A compressor is one of those devices better classed as a tool than a toy. A bad one is worse than useless and a good one can become invaluable. The Vesta Fire SL-020 dual compressor/limiter is definitely not a toy although this writer had great fun playing with one.

The original form of compressor was the human hand whipping down a fader when things got too loud. This method is okay as far as slower, scored music is concerned, where the recording engineer is able to predict a loud passage in advance, but as we look to recording improvised jazz and unpredictable rock we see the human hand lagging behind on the draw. With compressors we now have electronic hands on the faders to do the job and the speed of the electronic hand can deceive the ear.

A good compressor can be put to more uses than simply reducing the power of peaks in an audio signal in order to prevent tape saturation or circuitry overload, and we will mention some of these as we look at the SL-020.

Housed in the slimline 1U 19" rack-mount style with additional feet for free-standing use, this professional piece of equipment would not look out of place in any studio.

As the name says, it is a 'dual' compressor, meaning it can be used as two totally independent compressors or in stereo mode selected by a slide switch on the far left of the front panel. By selection of the stereo mode and equal setting of the controls on both channels the stereo image is maintained on any stereo programme material regardless ot peaks in either channel which, if using a pair of mono units, would cause the stereo image to shift to one side. Also with the unit switched to stereo, but the controls not necessarily set equally on each channel and often with the outputs summed back to mono, this facility can also be used to advantage for voice-overs or when the appearance of a specific signal is required to be dominant in a mix.

Complete Control

Each of the two identical channels on the SL-020 has a comprehensive set of compression controls.

First is the 'Attack' control. This sets the time that the circuitry takes to act on the input signal. It is rotary adjustable from 0.5 to 50 milliseconds, a range to suit the vast majority of applications. The partner to the attack control is the 'Release' which sets the time taken for the compression to recover and is adjustable from 0.3 to 5 seconds. A range as wide as this is necessary as the release timing must be carefully set to match any type of programme material be it a single instrument, voice or complete mix.

The setting of 'Attack' and 'Release' times on a compressor is a technique that is learned through experience. It is more subtle than deciding how much chorus or echo you require on a sound and the scope offered by the SL-020 should enable you to gain this experience as independent control of all the compression parameters are available.

The third pot along sets the 'Threshold' - this is the level of signal above which the compression comes into operation. There are two ranges of threshold setting on this unit and they are selectable via a slide switch adjacent to the jack sockets corresponding to each channel on the rear panel.

With the switch in the Low Input Attenuation position, the threshold range is adjustable from -40dB to -20dB. In the High position it operates from -20dB to 0dB. It can be seen from these figures, and is also borne out in practice, that the SL-020 will operate effectively on signals from microphone to line levels. Using it on a bass guitar I was able to get that satisfying punching sound, with the release carefully timed to the decay of the strings to give a degree of extra sustain.

The next pot on each channel is the 'Ratio' control - this sets the degree of compression applied to the signal once it has passed above the threshold level. The ratio is adjustable from 1:1 to Infinity:1. A compression of 1:1 is no compression at all, whilst a compression of infinity:1 is known as limiting, hence the SL-020 being called a compressor/limiter.

Limiting implies that for any increase in input signal amplitude that occurs above a given threshold, no increase in output will result. Limiters will often be seen in use with PA systems to ensure that signals of sufficient size to blow the speakers do not get as far as the amplifiers. In this way, a limiter can be used as a protection device.

Applications

A far more suitable use for a compressor with the capabilities of the SL-020 is that of attenuating just the peaks of a given musical signal. With its threshold set just above the average level of some background vocals on a multitrack, and a compression of about 10:1, the SL-020 enabled their level in a mix to be more easily positioned without the level of their corresponding fader having to be adjusted to compensate for the natural variations in volume with pitch.

With reference to the earlier passage concerning careful setting of 'Attack' and 'Release' times, this compressor was also admirably capable of adding a degree of punch to a previously recorded rhythm track without the results sounding at all unnatural. The SL-020 is capable of more than 30dB of compression which proved to be ample in each case. It also has a quoted signal-to-noise ratio of 90dB. A good S/N ratio is particularly important in a compressor because as peaks are attenuated enabling overall level to be increased, the noise level must also be raised.

The final control on each channel of the unit is the 'Output' which provides a lot of signal gain, but noise never becomes a problem.

After the output control on each channel is the effect on/off toggle switch. The switch itself is flattened and tapers outwards from its base. Silly point, I know, but I love it and I am not being facetious when I say it is one of the most satisfying toggles I have come across! The switch has an associated greed LED to show when the compressor is either in circuit or being bypassed.

Above this switch is a row of six small red LEDs which indicate the amount of gain reduction being applied to the input signal at any given moment. They range from 4 to 24dB and are very effective in confirming what you are hearing in conjunction with A/B comparison using the toggle switch, when setting up the compressor.

The mains on/off switch on the far right completes the front panel controls of the SL-020. It is an illuminated square red pushbutton protected against accidental operation by a raised surrounding guard. This is also a nice design touch.

Round The Back

On the rear of the SL-020, each channel has a row of four standard quarter inch jack sockets. The outside pair are the input socket (with its associated high or low attenuation slide switch) and the output socket. Between these two are the 'Detector Loop' input and output sockets. If an equaliser is connected here the compressor's frequency response can be influenced by the equaliser's dictated frequency characteristics.

For example, with the bass end cut on the equaliser the compressor will be triggered less readily by the bass drum in a piece of music. Alternatively, with the top end boosted, the SL-020 can be made to operate akin to a de-esser unit, reducing the power of sibilants ('s' sounds) in a vocal.

The final item on the rear panel is a ground lift switch to remedy any earth loop which may occur when the SL-020 is used with other auxiliary equipment.

Appraisal

It is always a pleasure to use a piece of equipment that is so capable of doing its job that it makes life in the home studio easier. It can so often be a battle with processing equipment; the more you use, the more you need. The Vesta Fire SL-020 dual compressor/limiter, however, is one piece of equipment that can solve problems without creating new ones. It can give punch and control in all the right places and with the minimum of fuss. It is wired to be used either balanced or unbalanced simply according to the configuration of the input and output jack plugs you use.

At a RRP of £294, whilst not being the cheapest on the market, it turns in a performance that gives some of the more expensive competition a very good run for its money.

If you are in the market for a good quality compressor this Vesta Fire offering is certainly one to try. I think you will like it.

The SL-020 retails for £294 including VAT.

Further details from: MTR, (Contact Details).