The 101 of the CompIQ Line of Compressors

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First developed by David Blackmer of dbx Inc., the original dbx 202 “Black Can” VCAs, can still be found in operating consoles today. These first VCAs suitable for pro audio equipment were built with a gain cell of eight discrete transistors. Later development of these IC’s surpassed all inconveniences of earlier designs, now rendering superior performances. CompIQ series of compressors use THAT Corporation Blackmer^® VCAs which are characterized by an exponential control characteristic (gain varies directly in decibels), extremely wide dynamic range, and low signal distortion. They are particularly neutral in sound, adding little or no coloration to audio signals.

Invented by David Blackmer of dbx Inc., the RMS-level detector computes Root Mean Square level of input signals in a logarithmic form, similar to how human ear perceives sound. The sound envelope decoded by this accurate detector is used to apply dynamic processing of the sound – the variation of signal level precisely controlled by the VCA according to parameters set by the user (Ratio, Threshold, Attack, Release, and Gain).

The threshold’s 50dB of range is designed to accommodate very weak signals up to pro audio level signals. The +4dB pro signal line level is at the top of this range, and consequently at the far clockwise end of the threshold knob. This gives a huge amount of headroom for the instrument’s generated signals, which ensures the compressor is not distorting with high-level spikes. We didn’t want that, by design. Some other compressors distort a lot, for various reasons, and headroom (or lack of it) is one of them. The CompIQ line of compressors is made to accommodate either lower OR hotter signals, and that is the reason why it can also be used with synths or other line-level devices. We wanted that so that we cover a broad spectrum of usage. As opposed to this kind of design are the compressors which have a very low threshold set hard within the circuit, and they would be controlled through “compression variation” only. This is equivalent to setting our compressors with the lowest threshold, and then vary the compression amount with the Ratio control.

One other fact to point out is the range of a pickup signal, which is in the lower 25% of the pro signal level. This falls as well within the first quarter of the Threshold knob’s rotation range. The compression, or limiting, should only occur on peaks and for that matter, the optimum threshold point for a pickup is also in the lower setting on the Threshold knob, maybe between -30 to -20dBu (by design, this is also the reference level of the internal circuit). Around 8 or 9 o’clock, you are more than halfway within a pickup signal range. If you are to compress just peaks, you would set the threshold knob at about 9 o’clock or slightly above. At this level, a higher compression ratio makes no sense, unless is limiting you’re after. If you want to have a more audible feel of the compression, you would set the threshold knob below 9 o’clock, and the lower you go counterclockwise the smaller compression ratios you should use so that the pickup signal is not squashed too hard. Unless you want to use the squash as an effect! And here comes the “New York compression style” which means compress with a high ratio and low threshold, and mix the compressed wet signal with the dry signal.

The Side Chain Filter is a frequency-dependent feature which allows a change in the compression triggering behavior.  The purpose of it is to delay the start of compression triggered by the high-amplitude low frequency content of the audio source and clamp higher frequencies too early, which may result in a muffled sound with less highs. This happens by the way the high-pass filter in the side chain modifies the compressor threshold, which is no longer a straight line applied uniformly to the entire frequency spectrum, but a sinuous curve which applies different thresholds to different frequencies in the audio spectrum. When using this feature, the same frequencies filtered out in the side chain control circuit end up being louder and less compressed at the output of the compressor. This results in a type of compression which has a particular sound characteristic perceived as more “full and natural” or “fatter and punchier” to the ear.

In CompIQ Stella, CompIQ Mini, and CompIQ Twain the SCF is switchable for a pre-set amount of low-cut of the triggering frequencies. The Normal option provides a general-purpose type of compression response which is frequency-compensated to resemble the human ear perception of sound, while Low and Deep options add a cut of -12dB@90Hz and -12dB@200Hz (-12dB@130Hz for the CompIQ Mini) on top of the Normal side chain curve, making it more suitable for bass-rich input signals.

