1-Band Parametric EQ — Build Documentation

Overview & Features

The 1-Band Parametric EQ is a compact, single-supply gyrator-based equaliser designed to fit inside a 1590A enclosure. Unlike a graphic EQ with fixed frequency bands, a parametric EQ lets you dial in the exact frequency you want to boost or cut, and adjust the bandwidth (Q) to taste — from a narrow surgical notch to a broad shelving lift.

The heart of the circuit is a gyrator: a network that simulates an inductor using an op-amp and passive components, without any real inductors. This gyrator forms a resonant LC-style bandpass whose centre frequency is set by the FREQ trimmer.

Parametric centre frequency

FREQ trimmer sets the target frequency. Default: 144 Hz. Swap C5, C6, R8 for other ranges.

Adjustable Q

Q trimmer controls resonance sharpness. Higher Q = narrower bandwidth. Default Q ≈ 11 (¼ octave).

Boost / Cut (±)

10k linear pot sweeps from full cut to full boost. Centre = flat response.

Output level (Pegel)

100k linear volume pot compensates for any level change introduced by the EQ.

Polarity protection

1N4001 diode on the power rail guards against accidental reverse-polarity connection.

1590A form factor

92 × 38 mm. One of the smallest standard pedal enclosures — careful layout required.

Connection overview

Circuit Theory

Signal path overview

Signal enters at IN through a 1 μF film coupling capacitor (C2) which blocks DC, with a 1M resistor (R2) pulling the node to virtual ground (VR = 4.5 V). The high-impedance input presents minimal load to the preceding effect or instrument.

Stage 1 — Input buffer (IC1A)

IC1A is a unity-gain voltage follower: its output is tied directly to its −IN pin, and the audio signal arrives at +IN via R4 (100k). This buffer presents high input impedance and drives the gyrator network at low impedance. R4 also forms a gentle high-pass corner with C2 well below audio range, removing DC offsets before IC1A.

Stage 2 — Gyrator EQ core (IC2A)

IC2A together with R8 (250k), R5 (470 Ω), C5 (47n) and C6 (220n) synthesises the behaviour of an inductor. The effective inductance is:

L = R8 × R5 × C5

This synthetic inductor in parallel with C6 forms a resonant bandpass filter. The FREQ trimmer varies R8, changing the synthesised inductance and therefore the centre frequency. IC2A is a unity-gain follower within the gyrator loop. The Q trimmer (5–10k) adjusts loop damping: higher resistance raises Q, narrowing the bandwidth.

Stage 3 — Boost/cut blend and output (IC1B)

IC1B sums the direct buffered signal and the gyrator resonant output. The ± pot (10k linear) controls the blend. At centre position the response is flat; towards boost it adds the resonant peak; towards cut a phase-inverted contribution creates a notch at the same frequency. R3 and R10 (both 10k) are the summing resistors. The output passes through C4 (1 μF) to remove the 4.5 V DC bias, then to the PEGEL volume pot (100k linear).

Virtual ground and power supply

R13 and R14 (both 10k) divide +9V to produce VR ≈ 4.5 V. C3 (10 μF) filters this reference node. IC2B buffers VR at very low impedance so the signal path cannot load the reference. D1 (1N4001) provides reverse-polarity protection. C1 (100 μF) decouples the supply rail at the ICs.

Gyrator vs. inductor: A real inductor of several henries would be physically impractical at audio frequencies. The gyrator uses an op-amp and RC network to simulate the same impedance–vs–frequency behaviour — it “looks like” an inductor to the rest of the circuit, with no magnetic component.

Schematic — 1-Band Parametric EQ V1.0

Gyrator Analysis

The gyrator parameters depend on R8 (FREQ trimmer, 250k maximum), R5 (470 Ω fixed), C5 (47n) and C6 (220n). The values below apply to the default component set shipped on the PCB.

Default configuration — bass (144 Hz)

Synthesised inductance

L = R8 × R5 × C5

R8 (max)

250k Ω

470 Ω

47 nF

Inductance L

5522 mH

Rotating FREQ clockwise reduces R8, lowers L, and raises f₀. Counter-clockwise increases R8 and lowers f₀.

