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F22X REV A

Interactive Assembly, Calibration Guide & Bill of Materials for the 500 / 51X Series Equalizer Kit

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1. Introduction

Welcome to the build manual for the F22X Filter Module.

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The F22X is a high-performance active High Pass / Low Pass filter module engineered for the popular 500 / 51X series rack format (+16V / -16V rails). Built around precision Sallen-Key filter topology, it combines flexible tone-shaping, continuously variable Q-factor/resonance, and dry/wet blend control with high-grade balancing stages.

In this revision, a dedicated Master Output Trim control has been added to give you full flexibility when driving and coloring the sound through the filter circuit.

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Key Features of the F22X:

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  • Flexible Sallen-Key Filters: Independent High Pass (HPF) and Low Pass (LPF) filter sections with switchable range multipliers.

  • Continuously Variable Controls: Dedicated Frequency, Resonance (Q-Factor), and Blend (Dry/Wet) controls per filter section.

  • Master Output Trim Control: Clean level adjustment in a range of ±20 dB Gain.

  • High-Grade Interfacing: THAT 1246 balanced line receiver input stage and THAT 1646 balanced driver output stage.

  • True-Bypass Relay Switching: Silent hardware circuit engagement when bypassed.

  • Streamlined Multi-Board Layout: Engineered for intuitive assembly and precise mechanical front-panel fitment.

 

This manual will guide you step-by-step through component selection, board assembly, initial voltage checks, and precision alignment. Please take your time, read all instructions carefully before soldering, and double-check component polarities at each stage.

Finished Module: F22X Rev A 500-series filter module featuring black aluminum front panel

2. Step-by-Step Assembly Guide

2.1 Resistors and Diodes

  • Place and solder all low-profile resistors on the main board and subboards according to the Bill of Materials.

  • Install all diodes.

  • Important: Double check the orientation and polarity (cathode stripe matching PCB silkscreen) of all diodes before soldering.

Main PCB with color-coded resistors and diodes positioned and soldered

2.2 IC Sockets and Ceramic Capacitors

  • Install all IC sockets (ensure notch orientation matches the PCB silkscreen).

  • Solder low-profile ceramic capacitors (100nF, etc.).

PCB assembly showing low profile ceramic capacitors

2.3 Film Capacitors and Inter-Board Connectors

  • Place and solder all WIMA film capacitors.

  • Install the PCB board connectors as shown in the reference photos:

    • On the small subboard, ensure the short pins are seated flush against the PCB before soldering.

  • Crucial: Press the headers firmly onto the board during soldering to guarantee they sit perfectly straight and level. This is vital for flawless mechanical alignment during final assembly.

PCB assembly showing WIMA capacitors, connectors and bridge rectifier

2.4 Relays, Voltage Regulators and Electrolytics

  • Solder the signal relays and voltage regulator.

  • Install the electrolytic capacitors:

    • Attention: CP1, CP2, CT3, and CT4 are polarized. Verify correct alignment (+ / -) before soldering.

    • All other electrolytic capacitors in this kit are bipolar or non-polarized, so orientation does not matter for those.

PCB assembly showing electrolytic capacitors, relais and potentiometers

2.5 Board Cleaning and Trimpots

  • Clean the PCB thoroughly using Isopropyl Alcohol or flux remover to clear away all solder residue.

  • Allow to dry, then install and solder the trimpots (0DbTrim, HP_RESOTRIM1, LP_RESOTRIM1).

  • Warning: Do NOT clean the PCB with liquids AFTER installing trimpots, potentiometers, or switches. Solvents can penetrate component casings, wash away internal conductive grease, and ruin electrical contact permanently.

2.6 Potentiometers Switches and Subboard Assembly

Potentiometer Taper Guide

Before soldering, verify the correct placement of all four potentiometer types included in this kit:

  • 2x A10K (Logarithmic): Used for HPF & LPF Resonance (Q-Factor).

  • 2x B10K (Linear): Used for HPF & LPF Blend controls.

  • 2x C10K (Reverse Logarithmic): Used for HPF & LPF Frequency sweeps.

  • 1x B50K (Linear with Center Detent): Used for Output Trim to feel the 0 dB center position.

 

Lower Subboard:

  • Trim all pins of previously soldered pin rows on the bottom of the lower subboard as short and flush as possible to prevent shorts in your rack.

  • Insert all potentiometers into the subboard simultaneously. Verify that C10K, A10K, B10K, and the B50K Center Detent Trim pot are in their designated locations.

  • Solder only one middle pin per potentiometer first.

  • Press each potentiometer flat against the board and verify its body is perfectly aligned with the printed silkscreen outlines.

  • Attach the mounting bracket to the pots, tighten the nuts, and solder all remaining pins.

  • Trim all potentiometer pins short, then temporarily remove the mounting bracket.

 

Upper Subboard:

  • Trim all pre-soldered leads on the underside as short as possible.

  • Insert all potentiometers and switches. Secure each part with a single solder joint first.

  • Check visual alignment against the silkscreen, install the blank bracket, and solder all remaining pins. Trim all leads short.

