Voltage Controlled Equalizer

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.

Features
- 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

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.

HIGH 10 kHz shelf
HI-MID 4 kHz peak
MID 1 kHz peak
LO-MID 400 Hz peak
LOW 100 Hz shelf

Adjustments to these bands can make both subtle and dramatic changes to the frequency characteristics of the source audio signal(s). These adjustments can be useful in many scenarios such as attenuating problem frequencies and accentuating others, or strategically making “space” in the mix for specific sounds.

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.

Complete Voltage Control
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.

Input & Output
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.