The 101 of the CompIQ Line of Compressors

Do you want to know more about mini compressor pedals? We compiled a Technical Shootout for the most performance and popular mini compressor pedals available. Find out how our CompIQ MINIs stand out.

The dbx 202 “Black Can” Voltage Controlled Amplifiers (VCAs), designed by David Blackmer, founder of dbx Inc., were the first professional-grade VCAs and are still used in audio consoles today. Built with a gain cell of eight transistors, they were groundbreaking for their time. However, advancements in integrated circuit technology have since led to better performance.

For example, the CompIQ series of compressors uses THAT Corporation’s Blackmer® VCAs, known for their unique exponential control, where gain changes directly in decibels. These VCAs offer a wide dynamic range, low distortion, and maintain a neutral tonal character, ensuring transparent audio processing without coloration. This makes them ideal for professionals seeking high-quality, transparent signal handling.

David Blackmer, the founder of dbx Inc., is known for inventing the RMS-level detector. It calculates the Root Mean Square level of input signals in a way that mimics how our ears perceive sound, which is in a logarithmic format.

This detector’s exact envelope is then used to control the Voltage Controlled Amplifier (VCA) based on user-defined settings like Ratio, Knee, SCF, Threshold, Attack, Release, and Gain.

The CompIQ compressors have a versatile Threshold control that can handle a wide range of input signals, from weak to pro-audio levels, preventing distortion from high-level spikes. The threshold range scale is logarithmic and spans from approximately -40dBu to +10dBu, suitable for various applications. Typically, the optimal threshold for pickup signals falls between -30dBu to -20dBu, with brief spikes that can reach higer, up to line level at +4dBu.

For desired peak compression, set the Threshold knob at around 9 o’clock or slightly higher. Higher compression ratios are unnecessary unless you’re aiming for limiting. If you want more noticeable compression, lower the threshold, but use smaller ratios to avoid excessive signal compression, unless you intend to achieve a specific effect, like the “New York compression style,” which blends compressed and unprocessed signals for a balanced dynamic range.

The Side-Chain Filter (SCF) is a feature that affects compression based on frequency. It prevents high-amplitude low-frequency content from triggering compression. This is achieved by filtering the side-chain downwards from 1kHz, as shown in the graph. By doing this, compression doesn’t affect those specific low frequencies that much at the output of compressor. As a result, the low frequencies come out louder, less compressed, creating a distinct and fuller sound.

The Side-Chain Filter also helps reduce unwanted pumping, a common issue with “high ratio / low threshold” compression settings. The SCF approach differs from methods like threshold or ratio adjustments, soft-knee compression, or blending dry and wet signals. When combined with these controls, the SCF offers more flexibility for handling high amplitude audio content.

After 2022, the SCF circuitry in compressors like the MK2 Mini, Stella, and Twain was upgraded. The Mini’s SCF now variably adjusts low frequencies, while the Stella and Twain models handle both low and high frequencies. In these compressors, rotating the SCF control counterclockwise (CCW) boosts, clockwise (CW) cuts, and the center position leaves frequencies unchanged in the side-chain. This variable control adds more versatility than usual cut-only filters.

The Stella and Twain MK2 models include a Highs Side-Chain Filter for frequencies above 3kHz with cut and boost. This feature helps control overly bright signals by fine-tuning compressor sensitivity to selectively target high frequencies.

For a detailed technical article on side-chain filtering, including frequency response plots, see the following post:

In our designs, “frequency compensation” refers to a pre-conditioning of the signal in the side-shain detector. In music, dominant frequencies and harmonics are present in each note. When notes have lower dominant frequencies, like in low guitar strings or bass, they can trigger compression too soon, over-compressing harmonics and higher notes. To tackle this, we use gradual pre-filtering starting at 20KHz and moving downwards with additional high-pass filter at 2KHz. This progressive approach results in a more natural, dynamic processing, especially noticeable in percussive or bass-rich instruments. Our compressors use a standard side-chain roll-off curve that aligns with human hearing, reducing -12dB at 2KHz compared to 20KHz.

