Parseq
It's a focused sort of EQ.
Parseq (Parametric Equalizer)
(Par)ametric + (s)helving (eq)ualizer ⇒ Parseq
An equalizer (EQ) effect is an understated workhorse. It expands the capabilities of your existing pedals or simply add a different color to your palette.
Graphic EQs have an easy-to-use interface, but all those sliders take up a lot of space and cutting long slots in an enclosure isn't as DIY-friendly as a design with rotary knobs. The frequency bands are also fixed so it's a matter of luck if the graphic EQ bands line up with a target frequency.
Fully parametric EQs are still EQs but in practice they fill a different niche than a graphic EQ. Rather than constructing a frequency response graph, a fully parametric EQ excels at pinpointing a specific frequency to cut or boost with targeted precision.
This design takes its inspiration from the Core Shaper, another EQ with a parametric midrange. The Core Shaper is a simpler, perhaps more elegant design with a semi-parametric midrange control based on a Wein bridge filter. It only uses 4 op-amp sections compared to the Parseq's 6. The mid sweep of the Core Shaper extends from about 310Hz to 3.2kHz. The Q in the Core Shaper is switchable from low (~0.3) to high (~0.8)
Figure 1: Parseq schematic
NOTE: table contains simulated values, I have not yet built this pedal yet so it is currently untested and unverified. If you decide to build one, proceed with caution.
| mid parameter | Core Shaper | Parseq |
|---|---|---|
| min freq (Hz) | 311 | 155 |
| max freq (Hz) | 3220 | 4180 |
| min Q | 0.3 | 0.3 |
| max Q | 0.8 | 2.4 |
| max boost (dB) | 16 | 15 |
| max cut (dB) | -16 | -16 |
By contrast, the Parseq uses a state variable filter for the midrange which gives it a wider bandwidth sweep as well as continuous control over the Q factor instead of a 2-mode switch. The downside of this design is it uses more components.
Besides the different approach to the midrange filter, the Parseq traces all of its features back to the Core Shaper:
- fixed gain input stage
- parametric midrange
- Baxandall treble/bass shelving filters
- passive output attenuator for volume control
My recommendation: use the Parseq when you want more precision than the Core Shaper and don't mind the extra circuit complexity.