By Phil O'Keefe |
DBX 676 Channel Strip with Tube Mic Preamp
A tubular channel strip? Well, sort of...
by Phil O'Keefe
What You Need To Know
- The Dbx 676 includes a transformer-balanced tube mic preamp, three-band EQ, and compressor / limiter, all in one very stylish looking, well-built housing that's a reasonable 8" deep and weighs 11 pounds. When you power up the 676 (which takes about 2 seconds), the first thing that catches your eye is an illuminated VU meter right in the center. It can monitor input or output levels, as well as the compressor's gain reduction. All the switches illuminate when active, making it very easy to see what's going on, and the knobs are comfortably spaced, large in size, smooth-turning and clearly labeled. The 676's overall vibe is one of quality.
- The mic preamp uses a 12AU7 tube and unlike some "tube" preamps, it uses a 250V plate voltage and is not a "starved plate" design. The Gain and Post Tube Attenuation knobs let you adjust the sound from clean (Gain control set lower and the Post Tube Attenuation set higher) to gritty, where you add tube coloration by setting the Gain higher, and lower the Post Tube Attenuation to compensate. A red Peak LED indicates when the tube is clipping. Up to 60dB of gain is available, and the Dbx 676 can go from clean sounding to quite warm - even fairly dirty, although it doesn't really reach highly distorted territory.
- Other goodies in the preamp section include switches for 48V phantom power, a 20 dB pad, polarity invert, and an 80 Hz low cut with a 12 dB / octave slope. A front panel instrument input and enable switch make the Dbx 676 suitable for use as a instrument DI.
- The 676's signal flow is well-thought-out and makes sense (see the diagram below). The only slight downer here is that there's no way to put the EQ after the compressor - it's hardwired pre-compressor in the audio path. It would be great to have a switch that let you run it in front of, or after, the compressor but before the limiter.
- The EQ Enable switch can bypass the EQ for easy comparisons with the unprocessed sound. Of the three bands, the high (10 kHz) and low (100 Hz) bands are fixed shelving with 12 dB / octave slopes and 15 dB of boost or cut. The midrange is a peaking filter with a two-position bandwidth switch (narrow setting with a Q of 2.9, or normal setting with a Q of 0.9). The midrange band's center frequency is sweepable from 100 Hz to 8 kHz, with a boost or cut range of 15 dB. The EQ sounds sweet, and is well-suited for the kinds of gentle tonal shaping that are often helpful when recording.
- Dbx is well known for their compressor-limiters, and the one in the 676 will further that reputation. It's a VCA-type unit based on the very popular and highly capable Dbx 162SL. Just about anything you could want from a VCA compressor is here, including standard hard knee and the famous Dbx OverEasy soft-knee compression, user-adjustable attack and release controls, and an auto mode that bypasses them in favor of fully program-dependent auto-compression.
- The compression ratio is variable from 1:1 to infinity:1, and as you'd expect, there's a make up gain control as well as variable threshold that's adjustable from -40 dBu to +20 dBu. Three LEDs assist you with setting the threshold level by showing when the signal is below, at, or over the current threshold setting. A contour switch helps reduce compression side effects on some sources by preventing frequencies below 180 Hz from triggering the compressor, and a compressor enable button allows for easy A/B comparisons. There's even a sidechain jack on the rear panel, along with a corresponding front panel enable switch. This lets you do frequency-dependent compression and de-essing with the help of an outboard equalizer.
- A single-knob PeakPlus limiter acts independently of the compressor. It's great for catching those occasional unexpected loud peaks without having to set the compressor for too much squish. Turning this knob up all the way bypasses the limiter. An LED indicates when the signal is triggering the limiter, making it easy to set, and you can adjust it to kick in anywhere from -4 dBu to + 22 dBu.
- The rear panel features an XLR mic input, as well as XLR and 1/4" balanced / unbalanced mic preamp outputs. A 1/4" TRS insert jack is also included, allowing you to patch other processors into the chain post-preamp and pre-EQ and compression. Unusual (and quite welcome) are the separate output jacks for the preamp and the compressor sections. With separate outputs you can do some very cool tricks such as parallel compression, by recording the output of each and blending them in the final mix.