The CompIQ Mini, Stella, and Twain models produced after 2022, 2024, and ongoing, offer selectable side chain processing. In Feed-Forward mode, the control signal is pre-VCA, resulting in an fast “in your face” compression feel. The new Feed-Back mode, however, uses the control signal post-VCA for a softer processing feel due to inherently longer attack and release times and the particularity of this way of controlling the side chain. In Feed-Back mode, an adjustable gain control within the VCA reduces overall noise from Make-Up Gain. This way of processing is different from the classical feed-back circuits which usually employ a fixed high gain placed after the gain reduction element.

Though Feed-Back mode limits compression ratio due to finite gain control, our compressors can combine Hard Knee with Fast Auto or Manual Timing (in Stella) for enhanced compression speed and amount. With a higher variable Threshold targeting high-amplitude frequencies, our compressors can effectively function as soft limiters, leaving the core sound intact. Notably, the RMS level sensor also adds to the natural feel of dynamic processing of our compressors.

For a detailed technical article on side-chain filtering, including frequency response plots for both Feed Forward and Feed-Back compression modes, see the following post:

The CompIQ Twain features a variable-range Linkwitz-Riley crossover (70Hz to 1KHz), which splits the input signal into two separate frequency bands processed by dual compression engines. The crossover output also feeds the Dry Line, allowing seamless blending of Dry and Wet signals without phase cancellations, no matter where the crossover is set.

To illustrate, a chart shows matched levels of internal and external circuits with the crossover at 1KHz, output set to buffer level, and Mix at 100% Wet. The chart demonstrates nearly perfect alignment of phase across the audio spectrum.

The input signal’s phase remains a straight line, but the output signal’s phase gradually shifts from 0° at the lows to 400° at the highs. This is a normal result of the signal separation and recombination by the crossover’s band filters and compression engines.

The following drawing illustrates the Crossover Knob Frequency Scale for Twain MK1 and the most suitable setting for utilizing the Saturation feature.

CompIQ Twain Crossover Frequencies & Best Saturation Range

For a detailed technical article on Twain MK2 frequency response, see the following post:

Both the CompIQ Stella and CompIQ Twain MK1 compressors offer an analog Tape Saturation circuit that exclusively affects the Dry signal. This lets you add optional saturation to your signal, which can then be blended with the compressed Wet signal to introduce harmonic distortion and warm up the audio while preserving the compressed signal’s dynamics. Note that you might need to dial in some saturation before it becomes audible due to the high headroom of the saturation circuit.

For the CompIQ Stella, you can activate the LPF and HPF by removing the internal jumpers. The CompIQ Twain has variable filters accessible through small trim knobs. The HPF is for the Lows band, and the LPF is for the High band. In both compressors, the HPF is placed before the Saturation engine, while the LPF is positioned after it.

These filters are recommended for use in conjunction with the Tape Saturation feature, as they may affect the clean, dry signal otherwise. The filters were introduced to provide flexibility when processing different audio sources while maintaining musicality and avoiding undesirable artifacts like muddiness in the low end (especially with bass) or harshness with bright guitar pickups.

The X-EQ section comes after the compressor and before the Mix control, affecting only the wet signal. When mixing dry and processed signals, the X-EQ effect gradually diminishes.

In the Stella, the X-EQ has two frequency pivot points for bass (at 330Hz) or guitar (at 1KHz). At extreme knob settings (fully clockwise or counterclockwise), there’s a total 12dB difference between low and high frequencies. When the X-EQ knob is in the central position, no frequency alterations occur. You can bypass the X-EQ section by adjusting a jumper within the pedal (only available for Stella up to V1.3; for V1.4 the jumper is not physically installed but it can be installed by the user if desired, and starting with V2.0 the jumper is no longer available).