The SCF processing allows for removing the pumping usually associated with the usage of “high ratio / low threshold” type of compression. The SCF provides a slightly different compression “feel” when compared with other methods of pumping removal such as raising the threshold, lowering the ratio, using soft-knee compression, blending dry over wet signal, or a combination of all or any of these controls. Of course, the SCF can be combined with all the other controls, giving an even greater flexibility in the way the compression is applied to different audio content.

Frequency Compensation  refers  to  the shape of the audio spectrum presented to the Side Chain Detector. Due to the nature of audio in general and musical instruments in particular, each musical note has a dominant frequency plus harmonics. The dominant frequency is always higher in amplitude than its harmonics. As musical notes fall lower in the audio spectrum (say notes on the lower strings in a guitar), their dominant frequency have bigger and bigger amplitudes (as opposed to notes on higher strings). That amplitude has the potential to trigger compression too early, and as a result, they may over-compress the harmonics or higher notes. This is usually heard by the human ear. To overcome this, we compensate for the low-frequency triggering potential, by progressively delaying it towards 1KHz, as the graph below shows. This is similar to say we apply a progressive threshold, where lower frequencies see a higher threshold than higher frequencies which see a lower threshold, proper for their lower amplitude. This type of progressive compensation helps prevent the “pumping” often encountered in compressors and makes the dynamic processing feel more natural. That is especially true for percussive instruments or for instruments rich in low frequencies, like bass.

The Normal side-chain roll-off curve above is particular to all our compressors,  providing a general-purpose type of compression response that corresponds to how the human ear perceives the sound. In CompIQ Stella and CompIQ Twain the Side Chain Filter has two additional options: Low (-12dB per octave at 90Hz) and Deep (-12dB per octave at 200Hz) on top of the Normal curve. The Deep SCF on the CompIQ Mini is set to free up -12dB per octave at 130Hz – a somewhere in between setting which should serve well both guitar and bass.

When SCF switchable options are combined with Knee selection, the variable Threshold, and the dry/wet Mix, they can all fully compensate for the absence of a variable control (very difficult to fit into such small enclosures which we want to preserve).

The CompIQ Twain features a variable 70Hz to 1KHz Linkwitz–Riley type of crossover which splits the input signal into two frequency bands which are processed independently by the two compression engines. The output of the crossover also feeds the Dry Line, so mixing the Dry and Wet signals is possible without phase cancellations, regardless of the crossover’s set point.

Below is a plot showing matched external and internal circuit levels with the crossover set at 1KHz, the output set at the buffer level, and Mix set to 100% Wet. As you can see, the phase of each signal component is almost perfectly aligned in the audio spectrum.

It is worth noting that while the input signal’s phase (dotted green) is a straight line, the output signal’s phase (dotted red) is progressively twisted from lows to highs (from almost 0° on the extreme lows up to 400° on the extreme highs). This is normal and is the result of the signal being separated by the crossover’s band filters, and then being re-combined at the output, after passing the compression engines. As a result, when switching from Bypass to Effect, the ear perceives the frequency delays although there is no audible loss of frequency throughout the audio spectrum.

Bellow is a drawing showing the Crossover Knob Frequency Scale and the most appropriate setting for using the Saturation feature.

The Tape Saturation analog circuit is available for the CompIQ Stella and CompIQ Twain compressors. It is designed to act only on the Dry signal. This optionally-saturated signal can then be mixed with the wet, compressed signal, to infuse harmonic distortions and warm-up the audio, without affecting the dynamics of the compressed signal. The headroom of the saturation circuit is pretty high, so you need to dial in some saturation before the effect is audible. For CompIQ Stella, the LPF and HPF can be engaged by removing the two corresponding internal jumpers. For CompIQ Twain, the filters are variable and directly accessible as small trim knobs – the HPF is available for the Lows band and the LPF is available for the High band. On both compressors, the HPF is placed before the Saturation engine and the LPF is placed after the Saturation engine.

Below you can see how the filters affect the frequencies of the Dry line.

