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Dbx 676

Mic Pre Valve Channel Strip
By Bob Thomas

Dbx 676

If you’re seeking a 'warm’ sonic character and effortless compression, this channel strip could be just the thing.

It has been around 14 years since Harman’s Dbx division released the 376 valve-based channel strip. In that time the channel-strip market has expanded considerably, as home and project studios have continued to eschew real mixing consoles — once their only source of microphone preamplifiers — moving instead to hardware channel strips and virtual mixers. Although the expansion in the channel-strip market has been driven largely by solid-state electronics, valves have continued to exert their sonic and romantic appeal, to the extent that you can now spend anything from ‘not so much’ to ‘one heck of a lot’ on a channel strip that has a valve sitting somewhere in its circuitry.

The new Dbx 676 Mic Pre Channel Strip is a handsome, sturdy, 2U rackmount box in basic black, featuring a Class-A preamp, based around a 12AU7 (ECC82) valve and a three-band EQ, and incorporating the compressor/limiter design from the Dbx 162SL (reviewed almost a decade ago, in SOS December 2004).

The Grand Tour

The Dbx 676 certainly looks the part with its tank-like build, chunky controls, switches that illuminate when active, plus the seemingly mandatory (where valves are involved) centrally positioned analogue VU meter. There’s also a front-panel grille, behind which the unit’s solitary 12AU7 sits in all its dim and orange glory.

Sitting next to the grille are the gain and post-tube attenuation controls, a peak LED and, underneath these, the quarter-inch jack instrument input and switches for the 48V phantom power, 20dB pad, polarity (‘phase’) invert and 80Hz low cut. The three-band EQ section fills the remainder of the left-hand side, with its cut and boost controls for high, low and mid ranges, the EQ enable switch and the mid-range frequency sweep from 100Hz to 8kHz. The Narrow switch pulls in the sweep Q from 1.5 to 0.5 octaves.

The illuminated analogue meter (calibrated in decibels) sits behind a large, primarily decorative, central fascia that’s held in place by four industrial-sized Allen bolts, and carries the meter display select switches — Input, Output and Gain Reduction.

Almost the entire right-hand side of the front panel is occupied by the comprehensive compressor and limiter controls, arranged in two rows and paralleling those of the 162SL, on which the 676’s circuits are based. The switch that enables the compressor/limiter precedes a three-LED ladder that displays the relationship between the incoming signal and the compressor’s threshold setting. This row is completed by the threshold, ratio and make-up gain controls and the switch that brings in Dbx’s famous ‘OverEasy’ mode.

Underneath these, you’ll find controls setting the compressor’s attack and release in manual mode and the limiter’s maximum output level. There are switches for the Contour (a 180Hz high-pass filter in the detector circuitry that stops low frequencies from triggering compression), to engage the external side-chain and automatic mode that makes the compressor’s attack and release times programme dependent.

The 676 features a  side-chain insert, which allows you to refine the compressor’s response via an external EQ or potentially to trigger the compressor using another signal entirely.The 676 features a side-chain insert, which allows you to refine the compressor’s response via an external EQ or potentially to trigger the compressor using another signal entirely.The front-panel roll call is completed with the off/on power switch and, below that, the switch that sets the sampling frequency for the 676’s optional (but not-yet-available) digital interface.

The rear panel is sparse and functional. There’s a mains inlet and a blanking panel where, eventually, the forthcoming optional digital interface will sit. Opposite these sit the balanced XLR microphone input, the TRS jacks that allow access to the side-chain and preamp insert points and the balanced XLRs and TRS jacks for the preamp (post-EQ, pre-compressor) and compressor/limiter line-level outputs.

Heating The Hall

Powering up the 676 results in a short wait, while the 12AU7’s heaters come up to operating temperature. The 676’s 12AU7 is run at a plate voltage of 250V, which is well inside the specified maximum for that valve of 330V and which will contribute to keeping unwanted valve noise and distortion down and valve life up. This also allows you to substitute an alternative valve with a lower maximum plate voltage, such as the 5963, which is often sold as a 12AU7 equivalent, but specifies a maximum of 250V on its plates.

The first stage of the 676’s Class-A microphone preamp is transformer balanced, giving the incoming signal a 15dB boost in the process. Both the 20dB pad and the polarity-invert precede the transformer, the output of which runs to the gain control via the front-panel mic/instrument selector. It’s worth remembering that the instrument input can handle +21dB unbalanced signals, so you can happily feed it with the line-level outputs from guitar and bass preamps, keyboards, effects units and so forth.

