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TK Audio BC1 Ltd

Stereo Compressor
Published August 2018
By Bob Thomas

TK Audio BC1 Ltd

More than a mere makeover, this 10th anniversary limited edition processor adds some useful functionality to the original BC1 design.

It’s now over three decades since Solid State Logic (SSL) wowed the world of audio production with the release of their 4000 G-Series console. That mixer’s central section featured, amongst other things, a certain VCA quad bus compressor which went on to earn an enviable reputation in its own right. Some 20 years later, electronics engineer, designer and music lover Thomas ‘TK’ Kristiansson launched a new brand, TK Audio — and the first product in the range was a hardware stereo VCA compressor, based on the earlier SSL design and called the BC1.

Designed and assembled in TK Audio’s facility in Halmstad on Sweden’s southwest coast, this stylish device wasn’t the first compressor to draw inspiration from the SSL G-Series compressor, and neither was it Kristiansson’s first venture in audio electronics; he’d been making Class-A preamps and processors under his Vintagedesign brand for a while. One reason the BC1 proved very successful for TK was that it wasn’t a simple clone — not only was it competitively priced, but it sported a number of feature enhancements, which I’ll discuss below.

The product I’m reviewing here is a limited edition, 10th anniversary version of the BC1. The BC1 has gone through a few iterations over the intervening years, and to understand just where this latest version, the BC1 Ltd, fits into TK Audio’s range, it’s helpful to understand what the previous revisions of the BC1 brought to the party.

Revisionism

As I mentioned, the original BC1 was more a thoughtful reworking of the SSL concept than a slavish clone, and the functionality it added opened up several mix and mastering applications that went well beyond the ‘magic sonic glue’ bus compression for which the G-Series compressor is rightly renowned.

A bank of three buttons controlled a switchable side-chain high-pass filter at 150Hz, allowed the selection of an external side-chain input, and enabled the user to engage L+R side-chain summing, which increased the BC1’s sensitivity to centrally panned signals by 6dB.

When it came to the compressor’s controls, the range of the continuously adjustable threshold was increased from the SSL’s ±15dB to ±20dB, and two additional ratios (1.5:1 and ‘Hard’) augmented the original’s menu of 2:1, 4:1 and 10:1. The original’s range of attack and release times was also expanded and, probably most importantly in my view, Kristiansson added both a dry/wet blend control and dry mute switch — it may seem a no-brainer now, given the number of compressors that feature such facilities, but at the time it was a rare feature; few hardware compressors had made parallel compression quite so quick and easy.

In late 2011 TK Audio introduced the BC1 MkII (reviewed in SOS January 2012: www.soundonsound.com/reviews/tk-audio-bc1-mkii), a design in which Kristiansson tweaked the THAT Corporation 2181 VCA-based gain-reduction element to slightly improve the new unit’s noise figures, and added a numerical scale around the threshold, make-up gain and blend controls, to aid recall and comparison of settings. Internal jumpers were introduced which allowed the BC1 MkII to be configured with the user’s choice of balanced or unbalanced inputs — a feature that made connection to a console’s unbalanced insert send/return loops much simpler. In 2013, the BC1 MkII was replaced by the BC1-S, which added detents to the threshold, make-up gain and blend controls, and saw the introduction of a large, SSL-style illuminated compressor in/out switch.

Although, strictly speaking, it was not a ‘BC1’, the BC2-ME Mastering Compressor released in early 2015 was nonetheless a variation on the same theme. Its main claim to fame was that its outputs could be switched between electronic and transformer balancing. From the point of view of the evolution of the BC1, though, the more important feature of the BC2 ME was the introduction of five selectable corner frequencies for its side-chain high-pass filter. This feature was also implemented in a reduced three-frequency format in the BC501, which was a 500-series repackaging of the BC1 introduced at the same time as the BC2-ME, and which was reviewed in SOS November 2015 (www.soundonsound.com/reviews/tk-audio-bc501). The BC501’s side-chain frequencies (80Hz, 150Hz and 220 Hz) are selected via an ingenious combination of two buttons, rather than a conventional three-position switch. The BC501 also featured the 6:1 compression ratio first seen on the BC2-ME, adding a slightly more aggressive option than offered by its rackmount predecessors.

However, the most interesting feature introduced on the BC501 was the THD button. This adds a fixed amount of second-order harmonic distortion that, depending on the source, is capable of introducing an attractive sense of enhancement.

You might have thought that, after 10 years of carefully crafted BC1 evolution, TK would already have reached the end of the BC1’s development path — but you’d be wrong. Because this latest 100-unit limited-edition model does more than celebrate its ancestors...

The front panel has been treated not only to a makeover but some new and updated controls, not least the Grab function, which hardens the knee at all ratio settings.The front panel has been treated not only to a makeover but some new and updated controls, not least the Grab function, which hardens the knee at all ratio settings.

The Box

Cosmetically, the BC1 Ltd pays homage to the classic SSL aesthetic, with its silver-grey front panel, large black compression (gain reduction) meter, and square, illuminated compressor in/out push-switch. Even the black-centred, grey-skirted rotary control knobs, a feature of BC1 front panels from the beginning, help to reinforce this processor’s heritage.

The rotary controls are carried over from the BC1-S, so you’ll find detented threshold, make-up and blend potentiometers and ratio, attack and release switches. However, as on the BC501, two long-standing BC1 family features — the ultra-fast ‘UF’ attack time and the ‘Hard’ ratio setting — have been omitted. The 1.5:1 compression ratio that first appeared on the original BC1 has survived the cull, but the 6:1 compression ratio of the BC2-ME and BC501 has not. The BC501’s THD button and the three-frequency, twin-button side-chain high-pass filter have been carried over in their entirety. Two front-panel buttons are the only switches to have survived from the original BC1 — the external side-chain selector and the dry signal mute.

