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TK Audio BC1 MkII

Stereo Compressor-Limiter
Published January 2012
By Matt Houghton

The concept behind TK's BC1 is simple: take a classic bus-compressor design and improve on it.

The BC1 MkII's main printed circuit board.The BC1 MkII's main printed circuit board.

Sitting beside me as I write this review is a second-edition (1990) SSL G-series mixing console manual, and in its 'applications guide', two quietly tucked-away lines suggest: "finally, before you commit the mix to tape, try the compressor on the main Quad output”. That's all they thought to include about how to use what's now considered a classic piece of gear. With a minimum ratio of 2:1, this compressor could be quite savage, so at that time most engineers wouldn't have considered tickling the meters with more than 2-3dB of gain reduction on the mix bus — which would provide the 'glue' for which the device has since become famous. You could patch the compressor into other channels, but you already had a compressor built into each channel, so even the temptation to use it as, say, a drum-bus compressor was less than it might be today, when we're working without consoles and often spend time searching for the 'right' compressor for each job in a mix.

SSL now make rackmount and modular versions of this compressor (and very nice they are too), but in recent years there have been plenty of hardware imitations and software models that aim to offer the same sonic result. A select few designers, though, have taken the original design and then tweaked and augmented it, either to improve the technical performance or to add useful functionality that increases the compressor's versatility. And this is where TK Audio come in, with their BC1 MkII.


In its aesthetics — the clean, uncluttered layout of the 1U front panel, and the white-on-black gain-reduction meter — and in its compression character (of which more later), this stereo compressor-limiter quite clearly tips its hat at the SSL G-series, but designer Thomas Kristiansson has made several thoughtful additions which extend its functionality and make it rather more versatile.

As with the BC1 MkI, the MkII includes a built-in switchable (on/off) 6dB/octave side-chain high-pass filter, with a turnover frequency of 150Hz. There's also an external side-chain input, and both features are useful in shaping the side-chain signal that dictates when the gain-reduction circuit operates. More obvious additions to the G-series concept are the wet/dry blend control, which gives you instant access to the world of parallel compression, and two new ratio settings in addition to the 2:1, 4:1 and 10:1 of the SSL: 1.5:1, and 'hard'. The threshold control runs from -20 to +20 dB, the attack from UF ('ultrafast') to 120ms, with eight available settings in total (two more than SSL's equivalent X-Logic processor), and the release control offers 50, 100, 300 and 600 ms and 1.2s settings, as well as an automatic release.

The 1:5:1 ratio and the blend control instantly make this compressor suitable for more subtle applications than the SSL. The lower ratio enables you to bring the threshold down further, so you can gently squeeze things, and the blend means that if you take things too far you can shift the balance towards the dry sound for a more natural result. The 'hard' setting, on the other hand, enables you to push the BC1 into aggressive limiting territory. Finally, a switchable L+R option sums the two channels' side-chain signals to mono before hitting the threshold detectors, which increases sensitivity to centre-panned sources by 6dB.

What's New In MkII?

There's little to see on the rear panel: just the balanced audio I/O, the IEC power inlet and the external side-chain input.There's little to see on the rear panel: just the balanced audio I/O, the IEC power inlet and the external side-chain input.

The Mk1 version of Kristiansson's design has been on sale for a while now, and my initial tests were carried out with that version. Just before I was due to write up the review for SOS, though, UK distributors ASAP Europe sent me the newer MkII. The changes aren't vast, but they're enough to make a useful difference. There's a new panel layout, with digits around threshold, make-up gain and blend controls, which makes it easier to recall and compare settings, and the gain-reduction meter responds more speedily than on the first design. TK have thoughtfully added internal jumpers so that you can configure the device for balanced or unbalanced use (the latter will be useful for anyone feeding the compressor from an unbalanced insert send/return loop).

Like many SSL-inspired designs (including, I believe, the current SSL XLogic version), the gain-reduction element in both the MkI and MkII BC1 designs is based on a pair of THAT Corporation 2181 VCA ICs. These THAT chips are placed in a Dbx 202 emulation circuit, and in the MkII TK have tweaked this circuit, resulting in slightly lower noise figures.

On Test

To test the BC1 MkII, I hooked it up to my DAW and ran several different sources, including single tracks and stereo stems and mixes through it, to see how it performed, and I was reminded of what I'd initially liked about the MkI. Other than the tickling-of-the-meters tactic I described earlier, I've usually found the SSL-style bus compressors to be rather blunt tools: they tend either to give me exactly the result I'm looking for, or come nowhere close to it. The wet/dry blend control offered here opened up a whole new world of options. For example, I could let the gain-reduction meter's needle hit down nearly as far -10dB on a  drum bus, to get a satisfyingly hard-hitting compression character, before backing off the blend control to the 12 o'clock position which retained elements of 'smack' but also restored some of the natural feel.

Used on the mix bus with the 1.5:1 ratio, the BC1 started to deliver on the promise made in the manual of it being "one of the most transparent bus compressors ever made”. In this role, the BC1 comes across as being much more 'honest' and 'polite' than the SSL, in a rather pleasant way: smoother and better behaved, if you like.

Whatever material I was deploying the BC1 on, the switchable side-chain filter seemed useful. It's one of those no-brainer additions that I'm surprised hasn't appeared in more software emulations, as it's so useful. Going back to the drum-bus example, the effect of switching in the high-pass filter is a cleaner, and yet more solid, hard-hitting drum sound. The kick sound and the low end of the snare seem to suffer less, and the signal as a whole doesn't tend to pump in time with the kick so much. It's a sound I like. My only criticism here is that I'd have liked to see more side-chain filter frequencies available on the front panel, but you always have the external side-chain input for that. Finally, for sanity-checking purposes, the switchable high-pass filter and both the compressor and hard bypass buttons were useful.


TK Audio BC1 MkII

The SSL G-series compressor is a tried, tested, and well-loved design, and the BC1 builds on it in a useful way. The wet/dry blend control is perhaps the most useful addition, and after a while, I found that I never wanted to have it set fully wet, such that I could only hear the compressed signal. The result just didn't sound as good to me that way.

Is the BC1 one of the most transparent bus-compressors around, as TK claim? I'm not sure 'transparent' is exactly the right word, given the compression character inherent in this design. 'Clean', 'fast' and 'good' would be perfectly apt, though, and whatever labels you pin to this device it's hard to find fault with it: it's capable of clean, gentle compression; it's capable of royally spanking a drum bus; and it's very much at home on the stereo bus in a range of genres. I have to say that the price, given the quality and functionality on offer here, was a pleasant surprise too. If you like that SSL bus compression sound, but are looking for something that can turn its hand to rather more tricks, the BC1 MkII is well worth an audition.   


The world isn't short of SSL-inspired options. Obviously, SSL's own X-Logic take on their own console bus compressor is a viable alternative, as is their modular X-rack version. Former SSL designer Al Smart has also created his own versions, in the form of the Smart C1 and C2, the latter tweaked in the direction of more 'attitude'. The Rolls Super Stereo compressor is another option, and DIY fans might be interested in building their own clone, with Gyraf's GSSL project seeming popular. However, the BC1 isn't an out-and-out clone: it's also capable of sounding rather smoother and of more finessed results. If I were to list alternatives on that criterion alone, though, this box would take up the whole magazine.

Published January 2012