TH Custom Effects V2.1 — 2020

3-Band Parametric EQ

Full Control — V2.1

Three independent gyrator-based parametric bands with variable frequency, Q and gain. On-board AC rectifier and ICL 7660S charge pump supply symmetric ±9 V rails.

3× TL072 ICL 7660S 9 V DC supply ±9 V rails Low / Mid / High bands 1590BB

01

Overview

This is the full-control implementation of the 3-Band Parametric EQ. Each band provides independent adjustment of centre frequency, bandwidth (Q) and cut/boost level via three dedicated pots. V2.0 introduced a revised layout with greater spacing between pots for easier panel mounting and operation.

The circuit uses op-amp gyrators to simulate inductors, forming bandpass resonators. An on-board bridge rectifier (polarity protection) and ICL 7660S charge pump provide symmetric ±9 V from a standard 9 V DC supply.

Three independent bands: Low (144 Hz), Mid (398 Hz), High (2720 Hz). Centre frequencies are set by fixed resistors and capacitors; the frequency pots provide a narrow sweep. Enclosure: 1590BB.

02

Schematic

3-Band Parametric EQ V2.1 full schematic. Three gyrator bandpass stages (LOW / MID / HIGH) share common ±9 V rails generated on-board.

03

Circuit Analysis

Signal Path

Signal enters through a 1 μF film coupling cap (C1) with a 1 MΩ pull-down (R1) to virtual ground (VR = +9 V mid-rail). Unity-gain buffer IC1A presents high input impedance and drives the three band-select pots at low impedance.

Each blend pot (LOW_X, MID_X, HIGH_X — all 10 kΩ linear) feeds the corresponding gyrator input from its wiper, with the two ends connecting the input bus to the summing output bus. Rotating the pot from centre progressively reduces the band contribution and increases the dry signal.

The three bandpass outputs sum into inverting amplifier IC1B (R21 = R22 = 10 kΩ, unity gain). Output passes through coupling cap C14 (1 μF) and the master VOL pot to the output jack.

Gyrator Bandpass Stage

Each band uses two op-amp gates (IC3A/B for Low/Mid, IC4A for High). The first gate is a unity-gain buffer with capacitive feedback forming a gyrator — a simulated inductor. Combined with the second capacitor this creates a bandpass resonator. The series Q resistor (R8/R13/R20 = 470 Ω) and Q pot (10 kΩ) control bandwidth. The frequency pot (1 MΩ) provides a narrow frequency sweep around the fixed centre.

Centre frequency and Q are given by:

f₀ = 1 / (2π × √(R_freq × R_series × C₁ × C₂))
Q = ½ × √(C₁/C₂) × (R_series + R_q wiper) / R_series

R_series = 470 Ω for all bands. R_q wiper ranges from 0 to 10 kΩ. Q is minimum (broadest) at pot maximum resistance, maximum (narrowest) at zero.

Calculated Centre Frequencies

Low Band

144Hz

R6 = 250k C4 = 220n C5 = 47n
R8 = 470 Ω
f sweep: 65–144 Hz Q range: 1.1–24

Mid Band

398Hz

R11 = 120k C8 = 86n C9 = 33n
R13 = 470 Ω
f sweep: 130–398 Hz Q range: 0.8–18

High Band

2720Hz

R16 = 47k C11 = 33n C12 = 4n7
R20 = 470 Ω
f sweep: 576–2720 Hz Q range: 1.3–30

C11 correction: The schematic parts list shows C11 = 47n, but all frequency annotations are based on C11 = 33n. Calculation confirms: f₀ = 1/(2π×√(47k×470×33n×4n7)) = 2719 Hz. With 47n the result is 2278 Hz — incorrect. Fit 33n for C11.

Power Supply

9 V DC enters through bridge rectifier B1, which provides polarity protection only — B1 can be omitted if polarity is guaranteed by the supply connector. C2 (100 μF) and C7 (100 μF) filter the supply rail. Schottky diodes D1/D2 and ICL 7660S charge pump (IC2) generate the negative rail; C6 (10 μF) filters the charge pump output. Resistors R17/R19 (2×10 kΩ) divide the +9 V supply to form the virtual ground reference VR. C3 and C13 (100 pF each) suppress HF noise on VR. C10 (100 nF) decouples the supply at the IC power pins.

