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Dangerous Music Compressor

Dual-channel Compressor
Published October 2014
By Hugh Robjohns

Dangerous Music Compressor

The Dangerous world of dynamics processing promises plenty of pleasant surprises.

Dangerous Music have become well known over the last 14 years for their range of analogue monitor controllers, source switchers, and summing boxes, all of which are great–sounding and thoughtfully designed products. More recently, though, they’ve begun to introduce signal processors into their range, starting with the BAX EQ and followed soon after with the Dangerous Compressor, which was announced at last year’s AES conference in New York.

The result of over two years of development and honing, the innocently named Compressor is designed to satisfy the quality expectations of the mastering fraternity, delivering a fundamentally transparent but musical character. However, as you might expect if you have any experience of Dangerous’s products, the design team introduced some unusual aspects to maximise the usability for project studios too. And I have to say up front that their ideas have resulted in the creation of a fantastic compression tool that fully justifies its price.

The Basics

The Compressor is described by Dangerous as a dual–channel compressor with provision to link the individual channel side–chain control signals for stereo operation. However, many of the configuration options apply to both channels simultaneously, so there are a few minor restrictions on true dual–channel operation. The Compressor is housed in a heavy–gauge 2U rackmounting case made of black–painted steel. The whole assembly weighs a chunky 5.25kg and stretches 310mm behind the rack ears.

A pair of distinctive edge meters claims pride of place in the centre of the front panel to show the input/output levels or the amount of gain reduction. Over on the right–hand side, 10 rotary controls govern all of the usual compressor parameters, starting with a switch which offers eight ratio options (1, 1.4, 1.7, 2, 3, 4, 6, and 20:1). A gain control allows attenuation as well as make–up gain over a ±10dB range, and is accompanied by threshold (±20dB), attack (1–100ms) and release (10–500ms) controls.

Leftfield Ideas

So far, so normal, then. But on the left–hand side are some less familiar facilities, controlled by 12 round push–buttons, each housing a status LED. The first button, labelled Engage (red LED), passes the signal through both channels of the compressor circuitry. When not engaged, a hard–wire relay bypass is applied. Next, a group of four buttons with orange LEDs configure the side–chain options (again, for both channels simultaneously). Normally of course, side–chain access is used to allow an external equaliser to manipulate the compressor’s sensitivity to certain frequency regions. However, the Dangerous Compressor features two basic EQ options in the side–chain signal path, in the form of separate bass–cut and HF–boost equalisers. These can be switched separately to reduce the compressor’s sensitivity to kick drums and to firmly control the sibilance region, respectively. The bass–cut option is a first–order (6dB/octave) high–pass filter reaching –3dB at 60Hz, while the sibilance boost is achieved with a 2dB shelving equaliser above 5kHz.

A shot of the interior. The VCA chips are the black rectangles to the left of the large black capacitors.A shot of the interior. The VCA chips are the black rectangles to the left of the large black capacitors.In cases where more sophisticated EQ or other external processing is required, the Compressor has fully balanced side–chain sends and returns, with the sends always being active provided the Compressor is ‘Engaged’. The return signals (for both channels) can be selected instead of the internal signal at the press of a button, and the side–chain signals can also be monitored through the main outputs, if desired, making it much easier to tune an external equaliser.

Three more buttons (green LEDs) make up the Contour section, which modifies the threshold, ratio and attack/release parameters. A button labelled ‘Soft Knee’ smooths the transition into compression when the signal exceeds the threshold, providing a gentler and more subtle or ‘transparent’ compression effect. By default, both the attack and release times are set to around 30ms, providing an automatic dynamic response which seems to work well with most transient–rich material. However, if the attack and release times need to be adjusted manually, pressing the ‘Manual Att/Rel’ button activates the front–panel rotary controls to allow just that, lighting a green LED between the controls to warn when they’re in service.

The rear panel sports dual side–chain send/return loops on XLR connectors, as well as meter trims.The rear panel sports dual side–chain send/return loops on XLR connectors, as well as meter trims.The most unusual configuration mode is controlled with a button labelled ‘Smart Dyn’, which effectively sets up the side–chain circuitry with two independent threshold detection circuits, one of which is only interested in the average level, and the other only in brief transients. The idea is to allow a higher average level to be maintained without occasional loud transients ‘punching holes’ in the sound. Dangerous Music recommend this mode for most material, and it does seem to be extremely effective and artifact–free. In essence, the system appears to provide separate limiter and compressor side–chains, controlling the same VCA at the same time — an approach I first noticed in the Audio & Design F760 Compex, back in the 1980s.

