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dbx 160S

Compressor/Limiter By Paul White
Published January 1998

dbx 160S

Is the dbx 160S pure technology, a work of art, or a little of both?

You can always tell when a valuable product comes in for review — it turns up in a flightcase, not a cardboard box! The 160S is dbx's top‑of‑the‑range analogue compressor/limiter, and it's pretty clear that the heavy, sculpted front panel and chunky metal knobs have been influenced by Focusrite's high‑end 'Red' styling, but the design aim seems to have been to capture the sonic signature of existing dbx classic products, rather than to go all out for sonic neutrality or attempt to create a new compressor characteristic.

As you would expect at this price point, the support circuitry and mechanical design are impeccable, and include the new and impressively specified high‑performance dbx V8 VCA to look after the gain control. This is a huge device potted in an aluminium heatsink‑style case to maintain an even internal temperature — this is important to maintain matching between the multiple transistors used in the gain‑control circuitry. Additionally, there are audiophile‑quality input and output stages, Jensen audio transformers, and a sophisticated power supply, but behind all this the aim is to give the user access to the compression characteristics that made the dbx 165 and 160 models famous during the '70s. To this end, both hard‑knee and soft‑knee compression modes are implemented and, according to dbx, it's possible to make the 160S sound like a 160, a 165, a 166 or a 1066. This feat is accomplished not by emulation, but by duplicating the key control elements from these different models and physically switching them.

Introduction

Housed in a 2U rackmounting case, the 160S is a 2‑channel compressor/limiter with the addition of dbx's proprietary PeakStopPlus limiter on the output. Metering is via moving‑coil meters rather than the more usual LED ladders, and full side‑chain access is available. The heavy aluminium front panel is blue anodised, and all the controls, buttons included, have a heavy, smooth feel that inspires confidence.

Before going further, it's probably helpful if I explain the presence of the PeakStopPlus limiter in a product that can already function as a limiter in its own right. The most obvious reason for including a separate limiter after a compressor is so that the user can apply gentle compression to the signal using the compressor section, but still have the limiter keeping watch for peaks that might otherwise exceed the safe limit for the next piece of equipment in line. This is particularly important with digital equipment, which doesn't tolerate any overload. However, even if you were to configure the main compressor as a limiter, by using a high ratio and a very fast attack time, you'd find this setting less than ideal for low‑frequency sounds, which are treated more kindly if the compressor attack time is set a little longer. Of course, setting anything other than the fastest attack time allows brief peaks to slip through the system unchecked, which is why a separate, very fast output limiter is so useful.

PeakStopPlus is actually a two‑stage limiter designed to arrest excessive peaks with the minimum of side‑effects, and it does this by first employing what dbx describe as their Instantaneous Transient Clamp, which controls the level using a soft logarithmic function to avoid harsh‑sounding clipping effects. This effectively prevents overshoots of more than around 2dB above the set threshold, but then stage two comes into action, introducing another new dbx term — Intelligent Predictive Limiting. I interpret this as atype of look‑ahead system that monitors the input level, providing a very short but still useful warning that a peak is about to hit the limiter. Apparently the top couple of dBs of the limiting process provide soft clipping rather than simple truncation which, again, helps produce a more natural sound. PeakStopPlus is provided as a kind of peak level safety net, so under normal circumstances the compressor output level would be set so that the limiter rarely operates (if ever). If desired, however, the limiter can be provoked into more frequent action, allowing its use as a creative effect.

Layout

Both channels are identical and operate independently unless the centre Stereo Couple button is pressed, in which case the side‑chains are linked for accurate stereo tracking. The channel has five rotary controls for the compressor/limiter, with a further single control to set the PeakStop threshold level. The latter operates on the signal after the compressor's Output Gain control and is calibrated from +4 to +30dBu, the maximum level the internal circuitry can handle. A separate bypass button is fitted for the PeakStopPlus section, and in common with every other switch on the front panel, a status LED is provided. A further LED adjacent to the Output Gain knob illuminates when the output signal level exceeds +27dBu — and that still leaves 3dB of headroom before clipping occurs.

To provide maximum flexibility, the compressor section can be switched to hard‑knee mode or dbx's soft‑knee OverEasy mode, and the threshold is fully variable, with three indicator LEDs: green shows when the input signal is below the threshold, yellow comes on to signify that the compressor is in its soft‑knee mode, and the red LED shows that the signal is above the threshold, and therefore subject to processing(providing the ratio is set to greater than 1:1). Ratio is fully variable, from 1:1 to hard limiting, via the large, sensibly calibrated Compression control. Between the Attack and Release controls is an Auto button for switching the unit into programme‑dependent mode, where both the attack and release time constants are adjusted on‑the‑fly according to the envelope characteristics of the input signal. Attack is variable from 400 to 1dB/ms and Release is from 4000 to 10dB/s. This is a slightly unconventional way of showing the response time but it is actually more accurate, as, in fact, response time depends on the signal level going through the unit. A simple time figure is easier to understand, but as most users set the controls by ear I don't suppose it makes much difference either way.

On a personal note, I think the VU meters should have been cream rather than white, to capture a true vintage feel, but they are switchable to monitor the input, output or gain‑reduction levels, with a VU‑style characteristic.