04

Bill of Materials

PCB component placement reference.

Reference	Qty	Value	Colour Code	Notes
Resistors
R1	1	1M	Brown · Black · Black \| Yellow · Brown	Input pull-down to VR
R2	1	100k	Brown · Black · Black \| Orange · Brown	VR bias divider
R3	1	10k	Brown · Black · Black \| Red · Brown	Input buffer feedback
R6	1	250k	Red · Green · Black \| Orange · Brown	Low band frequency resistor
R8	1	470R	Yellow · Violet · Black \| Black · Brown	Low band Q series resistor
R11	1	120k	Brown · Red · Black \| Orange · Brown	Mid band frequency resistor
R13	1	470R	Yellow · Violet · Black \| Black · Brown	Mid band Q series resistor
R16	1	47k	Yellow · Violet · Black \| Red · Brown	High band frequency resistor
R17	1	10k	Brown · Black · Black \| Red · Brown	Virtual ground divider
R19	1	10k	Brown · Black · Black \| Red · Brown	Virtual ground divider
R20	1	470R	Yellow · Violet · Black \| Black · Brown	High band Q series resistor
R21	1	10k	Brown · Black · Black \| Red · Brown	Summing amp feedback
R22	1	10k	Brown · Black · Black \| Red · Brown	Summing amp input
R23	1	33k	Orange · Orange · Black \| Red · Brown	IC4B feedback
R24	1	33k	Orange · Orange · Black \| Red · Brown	IC4B input
Capacitors — Film / MLCC
C1	1	1 μF		Film box — input coupling
C3	1	100 pF		MLCC — VR HF decoupling
C4	1	220 nF		Film box — Low band gyrator cap
C5	1	47 nF		Film box — Low band resonator cap
C8	1	86 nF		Film box — Mid band gyrator cap
C9	1	33 nF		Film box — Mid band resonator cap
C10	1	100 nF		Film box — +18 V supply decoupling
C11	1	33 nF		Film box — High band gyrator cap 33n not 47n
C12	1	4n7		Film box — High band resonator cap
C13	1	100 pF		MLCC — VR HF decoupling
C14	1	1 μF		MLCC — output coupling
Capacitors — Electrolytic
C2	1	100 μF		Electrolytic — +9 V filter, observe polarity
C6	1	10 μF		Electrolytic — charge pump output, observe polarity
C7	1	100 μF		Electrolytic — +18 V filter, observe polarity
ICs
IC1	1	TL072		Dual op-amp — input buffer (A) + summing amp (B)
IC2	1	ICL 7660S		Charge pump — negative rail MAX1044
IC3	1	TL072		Dual op-amp — Low band (A) + Mid band (B) gyrators
IC4	1	TL072		Dual op-amp — High band gyrator (A) + output stage (B)
Diodes
D1	1	1N5817		Schottky — charge pump diode, observe polarity
D2	1	1N5817		Schottky — charge pump diode, observe polarity
B1	1	Bridge rectifier		Polarity protection — can be omitted if supply polarity is guaranteed
Potentiometers
LOW_F	1	1 MΩ B		Low band frequency sweep (linear)
LOW_Q	1	10 kΩ B		Low band Q / bandwidth
LOW_X	1	10 kΩ B		Low band cut / boost
MID_F	1	1 MΩ B		Mid band frequency sweep (linear)
MID_Q	1	10 kΩ B		Mid band Q / bandwidth
MID_X	1	10 kΩ B		Mid band cut / boost
HIGH_F	1	1 MΩ B		High band frequency sweep (linear)
HIGH_Q	1	10 kΩ B		High band Q / bandwidth
HIGH_X	1	10 kΩ B		High band cut / boost
VOL	1	see PCB		Master output volume
Connectors / Hardware
X1, X2	2	Pad		+9 V power pads
AC1	1	DC connector		9 V DC supply input
TO_EFF / FROM_EFF	2	Pad		Effect loop send / return pads
GND / GND_OUT	2	Pad		Ground pads — input and output

05

Frequency Variants

Centre frequency is determined by the fixed resistor (R) and the two gyrator capacitors (C₁, C₂). Q range scales with the C₁/C₂ ratio and the 470 Ω series resistor. Replace R and/or the cap pair to reposition a band. Both caps must be changed together to maintain the Q range.

