BoredBrain EQx5 voltage controlled equalizer

• Five frequency bands with ±12 dB gain
• Post-EQ balance and level controls
• Control voltage inputs for all parameters
• Stereo I/O with discrete signal paths
• Additional mono sum output

EQx5 is an effective 5-band stereo equalizer for eurorack synthesizers. The module allows for quick and musical adjustments to the frequency content of audio signals, with voltage-control of all parameters for experimental EQ processing.

Five Frequency Bands

The module has five bands of graphic equalization, each with ±12 dB of level boost or cut. The first order filters employed for equalization have slopes of 6 dB/oct from their respective center frequencies.

Balance & Level

The BALANCE parameter is essentially an equal-power crossfader which simultaneously and inversely affects the gain of the Left and Right channels. When a stereo signal is connected to the module’s inputs, this parameter functions like a Pan control, shifting the stereo image Left and Right. When two mono signals are connected to the module’s inputs and the OUT L+R mono output is used, this parameter functions like a volume crossfader with no perceivable level drop in the center.

The LEVEL parameter is a post-eq volume adjustment, used primarily to compensate for level changes after frequency equalization. The module implements a pseudo-loudness feature, which gradually boosts the LOW and HIGH shelving filters as LEVEL is reduced. This helps to retain the signal’s “fullness” as it is attenuated, accounting for the equal-loudness contour. 

A key feature of EQx5 is the ability of all parameters to be controlled with external voltages, opening up a range of experimental possibilities in the eurorack environment.

The amount of boost or cut for each band can be modulated using control voltage signals, with the full range being ±5 V (10 Vpp). The frequency band’s gain knob then defines the bias point to offset the incoming control voltage.

Additionally, both BALANCE and LEVEL have their own respective CV inputs, also with a full range of ± 5 V. The LEVEL parameter can act very much like a VCA when used in combination with its CV input.

The two inputs, L and R, are designed for audio signals within the audible spectrum. These inputs are AC-coupled, and will block DC signals to reduce noise. R is normalled L, so if no plug is present, the channel will obtain a of copy of L.

In addition to the two discrete L and R outputs, the module has a mono output, OUT L+R, which is the sum of the two channels. As mentioned previously, this output is useful when crossfading between two signals using the BALANCE parameter, or to conveniently obtain a mix of the two audio signals.