Without doubt, this is a beautiful piece of equipment, both technically and aesthetically, and dbx have designed it to appeal to those countless engineers who already love the dbx sound but who'd like to get it all in one box, and with the ultimate in analogue performance.

Using The 160S

The 160S needs to be mounted fairly high in your rack if the control names aren't to be totally obscured by the knobs, but other than that the control layout is superb, the controls have a wonderful feel to them, and all the buttons are positive and solid. As suggested by the technical spec, the only noise you ever really hear from this unit is what you put into it, and the amount of headroom almost certainly exceeds that of the mixer you're connecting to. At a technical level, I have no complaints about this product at all, and the inclusion of status LEDs on every switch makes it clear what's going on, even from the other side of the studio.

Soundwise, the 160S is classic dbx, and the closest you'll get to transparent compression is by using the Auto mode. Even then, vocals sound as though they're being flattered in some way rather than simply being controlled in level — even in OverEasy mode, using more than a few dBs of compression gives the impression of fairly assertive gain control. Rock vocalists in particular will probably like what the 160S does, and existing 160 users can be assured that they can get their standard 160 effects from this box, but with an extra degree of audiophile gloss.

Using manual attack and release settings on vocals, I found that the release control had to be used in the last quarter of its travel, otherwise distortion was clearly audible, no doubt due to the compressor trying to respond to individual cycles of the low‑frequency components of the voice. Increasing the release time cures this completely, but for vocals Auto usually does the best job and never seems to get tripped up, regardless of what you throw at it. The faster manual release times will obviously be useful on percussion.

Setting up the PeakStopPlus limiter so that it rarely comes into operation results in a reasonably benign form of limiting, so the design strategy evidently works. If you deliberately push the levels so that the limiter is working more or less constantly, you hear the level flinch as it hits the limiter, then swell back up over half a second or so, producing an audible pumping effect that could be useful in some creative situations.

Summary

Without doubt, this is a beautiful piece of equipment, both technically and aesthetically, and dbx have designed it to appeal to those countless engineers who already love the dbx sound but who'd like to get it all in one box, and with the ultimate in analogue performance. I've used most of the dbx compressors at one time or another, though I didn't have them for direct comparison during this review, and the 160S is able to get very close to the sound of any of them. As to whether the super audio quality is necessary, this depends on what you want to use the compressor for. If you're mastering or compressing whole subgroups, it could be argued that you need the best signal path available, but if you're simply compressing one channel in a mix, a regular dbx 160A would be a more cost‑effective solution, and probably wouldn't make any perceptible difference to the quality of the end result. Whether you would use a dbx compressor for mastering is largely a matter of taste, but I've always thought of them more as 'effect' compressors than as a means of controlling gain without side‑effects.

The dbx 160S is certainly the ultimate realisation of the dbx compressor concept and, given the affection felt for dbx by certain mixing engineers, will almost certainly find its way into a number of prestigious studios. If you're a private studio owner, however, you have to look at the recording system as a chain and realise that adding one strong link will not necessarily increase the strength of the chain as a whole. If that is the case, the existing dbx compressor range will serve your sonic needs almost as well, albeit with rather less flexibility and finesse.

Feel The Quality

When I said at the start of this review that dbx had tried to build audiophile qualities into the 160S, I wasn't using the term loosely. The new VCA design has a dynamic range of 127dB, and where internal connectors are used they have gold‑palladium‑nickel contacts. The torroidal transformer used in the power supply is double‑shielded to keep spurious electromagnetic radiation to a minimum. Even the audio sockets are Neutrik with gold‑plated connectors, and the bypass relay sports even more gold, this time in an hermetically sealed unit. All the audio connectors, including the side‑chain send and returns, are balanced XLRs — there are no jack alternatives anywhere.The output transformers are feats of engineering in their own right, able to drive up to 1000 feet of cable without signal degradation at up to +30dBm, and the circuit boards themselves are heavy glass‑fibre. This really is a solidly engineered, beautifully thought‑out piece of kit.

The V8 VCA

The V8 VCA is a state‑of‑the‑art implementation of the original David Blackmer VCA, but whereas the original was a four‑transistor log amp multiplier, the new VCA version has 32 transistors used in the same basic topology. Whenever the number of devices is doubled in this circuitry, the headroom goes up by 6dB, but uncorrelated noise increases by only 3dB, which is how the 127dB dynamic range has been achieved.

However, to minimise THD (Total Harmonic Distortion), the transistors draw more current than in the earlier design, so an aluminium‑zinc package filled with a thermally conductive resin is used to keep the devices at an even temperature. This is necessary to prevent the performance drifting with temperature fluctuations.

Pros

  • Impeccable audio specifications.
  • Beautifully engineered and styled.
  • Flexible enough to emulate most of the classic dbx compressor sounds.
  • Effective limiter.

Cons

  • Hugely expensive.
  • Distortion is audible at short release times on some material. An built‑in hold time might have solved this, though the Auto mode seems pretty foolproof.

Summary

This is a flagship product, and its price will probably restrict sales to flagship studios, but for those wanting the dbx compressor sound in a technically advanced analogue form, the 160S has to be the one.

information

£2348.82 including VAT.

https://dbxpro.com

Published January 1998