The 12AU7 is configured as two cascading gain stages, delivering a total of 40dB of gain overall. As the 676’s raison d’être is the beneficial introduction of distortion arising as an artifact of increasing the level of gain into and within the 12AU7, the post-tube attenuation control allows you to drop the output signal from that valve, in order to obviate overload in succeeding stages.

An 80Hz high-pass filter comes after the attenuation to help deal with low-frequency rumble, and this is followed by the preamp insert, which precedes the three-band, swept-mid EQ section. With 15dB of cut or boost available to each EQ band, you can get quite radical with your treatments, though personally I’d be more inclined to use this facility for gentle reshaping more than for major surgery. Having said that, narrow the Q on the mid-range sweep and you can be quite precise in notching or boosting a bothersome frequency if you have to.

The manual is less than helpful when it comes to describing exactly where the input section’s metering and peak LED are being taken from. My experiments suggest that the input meter point is post the 80Hz filter and pre-EQ, whilst the peak LED sits post-EQ. The net result is that the meter and peak LED display the input level to the compressor and not the signals at the 676’s mic and line inputs. As long as you bear that in mind, you shouldn’t run into any problems, as the instrument/line input stage has plenty of headroom and the mic input has its 20dB pad available to tame any signal entering there.

The line-level, XLR preamp output is taken post-EQ, pre-compressor, giving you the ability not only to run a balanced signal to your console or DAW input (either directly or via an external processor) but also, in conjunction with the compressor output, to carry out parallel compression. Since the Preamp Insert sits pre-EQ you could patch that external processor in there if it can be run unbalanced. Another use for that insert’s return is running an unbalanced signal through the 676’s EQ and compressor, which adds another layer of versatility to the mix.

Next in the signal path comes the 676’s beating and versatile heart: its 162SL-derived VCA compressor/limiter. All the required controls are present and the panel arrangement reflects the anticipated operational ergonomics, with Threshold and Ratio being the largest knobs, sitting above the less prominent ones for Attack and Release. To my ears, a Dbx compressor in its OverEasy mode is the shortest cut I know to achieving transparent, unobtrusive compression and getting there is simply a matter of setting the threshold and ratio controls to achieve the sound that you want to hear.

Dbx 676There are two ways of watching, rather than listening to, what‘s going on in the compressor. Next to the compressor/limiter’s Enable switch sits the three-LED ladder that lets you know if the signal level going into the compressor is below (green), within (yellow, OverEasy mode only) or above (red) the onset of the compression that you’ve set with the threshold and ratio controls. The amount of actual gain reduction is displayed on the meter’s, with its Gain Reduction setting selected. It’s worth noting that the gain reduction being metered includes that not only of the compressor but also of the limiter, so you do need to turn the limiter off if you want to meter the compressor’s contribution.

The 676’s compressor is nothing if not malleable. In addition to setting the threshold and the level of compression, you can choose not only between soft-knee (OverEasy) and hard-knee modes, but also between manual and automatic attack and release times. You can then engage the Contour button which inserts a 180Hz high-pass filter in the compressor’s detection circuitry, thus preventing bass frequencies from triggering unwanted compression and pumping. If you need to introduce frequency-conscious compression, the Sidechain Enable switch brings the compressor’s side-chain insert point into play, where you can insert an EQ to emphasise the specific frequency area that you want to compress. De-essing is a common use for this, but you could also fine-tune the compressor’s response to different parts of the spectrum. Neither the Contour function nor any equalisation in the side-chain affect the audio signal, of course, their sole effect being on the operation of the compressor itself.

Next comes the make-up gain section, where any level drop resulting from compression can be compensated for, with the red peak LED reminding you when you’ve gone too far in this regard. Finally, the always-active PeakPlus limiter prevents the signal from overshooting the output level set by the Limiter control knob.

Turning Up The Heat

As any valve enthusiast would, the first thing I did was to plug in my trusty and clean-sounding AKG C414 condenser mic and start winding up the 676’s preamp gain — and the results were more than pleasing. Starting with the post-tube attenuation set as low as I could manage without the 12AU7’s self-noise becoming too apparent (somewhere around 11 o’clock in my system) increasing the gain produced a sound that got progressively warmer and finally moved into a slightly crunchier area as the gain increased. Obviously, as the gain went up so did the preamp output level and I had to increase the attenuation to keep the meter at round about 0dB. Singing gently close to the mic with the gain set at around 12 to one o’clock gave my voice a warm and intimate character that I really liked. The increase in gain produced by singing harder and louder started to push up the distortion level and I found a grittier edge appearing that I could get to like a lot.

