Groove Tubes CL1s

All the warmth of valve circuitry, and it's shiny on every surface too. Paul White tries out a hot piece of kit with tubes that groove...

Groove Tubes' main business is selecting and marketing valves for (mainly) musical applications, but they also build their own guitar amplifiers, valve microphones, power amplifiers and signal processors, and — with the exception of one forthcoming new microphone model — everything they do features a valve signal path. The CL1s compressor/limiter maintains this tradition, and features the characteristic Groove Tubes mirror‑chrome finish. Because of the valve circuitry and the inclusion of an on‑board power supply, this particular compressor is almost as deep as it is wide, and every square inch of the surface is chromed, not just the front panel. The result is a piece of equipment that is as weighty as it is visually imposing.

Judged solely on its control functions, the CL1s is a fairly conventionally set‑out dual‑channel compressor/limiter, with the option of channel linking for stereo use. The illuminated moving‑coil meters may be switched to read output level or gain reduction, and apart from the Ratio control, which is switched rather than continuously variable, everything looks comfortably familiar.

To provide maximum flexibility, the inputs are on combined jack and XLR connectors; the outputs feature discrete XLRs and jacks side by side. A two‑way sensitivity control on the front panel helps match the unit to any likely line‑signal source, and further rear‑panel jacks are provided as side‑chain insert points, allowing an equaliser to be patched in for de‑essing purposes. Special soft‑start circuitry has been included to extend tube life, and each channel uses three dual‑triodes: one 12AX7 and a pair of 12AU7s. The gain‑control element, though, is based around a photo‑electric circuit with a view to recreating the vintage '50s sound.

Interestingly, Groove Tubes founder Aspen Pitman has his own theory on the tube sound, and suggests that, except in guitar amplifiers, it has little to do with harmonic distortion. As he rightly points out, the designers of classic valve studio gear did their level best to design out distortion, yet it still sounds wonderful. Aspen puts this down to the valve's ability to handle steep transients more effectively than typical solid‑state circuits can, the argument being that this leads to a more detailed, lively and generally more transparent sound.

The input signal makes an unbalanced transformerless coupling to a 43kΩ impedance tube input stage, while the output feeds via a balanced Reichenbach transformer, to provide a fully floating output capable of feeding loads down to 600Ω. The nominal operating level is +4dBu; on the Hi setting this drops to ‑10dBV.

The CL1s has the advantage of immediacy, plus a stunningly flattering sound.

Each channel has its controls mounted on a separate sub‑panel, the reason being that a mono version is also available with a blanking panel fitted to one side. Other than the switched Ratio control (12, 8, 4, and 2:1), there's the usual complement of Threshold, Output (make‑up gain), Attack and Release knobs, none of which has calibration marks. There are recessed toggle switches to set bypass on/off, Low/High sensitivity, VU or GR meter switching and Stereo Link, this last fitted only to the left channel. All the knobs are tastefully vintage without being completely over the top, and the overall impression is one of sleek simplicity.

Sounding Groovy?

There's something distinctive about the sound of an opto‑driven compressor that makes it warm and punchy, yet obviously 'processed'‑sounding in a flattering way, and the CL1s manages to combine this character with the transparent warmth of valve circuitry. Applied gently, the compressor can be used for routine gain control with the minimum of artifacts, but it's only when you start to pile on the gain reduction — ideally using a fairly short release time — that things get really exciting. There's no attempt to disguise the pumping that occurs at high levels of compression, but the effect is wonderfully authentic and comes very close to the true vintage sound without the true vintage noise.

Used on vocals or guitar, a modest amount of gain pumping makes the sound seem more powerful and exciting, yet the valve circuitry seems to add a further dimension to the sound, which I find very appealing. It's probably fair to say that this is a compressor you'll want to use as an effect rather than as a routine gain‑control device, but it is capable of good behaviour if you want fairly transparent compression.

After the unit had been on for a couple of hours, the lid was almost too hot to touch, so it is important to leave an empty U or two above and below the CL1s to allow ventilation. I know that valve gear runs hot, but in this case you really do have to think carefully about where you put it.

Summary

For me, the Groove Tubes CL1s is up there with the best of the esoteric valve compressors; while some may have more features, the CL1 has the advantage of immediacy, plus a stunningly flattering sound. Operation is simple, and the lack of control graduations doesn't really cause any problems, as it's pretty obvious what's going on from the sound coming back over the monitors. Of course, a hand‑built valve unit such as this one is going to cost rather more than a hybrid design with just one or two valves in it, but top‑end equipment is never cheap. On the other hand, with a little care, a unit like this one is likely to last a lifetime — all you need to do is change the valves from time to time, and not very often at that.

That's probably as much as I can tell you without your hearing a unit for yourself, because, although the CL1s compares well with other premium‑priced valve compressors, that isn't the same thing as saying that they sound indistinguishable. Just as high‑end large‑diaphragm capacitor microphones sound different, so do valve compressors — but if you like the sound of valves, and you like to be able to hear your compressors working, this one is well worth spending some time getting to know.