- An optional digital daughterboard is planned for the 676. Based on the three-color sample rate switch on the 676's front panel, I assume it will support 44.1kHz, 48kHz and 96kHz sample rates, but no official word of the planned features and no news on its price or ETA is available at this time. An AES-EBU / S/PDIF output card for the 676 would be a very nice option!
- I've read ad copy online saying that the 676 has "parametric EQ with sweepable mids," but this is incorrect - or at least potentially confusing. The three-band EQ section has high and low shelving bands and a semi-parametric midrange with variable frequency, but not the fully adjustable bandwidth required for a true parametric - you have only the two-position bandwidth switch.
- The mic preamplifier is the only part of the signal path that uses a tube. The rest of the unit's circuitry is solid state. Some people might consider that as a negative, but others would consider this "the best of both worlds." Either way, the processing is high quality.
- A printed copy of the well-written manual isn't included with the unit. Instead, a PDF file is available for download. Some may consider this as a limitation, but on the other hand, it's more environmentally friendly, and allows the manuals to be easily updated. Do download the manual - there's lots of good information in it that isn't included in the Quick Start Guide you'll find in the box, including lots of suggested settings to get neophyte recordists started.
- You need to adjust the Gain and Post Tube Attenuation knobs carefully to avoid hitting the following stages with too much level, thereby causing them to distort. It's not difficult to avoid this, but you do need to be aware that it's possible - and unlike the tube distortion, it's not pleasant-sounding or musically useful.
- There's no dedicated Line Input, although there are a couple of workarounds; the unbalanced front panel instrument input can take levels of up to +21 dBu, or you can use the preamp insert's return, (although doing bypasses the tube gain stages).
While no single mic preamp or channel strip is going to be the dreamy ideal for every conceivable sound source you might encounter, the Dbx 676 is certainly a strong performer and an excellent choice for an all-purpose tube preamp / recording channel strip due to its sound quality and versatility. As with many channel strips it will probably see more use recording vocals than anything else, and it does clean or grungy vocals very well with its girthy-sounding tube mic preamp and excellent compressor.
But vocals aren't all it can do by any means. The EQ is generally well-suited to the kind of tone shaping you sometimes need to do while tracking, although it's not really ideal as a problem-solving EQ for surgical correction at specific frequencies using narrow bandwidths. Still, the inclusion of a sweepable midrange band with two switchable bandwidth settings does give it some flexibility, and having a smooth, good-sounding EQ at the ready beats having no EQ at all, which is the case with lots of other mic preamps. The onboard compressor is a real winner: easy to use, and it sounds superb. Auto mode and OverEasy make it simple for non-engineers to get a good sound, yet there's still the kind of precision control available that pros want too. You'd normally expect to pay at least the going price of the 676 for a compressor this good.
High plate voltage tube mic preamps are often expensive, and good channel strips tend to be too, but the 676 bucks both trends. The fact that Dbx is delivering so much with the 676 really makes it a terrific value. To sum up, not only do you get a true, high plate voltage tube preamp, but also sweet-sounding EQ and a very fast and capable VCA compressor - all in one well-built and attractive unit. Regardless of what other high-end gear you may have, you'll be proud to have the 676 in your rack, and will find plenty of occasions to use it.
Yes, I was very impressed by the sound of the 676 - not only on vocals, but also as a bass DI and for general-purpose use on a variety of sound sources. If you're looking for a bit of real tube warmth and character on your tracks, you have it - along with one of Dbx's best VCA compressors and very useable EQ. It's a solid channel strip with a subtly flattering sound, and a great buy. And not just "at this price" - while the price is very home studio-friendly, I suspect we'll see these being used by plenty of pros in the months and years to come. Classy and capable, it's a big winner from Dbx.
Dbx 676 channel strip ($1,248.99 MSRP, $999.95 "street")
Dbx's product web page
Dbx 676 Quick Start Guide (PDF file)
Dbx 676 manual (PDF file)
You can purchase the Dbx 676 from
Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Senior Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.