The Low & High cut filters should be used only when Tape Saturation is used, otherwise, they will affect the clean dry line, although, that might also be a desirable way of using the Dry/Wet Mix control. The filters were necessary so that they would accommodate different types of audio sources, and respond musically without introducing unwanted fuzziness on the low end (especially for bass), or make it sound brittle (especially on bright guitar pickups).

The X-EQ section of the circuit is placed after the compressor, just before the Mix control, which means it acts on the wet signal only. When mixing the dry unprocessed signal, with the wet, compressed, and processed signal, the effect of the X-EQ is washed out little by little, as you introduce more dry signal.

The X-EQ on Stella has two frequency pivot points so that it will accommodate either bass (pivot at 330Hz, which corresponds to the higher note on the highest note of a 4 or 5 strings bass), or guitar (pivot at 1KHz, which corresponds to the highest note on a 20-fret guitar). In extreme settings (CC or CCW), there is a total difference of 12dB between lows and highs. In the middle position of the X-EQ knob, the frequencies are not affected. The X-EQ section can be bypassed altogether by changing the position of a jumper inside the pedal.

The maximum input signal level without distortion is in between +5dBu and +10dBu, depending on the compressor model and powering voltage. All compressors have a 50dB threshold range, from -40dB up to +10dB which covers average magnetic pickup level and line-level signals as well. They all can be used on line-level FX Loops or feed the high impedance or line-level inputs on recording interfaces. Having an RMS-level detector, the compression is very accurate, and the LED indication is exact in that regard, as long as the input signal is at/around the calibrated reference level. CompIQ series “0dB reference input level” is internally set at -20dBu (77.5mVrms). The total amount of compression (inf:1 Ratio) depends on the input signal level, usually 20dB for input signals around -20dBu (77.5mVrms) and 36dB for +4dBu (1.23Vrms) input signal levels.

The re-amplification of a weak signal – as compressed signal is – is the main source for noise in compressors. Some compressors might be described as noisier than others, but the fact is they all introduce noise with amplification (the amount of noise is also dependent on the amplification circuit, FET-based amplification being potentially a bit noisier; in CompIQ compressors, amplification takes place in the VCA). To correctly compare compressor’s noise, they must be set for the same exact amount of threshold, ratio, and make-up gain, and be fed the same reference signal. Some manufacturers limit the Ratio of their compressors to 7:1 or as low as 3:1 and those indeed make for “very silent compressors”. Of course in this regard, the “silence” characteristic has a subjective meaning if it’s not a misleading statement. As far as CompIQ compressors go, these are the facts:

• at higher input signal levels, the makeup gain-related noise will be lower, because you deal with a bigger signal in the first place;
• if you set a higher threshold, hard knee, and inf:1 ratio and you affect only the peak of the signals – as this limiting setup makes sense to be used – the noise will be inaudible.
• for weak magnetic pickup signals, at the lowest set threshold and with ratios around 4:1 (which is a fair amount of compression), the CompIQ make-up gain will introduce noise similarly to studio-grade equipment.
• on top of Threshold, you have the MIX control which helps reducing noise by blending in the dry signal;
• using soft knee also contributes to reducing the need for make-up gain, so implicitly it reduces potential noise.

Pops or static noise may occur when switching the following settings with the pedal engaged:

• Knee
• Timing
• Side Chain Filter
• EQ Pivot
• Dual-Band/Stacked
• Power On/Off

The gain reduction meter is available for all compressors in the CompIQ line. They measure how much compression is applied to the input signal. The indication is in dB. Depending on the product, the metering ladder is differently configured. Keep in mind that due to the limited number of LEDs in the meter, the compression is “invisible” in between the LEDs. Ideally, a full meter scale would have a minimum of 20 LEDs, one for each dB of reduction.