The CompIQ compressor series can handle input signals ranging from +5dBu to +10dBu without distortion, depending on the model and power voltage. They offer a wide 50dB threshold range from -40dBu to +10dBu, making them suitable for magnetic pickups, line-level signals, line-level FX Loops, and high impedance or line-level inputs on recording interfaces. These compressors provide precise compression thanks to the RMS-level detector, and their LED indication is accurate when the input signal is around the calibrated reference level. The CompIQ series internally sets a “0dB reference input level” at -20dBu (77.5mVrms). The amount of compression (inf:1 Ratio) depends on the input signal level and is typically 20dB for input signals around -20dBu (77.5mVrms) and 36dB for +4dBu (1.23Vrms) input signal levels.

Stella MK2 voicing adds subtle shifts in the internal dynamic frequency processing that add nuanced coloring to the sound. This is not really an “EQ”. The Spark setting enhances the highs with polished brilliance, while Tight slightly dips the low-mids for added depth. Punch, on the other hand, delivers a tube-like compression feel with rich lows and warm highs, perfectly complementing the DITOS transformer voicing. While “punch” often suggests action, in this context it specifically relates to tonal sound perception. If you’re aiming for action-punch, we recommend using the Punch voicing along with Short (S) auto timing or manual (M) timing set with longer Attack and shortest Release settings. Additionally, Hard Knee and Feed-Forward side-chain configuration are ideal (Feed-Back will also work for punchiness, with short timings). During manufacturing, we may set the Spark or Punch options as default so you can start experimenting immediately. However, if you prefer to maintain the most transparent sound response that preserves the instrument’s natural character, you can simply turn off the voicing switches.

All our compressors are designed to provide up to +20dB of gain to compensate for volume loss during compression. They also handle signals up to +10dBu (that’s 6.9Vp-p of headroom when powered at 9V)—double the amount of pro line level at +4dBu—with a max of 0.5% Total Harmonic Distortion. A passive instrument pickup can peak at 2Vp-p, but only briefly, when the string is plucked hard. Most of the time, it sits just above an average of -20dBu (around 0.3Vp-p), which is why instrument levels are calibrated to that range.

But let’s say your signal source consistently peaks at +4dBu (3.47Vp-p) and you apply compression. How much gain can you add before hearing clipping in our compressors? Quite a lot—up to +20dB (with the gain knob maxed out)—relative to what’s compressed, aiming to bring the average signal back to input level. However, if the signal is below the threshold, maxing the gain will also reveal more noise.

Now, if you want to boost this strong signal further, how much gain can be applied before clipping? Up to about +6dB you’re still within 0.5% THD (which is inaudible), but if you keep adding gain, you’ll start hearing distortion around 10% THD, as you approach the circuit’s headroom limit. You’ll get slightly more headroom if you power the compressors at 12-18V, but headroom doesn’t increase proportionally with voltage.

For reference, if you boost a +4dBu signal by +20dB, the peak-to-peak voltage becomes 34.7Vp-p. Can that pass cleanly through something powered at even 24VDC? No.

What can we learn from this? First, there’s always a limit to how much you can push a signal in any system, so check the tech specs to know that limit. Second, if you feed a steady, strong signal into the CompIQ compressor circuits, you won’t be able to max the gain knob without hitting distortion at some point—it’s not designed to boost just any signal all the way up. Third, the Make-up Gain control in our circuits (and most compressors) is for recovery gain, not as a general boost. Finally, ask yourself: why would you need that much gain/boost in a compressor?

Compressors can introduce noise due to the amplification of make-up gain. As compression increases, more make-up gain is needed, adding noise to the signal. This noise can be further amplified by subsequent pedals or amplifiers in the signal chain. Additionally, any device before the compressor may introduce noise, which gets amplified by the make-up gain circuit.

