Dbx 376

Tube Channel Strip

Published in SOS May 2001
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Reviews : Processor

Dbx's new input channel combines tube processing with their renowned Type IV digital conversion. Paul White turns up the volume.

Voice channels with digital output options are no longer news — pretty much everybody seems to make one — but Dbx are one of the first companies to offer a digital output as standard on such a competitively-priced unit. The single-channel Dbx 376 is a 1U rackmount voice channel with hybrid solid-state/valve circuitry, which provides mic, line and instrument inputs followed by EQ, compression and de-essing. The analogue signal chain feeds Dbx's Type IV A-D conversion process and several digital output options are available.

Back To Front Tour

A glance at the rear panel reveals the balanced mic and line inputs, on XLR and quarter-inch TRS jack respectively, alongside a TRS jack for a pre-compressor insert point. The two line outputs, again on XLR and TRS jack, are balanced, though the jack output may be used unbalanced simply by inserting an unbalanced jack plug. The high-impedance instrument jack is unbalanced and is located on the front panel for easy access. Power comes in via a standard IEC mains connector, with an associated switch next to it which will be of little use if the equipment is mounted in your rack.

DBX 376 £550
pros
Good-sounding preamp and EQ.
Clear user interface.
Very attractive price.
Comprehensive digital output stage as standard.
cons
No bypass switches for compressor, EQ or de-esser.
No means to switch the EQ post-compressor.
summary
The Dbx 376 is an ideal front end to use with any system that has a digital input. By including Type IV conversion, Dbx have extended the useful dynamic range of digital audio while making it impossible to overload.

The rest of the connections are related to the digital audio interface. Both AES-EBU and co-axial S/PDIF connectors are provided, though these both stay active and carry whichever digital output format is selected on the front panel. Word clock I/O is alongside these connectors, on standard BNC connectors, and the word-clock generator is based around a Dbx VCXO chip designed for low-jitter performance.

The front panel is populated by sculpted metal knobs and, where possible, the controls have been simplified for ease of operation. The pots have a multi-detent 'clicky' feel that I rather like, though none have obvious centre detents, and all the buttons have a very positive action as well as being internally lit. The preamp section begins with a mic/line selector switch — the instrument input overrides the line input when a jack is plugged into the front panel. The Drive control which follows these applies gain and also regulates the amount of tube 'warmth'. A four-section LED meter above this control shows the input signal level and warns of clipping. Below the valve's ventilation slots, through which the dual-triode can be seen glowing, there are switches for 48V phantom power, 20dB pad, phase inversion and a low-cut filter (12dB/octave at 75Hz).

Next in line is the equaliser, which comes before the compressor. I'd rather it be post-compressor or, better still, switchable pre/post, as the two options can sound very different. You get a fairly simple three-band processor with shelving high and low controls (fixed at 80Hz and 12kHz) complementing a swept mid-band variable between 100Hz and 8kHz — a usefully wide range. The EQ is not, however, parametric as is erroneously claimed on the packaging. All three sections offer ±15dB of gain and a warning LED indicates if the signal is clipping the equaliser, in which case you'll probably have to back off the preamp Drive control.

The only variable controls in the compressor section are Threshold and Ratio, with combined attack/release response times switchable to a slower setting if necessary. A neat trio of LEDs shows whether the input is above or below the threshold value and a further eight-section LED meter monitors the amount of gain reduction. Dbx are famous for their gentle Overeasy compression (a type of soft-knee design), so it's no surprise that the 376 offers an Overeasy mode. When Overeasy is selected, the yellow LED in the middle of the threshold meter remains lit while the signal is in the Overeasy region. There is no dedicated compressor make-up gain control, though the main output gain control has enough range to compensate for this.

  What Is Type IV Conversion?  
  The 376 uses the proprietary Dbx Type IV A-D conversion system, which is designed to extend the headroom for any given digital bit-depth by introducing a non-linear region that affects only high-level signals. In this case, the top 4dB of the dynamic range is made logarithmic rather than linear — so that the further signals push into this region, the more they are squashed. In fact, the log law means that you can never get to the point of digital clipping and, because the logarithmic law mimics tape and tube distortion, it can actually enhance the signal to make it sound more 'analogue'. A further benefit is that, by avoiding clipping, high frequency detail is preserved, albeit in a slightly distorted form, whereas clipping would obliterate it altogether. As Type IV is unclippable, a separate limiter is also unnecessary.  
De-essing is a common requirement when recording vocals, especially when using capacitor mics that can exaggerate sibilance.This de-esser is a threshold-independent design and provides control over the de-essing frequency and the amount of high-frequency gain reduction that takes place when the de-esser is operating. The gain-reduction meter comprises only two LEDs calibrated at 1dB and 6dB. That leaves the Output Level control and its associated meter, which can be switched to monitor either the analogue or digital output levels via an eight-section display.

