5-Band EQ — Build Documentation

01

Overview & Features

A compact 5-band graphic equalizer built around the Rohm BA3812L — a single-chip gyrator-based EQ that integrates five bandpass filters and a summing amplifier in one 20-pin SIP package. Each band has ±12 dB of boost or cut via its own 100 kΩ slider/pot, with frequency centres set entirely by external capacitor pairs.

About this kit: This is the standard 5-Band EQ with Rohm's reference centre frequencies — 100 Hz, 300 Hz, 1 kHz, 3 kHz, 10 kHz. A general-purpose hi-fi / pedalboard EQ where the bands sit one octave-and-a-half apart and cover the whole audio range. For a guitar-focused alternative with bands packed into the midrange, see the METAL variant build doc.

Five bands

Each tuned by two caps (C0 and C). Change the caps to move any band anywhere in the audio range — there's no hidden RC network to fight against.

±12dB per band

Centre-detent linear pots cut and boost equally either side of unity gain. Pot at centre = flat response.

Stackable

You can wire the OUT of one board into the IN of another for 10, 15 or more bands — only the chip count and panel space limit you.

Slim profile

The BA3812L is a SIP package — sits flat on the board with all caps under the pots. Designed to fit a 1590B enclosure with five 9 mm Alpha pots in line.

Five 9 mm pots fit a 1590B enclosure across the long axis. The BA3812L SIP package sits on the PCB underneath the pot row.

02

Circuit Theory

The BA3812L combines five gyrator-based bandpass filters with an internal pre-amplifier and a summing output stage. The gyrator topology synthesises an inductor electronically using a capacitor and an op-amp, so the bandpass tank circuits inside the chip behave like ideal LC filters without needing real inductors. Each band's centre frequency and Q are set by two external capacitors (C0 and C) that complete the synthetic-LC resonant tank.

5-Band EQ V1.3 schematic — BA3812L SIP-20 chip with the five frequency-determining cap pairs around it. Note: the schematic image was rendered with the METAL variant cap values printed on it; in this kit C12 = 27n, C9 = 330n, C8 = 8n2, C7 = 100n, C6 = 2n7, C11 = 33n, C10 = 820p, C15 = 10n, C14 = 270p, and C13 = 1µ as in the BOM table below.

Signal path

Audio enters the chip through the input AC-coupling capacitor C2 (10µF) into pin 15. The internal pre-amplifier feeds pin 14 (the GAIN node), which drives R1 (6.8 kΩ) into the summing junction at pin 12 — this is also where C5 (1nF) provides high-frequency stability and where one end of every band's pot is connected. The five band-filter outputs are summed at pin 10 via the wipers of F1–F5; R2 (6.8 kΩ) feeds the summed signal to pin 13 (output stage input). Pin 14 acts as the dry path, and the band filters add or subtract content at their centre frequencies depending on each pot's position. The chip's internal output amplifier drives pin 16, which is AC-coupled out via C3 (10µF) to the OUT jack.

Per-band gyrator tank

Each band uses two external caps connected between two adjacent BA3812L pins:

C0 — the larger cap, sets the resonant centre frequency
C — the smaller cap, controls the Q (selectivity) of the band

f₀ ≈ 1 / (2π × √(R² × C0 × C)) where R ≈ 9.7 kΩ (internal gyrator resistance)

Q ≈ ½ × √(C0 / C)

Because both caps are external, you can move any band anywhere in the audio range simply by swapping the cap pair. See section 06 for guidance on retuning.

Pot wiring per band

Each 100 kΩ linear pot (F1–F5) is wired with pin 1 going to the chip's filter summing node (pin 10) and pin 3 going to the GAIN node (pin 12). The wiper (pin 2) connects through that band's small cap (C) to the BA3812L gyrator pin. Pot at centre = wiper sees neither path dominantly = unity gain (flat). Pot toward pin 1 = filter output adds = boost. Pot toward pin 3 = filter output subtracts = cut.

Power supply

D1 (1N4001) provides reverse polarity protection on the +9 V rail. C1 (100µF) is the main supply bulk decoupling cap. C4 (100µF) decouples the chip's internal bias reference at pin 16 — this is essential for low noise. C2 and C3 (10µF each) AC-couple the input and output. With a 9 V supply, the BA3812L draws around 5–8 mA — this is a low-current, low-noise IC.

Hz

Q factor

3.04

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Reading the Q numbers: A Q of about 3 means each band is ~⅓ octave wide at the −3 dB points. Higher Q = narrower, more surgical band; lower Q = wider, gentler shape. This kit uses Q ≈ 2–3 throughout — typical for a musical graphic EQ where you want the bands to overlap a little, not behave like surgical notches.

04

Bill of Materials

BA3812L availability: The BA3812L is an obsolete 5-band gyrator EQ chip. New stock is hard to find — most BA3812L sold today comes from old-stock distributors. Verify the chip is genuine before powering up. Fortunately the rest of the circuit is just supply decoupling and frequency-determining capacitors, so once you have the chip the build is straightforward.