It’s important to understand that if a compression setting requires a significant amount of make-up gain, noise will become more noticeable during silent parts. The signal-to-noise ratio drops during pauses, where noise may surpass the signal, leading to a negative SNR. Expecting complete silence when applying a +20dB gain is unrealistic. However, compressing at -20dB and then restoring with +20dB results in minimal but still noticeable amplification noise, especially during pauses. If maximum make-up gain is needed, it’s best to use the compressor for peak limiting rather than compressing the entire signal.

To accurately compare compressors for noise, they should be set with the same threshold, ratio, and make-up gain, fed the same reference signal. Some compressors have lower ratio limits, like 3:1 or 7:1, which makes them “quieter” because they require less re-amplification. Note that the term “quiet” is subjective and can be misleading.

For the CompIQ Twain, improper configuration in Stacked Mode can generate extra noise due to multiple amplification stages. The input preamp can boost the signal by up to 12dB, and each of the stacked compressor engines can add up to 20dB, resulting in a total boost of 32dB in dual-band mode or up to 52dB in stacking mode. This amplification can easily introduce noise or distortion, especially with strong input signals.

Given the pedal’s complexity, visual indicators for signal levels are included. The Twain MK2’s peak-level indicator shows Green at -2dBu, reflecting input strength after adjustments like preamp boost or external input settings. It turns Red when the output reaches +2dBu, which can happen when preamp trim and make-up gain combine. A balanced signal, typically around +4dBu, will flash between Green and Red. If both thresholds are hit at once, the indicator turns Orange. Steady Orange or Red signals suggest adjustments are needed for optimal processing.

The Twain Settings Examples offer tips for reducing amplification noise in stacked mode. Focus on adjusting compression and make-up gain more in the second (highs) engine than in the first (lows) engine. You can also control which frequencies are compressed in the second engine by increasing the lows threshold, using the Low or Deep side-chain filter, and applying a soft knee to lessen noise or the compression effect. Balancing these controls across both engines helps achieve the best settings for your needs. As a general guideline, aim to light up only the first three Green LEDs on each engine, with the fourth barely clipping occasionally.

Dual-band processing is more complex and requires specialized compression. What works for full-band compression doesn’t directly apply to dual-band compression. The dual-band compressor’s always-on crossover has passive components that inherently generate thermal noise, which is also amplified by make-up gain.

A practical approach is to understand how compression controls (threshold, ratio, knee, timing, blend, side-chain filter, and gain) affect compression and your desired outcome. Adjust these parameters to minimize the need for high make-up gain.

Noise generated by make-up gain is typically lower with higher input signal levels because the signal is larger. When using a limiting setup with a higher threshold, hard knee, and inf:1 ratio that affects only the signal peaks, any noise is usually inaudible.

For weak magnetic pickup signals, using a 4:1 ratio and a low threshold on the CompIQ can provide compression with noise levels similar to studio-grade equipment. You can further reduce noise by blending in dry signal with the MIX control and using a soft knee to minimize the need for make-up gain.

Power sources can introduce noise to electronics. Switching power supplies, in particular, are known for introducing hissing sounds. To minimize this, use well-filtered and regulated power sources. Most pedals aren’t designed for extensive power conditioning and filtering, so it’s essential to use quality power sources separately.

All our pedals operate on 9-18VDC, with current requirements provided in each product’s specifications. The cleanest DC power comes from batteries connected directly to the circuit. While only some pedals support internal 9V batteries, all can be powered with external supplies that deliver the required voltage and current.

Not all power supplies are built equally though. Most pedalboard or stand alone power supplies use switching designs that convert and regulate DC through high-frequency PWM, which can introduce noise that affects analog audio devices more than digital ones. High-quality power supplies however, like those from Voodoo Lab, Strymon, and Cioks, use isolated ports, multi-stage regulation, and  effective filtering and protections to prevent noise propagating in your audio signal chain. These are more expensive but worth the investment. It doesn’t make sense to invest in high-quality audio gear only to power it with cheap, poorly made supplies.

