You are here

Dbx 566

Dual Vacuum Tube Compressor By Hugh Robjohns
Published December 1999

Dbx 566

Although the Dbx product portfolio is growing rapidly with new digital products, they haven't forgotten about the sonic advantages of more traditional technology — hence their new Silver series of affordable 'classic' processors. Hugh Robjohns gets to grips with the 566 valve compressor.

Dynamics processing and Dbx go together like Volvo drivers and Trilby hats — you don't always see them together, but it's reassuring when you do! Over the years, there have been several aspects of dynamics processing where Dbx have made improvements which we now take for granted. Perhaps the most obvious is the classic 'Over Easy' compression characteristic, which provides the now‑familiar soft‑knee junction between linearity below the threshold and dynamic compression above it.

The Dbx Silver series is a deliberately 'retro' product line‑up, in both its looks and its inclusion of valve amplification stages using 12AU7 double triodes. The first in the series was the 586 stereo mic preamp, and this has been followed by the subject of this review, the 566 dual compressor. Each channel of the 566 is equipped with a Class A valve amplification stage combined with Dbx's new 'V1' VCA module, which performs the dynamic gain changes required. The two channels can be used independently or linked for stereo operation, and all the classic features such as the Dbx Over Easy compression characteristic can be found, along with totally independent protective 'Peak Plus' limiter sections at the output of each channel. The machine is also capable of accepting an optional 'Type IV' stereo A‑D converter module, allowing it to provide a high‑quality digital output.

The Tour

The brushed‑metal front‑panel features all the usual controls and a comprehensive metering section.The brushed‑metal front‑panel features all the usual controls and a comprehensive metering section.

The 566 is a rackmounting 2U box with a brushed‑metal finish, front‑panel handles, a set of supplied rackmount bolts, and slotted cooling vents. The rear panel is well populated with connectors, each channel having balanced XLR inputs and outputs as well as quarter‑inch jack sockets. Buttons adjacent to each input and output connector cater for +4dBu or ‑10dBV operating levels. Jack sockets are also provided for side‑chain insert points, with separate connectors for the send and receive signals operating at a sensible nominal level of +4dBu. There is also a high‑impedance instrument input, although it is a shame that this is not on the front panel. An integrated IEC mains socket, voltage selector and fuse holder completes the interconnections, and is accompanied by a rocker switch for mains isolation (there being no front‑panel power switch).

The operational controls are all entirely logical, familiar, and clearly labelled, with the two channels well delineated. All of the buttons illuminate when selected and the rotary controls have nicely defined pointers, so the status of the machine is obvious at a glance. Normally, the balanced rear‑panel input is the programme source, but the instrument input can be selected instead via a button in the bottom left‑hand corner of each channel's control section. This is adjacent to the 'Input Drive' control, which offers a +/‑15dB range around the nominal operating level for the valve amplifier stage. Directly above the Drive control, a 'Threshold' knob determines the point at which dynamic control starts, ranging between +20 and ‑40dB (relative to the nominal operating level). A set of 'traffic lights' alongside this control shows the instantaneous level of the signal, a green LED illuminating when the signal is below the threshold, and a red one when it is above. If the 'Over Easy' mode is switched on (via a push button to the right of the Threshold control), a central yellow LED also illuminates when the signal level falls within the 'Over Easy' (soft‑knee) part of the compression curve.

The ratio control is adjustable between 1:1 (ie. no compression) and infinity:1, with 2:1 at the 12 o'clock position. Around 50 percent of the control's rotation covers the area between 1:1 and 2:1, allowing precise adjustment of very gentle slopes. The level‑detection circuitry in the 566 is of the RMS (averaging) type, and the two time‑dependent controls occupy the remaining positions at the top of the panel. Attack provides settings from a fast 3dB/mS down to a slow 0.04dB/mS, and the Release knob is scaled from 250dB/mS down to 5dB/mS. An 'Auto' button between the two time‑constant controls overrides both with settings which change continuously to match the programme's dynamics. This automatic mode has apparently been modelled on the characteristics of the classic Dbx compressors.

Below the ratio and attack controls are four buttons, the first two of which relate to the side‑chain: 'SC Enable', which activates the (normalled) send‑return processing loop, and 'SC Mon', which allows the side‑chain signal path to be monitored, allowing easier adjustment of any external processing. The side‑chain output is derived from a point before the valve amplifier stage, and can be used as a direct output if required. A third switch, labelled 'Contour', introduces a gentle low‑pass filter into the side‑chain signal to reduce the compressor's susceptibility to bass‑heavy signals. The last switch in this section provides a full bypass mode which connects the input directly to the output, bypassing not only the compressor section but also the valve amplifier stage too.

There are two remaining rotary controls on the bottom edge of the panel, the first of which sets the amount of make‑up gain (Level) over a +/‑15dB range. This is not only intended to compensate for the inherent loss in level through the compression process, but also for the increase in level when the valve amplifier is driven hard. Finally, a second Threshold control applies to the independent 'Peak Plus' limiter, and is calibrated from 0 to +22dBu (off). This circuit is claimed to provide 'intelligent predictive gain reduction' to maintain control of the maximum signal peak amplitude, and is placed in the signal path after the Level control, where it serves to protect the subsequent equipment in the programme chain. It can also be used to protect the optional Type IV A‑D converter module from excessive transient peaks.