The remaining buttons select the output options for the built-in A-D converter — their integral LEDs change colour to indicate which of the available options is selected in each case. Dbx's Type IV conversion system is used, controlling peak excursions to usefully increase dynamic range without clipping. Audio can be sampled at rates of 44.1, 48, 88.2 or 96kHz, and at resolutions of 16, 20 and 24-bit. Dithering noise can be added, with two algorithms available (SNR2 or TPDF), and you can choose to shape this noise with either of two curves.

Using The 376

There's nothing difficult about using the Dbx 376 and it sounds surprisingly good given its 'too good to be true' price. The mic pre is clean and detailed, but with just a hint of positive 'tube character', and the EQ sounds rather classier than I had expected. Like all the best EQs, the bass control is able to add power without muddying the sound, while the high end enhances air and detail without harshness. Even the mid control sounds solid and natural, and it only takes a fraction of the available ±15dB gain range to fix most problems. Annoyingly, the equaliser has no bypass switch and the pots have no centre detents — comparing the sound of your processing with the untreated signal is therefore not easy, and setting the EQ flat is a fiddly process.

  Dithering About  
  The two dither options offered by the Dbx 376 provide an alternative to the loss of low-level detail that arises when signals are reduced in bit depth. Essentially, dither adds a tiny amount of noise to the signal, which reduces perceived low-level distortion in exchange for a slight worsening of the signal-to-noise ratio. Unfortunately, the manual doesn't go into much detail on how the 376's two dithering algorithms differ, so you have to decide between them by ear.

In order to minimise the impact of the added dither noise, noise shaping is used to shift the added noise components into a part of the spectrum where the ear is least sensitive — usually above 15kHz. Two noise-shaping curves are provided, with the first using a mild psychoacoustic curve to spread the noise over a wide region, while the second option is more aggressive. Of course, noise shaping and dither are only relevant when reducing bit depth — if you're outputting in 24-bit format, dither is irrelevant.

 
Though I'm not always a fan of Dbx compressors, the two choices provided here can tackle most vocal jobs without difficulty. While it's often possible to hear that the signal is being compressed, the side-effects tend to be flattering, adding power and depth to the sound. Having no attack and release controls didn't prove too much of a restriction either, even with instruments — clean electric guitar or miked acoustic guitar sound great through the 376 and you can compress quite heavily before the result sounds over-squashed.

The de-esser is easy to adjust and it works as well as any basic split-band type can. Because the whole of the high end is reduced in gain during processing, rather than just the the relevant band, some dullness was evident when de-essing aggressively. In this respect, the result doesn't come close to one which only filters out a narrow range of frequencies, but it's still better than using a compressor with EQ in the side-chain and just as good as many commercial stand-alone de-essers. Unless the vocalist has extremely bad sibilance problems, the unit should get the job done without getting itself noticed.

One of the real surprises of this unit is its excellent digital output section. You need to run a few 'non-real-world' tests if you're to hear the improvements brought about by dither (under-recording by around 40dB can help here!) but anyone should be able to hear the benefits of Type IV conversion! At normal signal levels, the converter behaves as you'd expect, with peak signals coming close to digital full scale, but you can then turn up the output gain by another 10 or 12dB and still the signal doesn't go into digital clipping — it doesn't even sound particularly distorted. When the distortion does become audible, it's very similar to analogue tape compression, which means you can record percussive sounds at a pretty hot level without ever having to worry about clipping. I have to admit to liking this feature a lot — it's about as close as you can get to a foolproof digital converter stage and I think I'm right in saying the analogue stages feeding it would clip before the digital signal hit the end stops.

Summary

The Dbx 376 isn't without its irritations, the main ones being the lack of bypass switches, but at the price it would be a bargain in the UK even without the digital output stage. The preamp and EQ sections are particularly good, and the limitations of the compressor and de-esser won't cause any problems for the sort of routine work for which the Dbx 376 is designed.

Having a 24/96 output stage as standard is both the icing and the marzipan on the cake (and possibly one of those little plastic robins too!), and I can't speak too highly of the Type IV conversion system — a simple idea that makes the digital converter even harder to overload than analogue tape. If you're in need of a good all-rounder for interfacing directly with the digital input of your soundcard, the Dbx 376 is a very hard act to follow at anything like the price.

 information
£549.95 including VAT.
Arbiter Music Technology
+44 (0)20 8970 1909.
+44 (0)20 8202 7076.
Click here to email
www.arbitergroup.com
www.dbxpro.com

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