Ref	Qty	Value	Colour code	Notes
Resistors — Metal film, ¼ W, 1%
R1, R2	2	6k8	Blue · Grey · Black \| Brown · Brown	Set the gain of the pre-amp and output stage paths around the chip
Capacitors — Electrolytic (supply & coupling)
C1, C4	2	100µF		Supply decoupling (C1) and chip bias decoupling (C4) — 16 V or 25 V rated, observe polarity
C2, C3	2	10µF		Input (C2) and output (C3) AC-coupling — 16 V or 25 V rated, observe polarity
Capacitors — Stabilisation
C5	1	1n		Box film cap. The original prototype used 1µF here and worked fine — 1nF is the recommended value but the chip is tolerant.
Capacitors — Frequency-determining (per band)
C13, C12	2	1µ / 27n		F1 band — 100 Hz centre
C9, C8	2	330n / 8n2		F2 band — 300 Hz centre
C7, C6	2	100n / 2n7		F3 band — 1 kHz centre
C11, C10	2	33n / 820p		F4 band — 3 kHz centre
C15, C14	2	10n / 270p		F5 band — 10 kHz centre
Semiconductors
D1	1	1N4001		Reverse polarity protection — observe cathode band orientation
IC1	1	BA3812L		Rohm 5-band gyrator EQ chip, SIP-20 package — obsolete part, see notice above
Pots
F1, F2, F3, F4, F5	5	100k-B		9 mm Alpha PCB-mount, B-taper (linear). Centre-detent versions strongly recommended
Hardware (not on PCB)
Jacks	2	¼″ mono		IN and OUT — wire to the IN/OUT pads on the PCB
DC jack	1	2.1 mm		Centre-negative standard — wire to +9V and GND pads

05

Build Guide

PCB silkscreen — component placement reference. Note the long row of holes between C7..C15 is for the BA3812L SIP-20 package.

Populated PCB — note the SIP-20 BA3812L lying flat in the middle, IC pin 1 at the left.

Populate the board in order from lowest to highest profile. The pots and the BA3812L are the tallest components; do them last so everything else can sit flat against the board.

Resistors and diode

Mount R1 and R2 (both 6k8). Then D1 (1N4001) — polarity matters: the black band on the diode body marks the cathode, match it to the silkscreen.

Film capacitors

Fit C5 first (1n), then all the per-band film caps (C6 through C15). No polarity. Take care to put the correct value in each position — re-check against the BOM as you go. Mistakes here will move band frequencies, but won't damage anything.

Electrolytic capacitors

C1 (100µF), C2 (10µF), C3 (10µF), C4 (100µF). Polarity matters — long lead is positive, the body has a stripe marking the negative side. The PCB silkscreen marks the positive pad with a +.

BA3812L (no socket)

The BA3812L is a SIP-20 package — twenty pins in a single row. Pin 1 is on the left side when the chip is oriented with the marking facing up; this matches the PCB silkscreen. Insert the chip flat against the PCB. Solder one end pin to hold it in place, check it's flush, then solder the rest. SIP packages are usually not socketed — the BA3812L is hard to source so it's worth being careful. If you really want a socket, machined SIP-20 turned-pin sockets exist, but most builders solder directly.

Pots — F1 through F5

The five 9 mm Alpha pots mount along one edge of the PCB. Insert all five before soldering anything — this lets the pot bodies self-align flat against the PCB. Solder one pin of each pot first, check the row is straight and seated, then solder the rest. Centre-detent pots will have a small click at the middle position; this is the flat (unity gain) reference for that band.

External wiring & first power-up

Wire the off-board components: 2 audio jacks (IN, OUT) and the 2.1 mm DC jack. Before plugging in a guitar, apply 9 V and confirm the supply current is reasonable (5–10 mA). A short on the supply will be obvious — the diode will get hot and the regulator on your power supply may shut down. If everything looks normal, plug in a guitar and verify each pot affects the corresponding frequency band.

If you don't need C2/C3: If you are using this EQ inside a larger pedal that already has its own AC-coupling caps at the input and output, you can omit C2 and C3 and short their PCB footprints with a wire jumper. This removes 4 µF of series capacitance from the path and slightly extends low-frequency response. Only do this if you're certain there is no DC offset between the connected stages.

06

Tuning Your Own Bands

Because every band's frequency is set by an external cap pair, you can retune any band simply by swapping its caps. The frequency-determining ref pairs on this PCB are:

F1 — caps C13 (C0) and C12 (C)
F2 — caps C9 (C0) and C8 (C)
F3 — caps C7 (C0) and C6 (C)
F4 — caps C11 (C0) and C10 (C)
F5 — caps C15 (C0) and C14 (C)

Pick a target frequency, then choose a C0 value such that 2π × R × C0 ≈ 1/f₀ with R = 9.7 kΩ. Then pick C ≈ C0 / 36 to get Q ≈ 3. The two value tables below — common values from the Rohm datasheet, and a 10-band suggestion — are starting points. Mix and match across both tables to define your own band set.

Rohm common values (5-band)

Centre	C0	C
100 Hz	1µ	27n
300 Hz	330n	8n2
1 kHz	100n	2n7
3 kHz	33n	820p
10 kHz	10n	270p

10-band suggested set

Centre	C0	C
70 Hz	1µ	68n
100 Hz	1µ	27n
180 Hz	470n	22n
300 Hz	330n	8n2
700 Hz	150n	4n7
1.1 kHz	100n	2n7
1.7 kHz	47n	2n2
3 kHz	33n	820p
4.5 kHz	22n	680p
10 kHz	10n	270p

Order in the schematic ≠ order on the front panel: The frequency bands as labelled on the schematic (F1–F5) are not in any particular numerical-frequency order — they correspond to physical pot positions. If you want the leftmost pot on the panel to be the lowest-frequency band, choose your cap values accordingly. For example, to make F1 the 100 Hz band: install 1µF for C13 and 27nF for C12.

Stacking multiple boards

You can chain the OUT of one 5-Band EQ board into the IN of a second board to create a 10-band EQ. Each board operates independently with its own +9 V supply (or share one supply). The internal AC-coupling caps (C2, C3) on the second board will block any DC offset; if you've jumpered them per the build note above, ensure the first board's output is DC-coupled cleanly.

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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.