word-length reduction or alternative anti-alias filter options.

The details of the format conversion to 16-bit/44.1kHz to produce this two-CD compilation are not given in the sleeve notes, but the same POW-R algorithm was used as in the mic comparison disc. Inevitably, auditioning A-D converters through a CD in this way is somewhat harder than comparing the outputs of the actual converters live against the analogue input signal, since the inherent format conversion and D-A converters used in the CD monitoring chain will reduce the transparency or even colour the tests to some degree. Nevertheless, the differences between converters can still be heard and the ability to compare their sonic characteristics in this way is quite fascinating.

There are three different auditioning tracks provided for each converter, starting with a short bluegrass track performed by Hayseed Dixie (3D Audio are based in Tennessee after all). This multitrack recording was replayed with a completely static mix from a 24-bit/48kHz RADAR session through a DDA AMR24 console. The second test track is of a pair of stereo acoustic guitars plus a female vocal replayed from an analogue two-inch tape source through the same console. The final test track is of a live-to-DSD stereo recording, made by DMP, of the Bob Mintzer Big Band, replayed from a Sony SCD555ES SACD player, presumably straight into the converters.

For the majority of the auditioning tracks the converters were all running on their internal crystal word clocks. However, a fourth test track (using the Big Band recording once again) is also included for those converters with facilities for external clocking. A Lucid SSG192 master clock was used to supply either a x256 Super Clock reference (for those devices that accepted it) or standard 44.1kHz word clock.

Overall Impressions

It wouldn't be appropriate to comment here on my own impressions of and preferences for the specific equipment and microphones presented for audition on these discs. However, I can say that I found these discs to be extremely interesting and informative, and I would thoroughly recommend them for anyone interested in creating equipment purchasing shortlists, or anyone wanting to expand and hone their listening skills. All of these sets should be considered an essential tool for use on every Music Technology course across the land! Although these discs may be considered a little expensive and specialised for the casual listener, the costs could be reduced and the benefits enhanced by clubbing together with a small group of interested friends to share both the purchase costs and the listening experiences.

Critical listening can be both tiring and time-consuming, so I would recommend auditioning these discs when you are feeling fresh and able to guarantee a period free from interruptions — each disc took me a couple of hours to work through. Obviously, the better your monitoring equipment and listening room, the clearer the sound characteristics of each preamp, mic or converter will be, but in many cases significant quality and tonal differences can be heard even on lo-fi equipment.

Of course, whereas the overall range between the best and worst of the preamps and microphones is quite broad, the sonic differences between some units can still be very small and subtle. Interestingly, in the converter comparisons CD I found the overall range between best and worst to be very much smaller — the consistency and similarity between devices seemed much greater. In real life, I know that converter differences are generally more obvious than this CD suggests, and I can only put that down to the processes involved in producing this disc, combined with any audible signature that may be imposed by the D-A converters used in the replay system.

Whatever the reasons, these discs certainly make it very clear that there are far greater quality improvements to be obtained through the correct selection of microphone and preamplifier than A-D converter, although the very best converters can help to capture a little more sonic magic if the rest of the chain is up to the challenge.