Paul White checks out a modern compressor/limiter designed to emulate the warmth and transparency of cherished vintage models.

Though it's easy to comprehend what compressors do to your music, what is harder to figure out is why they all sound so different — after all, essentially all a compressor does is monitor a signal level and progressively turn it down when it starts to get louder than it should be. One factor that undoubtedly affects the sound of a given compressor is the type of gain reduction element used, and there are plenty of options available if you're designing a compressor, even before you start to look at digital methods of gain reduction. There are dedicated VCA (Voltage Controlled Amplifier and Voltage Controlled Attenuator) chips from a variety of different manufacturers, all using different circuit topographies — then there are discrete circuits using FETs, opto-electronics, transistor arrays and, of course, valves. Interestingly, the compressors that engineers seem to like best aren't the ones that are the most technically perfect but the ones that colour the sound with subtle distortions during the compression process.

Once the gain element has been decided upon, there's the question of whether the compression system should be hard-knee, soft-knee or both. Soft-knee systems generally produce a smoother, more musical gain control, but on the other hand, the more heavy-handed approach of a hard-knee device is very useful when you're trying to be creative with sound rather than simply trying to keep it under control.

The design of the sidechain circuitry which monitors the input signal and derives the control signal is absolutely crucial, and even in this area, there are plenty of options confronting the would-be designer. The control law of the sidechain circuitry will affect the way the compressor sounds — should the control be linear, logarithmic or somewhere in between? There's even a choice of whether to derive the sidechain signal from the compressor's output or its input, and then should any sidechain equalisation be introduced or should it be left flat?

If you monitor the output signal and use this as the reference for gain control, you have a kind of negative feedback system, and by varying the control parameters, it is possible to apply anything from a mild degree of compression right up to hard limiting, where the signal level can never exceed the threshold set by the user.

If, on the other hand, the input signal is chosen as a reference (Feed Forward gain control), you can go beyond limiting and set the controls so that the signals that were originally loud are now quiet and the original quieter signals come out loudest. This reversal of dynamics isn't particularly useful when applying routine gain control, but can provide the basis for some interesting, creative effects.

Having briefly outlined some of the variables, it remains only to point out that further tonal differences are evident between compressors with valve and solid-state amplification after the gain-control stage, and that models with transformer-coupled outputs sound different again.

The LA Classic

Housed in a solid-looking, 2U rack case, the Classic Compressor is fitted with two large moving-coil VU meters which helps create the 'vintage equipment' image. However, the giveaway is that all the knobs are modern plastic types, and I really feel that LA Audio have missed out on the opportunity to use fluted 'Bakelite'-style knobs, which really would have looked more in keeping with the retro styling.

The rear panel is simply appointed with balanced XLR input and output sockets, and sidechain access points are on stereo jacks, allowing equalisers to be inserted into the sidechain signal path for de-essing applications and so on. Mains power is via a conventional IEC mains socket and the signal outputs are transformer-balanced, which helps keep ground loop problems at bay. The system may be used unbalanced by connecting the cold pin (pin 3 on the input and output XLRs) to ground (pin 1).

The Classic Compressor's circuitry is entirely solid-state, but the circuit design, combined with the use of output transformers, does help it achieve that elusive warmth that is normally associated with valve designs. FETs (Field Effect Transistors) are used in the gain control circuitry and these can be coaxed into a mild but musically pleasant distortion (a little over half a percent) during compression, which undoubtedly adds to the charm.

The control layout is very simple, each channel having the bare minimum Input, Output, Attack, Release and Ratio controls. As is normally the case, the two channels may be used separately or as a stereo pair. A failsafe circuit links the input to the output in case of an unforeseen power failure or fault, and the Bypass button also links the input directly through to the output when activated. The failsafe function has a built-in timer to prevent the bypass from chattering in and out of circuit in the event of an arcing mains plug or similar fault — thoughtful!

"Comparing the processed and bypassed signal reveals a very subtle, but nevertheless evident, brightening of the sound, almost like the mildest touch of 'exciter'."

Classic Controls

Unlike many compressors which employ a variable threshold, this model has a fixed threshold and instead uses an Input Gain control to vary the input signal relative to the threshold. This system is fairly common and, from the user's viewpoint, operation is similar. Because varying the input level will also vary the output signal level, an Output Gain control is provided to allow the user to add the required amount of make-up gain.

The Attack control sets the rate at which the Gain Reduction Amplifiers respond to a signal peak exceeding the threshold, and this may be varied from 22 microseconds to 20mS. Percussive effects can be enhanced by selecting a long attack time — this will allow the leading edge of transients to pass through unchecked, producing a percussive click. Faster times are used when the gain needs to be brought under control virtually instantaneously.

Release sets the rate at which the gain returns to normal once the signal has fallen back below the threshold, and this may be varied from 40mS to 4S. Most normal applications require a Release time of between 0.5 and 1S.

