Features

True Stereo Operation

Using an analog EQ on a stereo bus can be quite tricky, as only very few units offer the possibility to link their two mono channels for true stereo processing. While there are so many compressors in the market that can be linked for stereo operation, why are there almost no EQs which can do this, too?

One of the main answers is component tolerances. Obviously, you want the two linked channels to behave exactly the same, but natural tolerances of potentiometers and capacitors counteract this ideal characteristic. xfilter solves this problem by using computer-selected dual and quad layer potentiometers as well as special low tolerance caps.

Resonance High- & Low-Pass​

The outer bands of the xfilter show a lot of flexibility. In their standard mode, they are set up as sweet sounding high and low shelf filters that can be used to tailor the airiness and fundamentals of your tracks fast and efficiently.
As a special feature, you can independently switch these to become high and low cut filters with 12 dB per octave and an additional resonance peak at the knee frequency.

Especially bass frequencies can benefit from a low cut with resonance by obtaining a clean and punchy character. In the high frequency range, you can use the resonance filter to put an accent on a selected frequency without boosting the complete HF spectrum.

Switchable Q-Factor

The filter quality of the two parametric mid bands can be switched between two characteristics: Wide (Q 0.5) and narrow (Q 1.0), based on the proportional Q principle.

When designing a parametric filter, you have quite a range of Q factors to choose from. After extensive listening, we decided to implement a choice of 0.5 or 1.0 in xfilter, as these parameters always result in what this machine is focused on: Very musical processing.
You will notice that very narrow Q factors are not offered here. These days, digital filters are used for notching narrow ranges to get rid of problematic frequencies anyway.

Additional Passive High-Band

In addition to its four flexible active bands, xfilter features a switchable fixed LC filter for polishing the high frequency range. The filter has a slight resonance peak at 12 kHz and starts to fall of at 17 kHz.

This way, the saturation-like storage effect of the coil focuses on the area around the peak without pushing the complete high frequency spectrum too much. The inductors used in xfilter have their own shields to reduce outer influences like hum and all kinds of unwanted noises.
Now you know what the ‘Passive Massage’ is all about: Opening up the top end of your signals by pushing just a single button!

Selected Potentiometers

In a linked stereo EQ, the potentiometers need a lot of layers. The pots for setting the mixers (gain) have two layers each, while the pots for setting the frequencies even have a total of four layers each!

The problem occurs that every single layer has its own component tolerance, and while layer A could be at the absolute minimum of the allowed tolerance, layer B could be at the absolute maximum. As a result, the filter stages of the left and right channels would not behave the same. To solve this, every single potentiometer in xfilter is measured and selected with an Audio Precision rig and computer routines written exactly for this task.

Ground Shield Layer

The PCBs of xfilter have a total of four layers: 

Two of them are used for the audio circuitry, one is for power supplying traces, and the last one is a dedicated ground shield layer. But what’s the idea behind this? The filter networks of the individual bands of the EQ become higher in resistance the lower their specific frequency is set. This makes them more vulnerable towards hum which is caused by electric fields. 

As the additional ground layer is placed extremely close beneath the traces which carry audio, it is very efficient as a shield against unwanted noise interferences.