The measurements were taken using FRA4PicoScope 64-bit software with a PicoScope 2204A oscilloscope and its synchronized signal generator, a -20dBu (0.22Vp-p) input signal positioned at average instrument-level dynamics, and a 9VDC power supply. The CompIQ Stella Pro Compressor MK2 controls were set to balance input and output levels across the Dry and Wet lines. The output was connected to a high-impedance load during measurement.

The Stella Pro offers extensive control over frequency dynamics. The tilting EQ and Voicing settings shift specific frequencies, adding controlled coloration to the signal. By adjusting the side-chain frequency filters (cut or boost), users can intentionally alter the compression of affected frequencies at the output. While we don’t provide frequency response plots for every possible configuration, the Stella offers a wide range of adjustments. Each setting creates a unique response, making it adaptable to individual instruments and user preferences. The measurements use a 4:1 Ratio, threshold set for the indicated compression in dB, Hard Knee, Slower Auto Timing, and highlight Feed-Forward (FF) and Feed-Back (FB) side-chain topologies. Note that the input signal is not shown, as it would appear as a straight line at 0dB, similar to the bypassed signal in both powered and non-powered states.

 

Frequency response without compression and in true bypass

Below are the frequency response plots for the compressor output, covering both Wet and Dry lines with no compression applied. The bypassed signal is also shown for both powered and non-powered pedal states.

 

Tilting X-EQ with two frequency pivot options

When the X-EQ knob is turned clockwise (CW), Highs are boosted and Lows are attenuated. Turning it counterclockwise (CCW) boosts the Lows and attenuates the Highs. Two tilting frequency point options are available: 330Hz for bass and 1kHz for guitar. With X-EQ knob at noon, there are no frequency changes. Small variations in the tilting point may result from Make-Up Gain settings and component tolerances (we use 1% tolerance thin-film resistors and 5% tolerance plastic film capacitors).

 

The “Normal” Side-Chain

The two plots below show the compressor’s output frequency response during compression with the Lows and Highs SCF knobs at noon in “Normal” mode. The side-chain signal mirrors the input without extra filtering. With output level matching the input, the CompIQ Mini maintains a nearly perfect flat frequency response from 30Hz to 30kHz, even under this high compression.

The slight subsonic adjustment at the start reflects the RMS sensor responding to the automatic low frequency sweep. The starting level is set by the Make-Up Gain for the chosen compression amount. Once compression settles in, the circuit’s frequency response at the output is linear. The plots show responses for both Feed-Forward and Feedback compression types.

 

 

Cutting and boosting Side-Chain Filters frequency

Cutting and boosting the Side-Chain Filters frequency allows users to manipulate the balance of specific frequencies within the side-chain, influencing how the compressor responds to those frequencies at the output.

When the Lows and Highs SCF knobs are turned fully clockwise (CW), the side-chain progressively attenuates lows and highs using first-order variable High-Pass Filters. This reduces the compressor’s sensitivity to these frequencies, resulting in less compression and making them louder at the output via Make-Up Gain, as shown in the plots. Turning the SCF knobs counterclockwise (CCW) boosts these frequencies in the side-chain, increasing compression and reducing their levels at the output. With the SCF knobs at noon, the filters leave the signal unchanged, which we refer to as “Normal” side-chain processing.

Although there are slight differences between Feed-Forward and Feed-Back compression modes, only Feed-Forward plots are shown here. Additionally, the side-chain plot is overlapped (in Green), illustrating how the output frequency response (in Blue) mirrors the filtering in the side-chain.

SCF measurements were taken after balancing input and output levels with the SCF in the neutral position for the initial measurement. The filters were then adjusted to their extremes (fully CW and CCW) to demonstrate their effects. In practical use, Make-Up Gain and other parameter adjustments are necessary to maintain a balanced input-output level, factoring in the SCF setup. Filtering combinations, including the tilting EQ and Voicing, create even more distinct compression tone-prints.