If you hear high-pitched noise (like a squeal or screech) or constant hiss above the normal noise floor with our or any pedals, it’s likely due to poor regulation and filtering from your power supply, an overload from exceeding its current capacity, or both. To check if your power supply is the problem, try powering the pedals with an external 9V battery temporarily. If the noise goes away, it confirms that your power supply isn’t up to standard.

We recommend using professional-grade power supplies and keeping usage within 70-80% of their current rating. For example, if a pedal requires 9VDC and 70mAh, power it from an isolated port that supplies 100mAh.

Avoid daisy-chaining power when possible to prevent ground loop noise.

The CompIQ line of compressors features a gain reduction meter to show the amount of compression applied to the input signal in dB. However, the number of LEDs in the meter varies between products and may lead to “invisible” compression between LEDs. To achieve optimal metering, a minimum of 20 LEDs is recommended.

Each product’s metering is designed and calibrated with reference to comparators at 9-12VDC, ensuring an accurate indication of gain reduction. However, the CompIQ Twain can also operate at 18VDC. At 18VDC, some calibrated thresholds for metering may shift, resulting in a metering indication of approximately -3dB less. While running an electronic circuit within a fixed voltage range (plus or minus some tolerance) is proper, operating at 80-100% voltage upshifts may alter certain calibrations within the circuit blocks. While this might have audible advantages, it impacts metering precision and introduces variation.

In rare cases, such as when powering the pedal at a higher voltage and switching the knee, the meter LEDs may appear “locked” and remain lit. This happens due to an electric spike caused by knee switching, briefly activating the LEDs even without an input signal. To prevent this, it’s advisable to switch the knee when no input signal is present but with the input and output plugs inserted into the pedal.

To turn off the remaining lit LEDs, the pedal must be powered off and then on again, or you can play a signal louder than the remaining lit LEDs, resetting the comparators. Alternatively, you can power the pedal with 9-12VDC instead of 18VDC.

The CompIQ pedal line can function within a power range of 9-18VDC. However, for optimal performance and circuit protection, certain components like the gain reduction meter are designed and calibrated conservatively to operate within the 9-12VDC range. It’s important to use high-quality, regulated power sources because exceeding the 18VDC maximum can potentially damage active components.

To prevent voltage spikes during pedal connection, it’s advisable to connect the pedals before powering on the power supply. Additionally, it’s a good practice to power your entire pedalboard simultaneously by switching on the AC switch on the power supply or plugging it into the AC wall wart. This helps control current draw and maintain stable voltages at each power output.

While our pedals incorporate reverse polarity protection, there are limits to what these safeguards can handle. Thus, using power supplies within the recommended voltage range is important to ensure optimal performance and protect the circuits’ long-term durability.

• American Songwriter Magazine – CompIQ MINI – Review
• Bass Gear Magazine – CompIQ STELLA MK2 + Ditos – Review-Demo
• Bass Magazine – CompIQ STELLA + Ditos – Review
• Bassplayer Magazine – CompIQ STELLA + Ditos – Review
• Guitar World Magazine – Ziffer Overdrive – Review
• Guitar World Magazine – CompIQ MINI – Review
• Sound on Sound Magazine – CompIQ TWAIN – Review
• Sound on Sound Magazine – CompIQ STELLA – Review
• Vintage Guitar Magazine – Ziffer Overdrive – Review
• Vintage Guitar Magazine – CompIQ MINI ONE – 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 ONE & Ziffer Overdrive – 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 Stella MK2 – Review
• Guitar Pedal X – Ziffer Overdrive & Solo Boost Master
• Guitar Pedal X – Ziffer Overdrive
• Guitar Pedal X – CompIQ Compressors Line-Up
• Compressor Pedal Reviews – CompIQ TWAIN – Review
• Compressor Pedal Reviews – CompIQ STELLA MK2 – 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