Electric and acoustic guitars and basses also benefitted from the warming effects of the 676’s 12AU7 valve and although you’d never use it as an outright distortion effect, warming up the output of a DI or a clean preamp would be, in my view, a very attractive proposition.

The EQ worked well, and the narrow Q enabled me to hone in on specific mid frequencies. As I said earlier, this is a tool to be used sparingly for gentle correction, rather than as a sonic sledgehammer.

However, it’s also the compression that we’re here to hear, and the 676 does not disappoint. Setting up the attack and release in auto mode, alongside the OverEasy soft-knee curve, gives you an unobtrusive, silky-smooth compression without any real effort on your part. It’s pretty much impossible to make a source sound bad. Vocals, together with acoustic and electric guitars and basses, were kept under control without losing their character, no matter how dynamic they might be. The automatic attack and release times never tripped up, no matter what I threw at them. If this unit were mine, I doubt that I’d ever switch it out of Auto/OverEasy mode.

Of course, there are occasions where only manually controlled, hard-knee compression will do. In the case of the 676, the manual control features Dbx’s AutoVelocity manual mode, in which the attack and release times remain programme dependent but the decibel-per-second rates of attack and release can be scaled from 400dB/ms to 1dB/ms (attack) and from 4000dB/sec and 1dB/sec (release). The very fast attack and release times (25 s and 2.5ms respectively) of which the 676 is capable will cause unwanted distortion in the audio signal at extreme settings and should be used with care. The Contour control’s 180Hz high-pass filter proved extremely effective at its allotted task of rolling off low frequencies in the compressor’s detector circuitry and thereby preventing unwanted pumping.

Keeping things calm at the exit, the 676’s PeakPlus limiter held a very effective ‘clamp’ on proceedings but was unobtrusive in operation when set up to catch the odd transient overshoot, which is how I normally use any output limiter.


I’ve got a lot of time for the Dbx 676 Tube Mic Channel Strip. It does exactly what it claims to be able to do and does it well. I couldn’t fault its performance and sound quality and I’d be more than happy to use it whenever I wanted to warm up a sound source with a bit of valve harmonic distortion.

That said, I was always aware of the presence of the 12AU7 valve — the design means that it’s not possible to bypass the valve when you might prefer to capture a completely clean, hiss-free sound, other than by patching in a mic preamp via the preamp insert return. It’s thus unlikely that the 676, even with its forthcoming optional digital interface, could become my primary channel strip. Mind you, since the stock Russian-made valve isn’t the quietest that I’ve ever come across, investing a few tens of pounds in a higher-quality NOS 12AU7 or equivalent should pay dividends. I tried out an old Mullard that I happened to have in my valve collection and that dropped the preamp’s noise floor at lower attenuation levels by around 5dB on the meters — a very worthwhile improvement indeed.

Having said all that, the Dbx 676 Tube Mic Channel Strip was a pleasure to use and its valve-warmed, OverEasy compressed sound made everything that I ran through it (even my voice!) sound bigger and better, and it did so seemingly without any effort. It’s going to be very difficult to send this particular channel strip back from whence it came. The Dbx 676 really is an attractive option, especially if you’re looking for that something special and you’re into the sound of valves.


I can think of no valve-based channel strip at a directly comparable price to the Dbx 676. Lower-cost alternatives are available from ART, PreSonus, Aphex and Dbx themselves, and slightly higher in price is the SPL Channel One. Higher up the food chain there’s stiffer competition from a variety of channel strips. Some, such as those from Tree Audio, Thermionic Culture and Manley feature tube compressors, but some solid-state designs can also offer a bit of character, such as the offerings from API, Neve, and SSL. Another option would be to assemble a custom channel strip from 500-series modules.


  • Preamp valve distortion adds warmth and edge.
  • OverEasy compression mode works so well and sounds so good.
  • Contour filter prevents low frequencies inducing unwanted pumping.
  • Intuitive control layout.
  • High build quality.
  • Good value for money.


  • Preamp valve is always in the circuit.
  • You might want to budget for adding higher-quality valves.


The Dbx 676 channel strip features a Class-A, valve-based preamp, a three-band EQ and a compressor design taken from Dbx’s Purple Series 162SL. Coupling the preamp’s ability to add warmth and edge to Dbx’s renowned OverEasy compression mode gives the 676 an attractive sonic character that’s capable of enhancing most sources.


£949 including VAT.

Sound Technology +44 (0)1462 480000

Published August 2015