Centre frequency

f₀ = 1 / (2π√(L × C6))

f₀

144

Q factor

≈11

¼ oct BW

C6 tank

220

At maximum R8 the centre is 144 Hz with Q ≈ 11. Reducing R8 raises f₀ continuously.

Tuning the FREQ trimmer: Counter-clockwise increases R8, raises L and lowers f₀. Clockwise decreases R8, lowers L and raises f₀. The Q trimmer operates independently.

Frequency presets

Swap C5, C6 and R8 to target a different frequency range. The table lists verified presets from the original design documentation. Feel free to substitute your own values — the Muzique gyrator calculator at muzique.com/lab/gyrator.htm generates component combinations for any target frequency. Q 10 ≈ ¼-octave bandwidth; Q 3–5 ≈ one octave.

Range	R8	C6	C5	f₀	Q
Bass
Low bass	250k	220n	47n	144 Hz	10
Mid bass	250k	100n	22n	313 Hz	10
Upper bass	100k	330n	47n	186 Hz	5
Mids
Low mid	120k	86n	33n	400 Hz	10
Mid	100k	47n	22n	722 Hz	10
Mid	68k	33n	22n	1045 Hz	10
Mid (broad)	68k	220n	22n	400 Hz	4
Mid	47k	150n	15n	715 Hz	3
Upper mid	68k	68n	10n	1080 Hz	4
Upper mid	68k	68n	8n2	1200 Hz	4
Highs
High presence	47k	33n	4n7	2700 Hz	4
Air	47k	47n	2n2	3333 Hz	2

R6 — boost enable

R6 boost resistor: Use 33k for cut-only operation (no boost above unity). Use 56k to allow a modest boost above unity. This resistor is not in the default BOM as it depends on intended use — choose before populating the board.

Bill of Materials

All resistors are ¼W metal film 1%. Colour bands shown are 5-band (3 significant digits + multiplier + 1% tolerance).

Ref	Qty	Value	Colour code	Notes
Resistors
R3, R10, R13, R14	4	10k	Black · Brown · Black \| Orange · Brown	VR divider (R13/R14) and summing resistors (R3/R10)
R2	1	1M	Black · Brown · Black \| Green · Brown	Input pull-down to virtual ground
R4	1	100k	Black · Brown · Black \| Yellow · Brown	IC1A input isolation
R5	1	470R	Black · Yellow · Violet \| Brown · Brown	Gyrator series R — sets L/Q relationship. Do not substitute.
R8	1	250k	Black · Red · Green \| Yellow · Brown	Fixed R in FREQ network
R6	1	33k or 56k		33k = cut only · 56k = boost enabled. See Section 03.
Capacitors
C1	1	100 μF		Polarised electrolytic, 5–8mm Ø, 8mm pitch. 47 μF minimum.
C2, C4	2	1 μF		MLCC or film. Input and output coupling caps.
C3	1	10 μF		Polarised electrolytic, 5mm Ø, 8mm pitch. VR filter.
C5	1	47n		Box film. Gyrator cap — change to tune frequency range.
C6	1	220n		Box film. Tank cap — change to tune frequency range.
Diodes
D1	1	1N4001		Reverse-polarity protection. Cathode stripe toward IC supply rail.
Potentiometers
+/−	1	10k lin		Boost/cut. 9mm Alpha or equivalent. B taper (linear in EU/JP convention).
PEGEL	1	100k lin		Output volume. 9mm Alpha or equivalent. B taper.
Q	1	5k–10k		Bandwidth trimmer. ACP 6mm or Piher PT-6.
FREQ	1	1M		Frequency trimmer. ACP 6mm or Piher PT-6.
Integrated circuits
IC1, IC2	2	TL072		Dual op-amp, DIP-8. Use IC sockets.JRC4558DNE5532
Hardware
IN, OUT	2	¼″ jack		Mono. Input jack switched for true bypass if used.
PWR	1	DC jack		2.1mm barrel, centre-negative (standard pedal PSU convention).

Pot taper note: In European and Japanese convention (Alpha, Bourns), B taper = linear and A taper = logarithmic — the reverse of US convention. Both the ± and Pegel pots must be B (linear) taper.

Build Guide

Work low to high. Populate the shortest components first. All components are through-hole. Use a temperature-controlled iron at 320–350°C.

