DBX Model 760X

Thinking of buying a new microphone? Maybe you should consider investing in a dedicated mic preamp instead, to improve the clarity and definition of your recordings without breaking the bank. Dominic Hawken plugs in the mic he likes and checks out a new contender from dbx.

In a studio world dominated by synthesizer and sampling technology, the humble microphone is often forced to take a back seat. Most modern mixing consoles are equipped with microphone‑level inputs and a phantom power supply as standard, and for many users these features deliver audio of a perfectly adequate quality. For the sonic purist, however, an external preamplifier can bring out the best in even the most basic of microphones. Depth and clarity are often improved, and the user is no longer left at the mercy of the console.

Mixing desks are designed to interface with any number of individual signals, all delivered at completely different levels, ranging from extremely quiet sources (a microphone, for instance) to very loud ones (the latest synthesizer or outboard output). In order to achieve this, the sound is first passed through a broad‑band preamplifier, which is then adjusted to maintain a consistent input level. It is extremely difficult to design a circuit capable of working with such a varied range of inputs without affecting the overall quality of the sound in the process. Some preamplifiers are optimised to perform best with high levels, and some with low, but it is generally impossible to create a unit capable of producing a flat response across the entire dynamic range. During most mixes, a greater number of channels will be fed by high‑level signals rather than low‑level ones, and console manufacturers, in general, tend to optimise their preamp circuitry to cater for this type of signal. The audio quality of channels fed by microphones then suffers as a result.

Hence the need for a dedicated preamplifier. Connecting the microphone directly to the preamp transforms the low‑level signal to a higher line‑level, removing the overhead from the mixing console and improving quality. In some situations, it is possible to bypass the mixing desk completely and connect the output of the preamp directly to the recorder. This offers the best performance, removing any noise or coloration that would otherwise be introduced into the chain. The addition of a dedicated outboard equaliser can also improve the overall sound, offering control over any unwanted frequencies that can creep into the signal. Most top engineers have a favourite combination of equaliser and preamp that they use for every recording, with Focusrite and Neve among the most popular. The cost of these combinations can often run into many thousands of pounds — prohibitively expensive for most users — so it's good to see dbx entering the market with a unit at under £400. If its performance matches its specifications, the 760X could find its way into many mid‑range studios and production suites.

Design

Dbx have a 26‑year history of making outboard equipment, concentrating in the main on compression and noise‑reduction systems. The 760X follows their previous design standards, and is housed in a 1U black metal casing, taking up half the width of a normal rack bay. All external connections to the unit are via the rear panel, which offers balanced XLR sockets for signal input and output, as well as unbalanced jack connections as an alternative output source. Mains power is provided by an external supply shipped with the unit. Unfortunately — for the intrepid audio explorers amongst you — the supply delivers 18V AC, so running the unit from batteries with a portable DAT machine in the jungle is a definite no‑no. On the plus side, however, removing the power components from the main circuit board reduces overall noise and helps improve the signal specifications.

The front panel offers basic control over the fundamental settings of the unit. 'Basic' is the operative word here (although preamplification circuitry in general is usually designed with the minimum of controls, signal purity being the priority). The 760X has two independent channels; each one is assigned a single Gain control and two switches offering phantom power and phase reversal. Two LED indicators on each channel display the current switch settings, and a third flashes to show clipping anywhere within the internal signal chain. Somewhat surprisingly there's no mute facility on either channel, which would have been a useful addition, though not vital for studio‑based use. There is also no provision for any frequency control (some preamps offer a basic low‑cut option to remove rumble and sub‑bass from the audio), although this, again, should not compromise the effectiveness of the unit.

Internally, the 760X offers up to 60dB of gain, utilising servo‑balanced outputs rather than any of the alternatives. Transformer‑ or valve‑based technology is often used in modern preamplifier designs to amplify the input signal, but both of these options can colour the sound. Although this is not necessarily a bad thing — valves, in particular, produce a warm, rich sound that can be used to great effect on vocals and guitars — for some recordings true sonic integrity is required. In cases where the recorded sound has to be as accurate as possible, the design of the 760X should help to achieve this.

The 760X can be used on a desk or table top, or can be rackmounted using the supplied mounting kit. As the preamplifier takes up only half a rack width, two individual units can be mounted side by side in a single space; dbx supply a half‑width blanking plate if only one unit is required. As a bonus, the system requires no special ventilation, and can therefore be mounted virtually anywhere without the need for a cooling system.

In Use

With so few controls available, setting up the 760X is a simple task. I tested the system with a single microphone connected to the balanced input on channel 1, with the balanced output routed directly to a recording system. Once the phantom power supply had been switched on (introducing no discernible noise into the signal), it was a simple matter to adjust the gain to a suitable level for recording. The 760X provided a very quiet and clean signal, with a depth and clarity that certainly added a new dimension to the performance of the microphone.

Most of the tracks I'm involved with tend to evolve from a production session at home. Basic arrangements and ideas are sequenced and sampled, and the vocals are recorded using the studio's faithful Microtech Gefell mic, which is plugged directly into the inputs of a Mackie 8‑buss console and routed to a Tascam DA88 digital 8‑track. When it's time for the full production, much of the recorded audio is often kept and used on the final master, so a lot of work goes into achieving the best possible recording quality, even when working on basic demos. Using the Gefell through the desk has so far produced perfectly acceptable results, and I was pleasantly surprised at the difference that the dbx made. In the past, I've tested a number of other preamps, most of which have added their own sound to the mix, with pleasing, if not highly accurate, results. The dbx system, however, remains refreshingly neutral, introducing very little coloration and degradation to the sound.

More sonic improvements are noticeable when a pair of microphones are used to record in stereo. Much of the background ambience that contributes to the realism of a stereo recording are present only at very low levels, and these signals are often lost on systems lacking a suitable preamp. The 760X goes a long way towards retrieving this lost data, and the resulting audio takes on an added depth and a wider soundstage.

Conclusion

The dbx 760X is released at a time when many manufacturers are aware of a growing demand for low‑priced, high‑quality microphone preamplification, to match the current spate of digital recorders and compatible mixing consoles. Focusrite have launched their Green series — albeit at twice the price — to fill this gap (see last month's SOS review), and many other companies are no doubt working on competitive designs. The 760X utilises the same transformerless design as its competition, and this has resulted in an effective product that delivers a crisp and clean response way ahead of that which can be obtained using a standard console. It's unfortunate that the system lacks any form of muting, which can be especially useful in live or broadcast situations, where it is often necessary to cut the signal without changing any of the panel settings, but this should not have too much impact on its usefulness in the studio.

Dbx have a name for solid, workmanlike products that perform their function with the minimum of noise and fuss, and this preamp certainly continues that ethic. If you're looking to improve your current vocal sound on tape, or you're seriously into sampling acoustic instruments and rhythms, the 760X is a worthy investment. It's an ideal choice for the first‑time buyer and makes a great entry point into the world of mic preamplification. While you're at it, hire some of the competition and compare the results for yourself — you may be pleasantly surprised.