So the BC1 Ltd does combine various features of the different versions that preceded it, but its features aren’t all borrowed. A welcome new addition is the Grab button, which switches in a harder knee across all compression ratios, adding an aggression that’s no doubt intended primarily to enhance the BC1 Ltd’s performance on a stereo drum bus.

Alongside the main analogue inputs and outputs is an external side-chain input for each channel. Internal jumpers can be used to set the compressor for balanced or unbalanced operation.Alongside the main analogue inputs and outputs is an external side-chain input for each channel. Internal jumpers can be used to set the compressor for balanced or unbalanced operation.On the BC1 Ltd’s rear panel, the balanced XLR audio I/O connectors are now paired together as separate channels, each with its own external side-chain input, rather than the shared side-chain input and separate input and output groupings found in earlier BC1 incarnations.

Insider Information

Removing the top of the BC1 Ltd reveals a lot of empty space, which is there simply because the main circuit board is so compact and beautifully laid out; it occupies not much more than a quarter of the chassis’ internal area whilst managing to carry the entire audio, compressor and power-supply circuitry — including a toroidal mains transformer that has been shielded around its circumference. The main board’s diminutive dimensions come, in part, from the extensive use of space-saving surface-mount devices, with full-size componentry being restricted to relays, trim pots, certain capacitors and the THAT Corporation VCA, line driver and receiver chips.

Two ribbon cables connect the main board to two daughter boards that sit vertically behind the front panel on either side of the compression meter. The larger of these carries the main compressor controls, whilst the smaller deals only with the make-up gain and blend functions.

The high quality of the BC1 Ltd’s components, board layout and hand assembly inspires confidence, and the presence of manual trim pots in the compressor circuitry confirms that, like every other BC1 before it, each BC1 Ltd has been individually aligned at TK Audio’s facility in Sweden.

In Use

The BC1 Ltd looks and feels seriously classy when it’s plugged in and switched on. The meter’s LED-illuminated, bright-white 0-20 dB scale contrasts beautifully with the matte-black background. And since the BC1 Ltd is able to handle input levels up to +26dB and output signals at up to +27dB, nobody should experience any problems in interfacing with other professional-level audio equipment in their studio — once you’ve connected it up, all you need to do is to select a compression ratio, set the amount of gain reduction required via the threshold knob, and then work with the side-chain high-pass filter frequency and attack and release controls to dial in compression to suit the track being processed.

As with all the BC1 family, the two lower ratios have quite soft knees, with a harder one at 4:1, and the hardest of all at 10:1, the last making it act more like a limiter than a compressor. But the aforementioned Grab function increases the hardness of the knee at all ratios, and this really does add to the BC1 Ltd’s versatility, making it especially useful on program material that calls for a harder knee — I’ve mentioned drum busses but any other very dynamic sources with a high transient content could benefit.

The effect of switching in second-order harmonic distortion via the THD switch is subtle, but I always felt that it introduced a pleasing sense of colour and character to the track — some might prefer a cleaner sound, but to me, the THD processing is one of those functions that you try switching in and end up leaving it there, because you like what it’s doing.

Away from its mix-bus duties, you’ll find that the BC1 Ltd is a useful studio tool more generally. At a 10:1 ratio with Grab engaged and a very fast attack setting, you can squash a drum track assertively, and yet, by using the side-chain HPF, you can reduce the pumping induced by the bass drum. Then, using the Blend control, you can balance the uncompressed transient-rich original with the squashed track to create a seriously impressive drum sound. On the other hand, if you’re actively searching for heavy pumping, you could send the bass drum’s close mic to the BC1 Ltd’s external side-chain inputs, to trigger compression on the drum bus.

When it comes down to it, the BC1 Ltd is wonderfully versatile and the decision to exchange its predecessor’s Hard ratio for the knee-hardening Grab function has contributed massively to that versatility. Of course, in amongst its other tricks, any VCA bus compressor that takes its DNA from the SSL G-Series model must be able to deliver that ‘magic glue’ effect. And, like all of its ancestors, the BC1 Ltd does that with ease — start with a 2:1 compression ratio, the side-chain filter set at 150Hz, a 10ms attack time, an automatic release, the threshold set to give 2-3 dB of compression (or thereabouts), the blend control set to fully wet, and that will give you one glued-together mix. Then, finely hone that result by adjusting ratio, attack time and release time to suit the character of the track.

Verdict

Once upon a time, if you wanted an SSL bus compressor you had to buy a G-Series SSL console! That has changed in recent years, with a proliferation of products that pay homage to the SSL VCA-based design, but they’re not all created equal. TK Audio’s BC1 Ltd 10th anniversary limited edition is, as far as I’m concerned, one of the very best of the current crop. Some companies might have chosen to celebrate such an anniversary simply by refreshing a product’s aesthetics — but TK chose instead to continue to refine and improve their most popular product. The thoughtful 10-year evolution from the original BC1 of 2008 to the present-day BC1 Ltd has resulted in a compressor which not only delivers the fabled ‘magic glue’, but also — thanks to its side-chain HPF, knee-hardening Grab function, and on-board parallel-processing capability — boasts a versatility that means it can do service in a range of different studio applications. And it’s available at a very reasonable price, considering the build quality, the performance and the feature set that’s on offer.

Alternatives

SSL’s XLogic G-Series Compressor is an obvious alternative but, like many of its imitators, it doesn’t offer the versatility of the BC1 Ltd. Other than TK Audio’s other products (discussed in the review), the closest competitor is probably the Serpent Audio SB4001, for which you’ll need a 500-series host chassis.

Published August 2018