Buttoned Up

Another trio of buttons (red LEDs again) configures the edge–meter display, with the first switching between gain reduction (on) and signal level. With the signal-level mode selected, the second button selects either output (on) or input level. The last button attenuates the level meter signal by 6dB to accommodate hot signal levels, so 0VU is aligned to +4dBu as standard or +10dBu in the ‘hot’ mode. The meters have a typical VU ballistic, so when switched to indicate gain reduction they react fairly slowly, displaying the average gain reduction and omitting any brief transients. Consequently, two bright-green LEDs are provided as well, flashing when any level of gain reduction is being applied — these respond to even the briefest transients.

Finally, a single button with a blue LED links the two channels together for stereo operation. When active, the control–voltage (CV) outputs (as opposed to the audio input signals) from the two independent side–chains are combined, with the largest one at any moment effectively taking control of both channels’ VCAs equally. This approach, which ensures a stable stereo image, is a standard practice in dual–mono compressors configured for stereo operation, and is the only sensible approach.

Dangerous Music say that some stereo–only compressors cut corners, using a single side–chain circuit working on a simple mono sum of the two audio inputs. Such an approach is riddled with problems if there are out–of–phase elements between the channels, or very strong central components, resulting in all sorts of unexpected and unhelpful gain-reduction effects. Thankfully, there are few such designs — in all my years of reviewing compressors, it’s something I’ve only ever noticed in one product.

Confirmation of the crosstalk specification at 1kHzConfirmation of the crosstalk specification at 1kHzIn stereo mode, the upper channel’s gain and threshold rotary controls affect both channels together, and the lower channel’s corresponding controls are disabled. However, the ratio and attack/release (if activated) controls remain completely independent on both channels, and normally need to be set to the same positions.

Moving briefly around the back, the electronically balanced line input and output connections are presented on XLRs with 20kΩ input and 50Ω output impedances. Four more XLRs provide electronically balanced side–chain sends and returns for the two channels. An IEC mains inlet incorporates the mains on–off switch, fuse, and voltage selector. A pair of recessed multi–turn trimmers allows the meter zero point to be adjusted when switched to the gain-reduction mode, compensating for any side–chain control–voltage drift that might occur over time.

Tech Specs

As I’d expected, the published technical specifications are exemplary, with a frequency response that’s flat within 0.25dB from 15Hz to 80kHz (not reaching –3dB until 6Hz). Given its maximum signal I/O level of +27dBu and a noise floor of –93dBu, the Compressor’s potential dynamic range is an impressive 120dB, while distortion is below 0.005 (THD+N) and 0.007 percent (IMD). Crosstalk between the two channels is also very impressive: it’s quoted as 115dB at 1kHz, but I measured it closer to 117dB, and better than 109dB at 10kHz. These figures confirm this product’s suitability for genuinely independent two–channel processing, if you don’t mind the channel–linked configuration modes.

Confirmation of the frequency responseConfirmation of the frequency responseTaking a peek inside the case, there’s more empty space than I was expecting. A linear power supply hugs the left–hand side, featuring two compact toroidal transformers. One provides ±18V DC audio power rails to maximise audio headroom, and the other provides the DC control and LED rails. The one mildly negative comment I’d level at the construction is a personal bug–bear: the IEC mains inlet safety earth is routed via the PSU circuit board and connects to the case via a PCB track, PCB mounting screw, and metal stand–off post. From a safety perspective, I’d prefer this to be connected directly to the chassis. The Compressor has a two–year warranty period.

All of the audio electronics are hosted on a main circuit board which abuts the rear panel and carries the XLR connections. Surface-mount components dominate the board, all apparently being selected on the basis of their (lack of) influence on the sound quality. The audio circuitry is DC-coupled, but a single Solen capacitor blocks DC to the VCA.

Confirmation of the Intermodulation Distortion performanceConfirmation of the Intermodulation Distortion performanceThe main input and side–chain return stages employ THAT 1246 balanced line receivers, with Analog Devices AD8510 high–performance J–FET op amps providing most of the buffering and gain–stage duties throughout the rest of the signal path. THAT 1646 balanced output drivers are employed for the main and side–chain send outputs.

Audio signal attenuation is courtesy of THAT 2181 Blackmer VCAs, controlled from a combination of THAT 2252 RMS level detector chips and some custom circuitry involving Analog Devices AD8513 J–FET op amps. I counted 18 sealed relays on the PCB, which configure the various operating modes, and 21 cermet trimmers in different circuit sections allow the circuitry to be aligned and tuned, all carefully sealed in position after factory calibration.A plot of the different ratios in the hard knee settingA plot of the different ratios in the hard knee setting

Put To The Test

Dangerous Music’s design goal with the Compressor was to make it simple to use for the novice, but with the manual controllability required by experienced users — and they’ve certainly achieved that. In most cases, you simply decide on how ‘squashed’ the track needs to be, set the ratio control accordingly, and then turn down the threshold control to achieve the required amount of gain reduction. The automatic attack/release times seem to work well with most material, as does the ‘Smart Dyn’ mode. Switching on the soft-knee option helps to maintain the inherently very transparent character.

