‘BootEQ mkII’ – LF shaping

BootEQ mkII - new LF controls

If you were a user of the very first BootEQ equalizer VST plug-in then you know about it’s limitations in the low frequency department: LF adjustment is limited to a +/-12dB gain control plus a HP switch at 40/60Hz. This is now reworked in BootEQ mkII which offers some new and probably surprisingly versatile (but still easy to use) control over the entire lower frequency region.

The newly added LF band of the mkII is now a frequency and gain adjustable peak filter as so is the ‘MF’ band but just with different (overlapping) frequencies and some minor adjustments to the specific lower frequency band (allowing some cool tricks but more on this later on).  The frequency range is selectable in a stepless fashion from 40Hz up to 250Hz and the curve is asymetrical (regarding cut/boost behaviour) as found in some analog designs.

Example peak curves at around 100Hz

The mkII versions design goal was to keep the overall workflow just exactly that simple and fast while providing more flexibility and LF detail control. To contribute to this the lowcut (aka highpass) option is now included into the LF peak filter as an switchable option which allows flexible cutting between 40 and 250Hz. Using the gain control of this band (which normally just applies to a peak filter) this now controls the so called ‘pass-band’ or ‘transition’ of the lowcut filter.

Low-cut transitions at around 100hz

This allows to obtain different charactaristics of low-cut filtering with different shapes and steepness. The leftmost gain position is the most gentle and less steep one, the rightmost gain position is the most steep and features a slight LF ‘bump’ before cutting.

BootEQ mkII - new LF controls

You may have noticed, that there is an additonal LF control knob on the right side of the plug-in as well. What is that exactly and why is that on the pre-amp side of the plug-in?

The right plate of this plug-in features the ‘pre-amp’ section and this contains some additional LF control but in a much different fashion. It basically controls the frequency range way below 200Hz but is capable of adding nice extra harmonic content in the low-mid frequency range. When boosting with the right side LF option then some additonal ‘transformer’ style harmonics are applied due to the nonlinear behaviour of the pre-amp simulation.

LF transfomer frequency response

Those plots where made in ‘vintage’ mode and this causes some slight dip in front of the LF boost but also slightly bumps the upper lower-mid region as well. This is often perceived as  ‘fat’ sounding and is quiet different to the ‘modern’ mode which behaves variable not only but particularly in the LF range.

Setting the right LF dial to zero (or below) leads to a more compact and tight dynamic response of the transformer simulation. Yes, right, this simulation does not only covers frequency and phase response plus distortion but handles some dynamic aspects as well.

The second order harmonics added from the transformer simulation are independent from the tube style ones and are variably dialed in. They made some important part of the color and sound perception of this simulation, especially in the lower frequency region.

Example 'transformer' saturation harmonics

Given this options we have now the ability to add some LF content while cutting them at the same time – does that makes sense? Yes! Here is one example: Boosting the LF region with the pre-amp LF section achieves some extra low-mid frequency harmonics as well. Low-cutting now with the left side LF filter (which is applied in serial since the routing internally is pre-amp –> EQ) we are actually just cutting off  some LF frequencies but the added low-mid frequency harmonics (generated by the pre-amp) remains! Due to psycho-accoustics this could also improve the bass perception on systems which are not able to reproduce LF content at all.

LF + MF filters

What we have missed so far is the existence of the ‘MF’ equalizer which actually ranges from 1.5kHz down to 100Hz which  means of course that you can manipulate some LF region with this one as well. I’m not going to talk here about standard usages but will explain some way cool trick with this one in combination with the LF filter.

Assure that the LF filter is set to peak mode and set the MF to +12dB, 100Hz and the LF to -12dB. Now we are able to perform some push/pull EQ mechanics just by altering the LF frequency (the blue ‘FRQ’ knob) which results in some very musical sounding shelving filters.  This resembles some vintage EQ designs which offers frequency boost and cut options at the same time and on (nearly) the same frequency.

While dialing the blue FRQ knob between 40 and 100Hz we obtain now a filter frequency response which cuts some sub-bass but in the same time boosts some part of the LF spectrum.

LF shelving 1

If we then dial in LF frequencies above 100Hz then the curve flips around and we obtain sub-bass boosting and some dipping at little higher frequencies.

LF shelving 2

In BootEQ mkII everything can be combined of course and I especially recommend to use this in combination with the pre-amp section to achieve a very natural and musical sounding frequency shaping while adding pleasant harmonic content and gently touching the dynamics of the audio signal.