2.7 Front Panel Fitment and LED Alignment

  • Attach the front panel to the subboard assembly using the provided spacers and hardware or clamps to align pushbuttons and switches within their panel cutouts.

  • Rearrange and center pushbuttons/switches in the panel cutouts, solder all switch pins completely, then trim their leads short.

  • Prepare LEDs: Trim LED leads at slightly different lengths so you can easily distinguish the anode (longer leg, +) from the cathode (shorter leg, -).

  • Solder LEDs: Insert LEDs into position so they sit flush with the front panel surface (using masking tape on the front panel helps hold them flush). Solder one leg first to adjust depth and height, verify fitment, then solder the second leg.

  • Disassemble the front panel and mounting bracket once alignment is complete to prepare for joining the boards.

3. Final Assembly and Voltage Check

3.1 Board Joining

  • Connect the mainboard and subboard using two M3 screws and nuts as temporary height spacers in the corners (or connect via ribbon cables depending on board revision).

  • Solder one pin per header row first, press the boards tightly together, then solder all remaining header pins and trim flush.

  • Remove temporary corner spacer screws.

3.2 Pre Flight Power On and Voltage Check without ICs

  • Ensure NO ICs are inserted into any sockets.

  • Power the unit via your 500 series rack or test rig using an extension jig/cable.

  • Set your multimeter to DC Volts and measure supply voltages at the socket pins relative to chassis ground:

  • NE5532 sockets: Supply rails equal ~ ±15.3 Volts DC (due to diode drop)

  • THAT 1246 socket: Supply rails equal ~ ±15.3 Volts DC

  • THAT 1646 socket: Supply rails equal ~ ±15.3 Volts DC

  • Verify that status LEDs light up and relays respond to switching commands properly (listen for relay clicks).

  • Power off the rack.

3.3 IC Installation and Hardware Assembly

  • Once correct power rails of approximately ±15.3 V DC are confirmed, insert all ICs (NE5532, THAT1246, THAT1646) into their respective sockets. Pay strict attention to the Pin 1 notch orientation on every IC package.

  • Re-attach mechanical hardware: brackets, ribbon cables, subboards, 24 millimeter inter-board spacers, and the front panel using potentiometer nuts and M3 front hardware.

  • Fit the color-coded knob caps to their respective controls:

    • Blue for HPF Band

    • Red for LPF Band

    • Grey for Master Output Trim

4. Calibration Guide

Calibration is straightforward and requires playing sine waves into the unit while measuring the output level.

 

Test Requirements:

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  • Headroom: Keep at least 20 dB of headroom on your audio interface to avoid clipping during boost adjustments.

  • Software: Any DAW with an oscillator and level meter works, but REW (Room EQ Wizard) is highly recommended.

  • Reference Level: Max level reference: +6 dBu (1.545 V RMS). Test signal level: -12 dBu (0.195 V RMS).

 

Recommended Resources:

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Initial Check:

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  1. Set all front panel gain and frequency controls to their center/detent positions.

  2. Engage the EQ in circuit.

  3. Send a White Noise signal through the unit and verify using an oscilloscope, analyzer, or monitoring system that all bands boost, cut, and sweep correctly.

  4. Switch generator output to a Sine Wave to begin precision calibration.

 

Output Trim Calibration (0DbTrim / 10k 0DbTrim):

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  1. Set all controls to center detent position. Set all filter bands to OFF.

  2. Set front panel Output Trim knob to center detent (0 dB position).

  3. Switch main Filter section IN.

  4. Send a 1 kHz Sine Wave at +6 dBu (1.545 V RMS) into the module input.

  5. Connect multimeter probe to test point TP1 and GND (reads ~1.32 V AC).

  6. Move probe to test point TP4 and GND.

  7. Adjust the 0DbTrim trimpot until the voltage at TP4 matches the input level exactly (unity gain / match input and output in REW).

 

High Pass Filter (HPF) Resonance Calibration (HP_RESOTRIM1):

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  1. Set front panel controls:

    • HPF Blend: Fully clockwise (100% Filtered)

    • HPF Resonance: Fully clockwise (Maximum Q)

  2. Set sine oscillator frequency to 80 Hz.

  3. Switch HPF ON and monitor the output level.

  4. Sweep the HPF Frequency knob on the unit until you locate the maximum output level peak.

  5. Fine-tune the generator frequency slightly to lock onto the exact resonance peak.

  6. Adjust the HP_RESOTRIM1 trimpot until the peak output reads exactly +15 dB of boost relative to the bypassed level.

 

Low Pass Filter (LPF) Resonance Calibration (LP_RESOTRIM1):

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  1. Set front panel controls:

    • LPF Blend: Fully clockwise (100% Filtered)

    • LPF Resonance: Fully clockwise (Maximum Q)

  2. Set sine oscillator frequency to 8 kHz.

  3. Switch LPF ON and monitor the output level.

  4. Sweep the LPF Frequency knob on the unit until you locate the maximum output level peak.

  5. Fine-tune the generator frequency slightly to lock onto the exact resonance peak.

  6. Adjust the LP_RESOTRIM1 trimpot until the peak output reads exactly +15 dB of boost relative to the bypassed level.

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5. Interactive Bom

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