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Two-channel Compressor
Published June 2013
By Paul White

ART Pro VLA II optical compressor.

This optical outboard compressor is cheaper than some plug-ins. What's the catch?

ART describe their Vactrol-based optical compressor circuit as being particularly natural sounding, and that claim is not far from the truth. Like any optical compressor, it tends to deliver a degree of sneaky flattery, and when pushed harder its contribution as an outright effect becomes rather more apparent, but it is capable of behaving very politely too.

One device that uses this circuit is the Art Pro VLA II compressor reviewed here. This device has a soft-knee compression characteristic that sounds pretty transparent at moderate gain-reduction levels, but with a subtle musical warmth added by a 12AT7 vacuum tube in the otherwise solid-state, transformerless signal path. Push it harder at higher ratios and it can inject a musical-sounding 'bounce' into the mix, so its applications can vary from transparent gain control to lively gain pumping.


Inputs and outputs are thoughtfully presented on both jack and XLR connectors, but there's no external side-chain input.Inputs and outputs are thoughtfully presented on both jack and XLR connectors, but there's no external side-chain input.

Constructed in a 2U rack chassis, with an elegantly sculpted front panel, the two-channel Pro VLA II incorporates a pair of large, backlit analogue VU meters, which are switchable to display input or output levels. There are separate 10-segment LEDs below, which monitor the compressor's gain reduction. Further LED meters in the centre of the unit follow both the average and peak output level, with the peak LED holding its value for a second or so. Both XLR and quarter-inch TRS balanced inputs and outputs are provided, with +4dBu/-10dBu switching for connecting to systems operating at either level, and both the inputs and outputs have enough headroom to operate at up to +20dBu when switched to the +4dBu setting. A stereo link button means that the unit can function either as two separate mono compressors or as a stereo compressor.

The control layout is pretty conventional, with threshold, ratio and output-level knobs positioned above smaller attack and release controls on both channels. Both the attack and release are always set manually — there's no auto mode. There's also no 'mix' or 'blend' knob for managing parallel compression — not an essential feature, but I expect to see it on more hardware compressors these days, given the interest in that particular processing technique, and the fact that fewer people are using consoles where this can be easily set up. You can usually rig up suitable routing within your DAW to do that, though, as long as your DAW is accurately compensating for your interface's inherent latency. Also absent is any form of external side-chain input or a switchable high-pass filter for the side-chain, so I suspect that the design brief was to come up with a straightforward, easy-to-use, no-frills compressor.

In stereo mode, the left-channel controls act as the master set. However, the output level controls on both channels are used: one governs the output level, and the other the stereo balance, which is a neat and economical way of doing things. Illuminated buttons select the VU source as input or output for each channel, with the bypass button being immediately to the right. In stereo mode, the same amount of gain reduction is always applied to both channels, to avoid any undesirable shifting of the stereo image. With a ratio range of 2:1 up to 20:1, the compressor can go from relatively gentle compression right up (virtually) to limiting, making it suitable for both individual track compression and bus processing applications, and potentially useful for DIY mastering duties too.


The controls on the left channel act as the master set in stereo operation, but the output level knobs on each channel are both used, one for level, and one for stereo balance.The controls on the left channel act as the master set in stereo operation, but the output level knobs on each channel are both used, one for level, and one for stereo balance.

I tried the Art Pro VLA II on a number of different sources and was pleasantly surprised. On vocals, it was possible to apply a surprisingly lavish amount of gain reduction before it became too obvious — the tonality and dynamics appeared natural, although the overall gain excursions were actually being contained very effectively. On bass, I was able quite easily to create a more solid, even sound without giving the game away, but using shorter release times, coupled with higher levels of gain reduction, I could also induce some gain pumping when I felt that was desirable. The situation was similar for drums — unless I deliberately got too heavy-handed, I was always greeted with a more even, more polished sound. The Pro VLA II's generous metering also deserves a mention, as it makes setting up very easy.

In stereo mode, I found this compressor to be very kind to complete mixes and submixes; it was only when I hit it very hard that any top-end loss started to become noticeable. At low to moderate gain-reduction levels, I was hard pushed to hear any difference between the general tonality of the compressed sound and the bypass position.

Going beyond about 15dB of gain reduction at higher ratios, a little top end loss did become evident, simply due to the bass end dominating the overall gain reduction. You don't usually need to apply that much gain reduction, of course, but the inclusion of an external side-chain input, or of a side-chain high-pass filter, perhaps, would have allowed you to do so. Similarly, although fast release times can, as I said, be used to create rhythmic gain pumping, the lack of external side-chain input means that this compressor can't be made to respond only to the kick-drum track when you're 'squashing' a dance mix. In its favour, the Pro VLA II has a very low noise floor, even when you're compressing hard.


For general compression duties, despite costing less than some software plug-in compressors, the Pro VLA II turns in a very capable performance. The soft-knee characteristic, combined with the optical gain-control circuit, make it capable of very transparent, natural-sounding gain control. The Vactrol gain-control element is somewhat less wayward than most of the vintage optical designs, which used discrete lamps and photocells, but it's still capable of imparting a very obvious optical character at higher ratios, when more gain reduction is being applied. Despite the one or two omissions mentioned above, then, its price, quality of performance and ease of use mean that the Pro VLA II must be considered something of a bargain.  


Affordable compressors are not in short supply, but there are fewer with optical gain controls, and while the 'esoteric' category is buoyant enough, there are fewer affordable designs than there used to be. If you can afford to be near the top of the food chain, check out Universal Audio's Teletronix LA2A, but for a good-quality device that won't break the bank, you could look at the various offerings from JoeMeek.

Published June 2013