We get inside Focusrite's new compressor to find out what really makes it tick.
The Focusrite Platinum series of processors was conceived to put Focusrite equipment within the financial reach of project studio operators, but rather than simplify their existing designs, they've taken the unusual step of developing completely new units that have a different sonic character to their high end models. All the models released so far also include novel facilities for adding character to the sound, and the new Compounder is no exception.
The Compounder is a dual‑channel compressor‑plus‑limiter with an integral expander/gate section, but there's also a Bass Expander section that significantly increases the low‑end punch of the material being processed (see the 'How Low Can You Go?' box below). This is the section that puts the Pound in Compounder and I'll be looking at it more closely a little later in the review. Other than the Bass Expander, the basic description of the Compounder sounds like that of many competing products, but there are several design points and philosophies worthy of note. For example, instead of using a single VCA to provide the gain control for the expander/gate, compressor and limiter sections, the designers have opted to use a different type of gain‑control element for each section, thereby allowing them to have more control over the sonic character of the device. Normally, stacking gain‑control elements in series is frowned upon because off‑the‑shelf VCAs tend to add distortion and noise, which becomes cumulative the more of them you cascade. In the Compounder, however, the limiter and expander/gate sections are handled by photo‑resistor circuits, which don't contribute to the sound until they start to operate. In other words, if you're not limiting or gating, the circuitry is effectively passive and linear, like a resistor. When the circuit does operate, it has a smoother response than a typical VCA, so the likelihood of clicks when the gate opens and closes is much reduced. There's also a 200kHz audio bandwidth, which makes a significant contribution to overall transparency and clarity.
Some of the more upmarket Focusrite dynamics processors use commercial VCA gain‑control elements, but getting ones that do the job properly without contributing unacceptable levels of distortion is expensive. Rather than compromise by using a low‑cost commercial VCA in the Compounder, Focusrite have come up with their own hybrid Class A design that involves the use of a matched multi‑transistor element. While a regular VCA produces odd‑harmonic distortion, which the ear finds offensive even in very small amounts, the Class A design produces predominantly second‑order harmonics when overdriven, and the human ear can tolerate quite large amounts of even‑harmonic distortion before noticing anything at all. Even so, the distortion figure for the Compounder has three noughts after the decimal point for signal levels up to 20dBu, so you'd have to drive this machine pretty hard to get any significant distortion out of it.
Styled with the same silver front panel as the previous two units in the Platinum series, the 1U Compounder comprises two identical channels that may be linked to operate as a stereo pair, or used independently to process mono signals. Ground‑compensated, unbalanced jacks and balanced XLRs are provided on the rear panel for both inputs and outputs; the output jacks operate at ‑10dBV and the XLRs at +4dBu. The inputs operate at the same level, which may be switched from ‑10dBV to +4dBu by means of rear‑panel push switches, one for each channel. Two further jacks provide side‑chain key inputs to the gate section to allow external signals to control the gate in more creative applications, such as chopping guitar chords keyed from a programmed click or drum part.
The control panel is divided into two main sections per channel: the expander/gate and the compressor‑plus‑limiter. Taking the expander/gate first, this has just two knobs for Threshold and Release/Hold, but also has six buttons and a four‑LED gain‑reduction meter. Threshold sets the level below which gating or expansion will take place, as selected by the Expand button (the circuit operates as a conventional gate when this button is out), and when the Hold button is pressed, the Release/Hold control sets the time the gate stays open after the input (or external Key signal) has fallen below the threshold, after which it closes fairly rapidly. This is the mode used to create the well‑known gated reverb sound, though it has other uses! There is no variable attack control, but a button allows the gate to be set to a fast attack for working with percussive sounds, while the next button along allows the gate attenuation to be set to either 15dB or 70dB. The 15dB setting enables the gate to quieten pauses without silencing them altogether, while the 70dB setting effectively mutes all the audio during pauses.
Expand puts the circuit into expander mode rather than gate mode, so that gain reduction is increased progressively as the signal falls further below the threshold. This functions rather like a soft gate and may be more forgiving on things like vocal tracks. Two further buttons select the Key input as the gate triggering source and allow the entire section to be switched in or out. All the main Compounder function buttons have integral red status LEDs that show through the switch cover when the switches are depressed.
The compressor section is pretty comprehensive, and most of the controls will be familiar to anyone who's ever used a compressor before. There are knobs for Threshold, Ratio, Attack, Release and Make‑up gain, which is pretty standard, but then there are two more knobs to control the bass enhancement and the threshold of the separate limiter. The limiter threshold range is variable between 12 and 26dB to allow more precise control, and a separate bypass switch is provided for this facility. There's also a red Limit LED that comes on when the signal hits the limit threshold.
As well as the compressor In button, there's a switch to select hard‑ or soft‑knee operation and another to select an auto‑release time mode — useful if your programme dynamics keep varying. A five‑section LED meter monitors the compressor input up to 20dB, and the Ratio control actually goes up to infinity and beyond so you can get those classic pumping effects where loud sounds are pushed down and quiet sounds are pulled up above them. A further seven‑section LED meter shows the compressor gain reduction up to 24dB.
Bass Expand is simply variable from Flat to Fat, and generally starts boosting at around 50Hz. However, pressing the button labelled Huge pulls this up to around 100Hz, so that a wider section of the low end gets boosted. As a rule, this knob gives all the punch you need without even exceeding the first half of its travel.