The metering in each product was designed and calibrated to reference the comparators to 9-12VDC for an accurate gain reduction indication. However, the CompIQ Twain can also be powered at 18VDC. At 18VDC, several thresholds that are calibrated for metering are a bit shifted, and as a result, the metering shows around -3dB less in the meter. Usually, a proper powering of an electronic circuit is with a fixed voltage +\- some tolerance. But 80-100% voltage up shifting, also shifts some calibrations within the blocks of circuits inside. While the audible side of the change is for the better and likable, the precision of the metering reacts to this shifting and introduces a variation.

There is a possibility that the meter LEDs remain “locked” lit in some conditions outside the normal usage of the pedal. For example, this may happen when powering the pedal at a higher voltage and switching the Knee in some particular circumstances such as when the knobs are set for compression but no input signal is present or input cable’s jack is not inserted in the pedal.

The gain reduction meter needs an input signal that varies up and down the thresholds set for each LED, and while a raising signal lits them, they must also be turned off by a decaying signal. The electric spike introduced by switching the knee (which is a change of the operation mode of a portion of a circuit while also setting the rest of the controls for compression) varies very shortly and it does trigger the LEDs although no signal on the input of the metering circuit is present so that the LED’s are reset. Nothing is broken and nothing breaks – is just a condition you put the circuits in, but that condition is different from the intended usage of the pedal.

To prevent that, switch the knee when you don’t play but you must have the input and output plugs inserted in the pedal. To switch off the LEDs that remain lit, power off the pedal and then on again OR, play your instrument with a signal higher than the LEDs on the display that remain lit. This way the circuit sees the decaying signal and the comparators are reset. Alternately, power the pedal with 9-12VDC instead of 18VDC.

Although all CompIQ line of pedals can be powered in the 9-18VDC range, we conservatively designed and calibrated some portion of the circuits (like the gain reduction meter) to be run in the 9-12VDC range. The headroom is more than enough at these voltages, and we can also protect the circuits in the long run from accidental failures of power supplies.

Please be sure you only use good quality and regulated power supplies because the 18VDC is the absolute maximum for some of the ICs inside. And although they might still support some minor voltage spikes, say at up to 10% you might still be safe, if the power supply spikes more or fails altogether and bursts a constant 20^ish Volts into the pedal, then those active components might fail.

The circuits have other protections as well, like reverse polarity, yet there is a limit these protections can handle. That is valid for all our compressors.

• Sound on Sound Magazine – CompIQ STELLA – Review
• Vintage Guitar Magazine – CompIQ STELLA – Review
• Gitarre und Bass Magazine – CompIQ TWAIN & STELLA – Review
• Gitarre und Bass Magazine – CompIQ MINI – Review
• Bass Professor Magazine – CompIQ STELLA – Review
• Guitar Magazine – CompIQ STELLA – Best Compressors in 2020
• Guitar Magazine – CompIQ TWAIN – Review
• Guitar Magazine – CompIQ STELLA – Review
• Premier Guitar Magazine – CompIQ TWAIN – Review & Sound Samples
• Premier Guitar Magazine – CompIQ TWAIN – Gear Radar
• Premier Guitar Magazine – CompIQ STELLA – Pedal Showcase
• Premier Guitar Magazine – CompIQ MINI – Quick Hit Review
• Guitar Pedal X – CompIQ Compressors Line-Up
• Compressor Pedal Reviews – CompIQ TWAIN – Review
• Compressor Pedal Reviews – CompIQ STELLA – Review
• Compressor Pedal Reviews – CompIQ MINI – Review
• Compressor Pedal Reviews – CompIQ MINI ONE – Review
• Onlybass Forum – CompIQ STELLA – Users Review (French language)
• Onlybass Forum – CompIQ STELLA – Review with sound samples (French language)
• Onlybass Forum – CompIQ MINI & STELLA – Reviews & Comments (French language)
• Talkbass Forum – CompIQ MINI – Reviews & Comments
• Talkbass Forum – CompIQ MINI ONE – Reviews & Comments
• Talkbass Forum – CompIQ STELLA – Reviews & Comments
• Basschat Forum – CompIQ STELLA – Review
• Basschat Forum – CompIQ STELLA – Reviews & Comments
• YouTube – Reviews and Demos