The same ratio slopes, but with the soft-knee option engaged, showing how the transition into compressor starts earlier and more gently.The same ratio slopes, but with the soft-knee option engaged, showing how the transition into compressor starts earlier and more gently.Whether working on a solo voice or a complete stereo mix, the Compressor reigns in the dynamics effortlessly and naturally and, in many cases, so well that I questioned whether it was switched in! The same was true of percussive and transient elements: no holes, no pumping, just less dynamic range. What more can you ask? On vocals, the Compressor is sublime — it’s one of the most natural and inaudible compressors I’ve ever used, even with quite heavy-handed ratio and threshold settings. The side–chain sibilance filter is pretty good too; using an external equaliser may allow a more tailored and aggressive approach, but the somewhat conservative generic setting seems to minimise most typical sibilance effects quite effectively on its own. I also found it useful in a mastering context, with a mix that was rather full–on through the high–mids.

As a bus compressor, the Compressor does a superb job of gluing all the elements together to form a cohesive sound, and it does this without ever sounding controlled or constrained; it always seems to sound open and transparent, crisp and clean. As I said before, I often found myself bypassing the compressor to convince myself that it really was wired in and doing what the meters said it was doing. And when I bypassed the Compressor, all the quieter details I could hear fell away — so it was was definitely doing something very useful, but in the most natural, unobtrusive way.

Confirmation of the THD+N distortion performance.Confirmation of the THD+N distortion performance.An oscilloscope plot showing the attack/release characteristics in the default mode with a  4:1 ratio. The test signal alternates between –20 and –10 dBFS. Note the initial overshoot as the VCA takes time to react, and the under–shoot as gain reduction is released. These are the default attack/release settings of 30msAn oscilloscope plot showing the attack/release characteristics in the default mode with a 4:1 ratio. The test signal alternates between –20 and –10 dBFS. Note the initial overshoot as the VCA takes time to react, and the under–shoot as gain reduction is released. These are the default attack/release settings of 30msAttack and release set to minimum manual times. Note reduced attack and recovery times compared to default.Attack and release set to minimum manual times. Note reduced attack and recovery times compared to default.The attack response appears much faster in this mode.The attack response appears much faster in this mode.Attack and release set to maximum manual times. Note lethargic attack and recovery times compared to default.Attack and release set to maximum manual times. Note lethargic attack and recovery times compared to default.I can certainly see (and hear) the mastering influence. The Compressor may not have expensive switched controls everywhere, but it’s easily of mastering quality from a sound point of view. The Smart Dyn mode is so effective at controlling transients that I found my usual down–stream limiter often became redundant, which is another big plus in terms of cost–effectiveness.

Doing The Dirty?

As a transparent compressor, then, this is a great tool, but sometimes compression artifacts are intended as an element in the mix. Impressively, the Compressor makes this possible too: you just switch off the soft–knee and Smart Dyn modes, and turn on the manual attack/release. Adjusting the release time to get the track bouncing along just right then becomes very easy.

The side–chain bass filter is even more conservative than the sibilance filter, but once more it works well, reducing any tendency for the kick drum to modulate the rest of the mix. A more targeted result might be achieved with an external EQ, but that doesn’t make it any less useful and the bass and sibilance filters are still available even when the external side–chain is active.

Cleaning Up

The over–riding sound character of the Compressor is well, there isn’t one! It’s very open, clean and natural at the top end, and tight, focussed and accurate at the bottom. Engaging the stereo mode doesn’t narrow the image at all — not that it should, but psycho–acoustically some compressors do seem to suffer in that way. The astonishing sense of transparency and complete absence of audible distortion, even when compressing hard, are seriously impressive. For those who like compressors to introduce grunge and dirt, the Dangerous Music Compressor might not be ideal, but for anyone who wants to reign in the dynamics and push up the average level subtly, musically and effectively, this unit does a very impressive job.

Alternatives

There are a surprising number of stereo and dual–channel compressors available for around the same cost as the Dangerous Compressor. Avalon’s AD2044 is a well–respected optical compressor, as is the Empirical Labs Distressor EL8S (and XS) dual–channel combo, and then there’s the classic Universal Audio 2–1176 FET compressor too. Moving into the less well-known alternatives, A–Designs’ HM2 Nail is a hybrid solid–state/valve design, with some very interesting functions, including a blend control for parallel compression, while the Looptrotter Monster is really a sound–shaping FET compressor with various tube-saturation modes. The Smart Research C2 also features a crush facility for really cranking up the mix level. For truly transparent compression, Pendulum Audio’s OCL–2 is an optical compressor/limiter, Dramastic Audio’s Obsidian is a stereo–only VCA–based unit, and the IGS Audio Tubecore Vari–Mu is a valve–based stereo mastering compressor. I could go on...

Published October 2014