For stereo operation, it's possible to link the compressor and limiter sections independently via two separate buttons. There's no link for the expander/gate, as in most circumstances you'll want these to remain independent.
As the Compounder is really three sections in one, it's probably best to look at the performance of each section separately. The expander/gate has a very positive action, and choosing a fast gate setting works perfectly on drums. As with many gates, setting a very fast attack time coupled with a fast release can cause the gate to chatter or to add distortion to low‑frequency sounds, but in practice, you'd rarely set the release time to its fastest. At its normal setting, the gate works without clicking on virtually any type of programme material, but for the gentlest results, the expander mode is the one to go for. This really handles your signal with kid gloves and is probably the best mode for vocals. The external key function works in combination with gate mode and Hold to produce gated reverb or chopped audio. This is a popular technique with dance music, where chordal guitar parts can be keyed from a programmed drum hit sound to turn the guitar into an almost percussive rhythmic instrument.
The compressor section sounds quite different depending on whether you use the hard‑ or soft‑knee setting. Apparently Focusrite modelled the soft‑knee characteristic after that of their high‑end Red compressors, so that it would sound smooth and transparent on complex mixes. This certainly seems to be the case — you have to use a lot of compression before any side‑effects are evident.
Hard‑knee has more of an attitude, and is the setting of choice for creating punchy kick drums or bass guitars. The sound is still rounded and musical, but punchier and more obviously compressed than in soft‑knee mode. Interestingly, the limiter seems very fast, despite the fact that opto circuits are known for being a little on the sluggish side, and checking with the designers confirmed that they have incorporated lots of feedback to sharpen up the response. Normally, the limiter is set just below the clipping threshold of the destination device to catch errant peaks, but this one sounds very '60s if you deliberately drive it into hard limiting. The effect may not be everybody's cup of tea, but it's another creative tool if you want it.
I've left the bass enhancement until last, because it's the one feature that you wouldn't expect to find on a compressor. On the normal setting, you'll need good monitors to hear the result properly, because the enhanced bass creeps in at around 50Hz and below — the point where many nearfield monitors lose sensitivity. This is the setting to use for dance mixes destined to be replayed over large systems with bass bins. Pressing the Huge button extends the effect further up the spectrum so that the results can be heard on typical small speakers, but in either case, what's added seems cleaner and better controlled anything you could manage using EQ alone. You need to be a little careful in how much you dial up, but used sensibly, it can add significant warmth and weight to the bottom of a mix without messing up what's happening in the mid‑range.
Compressors are personal things, but Focusrite have tried very hard to make this one suitable for transparent gain control as well as for more obvious 'beefing up'. The signal path is extremely clean and doesn't suffer the high‑end blurring of some designs, nor is the compression over‑obvious in soft‑knee mode. The separate limiter works very well, as does the expander/gate section, though with both the gate and the compressor, when using a fast attack, you have to be careful not to set such a short release time as to cause audible distortion. The auto‑compression mode handles inconsistent sources very well, including vocals and bass guitar, while the Bass Enhancer will appeal to anyone wanting to add weight to their sound.
Given the competitive price of the Compounder, there are no real weaknesses and it's certainly versatile. Because all compressors have their own character, there is no obvious equivalent, so if the Compounder fits your budget, make sure you give it a test drive to see what it can do for you.
So, what about that mysterious Bass Expansion circuit? When I first heard the Compounder, I thought the unit was generating artificial sub‑bass frequencies, but (as the designer explained to me) this is not what actually happens. Essentially, the enhancement circuit comprises an inductor (coil)‑based low‑frequency filter that allows some of the bass energy to bypass the compressor circuit and hence avoid gain reduction. This may seem to defeat the object of a compressor, as the bass sounds are the ones where all the energy resides, but you don't always want to use a compressor just to control gain — you may want it to make the material sound compressed in an interesting way. Normally, because the bass sounds contain so much of the energy in a typical mix, the compressor works hardest on bass notes or kick drums, and this can actually rob the mix of some of its punch. By sneaking some of the bass past the compressor, the low‑end energy can be made proportionally higher than the rest of the mix, which is still being compressed. The nature of the inductor‑based filter also causes hard‑driven bass sounds to pick up even‑harmonic distortion, much as they would in a heavily driven valve circuit. This increases the level of subjective bass.
Inductors also store energy, so the bass sounds may also get 'stretched', which would increase their average energy and hence their perceived level. The graphs provided with the unit show a slight dip in the frequency response an octave or so above the point at which the filter is adding bass energy, most likely due to phase cancellations that occur when the filtered (and hence phase‑shifted) signal is added back to the original. This helps keep the bass sounding tightly controlled, and is not unlike the effect of the bass control on an SPL Vitalizer. Furthermore, the effect of the filter combined with the compressor gives the bass end a degree of dynamic control, as though it's passing through an expander.
- Comprehensive control system, including bass enhancement.
- Clean, musical signal path.
- Separate expander/gate, compressor and limiter sections.
- The inexperienced user could get the unit to distort by setting very fast attack and release times. However, the manual does include typical settings for specific applications, so this shouldn't be a serious problem.
The Compounder is a powerful, flexible and well behaved addition to the Platinum range, and its bass enhancer is a genuinely useful bonus.