User Manual
21
PEQ Filter Section
History / Theory of Operation
The P-lter section is an homage to a classic Pultec™ EQ from the 1950s which has attenuation (cut) and
addition (boost) controls on separate knobs. This design is unique in that identical boosts and cuts do not cancel,
but rather deform the lter response in a special way. However we did not set out to make an exact replica. We
wanted to update this classic design and make it even better while maintaining its somewhat quirky feel, so
original users can feel right at home. The PEQ section is a passive lter, and we have included a make-up gain
stage which has slightly more gain than in the originals. It is also built with modern components which create a
faster response, resulting in sharper and more detailed transient reproduction. Our implementation of this lter
retains the original’s classic warm, resonant quality while still being lively and transparent for classical music or
audiophile jazz.
PEQ Topology/ Implementation
The P-lter low frequency section is a rst order RC lter network with additional resonance. The P-Filter
high frequency boost is built with an LC lter network. The inductor in this network is hand-wound for superior
sonics and custom sound characteristics. Both RC and LC lter networks are passive lters. The make-up gain
of the P-lter section is performed by a high-speed operational amplier to give clean, accurate, and consistently
reproducible gain. Since the digital encoders are speed-dependent, turning them slower results in a ner gain shift,
so it is possible to be very exact with this EQ, and our true-stereo implementation is perfectly matched channel-to-
channel with single control ergonomics.
PEQ Bandwidth/ Gain Ranges
The P-lter section is in four parts:
Low Frequency
1) Boost: a xed bandwidth shelf with a maximum boost of +14 dB
2) Cut: a xed bandwidth shelf with a maximum cut of -14 dB
High Frequency
3) Boost: a bell with adjustable slope. At its middle bandwidth of 7 its maximum gain is +14 dB.
As the bandwidth is increased, its maximum boost is reduced.
4) Cut: a xed bandwidth shelf with a maximum cut of -14 dB.
PEQ Low frequency band use
Simple Boosting or Cutting: Boosting or Cutting (attenuation) is marked on the front panel from 1 through
14, and further divided into tenths (e.g., a very delicate boost of 0.1 is possible). The equalizer is at at a setting
of 0 (knob indicator fully counterclockwise). Be aware that unlike modern shelving equalizers, the PEQ’s
boost frequency is specied at the frequency of maximum boost instead of the 3 dB down point. So 20 Hz is
a very useful choice with effect beyond 100 Hz, as shown in the curve below. As you can see, the 20 Hz boost
lter’s nominal center (3 dB down point) is about 50 Hz (so modern equalizers would probably call this a 50 Hz
shelf). In contrast, the 20 Hz cut lter’s nominal center is intentionally an octave higher—100 Hz. The 100 Hz
boost lter’s 3 dB down point is about 300 Hz, so it is really a very warm 300 Hz shelf extending well into the
midrange. Welcome to the world of the PEQ—enjoy its power and its quirks!