PCB top view — component placement (left) and layout reference (right)

Resistors

Populate all resistors first. Verify R5 (470 Ω) carefully — its bands look similar to R3/R10/R13/R14 (10k) at a glance. Use a multimeter if unsure.

Diode D1

Insert D1 (1N4001) observing polarity. The cathode stripe on the body must match the stripe on the PCB silkscreen. This diode provides reverse-polarity protection.

IC sockets

Fit 8-pin DIP sockets for IC1 and IC2. Align the notch with the PCB marking. Do not insert the TL072s yet.

Film capacitors (C2, C4, C5, C6)

Box film caps are non-polarised. Populate in value order: C5 (47n), C6 (220n), then C2 and C4 (1 μF). These are the gyrator and coupling capacitors.

Electrolytic capacitors (C1, C3)

Electrolytics are polarised — long lead (positive) to the pad marked + or toward +9V. C1 is 100 μF (supply decoupling), C3 is 10 μF (VR filter).

Trimmers Q and FREQ

Fit both ACP 6mm (or Piher PT-6) trimmers flush to the board. Q (5–10k) and FREQ (1M) will be adjusted during calibration.

Panel pots (± and Pegel)

Insert all pots before soldering to ensure co-planar seating. Verify shaft height against enclosure depth before committing.

Insert ICs

Insert TL072s into sockets last, after all soldering is complete. Align pin 1 (dot or notch on IC body) with the socket notch and PCB marking.

1590A clearance warning: Keep all component heights to a minimum — lay electrolytic caps on their side if necessary. Brian Guerreiro’s Baby Board Guide at madbeanpedals.com is the definitive reference for 1590A builds.

Enclosure & Drilling

Designed for a Hammond 1590A (92 × 38 × 31 mm) die-cast aluminium enclosure.

Recommended reference: Brian Guerreiro’s 1590A build guide at madbeanpedals.com/downloads/BabyBoardGuide.pdf includes drilling templates and detailed layout advice.

Front face: 2 × 7mm pot holes (10mm for Alpha 9mm bodies), 20–22mm centre-to-centre.

Sides: 2 × 9mm for ¼″ mono jacks. Input right, output left by convention.

Top: 1 × 7.5mm for the 2.1mm DC barrel jack.

Shaft length: The effective depth from PCB to enclosure face in a 1590A is typically 15–18mm. Use short-shaft (15mm) pots or trim shafts to match before drilling.

Usage & Variations

Setting up

Set the FREQ trimmer

Clockwise raises frequency; counter-clockwise lowers it. For the default 144 Hz bass voicing set the trimmer fully counter-clockwise.

Set the Q trimmer

Start at mid-range for a musical broad curve. Clockwise narrows the bandwidth (surgical). Counter-clockwise widens it to roughly one octave.

Boost or cut

Centre ± = flat. Clockwise = boost. Counter-clockwise = cut. Use Pegel to compensate overall level after adding boost.

Applications

Bass — low-end boost

Default 144 Hz adds warmth and fundamental weight. Broad Q (3–5) blends naturally; narrow Q (8–10) tightens on a specific note.

Guitar — presence

2.7 kHz or 3.3 kHz preset with broad Q (2–3) adds cut-through presence in a dense mix.

Feedback notch

Narrow Q (8–10) with maximum cut notches out a feedback frequency without affecting the rest of the spectrum.

Mid scoop / boost

400–1 kHz with medium Q shapes a classic mid-scoop or mid-boost as found in many amplifier voicings.

Octaver prep

The original design use-case: set a low-pass gyrator to feed only bass frequencies to an octaver for clean tracking.

Frequency range swap

Replace R8, C5, C6 with values from the presets table. The Muzique calculator generates custom values.

In the effects chain: Before drive stages it shapes the signal going into clipping. After drive it sculpts the post-distortion tone. At the end of the chain it acts as a master EQ. Its ability to boost or cut at a precise frequency makes it versatile in any position.

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Disclaimer & Licence

PCBs purchased from TH Custom Effects are intended for DIY and non-commercial use only. Redistribution of PCBs and artwork from this document is not permitted. You may use these instructions and PCBs to build and sell your own product based on PCBs ordered from TH Custom Effects.

The gyrator parametric EQ topology is based on the work of Jack Orman. Online gyrator calculator: muzique.com/lab/gyrator.htm. This circuit originated from a custom build request.