The ratio control is not fully variable as on most modern compressors but is, instead, a rotary switch providing eight choices of ratio plus an Off position. The available ratios are: 1.3:1; 2:1; 3:1; 5:1; 8:1; 12:1; 20:1; 30:1; and Off. The Off position switches out all gain reduction and can be thought of as a 1:1 ratio setting. If anyone is wondering why the Ratio switch needs to have an Off position, it will shortly be explained — the designers had a cunning plan!

Both channels may be ganged using the A/B switch, at which point a green status LED is illuminated and channel A's controls serve as master controls. The Input Gain controls remain independent, allowing any imbalance in the stereo signal to be corrected, but the sidechain signals are combined and the same degree of gain reduction is applied to both channels.

If the unit is set for linked operation and one of the channel ratio controls is set to the Off position, the channel controls become unlinked and both sets of Gain Reduction Amplifiers are controlled by the channel that is still on. This allows the unit to be used for ducking and similar applications, where the dynamics of the input on one channel control the level of the signal passing through the other channel. The signal being controlled (for example, music) is passed through the channel switched to Off while the controlling signal (usually a voice) is patched into the channel still switched on. When the voice signal exceeds the threshold, gain reduction will occur on both channels, the attack and release characteristics being determined by the settings of the Attack and Release controls. This will result in the music being 'ducked' to make way for the voice-over, the amount of ducking being set by means of the ratio and Input controls. However, setting up a ducking patch this way limits the processor to controlling a mono signal, so it may be more useful to feed the controlling signal into the sidechain insert returns. In linked mode, a signal appearing at one insert return point will influence both channels.

A single button switches the two meters as a pair to monitor either the gain reduction or the actual output level — again a pretty common feature.

Sound Effects

After only a little experimentation, it becomes evident that this is a compressor designed to make its presence heard. Even when the input/threshold relationship is set so that no gain reduction at all is taking place, comparing the processed and bypassed signal reveals a very subtle, but nevertheless evident, brightening of the sound, almost like the mildest touch of 'exciter'. This may well be due to the harmonic distortion around the FET circuitry in the gain reduction stage, but I must say I like it.

"When you're trying to create a high-energy dance track, the Classic Compressor certainly pulls the sound together and, properly set up, helps to lift the beat."

When the compression is brought into play, subtle settings keep the gain nicely under control without strangling the sound, whereas more severe settings can be used to really pump energy into a voice, instrument or mix. One of the factors that impressed me most was how effective the Attack control can be. When treating a mix with a heavy snare beat, setting a fast attack brought the level of the snare beat right down, but setting the Attack at around the 'quarter to' position lifted the snare quite significantly, making it really stand out. More considered listening tests showed that it did the same for all percussive events; the character of the hi-hats and bass drums in a mix can be changed to a surprising degree simply by varying the attack time. I know this is something that's supposed to be true of all compressors, but on this one, you can really hear it happening!

I feel that this unit is at its best when treating vocals, percussive sub-mixes or complete mixes. It's not usually my practice to compress finished mixes, but when you're trying to create a high-energy dance track, the Classic Compressor certainly pulls the sound together and, properly set up, helps to lift the beat.

Character Reference

Working with this compressor is a bit like watching a good magician doing a card trick — you know you're being fooled, but you're not quite sure how. There are cheaper compressors that do just as good a job on routine gain control tasks, but when it comes to a little showmanship, the Classic really comes into its own. It's not the only good 'character' compressor on the market by any means, but most of its rivals cost more or only offer a single channel of processing. In fact the only other stereo compressor that offers a challenge in this price bracket is Drawmer's 1960 model, which uses a valve stage rather than FETs to introduce the harmonic subtleties, and also has integral mic preamps and EQ. As with any of these things, you really need to hear one working, because me saying it sounds 'good' only tells you that I like it. And with compressors, as with microphones, there are many degrees and shades of 'good'.

Of course nothing is perfect and my main criticism of the Classic Compressor is that the Attack and Release controls have no time calibration marks on them. I know you're supposed to set these things up by ear, but I usually have a good idea of what value I'd like to start with, and not knowing which part of the dial represents a half second release time doesn't help. I also feel quite strongly about the knobs — buy real Bakelite ones please, not these tacky, modern plastic things!

Minor gripes aside, this is a very nice sounding processor, it's easy to use and it has a variety of applications aside from the obvious vocal levelling. It costs more than your average compressor, but then it does have a distinctive sound, and just as people will pay a lot for a mic with a particular sound, there are those who feel just as romantically attracted to compressors.

I was quite impressed by this compressor, and apparently I'm not alone. Several have been sent out for evaluation by top engineers and producers, and a surprising number have sent in cheques instead of returning their units. Whether LA's new product lives up to its name and eventually becomes a classic is something I can't predict, but if you're looking for a vocal compressor with character, you really should check this out.

Further Information
LA Audio Classic Compressor £934.13 including VAT.

LA Distribution Ltd, (Contact Details).