Pultec EQ

EQ — Position 3

The Pultec EQP-1A is one of the most revered equalizers in recording history. Its passive tube circuit enables a unique behavior: the low-frequency boost and cut controls can be applied simultaneously, producing a characteristic curve no single filter can reproduce. The original hardware’s inductor network also creates a resonant peak at the shelf corner frequency — this module models that resonance authentically.

Controls

Low Frequency Section

Control	Range	Description
LF Boost Freq	20–300 Hz (skewed)	Corner frequency for the LF boost shelf. 60 Hz is the classic kick/bass fundamental; 100–150 Hz for warmth on guitars or synth bass.
LF Boost	0–18 dB	Low shelf boost amount. At the corner frequency, the LCR resonant peak adds an additional bump (≈45% of boost dB, Q=1.8), characteristic of the original hardware inductor.
LF Boost Bandwidth	0.0–1.0	Shelf width. 0 = narrow (Q=1.0, boost concentrated just below corner). 0.67 = musical default (Q=0.5, boost spreads through 2–3× the corner frequency). 1.0 = wide (Q=0.25, broad low-end lift).
LF Cut Freq	20–400 Hz (skewed)	Corner frequency for the simultaneous LF cut. Typically set to the same frequency as the boost or slightly lower.
LF Cut	0–18 dB	Low shelf attenuation amount. Cuts the low-mid bloom while the boost adds sub energy — the source of the classic Pultec shape.
LF Cut Bandwidth	0.0–1.0	Cut shelf width. Same Q mapping as LF Boost Bandwidth.

High Frequency Section

Control	Range	Description
HF Boost Freq	3–20 kHz	High-frequency boost center. 8–12 kHz for presence and air; 3–5 kHz for upper-mid bite.
HF Boost	0–18 dB	HF boost. The Pultec HF boost is famous for being non-fatiguing at high amounts due to the gentle passive shelf shape.
HF Boost BW	0.0–1.0	HF boost bandwidth. Low = wide, shelf-like. High = narrow, bell-shaped.
HF Cut Freq	5–25 kHz	HF shelving cut center.
HF Cut	0–18 dB	HF shelf cut amount.

Saturation

Control	Range	Description
Tube Drive	0.0–1.0	Tube saturation via tanh soft clipping. Low values (0.1–0.25) add analog warmth without audible distortion. Higher values bring in harmonic density.
Bypass	On/Off	Bypasses all processing.

LCR Resonance

The original EQP-1A’s inductor network creates a resonant peak at the boost shelf corner frequency. This is not a bug — it is the reason the Pultec adds perceived punch even at modest boost levels. The resonant peak is modeled as a PeakingEQ at the corner frequency with:

Amplitude: 45% of the shelf boost dB
Q: 1.8

At +12 dB LF Boost / 60 Hz, you get approximately +5.4 dB of resonant peak right at 60 Hz, sitting on top of the shelf boost. This combination gives kick drums and bass instruments a tight, focused fundamental.

The LF Boost Bandwidth control interacts with this: a wider shelf (higher BW value) spreads the boost through more of the musical range (100–300 Hz), making it useful for guitar buses and synths in addition to bass-heavy material.

The Pultec Trick

The simultaneous boost/cut is the most celebrated Pultec technique:

Control	Value
LF Boost Freq	`60 Hz`
LF Boost	`10–12 dB`
LF Boost Bandwidth	`0.67`
LF Cut Freq	`60 Hz`
LF Cut	`5–7 dB`

What happens: The boost adds sub and low-end energy with an LCR resonant peak at 60 Hz. The cut attenuates low-mid bloom at a lower effective frequency (the cut shelf falls off more gradually at the same frequency). The result is a tight, punchy low end with real weight and no mud — a shape no single filter can produce.

Increase LF Cut for tighter, more controlled low end. Decrease it for a more open, roomy bottom.

Techniques

Master Bus Weight and Air

Control	Value
LF Boost Freq	`30 Hz`
LF Boost	`6 dB`
LF Boost Bandwidth	`0.67`
HF Boost Freq	`12 kHz`
HF Boost	`4 dB`
Tube Drive	`0.15`

Sub weight below 30 Hz with the LCR resonance providing a gentle 30 Hz focus, plus air above 12 kHz. Classic mastering move.

Guitar Bus Presence

Control	Value
LF Boost Freq	`100 Hz`
LF Boost	`5 dB`
LF Boost Bandwidth	`0.8`
HF Boost Freq	`8 kHz`
HF Boost	`6 dB`
HF Boost BW	`0.60`

Wide LF boost at 100 Hz adds body and weight to electric guitars. The resonant peak at 100 Hz gives that characteristic upper-bass warmth. HF presence boost at 8 kHz adds cut-through.

Bass Bus Fundamentals

Control	Value
LF Boost Freq	`60 Hz`
LF Boost	`10 dB`
LF Boost Bandwidth	`0.5`
LF Cut Freq	`60 Hz`
LF Cut	`6 dB`

Tight LF bandwidth (Q≈0.7) concentrates the boost and resonance right at the bass fundamental. The simultaneous cut removes low-mid bloat.