Formula: f₀ = 1 / (2π × √(R × 470 × C₁ × C₂)) — all frequencies calculated and verified against schematic annotations.

Q guide: Q < 2 = broad, programme-style shelf. Q 5–10 = classic parametric notch. Q > 15 = surgical cut or feedback notch. The Q pot sweeps the full range continuously.

Bass Band Variants — R6 / C4 / C5

R6	C4	C5	f₀	Q max	Notes
250k	220n	47n	144 Hz	24	Default — deep bass
250k	100n	22n	313 Hz	24	Upper bass / low-mid
100k	330n	47n	186 Hz	30	Sub-bass, higher Q potential

Mid Band Variants — R11 / C8 / C9

R11	C8	C9	f₀	Q max	Notes
120k	86n	33n	398 Hz	18	Default — low-mid
100k	47n	22n	722 Hz	16	Upper-mid presence range
68k	33n	22n	1045 Hz	14	Mid presence
68k	220n	22n	405 Hz	35	Low-mid, high Q potential
47k	150n	15n	714 Hz	35	Upper-mid, high Q potential
68k	68n	10n	1080 Hz	29	Upper-mid notch
68k	68n	8n2	1192 Hz	32	Upper-mid / low-treble

High Band Variants — R16 / C11 / C12

R16	C11	C12	f₀	Q max	Notes
47k	33n	4n7	2720 Hz	30	Default — presence / upper-mid
47k	47n	2n2	3330 Hz	51	Air / brilliance range, very high Q

Note on Q values: Q max figures assume Q pot at minimum resistance (fully clockwise). Q min (pot at maximum, 10 kΩ) is given in the Circuit Analysis section filter cards. The 470 Ω series resistor sets the absolute Q floor; reducing it below 470 Ω is not recommended.

06

Build Steps

1

Rectifier (bottom side). Fit bridge rectifier B1 on the underside of the PCB. B1 provides polarity protection only and can be omitted if the supply connector guarantees correct polarity.

2

Diodes. Fit D1 and D2 (1N5817 Schottky). Cathode band matches PCB silkscreen stripe.

3

Resistors. Fit all 15 resistors. Pay particular attention to: R6 (250k), R11 (120k), R16 (47k) — band centre frequency; R8, R13, R20 (all 470 Ω) — Q baseline.

4

IC sockets. Fit four DIP-8 sockets for IC1–IC4. Align the notch to silkscreen pin-1 marker.

5

Small capacitors (MLCC / ceramic). Fit C3, C13 (100 pF) and C14 (1 μF MLCC). No polarity.

6

Film capacitors. Fit C1 (1 μF), C4 (220n), C5 (47n), C8 (86n), C9 (33n), C10 (100n), C11 (33n — not 47n), C12 (4n7). No polarity.

7

Electrolytic capacitors. Fit C2 (100 μF), C6 (10 μF), C7 (100 μF). Longer lead to + as marked.

8

ICs. Insert IC1, IC3, IC4 (TL072) and IC2 (ICL 7660S or MAX1044) in their sockets. Pin 1 notch to silkscreen marker.

9

Potentiometers. All pots mount from the rear (solder side) of the PCB. Fit all nine EQ pots and the VOL master pot. Confirm shaft direction before soldering.

10

Connectors and wiring. Solder the DC supply connector (AC1) and +9 V pads X1/X2. Wire effect loop pads TO_EFF / FROM_EFF to jacks or bypass switching as required. Connect GND and GND_OUT to enclosure ground.

11

Test before powering with ICs. With ICs removed, check for shorts between +18 V, VR (+9 V), GND, and the −9 V rail with a multimeter. Power up and verify rail voltages. Insert ICs, repower and test signal path.

07

Build Photos

Completed PCB — component side

Completed PCB — pot side

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