The central portion of the machine, between the two audio channel control sections, contains three further illuminated buttons. The upper switch activates the Stereo Linking function, which combines the output from the level‑detection circuitry of each channel and then applies the resulting control signal equally to both channels. Only the controls of Channel 1 affect the compression parameters of both channels in this mode, with Channel 2's controls becoming entirely redundant except for the Bypass, Contour, Side Chain Enable and Side Chain Monitor buttons.

The final two buttons are active only if the Type IV A‑D module is installed. The Dither switch then selects one of two probability functions for the dither (SNR2 or TPDF), and the Shape switch determines a noise‑shaping algorithm (Shape 1 or 2).

Quality is expected of a Dbx compressor, and the 566 does not fail to meet expectations. Its frequency response extends from 10Hz to 200kHz at the 3dB points, and the THD + Noise (distortion and noise) figure varies from 0.04 percent, with minimum valve drive and maximum output level, to around 3.5 percent with drive and level controls both at their 50 percent settings.

Valve processors aren't really about clean sounds, and cranking the drive level up adds a subtle warmth and substance to the sound — it is not perceived as distortion but it is not entirely clean either.

In Use

The Dbx 566 boasts balanced audio inputs and outputs via both XLR and TRS‑jack connections, with reference‑level selector switches for both channels. Instrument inputs and side‑chain insert points are also provided.The Dbx 566 boasts balanced audio inputs and outputs via both XLR and TRS‑jack connections, with reference‑level selector switches for both channels. Instrument inputs and side‑chain insert points are also provided.

The Dbx 566 is all that you might expect it to be. It has that classic Dbx characteristic, with the Over Easy soft‑knee action and all the sonic qualities of a typical 12AU7 double‑triode valve stage adding warmth and body to the sound. Whether you're using it with solo instruments or compressing an entire mix, the Dbx works very well and is certainly fast and easy to set up.

There are two schools of thought when it comes to dynamics processing: some prefer the compressor to be as transparent as possible, providing dynamic control without adding anything else. Others like their dynamics processing to introduce a recognisable character, using compression as an effect. There is room for both approaches, but the Dbx 566 definitely falls into the latter category, although minimal drive settings also allow a pretty clean through‑signal if required. But valve processors aren't really about clean sounds, and cranking the drive level up adds a subtle warmth and substance to the sound — it is not perceived as distortion but it is not entirely clean either.

The Attack and Release time constants cover useful ranges and the programme‑dependent mode is very effective too. In fact, I found I tended to leave the unit switched to Auto for most things because it coped so well — adjusting the Ratio and Threshold controls was normally all that was required to obtain the desired sound quality without too much fiddling around. The level‑detection and VCA circuitry are based around Dbx's own ICs, and coped admirably with a wide variety of demanding material. The 566 is ideally suited for creative compression effects — particularly for adding that dynamic punch and bite to solo instruments by using the slower attack times — although as a general‑purpose mix compressor it also fared extremely well in most circumstances. It even worked successfully in bringing up the warmth and ambience on a light classical music recording.

The problem with compression techniques involving slowed attack times is the lack of control over the high‑energy transients that can cause real problems with digital recording formats. Fortunately, the 'Peak Plus' output limiter is extremely effective at catching even the briefest of high‑level transients, yet remains completely transparent. Its position after the Output Level control in the signal path allows it to be configured to safeguard subsequent downstream equipment regardless of any of the compressor's other settings.

At a shade under a thousand pounds, the Dbx 566 is around £150 cheaper than the Drawmer 1960 valve compressor (at list price), with which it compares very favourably. It is also less than half the price of Dbx's own classic 160S dual compressor/limiter, but performs impressively well. The sonic character of the Dbx 566 remains consistent with its ancestral line, yet the valve adds a new quality not previously associated with the marque. The Dbx Silver Series is off to a very good start!

Meter Matters

The 566's metering is based around a large backlit VU display on the right‑hand side of the channel control section, with three different modes selected by associated push buttons below. These determine the signal routed to the meter, with options for Drive Level (ie. input to the valve amplifier stage), Gain Reduction (normal mode) and Output Level. In the gain‑reduction mode the needle sweeps in the reverse direction, as is typical of such devices but, unusually, its rest position is at +5.5dB VU instead of the more common 0dB mark. However, the handbook confirms this to be correct, even though it makes assessing the amount of gain reduction less intuitive! The meter calibration in the output‑level mode is referenced to the nominal operating level, with 0VU corresponding to either +4dBu or ‑10dBV depending on the setting of the relevant switch on the rear panel.


  • A flexible processor capable of a wide range of creative and corrective dynamics effects.
  • Easy and fast to set up and use.
  • Sensible side‑chain access and interfacing.


  • Instrument input only available on rear panel.
  • Gain‑reduction meter calibration rather unusual.


The 566 brings a new dimension to the familiar Dbx sound by combining the latest VCA technology with a classic triode valve amplifier stage.