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Bellari RP583

Valve Compressor
Published June 2004
By Paul White

Bellari RP583Photo: Mike Cameron

This dual-channel compressor combines valve circuitry with an opto-compressor design. But does it give the best or the worst of both worlds?

Outwardly, the Bellari RP583 looks like a very standard two-channel compressor. Its brushed-gold, 2U front panel sports the familiar Output Level, Threshold, Ratio, Attack and Release controls for each channel, there are two moving-coil meters (that can be switched to read gain reduction or output level), and there's a Link button for compressing stereo signals. Each channel has its own bypass button and there's an illuminated rocker-style power switch.

A glance around the back confirms that life is pretty ordinary there too, except that the mains comes in via a captive cable rather than an IEC connector. There's balanced I/O on XLRs plus unbalanced I/O on quarter-inch jacks, with two further jacks per channel providing an insert point for the side-chain and also allowing the compressor to be triggered from an outside source — to produce ducking effects, for example. The construction is tidy and proficient without being overly fancy, and the use of large knobs and clear legending will be welcomed in dimly lit studios.

While the outside might suggest a very ordinary compressor, the circuitry reveals a number of 'classic' ingredients, specifically an optical gain-control element, that's used within a tube circuit with the aim of providing smooth and musical gain control rather than clinical accuracy. Each channel includes a 7025 dual-triode tube, where the opto circuit comes between the two tube stages.

The quoted frequency response for the unit is a healthy 20Hz-40kHz, where the balanced input impedance is 10kΩ and the unbalanced a rather high 1MΩ, which means you could plug guitars and basses directly into the jack input without getting an impedance mismatch. The signal-to-noise ratio is 90dB, and the value for total distortion plus noise is less than 0.1 percent under typical operating conditions. In use, no additional noise was evident when using this unit, other than where the act of compression brought up noise already present in the input signal.

Getting To Grips With The Controls

There's no in-depth explanation of the characteristics of this compressor — indeed, the manual comprises little more than a couple of lines covering each of the controls — but given that the gain control circuit is optical it's probably also somewhat nonlinear, which is what gives optical compressors their unique character. Once the channel is active, Threshold sets the level above which compression will take place, while the Output Level control is used to make up any gain lost due to the compression process. Ratio works in the usual way by determining how many decibels the input needs to rise in order to achieve a 1dB rise in output, but if, as I suspect, the opto-compressor has a somewhat soft-knee characteristic, then this will only be approximate. The Attack and Release controls set the time the compressor takes to respond and the time the gain takes to return to normal after the signal has fallen back below the threshold level, so there's nothing out of the ordinary there.

The Link switch combines the side-chain signals and puts both channels under the control of the lower channel's knobs (other than the output gains, which remain independent) so that the same amount of gain reduction will be applied to both channels even if the signals passing through them differ (as they do when the source is stereo). This prevents the stereo image shifts that occur when stereo signals are processed via independent compressors.

Inserting an equaliser into the side-chain makes the compressor frequency selective, which can be useful for setting up simple de-essing or de-popping, or other more subtle frequency-dependent effects. The insert send may also be used as an output before the Output Level control, while the side-chain insert can be used to feed in an external signal to control the compressor. The most common application of a side-chain input is ducking, where the level of voice fed in via the side-chain reduces the level of any music passing through the unit. Ducking is commonly used by radio DJs to ensure their inane chatter is heard above the music at all times, but it is also useful in the studio for pulling down the levels of instruments in the presence of vocals or solo instruments. As a rule, setting the threshold and ratio so that the ducked material drops in level by only a decibel or two is enough, and prevents the effect from sounding too obvious.

Cleanly laid-out Bellari RP583 rear panel.Cleanly laid-out Bellari RP583 rear panel.Photo: Mike Cameron

Golden Tone

I have to admit that I was pleasantly surprised by this compressor, because not only could it deliver the 'opto-compressor with attitude' effect reminiscent of earlier Joe Meek designs, but it was also capable of very tasteful mix compression. The secret to setting how much attitude you hear is really a matter of how high a ratio you use (and of course how far the input rises above the threshold) combined with the attack- and release-time settings — higher ratios combined with a slowish attack and a fast release results in an audible, but still rather musical, pumping effect. Lengthening the release time tames the pumping, while shortening the attack brings transients under control more quickly.

The only real way to evaluate how a compressor is affecting the sound is to set the output levels so that when you hit bypass there's no obvious level change. Having done this, I found the unit sounded rather better than expected for mix levelling — adjusting the gain so that the peak sections matched the bypass level resulted in the quieter sections becoming noticeably louder. There was virtually no tonal change when hitting bypass, which is also very important, and though compressing a mix hard does introduce some obvious compression artefacts, they are on the whole warm and musical. A little mild pumping can be useful to inject a sense of energy into an otherwise tame-sounding mix!

Used to process individual voices and instruments, the RP583 injects an endearing warmth and solidity into the sound, but again without compromising the original tonal integrity. It works just as well on drums and bass as it does on vocals, and, though not as transparent sounding as some compressors I've used when you start to push it hard, it still manages to enhance musicality, and it does so without deadening transients or adding muddiness. At more sensible gain-reduction levels, it sounds far more benign, but it's difficult to tell exactly how much gain reduction you are applying when the meters are moving-coil types rather than peak-reading LED meters.

Beauty Or Beast?

On balance I really liked the RP583, because it delivered what I wanted to hear with very little effort on my behalf, and it was also versatile enough to be used both for overt compression effects and for more subtle gain management. It isn't completely transparent, of course, and probably isn't designed to be, but it has the kind of smooth, musical character that I tend to associate with more costly units, and it is far more versatile than its few controls might lead you to believe. It works well on mixes, submixes, and individual instruments, as well as on voices. Although it's hard for a unit such as this to be a Jack of all trades and master of them all, the RP583 tries very hard to please. A good all-rounder at a sensible UK price.

Published June 2004